COURSE CONTENT

Computer Aided Design And Manufacturing

Module Topic Page

Module E – Computer Numerical Control of Machine Tools

Module E(1) An Overview of CNC Machines

Module E(2) Classification of CNC Machines

Module E(3) Major Components of a CNC System

Module E(4) CNC System (Electrical Components)

Module E(5) CNC System (Mechanical Components)

Module E(6) CNC Tooling

ModuleE(7) CNC Workholding

Module F – CNC Part Programming and CNC Software

Module

F(1) CNC Part Programming I

Module

F(2) CNC Part Programming II

Module

F(3) CNC Part Programming III

Module

F(4) Machining of Free-form Geometries

Module

F(5) CNC Program Verification Methods

Module G – CAD/CAM Applications

Module

G(1) Computer Aided Assembly Planning

Module

G(2) Computer Aided Inspection

Module

G(3) Reverse Engineering

Module

G(4) Rapid Prototyping

Module

G(5) Computer Aided Process Planning

AN OVERVIEW OF CNC MACHINES

( 1 ) Historical Perspective

The word NC which stands for numerical control refer to control of a machine or a

process using symbolic codes consisting of characters and numerals. The word CNC

came into existence in seventies when microprocessors and microcomputers replaced

integrated circuit IC based controls used for NC machines. The development of

numerical control owes much to the United States air force. The concept of NC was

proposed in the late 1940s by John Parsons who recommended a method of automatic

machine control that would guide a milling cutter to produce a curvilinear motion in

order to generate smooth profiles on the work-pieces. In 1949, the U.S Air Force

awarded Parsons a contract to develop new type of machine tool that would be able to

speed up production methods.

Parsons sub-contracted the Massachusetts Institute of Technology (MIT) to develop a

practical implementation of his concept. Scientists and engineers at M.I.T built a

control system for a two axis milling machine that used a perforated paper tape as the

input media. This prototype was produced by retrofitting a conventional tracer mill

with numerical control servomechanisms for the three axes of the machine. By 1955,

these machines were available to industries with some small modifications.

The machine tool builders gradually began developing their own projects to introduce

commercial NC units. Also, certain industry users, especially airframe builders,

worked to devise numerical control machines to satisfy their own particular

production needs. The Air force continued its encouragement of NC development by

sponsoring additional research at MIT to design a part programming language that

could be used in controlling N.C. machines.

In a short period of time, all the major machine tool manufacturers were producing

some machines with NC, but it was not until late 1970s that computer-based NC

became widely used. NC matured as an automation technology when electronics

industry developed new products. At first, miniature electronic tubes were developed,

but the controls were big, bulky, and not very reliable. Then solid-state circuitry and

eventually modular or integrated circuits were developed. The control unit became

smaller, more reliable, and less expensive.

(2) Computer Numerical Control

Computer numerical control (CNC) is the numerical control system in which a dedicated

computer is built into the control to perform basic and advanced NC functions. CNC controls are

also referred to as soft-wired NC systems because most of their control functions are

implemented by the control software programs. CNC is a computer assisted process to control

general purpose machines from instructions generated by a processor and stored in a memory

system. It is a specific form of control system where position is the principal controlled variable.

All numerical control machines manufactured since the seventies are of CNC type. The computer

allows for the following: storage of additional programs, program editing, running of program

from memory, machine and control diagnostics, special routines, inch/metric,

incremental/absolute switchability.

CNC machines can be used as stand alone units or in a network of machines such as flexible

machine centres. The controller uses a permanent resident program called an executive program

to process the codes into the electrical pulses that control the machine. In any CNC machine,

executive program resides in ROM and all the NC codes in RAM. The information in ROM is

written into the electronic chips and cannot be erased and they become active whenever the

machine is on. The contents in RAM are lost when the controller is turned off. Some use special

type of RAM called CMOS memory, which retains its contents even when the power is turned

off.

Figure 21.1: CNC milling machine

( 1.3 ) Direct Numerical Control

In a Direct Numerical Control system (DNC), a mainframe computer is used to coordinate the

simultaneous operations of a number NC machines as shown in the figures 21.2 & 21.3. The

main tasks performed by the computer are to program and edit part programs as well as

download part programs to NC machines. Machine tool controllers have limited memory and a

part program may contain few thousands of blocks.So the program is stored in a separate

computer and sent directly to the machine, one block at a time.

First DNC system developed was Molins System 24 in 1967 by Cincinnati Milacron and General

Electric. They are now referred to as flexible manufacturing systems (FMS). The computers that

were used at those times were quite expensive.

Figure 21.2: DNC system

Figure 21.3: DNC system

21.4 Advantages & Disadvantages of CNC machine tools

Figure 21.4 (a) Manually operated milling

Figure 21.4 (b) Computer controlled

machine milling machine

Some of the dominant advantages of the CNC machines are:

CNC machines can be used continuously and only need to be switched off for occasional

maintenance.

These machines require less skilled people to operate unlike manual lathes / milling

machines etc.

CNC machines can be updated by improving the software used to drive the machines.

Training for the use of CNC machines can be done through the use of 'virtual software'.

The manufacturing process can be simulated virtually and no need to make a prototype or

a model. This saves time and money.

Once programmed, these machines can be left and do not require any human

intervention, except for work loading and unloading.

These machines can manufacture several components to the required accuracy without

any fatigue as in the case of manually operated machines.

Savings in time that could be achieved with the CNC machines are quite significant.

Some of the disadvantages of the CNC machines are:

CNC machines are generally more expensive than manually operated machines.

The CNC machine operator only needs basic training and skills, enough to supervise

several machines.

Increase in electrical maintenance, high initial investment and high per hour operating

costs than the traditional systems.

Fewer workers are required to operate CNC machines compared to manually operated

machines. Investment in CNC machines can lead to unemployment.

( 5 ) Applications of NC/CNC machine tools

CNC was initially applied to metal working machinery: Mills, Drills, boring machines, punch

presses etc and now expanded to robotics, grinders, welding machinery, EDM's, flame cutters

and also for inspection equipment etc. The machines controlled by CNC can be classified into

the following categories: CNC mills and machining centres.

CNC lathes and turning centers

CNC EDM

CNC grinding machines

CNC cutting machines (laser, plasma, electron, or flame)

CNC fabrication machines (sheet metal punch press, bending machine, or press brake)

CNC welding machines

CNC coordinate measuring machines

CNC Coordinate Measuring Machines:

A coordinate measuring machine is a dimensional measuring device, designed to move the

measuring probe to determine the coordinates along the surface of the work piece. Apart from

dimensional measurement, these machines are also used for profile measurement, angularity,

digitizing or imaging.

A CMM consists of four main components: the machine, measuring probe, control system and

the measuring software. The control system in a CMM performs the function of a live interaction

between various machine drives, displacement transducers, probing systems and the peripheral

devices. Control systems can be classified according to the following groups of CMMs.

1. Manually driven CMMs

2. Motorized CMMs with automatic probing systems

3. Direct computer controlled (DCC) CMMs

4. CMMs linked with CAD, CAM and FMS etc.

The first two methods are very common and self explanatory. In the case of DCC CMMs, the

computer control is responsible for the movement of the slides, readout from displacement

transducers and data communication. CMM are of different configurations-fixed bridge, moving

bridge, cantilever arm figure 21.5(a), horizontal arm and gantry type CMM as shown in figure

21.5(b).

Figure 21.5(a) Cantilever type CMM

Figure 21.5(b) Gantry type CMM

(6 ) CNC welding machines:

Figure 21.6 4 axis CNC Tig welding machine

The salient features of CNC welding machines are:

Superior quality and weld precision.

These machines are also equipped with rotary tables.

Weld moves, welding feed rate, wire feed, torch heights & welding current can be

programmed.

CNC welding machines are used for laser welding, welding of plastics, submerged arc

welding, wire welding machines, butt welding, flash butt welding etc.

These machines are generally used in automobile work shops

Cost of these machines will be twice than the conventional welding machines.

CNC EDM & WEDM machines:

EDM is a nontraditional machining method primarily used to machine hard metals that could not

be machined by traditional machining methods. Material removal will be taking place by a series

of electric arcs discharging across the gap between the electrode and the work piece. There are

two main types- ram EDM & wire cut EDM. In wire-cut EDM, a thin wire is fed through the

work piece and is constantly fed from a spool and is held between upper and lower guides. These

guides move in the x-y plane and are precisely controlled by the CNC. Wire feed rate is also

controlled by the CNC.

Figure 21.6 (a) Ram EDM Figure 21.6 (b) Wire cut EDM

CLASSIFICATION OF CNC MACHINE TOOLS

( 1) Based on the motion type ' Point-to-point & Contouring systems

There are two main types of machine tools and the control systems required for use with them

differ because of the basic differences in the functions of the machines to be controlled. They are

known as point-to-point and contouring controls.

( 1.1) Point-to-point systems

Some machine tools for example drilling, boring and tapping machines etc, require the cutter and

the work piece to be placed at a certain fixed relative positions at which they must remain while

the cutter does its work. These machines are known as point-to-point machines as shown in

figure 22.1 (a) and the control equipment for use with them are known as point-to-point control

equipment. Feed rates need not to be programmed. In theses machine tools, each axis is driven

separately. In a point-to-point control system, the dimensional information that must be given to

the machine tool will be a series of required position of the two slides. Servo systems can be

used to move the slides and no attempt is made to move the slide until the cutter has been

retracted back.

( 1.2) Contouring systems (Continuous path systems)

Other type of machine tools involves motion of work piece with respect to the cutter while

cutting operation is taking place. These machine tools include milling, routing machines etc. and

are known as contouring machines as shown in figure 22.1 (b) and the controls required for their

control are known as contouring control.

Contouring machines can also be used as point-to-point machines, but it will be uneconomical to

use them unless the work piece also requires having a contouring operation to be performed on

it. These machines require simultaneous control of axes. In contouring machines, relative

positions of the work piece and the tool should be continuously controlled. The control system

must be able to accept information regarding velocities and positions of the machines slides.

Feed rates should be programmed.

Figure 22.1 (a) Point-to-point system Figure 22.1 (b) Contouring system

Figure 22.1 (c) Contouring systems

22.2 Based on the control loops ' Open loop & Closed loop systems

22.2.1 Open loop systems: Programmed instructions are fed into the c

ontroller through an input device. These instructions are then converted to electrical pulses

(signals) by the controller and sent to the servo amplifier to energize the servo motors. The

primary drawback of the open-loop system is that there is no feedback system to check whether

the program position and velocity has been achieved. If the system performance is affected by

load, temperature, humidity, or lubrication then the actual output could deviate from the desired

output. For these reasons the open -loop system is generally used in point-to-point systems where

the accuracy requirements are not critical. Very few continuous-path systems utilize open-loop

control.

Figure 22.2 (a) Open loop control system Figure 22.2 (b) Closed loop control system

Courtesy: http://jjjtrain.kanabco.com/vms/Media/glossary_o/cnc_opencloseloop.gif

Courtesy: http://jjjtrain.kanabco.com/vms/Media/glossary_o/cnc_opencloseloop.gif

Figure 22.2 (c) Open loop system

22.2.1 Closed loop systems:

The closed-loop system has a feedback subsystem to monitor the actual output and correct any

discrepancy from the programmed input. These systems use position and velocity feed back. The

feedback system could be either analog or digital. The analog systems measure the variation of

physical variables such as position and velocity in terms of voltage levels. Digital systems

monitor output variations by means of electrical pulses. To control the dynamic behavior and the

final position of the machine slides, a variety of position transducers are employed. Majority of

CNC systems operate on servo mechanism, a closed loop principle. If a discrepancy is revealed

between where the machine element should be and where it actually is, the sensing device

signals the driving unit to make an adjustment, bringing the movable component to the required

location.

Closed-loop systems are very powerful and accurate because they are capable of monitoring

operating conditions through feedback subsystems and automatically compensating for any

variations in real-time.

Figure 22.2 (d) Closed loop system

(3 ) Based on the number of axes ' 2, 3, 4 & 5 axes CNC machines.

( 3.1) 2& 3 axes CNC machines:

CNC lathes will be coming under 2 axes machines. There will be two axes along which motion

takes place. The saddle will be moving longitudinally on the bed (Z-axis) and the cross slide

moves transversely on the saddle (along X-axis). In 3-axes machines, there will be one more

axis, perpendicular to the above two axes. By the simultaneous control of all the 3 axes, complex

surfaces can be machined.

( 3.2 ) 4 & 5 axes CNC machines:

4 and 5 axes CNC machines provide multi-axis machining capabilities beyond the standard 3-

axis CNC tool path movements. A 5-axis milling centre includes the three X, Y, Z axes, the A

axis which is rotary tilting of the spindle and the B-axis, which can be a rotary index table.

Figure 22.3 Five axes CNC machine

Importance of higher axes machining :

Reduced cycle time by machining complex components using a single setup. In addition to time

savings, improved accuracy can also be achieved as positioning errors between setups are

eliminated.

Improved surface finish and tool life by tilting the tool to maintain optimum tool to part

contact all the times.

Improved access to under cuts and deep pockets. By tilting the tool, the tool can be made

normal to the work surface and the errors may be reduced as the major component of

cutting force will be along the tool axis.

Higher axes machining has been widely used for machining sculptures surfaces in

aerospace and automobile industry.

(3.3) Turning centre:

Traditional centre lathes have horizontal beds. The saddle moves longitudinally and the cross

slide moves transversely. Although the tools can be clearly seen, the operator must lean over the

tool post to position them accurately. Concentration of chips may be creating a heat source and

there may be temperature gradients in the machine tool. Keeping the above points in view,

developments in the structure of the turning centres lead to the positioning the saddle and the

cross slide behind the spindle on a slant bed as shown in the figure 22.4. Chips fall freely

because of slant bed configuration which is more ergonomically acceptable from operator's point

of view.

Figure 22.4 Slant bed turning centre

22.4 Based on the power supply ' Electric, Hydraulic & Pneumatic systems

Mechanical power unit refers to a device which transforms some form of energy to mechanical

power which may be used for driving slides, saddles or gantries forming a part of machine tool.

The input power may be of electrical, hydraulic or pneumatic.

22.4.1 Electric systems:

Electric motors may be used for controlling both positioning and contouring machines. They

may be either a.c. or d.c. motor and the torque and direction of rotation need to be controlled.

The speed of a d.c. motor can be controlled by varying either the field or the armature supply.

The clutch-controlled motor can either be an a.c. or d.c. motor. They are generally used for small

machine tools because of heat losses in the clutches. Split field motors are the simplest form of

motors and can be controlled in a manner according to the machine tool. These are small and

generally run at high maximum speeds and so require reduction gears of high ratio. Separately

excited motors are used with control systems for driving the slides of large machine tools.

22.4.2 Hydraulic systems:

These hydraulic systems may be used with positioning and contouring machine tools of all sizes.

These systems may be either in the form of rams or motors. Hydraulic motors are smaller than

electric motors of equivalent power. There are several types of hydraulic motors. The advantage

of using hydraulic motors is that they can be very small and have considerable torque. This

means that they may be incorporated in servosystems which require having a rapid response.

( 1 ) Different components related to CNC machine tools

Any CNC machine tool essentially consists of the following parts:

( 1.1 ) Part program:

A part program is a series of coded instructions required to produce a part. It controls the

movement of the machine tool and on/off control of auxiliary functions such as spindle rotation

and coolant. The coded instructions are composed of letters, numbers and symbols.

( 1.2 ) Program input device:

The program input device is the means for part program to be entered into the CNC control.

Three commonly used program input devices are punch tape reader, magnetic tape reader, and

computer via RS-232-C communication.

( 1.3 ) Machine Control Unit:

The machine control unit (MCU) is the heart of a CNC system. It is used to perform the

following functions:

To read the coded instructions.

To decode the coded instructions.

To implement interpolations (linear, circular, and helical) to generate axis motion

commands.

To feed the axis motion commands to the amplifier circuits for driving the axis

mechanisms.

To receive the feedback signals of position and speed for each drive axis.

To implement auxiliary control functions such as coolant or spindle on/off and tool

change.

( 1.4 ) Drive System:

A drive system consists of amplifier circuits, drive motors, and ball lead-screws. The MCU feeds

the control signals (position and speed) of each axis to the amplifier circuits. The control signals

are augmented to actuate drive motors which in turn rotate the ball lead-screws to position the

machine table.

( 1.5 ) Machine Tool:

CNC controls are used to control various types of machine tools. Regardless of which type of

machine tool is controlled, it always has a slide table and a spindle to control of position and

speed. The machine table is controlled in the X and Y axes, while the spindle runs along the Z

axis.

( 1.6 ) Feed Back System:

The feedback system is also referred to as the measuring system. It uses position and speed

transducers to continuously monitor the position at which the cutting tool is located at any

particular instant. The MCU uses the difference between reference signals and feedback signals

to generate the control signals for correcting position and speed errors.

( 2 ) Machine axes designation

Machine axes are designated according to the "right-hand rule", When the thumb of right hand

points in the direction of the positive X axis, the index finger points toward the positive Y axis,

and the middle finger toward the positive Z axis. Figure 10 shows the right-hand rule applied to

vertical machines, while Figure 23.1 applies to horizontal machines.

Figure 23.1: Right hand rule for vertical and horizontal machine

CNC SYSTEMS - ELECTRICAL COMPONENTS

(1) Power units

In machine tools, power is generally required for

For driving the main spindle

For driving the saddles and carriages.

For providing power for some ancillary units.

The motors used for CNC system are of two kinds

Electrical - AC , DC or Stepper motors

Fluid - Hydraulic or Pneumatic

Electric motors are by far the most common component to supply mechanical input to a linear motion system.

Stepper motors and servo motors are the popular choices in linear motion machinery due to their accuracy and

controllability. They exhibit favourable torque-speed characteristics and are relatively inexpensive.

(1.1) Stepper motors

Stepper motors convert digital pulse and direction signals into rotary motion and are easily

controlled. Although stepper motors can be used in combination with analog or digital feedback

signals, they are usually used without feedback (open loop). Stepper motors require motor

driving voltage and control electronics. The rotor of a typical hybrid stepper motor has two soft

iron cups that surround a permanent magnet which is axially magnetized. The rotor cups have 50

teeth on their surfaces and guide the flux through the rotor- stator air gap. In most cases, the teeth

of one set are offset from the teeth of the other by one-half tooth pitch for a two phase stepper

motor.

Figure 24.1 Unipolar and Bipolar Stepper Motor

The stator generally has the same number of teeth as the rotor, but can have two fewer depending

upon the motor's design. When the teeth on the stator pole are energized with North polarity, the

corresponding teeth on the rotor with South polarity align with them. Similarly, teeth on the

stator pole energized with South polarity attract corresponding teeth on the rotor that are

energized with North polarity. By changing the polarity of neighbouring stator teeth one after the

other in a rotating sequence, the rotor begins to turn correspondingly as its teeth try to align

themselves with the stator teeth. The strength of the magnetic fields can be precisely controlled

by the amount of current through the windings, thus the position of the rotor can be precisely

controlled by these attractive and repulsive forces.

There are many advantages to using stepper motors. Since maximum dynamic torque occurs at

low pulse rates (low speeds), stepper motors can easily accelerate a load. Stepper motors have

large holding torque and stiffness, so there is usually no need for clutches and brakes (unless a

large external load is acting, such as gravity). Stepper motors are inherently digital. The number

of pulses determines position while the pulse frequency determines velocity. Additional

advantages are that they are inexpensive, easily and accurately controlled, and there are no

brushes to maintain. Also, they offer excellent heat dissipation, and they are very stiff motors

with high holding torques for their size. The digital nature of stepper motors also eliminates

tuning parameters.

There are disadvantages associated with stepper motors. One of the largest disadvantages is that

the torque decreases as velocity is increased. Because most stepper motors operate open loop

with no position sensing devices, the motor can stall or lose position if the load torque exceeds

the motor's available torque. Open loop stepper motor systems should not be used for high-

performance or high-load applications, unless they are significantly derated. Another drawback

is that damping may be required when load inertia is very high to prevent motor shaft oscillation

at resonance points. Finally, stepper motors may perform poorly in high-speed applications. The

maximum steps/sec rate of the motor and drive system should be considered, carefully.

( 1.2) Servo Motors

Servo motors are more robust than stepper motors, but pose a more difficult control problem.

They are primarily used in applications where speed, power, noise level as well as velocity and

positional accuracy are important. Servo motors are not functional without sensor feedback.

They are designed and intended to be applied in combination with resolvers, tachometers, or

encoders (closed loop). There are several types of servo motors, and three of the more common

types are described as follows. The DC brush type servo motors are most commonly found in

low-end to mid-range CNC machinery. The "brush" refers to brushes that pass electric current to

the rotor of the rotating core of the motor. The construction consists of a magnet stator outside

and a coil rotor inside. A brush DC motor has more than one coil. Each coil is angularly

displaced from one another so when the torque from one coil has dropped off, current is

automatically switched to another coil which is properly located to produce maximum torque.

The switching is accomplished mechanically by the brushes and a commutator as shown below.

There are distinct advantages to using DC brush servo motors. They are very inexpensive to

apply. The motor commutates itself with the brushes and it appears as a simple, two-terminal

device that is easily controlled. Among the disadvantages it is the fact that they are thermally

inefficient, because the heat must dissipate through the external magnets. This condition reduces

the torque to volume ratio, and the motor performance may suffer inefficiencies. Also, the

brushed motor will require maintenance, as the brushes will wear and need replacement. Brushed

servo motors are usually operated under 5000 rpm.

The DC brushless type offers a higher level of performance. They are often referred to as "inside

out" DC motors because of their design. The windings of a brushless motor are located in the

outer portion of the motor (stator), and the rotor is constructed from permanent magnets as

shown below. DC brushless motors are typically applied to high-end CNC machinery, but the

future may see midrange machinery use brushless technology due to the narrowing cost gap.

AC servo motors are another variety that offers high-end performance. Their physical

construction is similar to that of the brushless DC motor; however, there are no magnets in the

AC motor. Instead, both the rotor and stator are constructed from coils. Again, there are no

brushes or contacts anywhere in the motor which means they are maintenance-free. They are

capable of delivering very high torque at very high speeds; they are very light and there is no

possibility of demagnetization.

.However, due to the electronic commutation, they are extremely complex and expensive to

control. Perhaps the largest advantage of using servo motors is that they are used in closed loop

form, which allows for very accurate position information and also allows for high output torque

to be realized at high speeds. The motor will draw the required current to maintain the desired

path, velocity, or torque, and is controlled according to the requirements of the application rather

than by the limitations of the motor. Servo motors put out enormous peak torque at or near stall

conditions. They provide smooth, quiet operation, and depending upon the resolution of the

feedback mechanism, can have very small resolutions. Among the disadvantages of servo motors

are the increased cost, the added feedback component, and the increased control complexity. The

closed loop feature can be a disadvantage for the case when there is a physical obstacle blocking

the path of motion. Rather than stalling, the servo motor will continue to draw current to

overcome the obstacle. As a result, the system hardware, control electronics, signal amplifier and

motor may become damaged unless safety precautions are taken.

( 2 ) Encoders

An encoder is a device used to change a signal or data into a code. These encoders are used in

metrology instruments and high precision machining tools ranging from digital calipers to CNC

machine tools.

( 2.1) Incremental encoders

With incremental linear encoders, the current position is determined by stating a datum and

counting measuring steps. The output signals of incremental rotary encoders are evaluated by an

electronic counter in which the measured value is determined by counting "increments". These

encoders form the majority of all rotary encoders. Incremental rotary encoders with integral

couplings used for length measurement are also in the market.

The resolution of these encoders can be increased by means of electronic interpolation. There

are, of course, the precision rotary encoders specifically designed for angle measurement. If finer

resolution is required, standard rotary encoders often utilize electronic signal interpolation.

Rotary encoders for applications in dividing heads and rotary tables, with very small measuring

steps (down to 0.36 arc second) have in principle the same basic design features as standard

rotary encoders, but incorporate some overall varying construction.

Figure 24.2 Rotary encoders

( 2.2 ) Absolute encoders

Absolute linear encoders require no previous transfer to provide the current position value.

Absolute rotary encoders provide an angular position value which is derived from the pattern of

the coded disc. The code signal is processed within a computer or in a numerical control. After

system switch-on, such as following a power interruption, the position value is immediately

available. Since these encoder types require more sophisticated optics and electronics than

incremental versions, a higher price is normally to be expected. Apart from these two codes, a

range of other codes have been employed, though they are losing their significance since modern

computer programs usually are based on the binary system for reasons of high speed. There are

many versions of absolute encoders available today, such as single-turn or multi-stage versions

to name only two, and each must be evaluated based on its intended application.

( 2.3 ) Rotary and Linear encoders

A linear encoder is a sensor, transducer paired with a scale that encodes position. The sensor

reads the scale in order to convert the encoded position by a digital readout (DRO). Linear

encoder technologies include capacitive, inductive, eddy current, magnetic and optical.

A rotary encoder, also called a shaft encoder, is an electro-mechanical device used to convert the

angular position of a shaft to a digital code, making it a sort of a transducer.

Rotary encoders serve as measuring sensors for rotary motion, and for linear motion when used

in conjunction with mechanical measuring standards such as lead screws. There are two main

types: absolute and relative rotary encoders. Incremental rotary encoder uses a disc attached to a

shaft. The disc has several radial lines. An optical switch, such as a photodiode, generates an

electric pulse whenever one of the lines passes through its field of view. An electronic control

circuit counts the pulses to determine the angle through which the shaft has turned.

As the present trend of machine tools evolves toward increasingly higher accuracy and

resolution, increased reliability and speeds, and more efficient working ranges, so too must

feedback systems. Currently, linear feedback systems are available that will achieve resolutions

in the submicron range.

Figure 24.3: Exposed and sealed linear encoders

Submicron resolutions, for example, are required in the semiconductor industry and in ultra-

precision machining. Achieving these resolutions is possible with the use of linear scales which

transmit displacement information directly to a digital readout. As in rotary, linear scales operate

on the same photoelectric scanning principle, but the linear scales are comprised in an overall

straight construction, and their output signals are interpolated or digitized differently in a direct

manner. One of these signals is always used by the accompanying digital readout or numerical

control to determine and establish home position on the linear machine axis in case of a power

interruption or for workpiece referencing. Overall, there are two physical versions of a linear

scale: exposed or enclosed as shown in the figure 24.3. With an enclosed or "sealed" scale, the

scanning unit is mounted on a small carriage guided by ball bearings along the glass scale; the

carriage is connected to the machine slide by a backlash-free coupling that compensates for

alignment errors between the scale and the machine tool guide ways.

A set of sealing lips protects the scale from contamination. The typical applications for the

enclosed linear encoders are primarily machine tools. Exposed linear encoders also consist of a

glass scale and scanning unit, but the two components are physically separated. The typical

advantages of the non-contact system are easier mounting and higher traversing speeds since no

contact or friction between the scanning unit and scale exists. Exposed linear scales can be found

in coordinate measuring machines, translation stages, and material handling equipment.

Another version of the scale and scanning unit arrangement is one that uses a metal base rather

than glass for the scale. With a metal scale, the line grating is a deposit of highly reflective

material such as gold that reflects light back to the scanning unit onto the photovoltaic cells. The

advantage of this type of scale is that it can be manufactured in extremely great lengths, up to 30

meters, for larger machines. Glass scales are limited in length, typically three meters. There are

several mechanical considerations that need to be understood when discussing linear encoders. It

is not a simple matter to select an encoder based just on length or dimensional profile and install

the encoder onto a machine. These characteristic considerations include permissible traversing

speeds, accuracy and resolution requirements, thermal behaviour and mounting guidelines.

Figure 24.4: Principle of rotary and linear encoders

( 3 ) CNC Controller

There are two types of CNC controllers, namely closed loop and open loop controllers. These

have been discussed in details in section 22.2.

( 3.1 ) Controller Architecture:

Most of the CNC machine tools were built around proprietary architecture and could not be

changed or updated without an expensive company upgrade. This method of protecting their

market share worked well for many years when the control technology enjoyed a four-to-five

year life cycle. Now a day the controller life cycle is only eight-to-twelve months. So CNC

manufacturers are forced to find better and less expensive ways of upgrading their controllers.

Open architecture is the less costly than the alternatives. GE Fanuc and other manufacturers

introduced control architecture with PC connectivity to allow users to take advantage of the new

information technologies that were slowly gaining acceptance on the shop floor. They created an

open platform that could easily communicate with other devices over commercially available MS

Windows operating system, while maintaining the performance and reliability of the CNC

machine tool.

CNC SYSTEMS - MECHANICAL COMPONENTS

The drive units of the carriages in NC machine tools are generally the screw & the nut

mechanism. There are different types of screws and nuts used on NC machine tools which

provide low wear, higher efficiency, low friction and better reliability.

(1) Recirculating ball screw

The recirculating ball screw assembly shown in figure 25.1 has the flanged nut attached to the

moving chamber and the screw to the fixed casting. Thus the moving member will move during

rotational movement of the screw. These recirculating ball screw designs can have ball gages of

internal or external return, but all of them are based upon the "Ogival" or "Gothic arc".

In these types of screws, balls rotate between the screw and nut and convert the sliding friction

(as in conventional nut & screw) to the rolling friction. As a consequence wear will be reduced

and reliability of the system will be increased. The traditional ACME thread used in

conventional machine tool has efficiency ranging from 20% to 30% whereas the efficiency of

ball screws may reach up to 90%.

Figure 25.1: Recirculating ball screw assembly

Figure 25.2: Preloaded recirculating ball screw

There are two types of ball screws. In the first type, balls are returned through an external tube

after few threads. In another type, the balls are returned to the start through a channel inside the

nut after only one thread. To make the carriage movement bidirectional, backlash between the

screw and nut should be minimum. One of the methods to achieve zero backlash is by fitting two

nuts. The nuts are preloaded by an amount which exceeds the maximum operating load. These

nuts are either forced apart or squeezed together, so that the balls in one of the nuts contact the

opposite side of the threads.

These ball screws have the problem that minimum diameter of the ball (60 to 70% of the lead

screw) must be used, limiting the rate of movement of the screw.

(2) Roller screw

Figure 25.3: Roller screw

These types of screws provide backlash-free movement and their efficiency is same as that of

ball screws. These are capable of providing more accurate position control. Cost of the roller

screws are more compared to ball screws. The thread form is triangular with an included angle of

90 degrees. There are two types of roller screws: planetary and recirculating screws.

Planetary roller screws:

Planetary roller screws are shown in figure 25.3. The rollers are threaded with a single start

thread. Teeth are cut at the ends of the roller, which meshes with the internal tooth cut inside the

nut. The rollers are equally spaced around and are retained in their positions by spigots or spacer

rings. There is no axial movement of the rollers relative to the nut and they are capable of

transmitting high load at fast speed.

Recirculating roller screws:

The rollers in this case are not threaded and are provided with a circular groove and are

positioned circumferentially by a cage. There is some axial movement of the rollers relative to

the nut. Each roller moves by a distance equal to the pitch of the screw for each rotation of the

screw or nut and moves into an axial recess cut inside the nut and disengage from the threads on

the screw and the nut and the other roller provides the driving power. Rollers in the recess are

moved back by an edge cam in the nut. Recirculating roller screws are slower in operation, but

are capable of transmitting high loads with greater accuracy.

CNC Tooling

(1) Tool changing arrangements

There are two types of tool changing arrangements: manual and automatic. Machining centres

incorporate automatic tool changer (ATC). It is the automatic tool changing capability that

distinguishes CNC machining centres from CNC milling machines.

(1.1) Manual tool changing arrangement:

Tool changing time belongs to non-productive time. So, it should be kept as minimum as

possible. Also the tool must be located rigidly and accurately in the spindle to assure proper

machining and should maintain the same relation with the work piece each time. This is known

as the repeatability of the tool. CNC milling machines have some type of quick tool changing

systems, which generally comprises of a quick release chuck. The chuck is a different tool

holding mechanism that will be inside the spindle and is operated either hydraulically or

pneumatically. The tool holder which fits into the chuck can be released by pressing a button

which releases the hydraulically operated chuck. The advantage of manual tool changing is that

each tool can be checked manually before loading the tools and there will be no limitation on the

number of tools from which selection can be made.

(1.2) Automatic tool changing arrangement

Tooling used with an automatic tool changer should be easy to center in the spindle, each for the

tool changer to grab the tool holder and the tool changer should safely disengage the tool holder

after it is secured properly. Figure 27.1 shows a tool holder used with ATC. The tool changer

grips the tool at point A and places it in a position aligned with the spindle. The tool changer will

then insert the tool holder into the spindle. A split bushing in the spindle will enclose the portion

B. Tool changer releases the tool holder. Tool holder is drawn inside the spindle and is

tightened.

Figure 27.1: Tool holder

( 2) Tool turrets

An advantage of using tool turrets is that the time taken for tool changing will be only the time

taken for indexing the turret. Only limited number of tools can be held in the turret. Tool turrets

shown in figure 27.2 a, b & c are generally used in lathes. The entire turret can be removed from

the machine for setting up of tools.

Figure 27.2(a): Six station tool

turret Figure 27.2(b): Eight station tool turret

Figure 27.2(c): Twelve station tool

turret

( 3 ) Tool magazines

Tool magazines are generally found on drilling and milling machines. When compared to tool

turrets, tool magazines can hold more number of tools and also more problems regarding the tool

management. Duplication of the tools is possible and a new tool of same type may be selected

when ever a particular tool has been worn off. Though a larger tool magazine can accommodate

more number of tools, but the power required to move the tool magazine will be more. Hence, a

magazine with optimum number of tool holders must be used. The following types of tool

magazines exist: circular, chain and box type.

( 3.1 ) Chain magazine:

These magazines can hold large number of tools and may hold even up to 100 tools. Figures 27.3

a & b show chain magazines holding 80 and 120 tools respectively. In these chain magazines,

tools will be identified either by their location in the tool holder or by means of some coding on

the tool holder. In the former it is followed for identifying the tool and then the tool must be

exactly placed in its location. The positioning of the magazine for the next tool transfer will take

place during the machining operation.

Figure 27.3 (a) 80-tool chain magazine Figure 27.3 (b) 120-tool chain magazine

( 3.2) Circular magazine:

Circular magazines shown in figure 27.4 will be similar to tool turrets, but in the former the tools

will be transferred from the magazine to the spindle nose. Generally these will be holding about

30 tools. The identification of the tool will be made either by its location in the tool magazine or

by means of some code on the tool holder. The most common type of circular magazine is

known as carousel, which is similar to a flat disc holding one row of tools around the periphery.

Geneva mechanism is used for changing the tools.

Figuure 27.4: Circular magazine

( 3.3 ) Box magazine:

In these magazines, the tools are stored in open ended compartments. The tool holder must be

removed from the spindle before loading the new tool holder. Also the spindle should move to

the tool storage location rather than the tool to the spindle. Hence, more time will be consumed

in tool changing. Box magazines are of limited use as compared to circular and chain type of tool

magazines.

( 4 ) Automatic tool changers :

Whenever controller encounters a tool change code, a signal will be sent to the control unit so

that the appropriate tool holder in the magazine comes to the transfer position. The tool holder

will then be transferred from the tool magazine to the spindle nose. This can be done by various

mechanisms. One such mechanism is a rotating arm mechanism.

Rotating arm mechanism:

Movement of the tool magazine to place the appropriate tool in the transfer position will take

place during the machining operation. The rotating arms with grippers at both the ends rotate to

grip the tool holders in the magazine and the spindle simultaneously. Then the tool holder

clamping mechanism will be released and the arm moves axially to remove the tool holder from

the spindle. Then the arm will be rotated through 180 degrees and the arm will then move axially

inwards to place the new tool holder into the spindle and will clamped. Now the new tool holder

is placed in the spindle and the other in the magazine. Figure 27.5 and 27.6 show various stages

during tool change with a rotating arm mechanism.

Figure 27.5: Rotating arm mechanism

Figure 27.6: Rotating arm mechanism

( 5 ) Tool wear monitoring :

Most of the modern CNC machines now incorporate the facility of on-line tool wear monitoring

systems, whose purpose is to keep a continuous track of the amount of tool wear in real time.

These systems may reduce the tool replacement costs and the production delays. It is based on

the principle that the power required for machining increases as the cutting edge gets worn off.

Extreme limits for the spindle can be set up and whenever it is reached, a sub-program can be

called to change the tool. Following figures show some typical tool wear monitoring systems.

Figure 27.7: ON-line tool wear monitoring system Figure 27.8 : Graphical display of tool wear monitoring system

28. CNC WORK HOLDING DEVICES With the advent of CNC technology, machining cycle times were drastically reduced and the

desire to combine greater accuracy with higher productivity has led to the reappraisal of work

holding technology. Loading or unloading of the work will be the non-productive time which

needs to be minimized. So the work is usually loaded on a special work holder away from the

machine and then transferred it to the machine table. The work should be located precisely and

secured properly and should be well supported.

28.1 Turning center work holding methods: Machining operations on turning centers or CNC lathes are carried out mostly for axi-

symmetrical components. Surfaces are generated by the simultaneous motions of X and Z axes.

For any work holding device used on a turning centre there is a direct "trade off" between part

accuracy and the flexibility of work holding device used.

Work holding

methods Advantages Disadvantages

Automatic Jaw &

chuck changing

Adaptable for a range of work-

piece shapes and sizes

High cost of jaw/chuck changing

automation. Resulting in a more

complex & higher cost machine tool

Indexing chucks

Figure 28.1

Very quick loading and

unloading of the workpiece can

be achieved. Reasonable range

of work piece sizes can be

loaded automatically

Expensive optional equipment. Bar-

feeders cannot be incorporated.

Short/medium length parts only can be

incorporated. Heavy chucks.

Pneumatic/Magnetic

chucks

Figure 28.3

Simple in design and relatively

inexpensive. Part automation is

possible. No part distortion is

Limited to a range of flat parts with

little overhang. Bar-feeders cannot be

incorporated. Parts on magnetic chucks

caused due to clamping force must be ferrous. Heavy cuts must be

avoided.

Automatic Chucks

with soft jaws

Adaptable to automation. Heavy

cuts can be taken. Individual

parts can be small or large in

diameter

Jaws must be changed manually &

bared, so slow part change-overs. A

range of jaw blanks required.

Expanding mandrels

& collets

Figure 28.2

Long & short parts of reasonably

large size accommodated.

Automation can be incorporated.

Clamping forces do not distort

part. Simple in design

Limitation on part shape. Heavy cuts

should be avoided.

Dedicated Chucks

Excellent restraint & location of

a wide range of individual &

irregular -shaped parts can be

obtained.

Expensive & can only be financially

justified with either large runs or when

extremely complex & accurate parts

are required. Tool making facilities

required. Large storage space.

( 2) Work holding for Machining Centres:

Workholding methods Advantages Disadvantages

Modular Fixtures

Figure 28.6

Highly adaptable. Can be purchased in

stages to increase its sophistication.

Reasonable accuracy. Speedily assembled.

Small stores area is required. Can be set-up

to a machine more than one part. Proven

technology

Costly for a complete

system. Difficult to

automate. Skills required in

kit assembly

Automatic Vices

Relatively inexpensive. Can be operated by

mechanical, pneumatic, or by hydraulic

control. Quick to operate with ease of set-

up. Reasonable accuracy. Easily automated.

Simplicity of design. Using multi-vices

allows many parts to be machined. Proven

Technology

Work holding limitations.

Clamping force limitations.

Jaws can become strained.

Work location problems.

Limitations on part size.

Pneumatic/Magnetic

Work holding devices

Relatively inexpensive. Reasonable

accuracy. Can machine large areas of the

work piece. Quick setups. Easily

automated. Simplicity of design. Many

parts can be machined at one set up.

Large surface area is

required. Swarf can be a

problem. Nonferrous

material limitation on

magnetic devices.

4/5 axis CNC work

holding devices

Allows complex geometric shapes to be

machined. High accuracy. Opportunity for

"one hit" machining. Easily automated.

Costly & limited part

geometry clamping. Part

size limitations. Usually

only one part can be

machined. Cannot be fitted

to all machines.

Dedicated Fixturing

Large & small parts are easily

accommodated. High accuracy of part

location. Easily automated. Simplicity of

design. Proven technology. Many parts can

be machine at one setup good vibration

damping capacity

Large storage space

required. No part flexibility.

Heavy fixtures. Tool

making facilities required.

Figure 28.1: Indexing chucks

Figure 28.2: Mandrels

Figure 28.3: Magnetic chucks

Figure 28.4: Vise

Figure 28.5(a): Pallets

Figure 28.5(b) Figure 28.5(c)

Figure 28.6 : Modular fixture

Figure 28.7 : Chucks

Module F (1) : CNC Part Programming I

(1) Programming fundamentals

Machining involves an important aspect of relative movement between cutting tool and workpiece. In machine tools

this is accomplished by either moving the tool with respect to workpiece or vice versa. In order to define relative

motion of two objects, reference directions are required to be defined. These reference directions depend on type of

machine tool and are defined by considering an imaginary coordinate system on the machine tool. A program defining

motion of tool / workpiece in this coordinate system is known as a part program. Lathe and Milling machines are

taken for case study but other machine tools like CNC grinding, CNC Hobbing, CNC filament winding machine, etc.

can also be dealt with in the same manner.

(1.1) Reference Points

Part programming requires establishment of some reference points. Three reference points are

either set by manufacturer or user.

a) Machine Origin The machine origin is a fixed point set by the machine tool builder. Usually it cannot be

changed. Any tool movement is measured from this point. The controller always remembers

tool distance from the machine origin.

b) Program Origin

It is also called home position of the tool. Program origin is point from where the tool starts

for its motion while executing a program and returns back at the end of the cycle. This can be

any point within the workspace of the tool which is sufficiently away from the part. In case of

CNC lathe it is a point where tool change is carried out.

c) Part Origin The part origin can be set at any point inside the machine's electronic grid system.

Establishing the part origin is also known as zero shift, work shift, floating zero or datum.

Usually part origin needs to be defined for each new setup. Zero shifting allows the relocation

of the part. Sometimes the part accuracy is affected by the location of the part origin. Figure

29.1 and 29.2 shows the reference points on a lathe and milling machine.

Figure 29.1. Reference points and axis on a lathe

Figure 29.2. Reference points and axis on a Milling Machine

(1.2 ) Axis Designation

An object in space can have six degrees of freedom with respect to an imaginary Cartesian

coordinate system. Three of them are liner movements and other three are rotary. Machining of

simple part does not require all degrees of freedom. With the increase in degrees of freedom,

complexity of hardware and programming increases. Number of degree of freedom defines axis

of machine.

Axes interpolation means simultaneous movement of two or more different axes to generate

required contour.

For typical lathe machine degree of freedom is 2 and so it called 2 axis machines. For typical

milling machine degree of freedom is , which means that two axes can be interpolated at a

time and third remains independent. Typical direction for the lathe and milling machine is as

shown in figure 12 and figure 13.

(1.3 ) Setting up of Origin

In case of CNC machine tool rotation of the reference axis is not possible. Origin can set by

selecting three reference planes X, Y and Z. Planes can be set by touching tool on the surfaces of

the workpiece and setting that surfaces as X=x, Y=y and Z=z.

(1.4 ) Coding Systems

The programmer and the operator must use a coding system to represent information, which the

controller can interpret and execute. A frequently used coding system is the Binary-Coded

Decimal or BCD system. This system is also known as the EIA Code set because it was

developed by Electronics Industries Association. The newer coding system is ASCII and it has

become the ISO code set because of its wide acceptance.

(2) CNC Code Syntax

The CNC machine uses a set of rules to enter, edit, receive and output data. These rules are

known as CNC Syntax, Programming format, or tape format. The format specifies the order and

arrangement of information entered. This is an area where controls differ widely. There are rules

for the maximum and minimum numerical values and word lengths and can be entered, and the

arrangement of the characters and word is important. The most common CNC format is the word

address format and the other two formats are fixed sequential block address format and tab

sequential format, which are obsolete. The instruction block consists of one or more words. A

word consists of an address followed by numerals. For the address, one of the letters from A to Z

is used. The address defines the meaning of the number that follows. In other words, the address

determines what the number stands for. For example it may be an instruction to move the tool

along the X axis, or to select a particular tool.

Most controllers allow suppressing the leading zeros when entering data. This is known as

leading zero suppression. When this method is used, the machine control reads the numbers from

right to left, allowing the zeros to the left of the significant digit to be omitted. Some controls

allow entering data without using the trailing zeros. Consequently it is called trailing zero

suppression. The machine control reads from left to right, and zeros to the right of the significant

digit may be omitted.

(3) Types of CNC codes

(3.1) Preparatory codes

The term "preparatory" in NC means that it "prepares" the control system to be ready for

implementing the information that follows in the next block of instructions. A preparatory

function is designated in a program by the word address G followed by two digits. Preparatory

functions are also called G-codes and they specify the control mode of the operation.

(3.2) Miscellaneous codes

Miscellaneous functions use the address letter M followed by two digits. They perform a group

of instructions such as coolant on/off, spindle on/off, tool change, program stop, or program end.

They are often referred to as machine functions or M-functions. Some of the M codes are given

below.

M00 Unconditional stop

M02 End of program

M03 Spindle clockwise

M04 Spindle counterclockwise

M05 Spindle stop

M06 Tool change (see Note below)

M30 End of program

In principle, all codes are either modal or non-modal. Modal code stays in effect until cancelled

by another code in the same group. The control remembers modal codes. This gives the

programmer an opportunity to save programming time. Non-modal code stays in effect only for

the block in which it is programmed. Afterwards, its function is turned off automatically. For

instance G04 is a non-modal code to program a dwell. After one second, which is say, the

programmed dwell time in one particular case, this function is cancelled. To perform dwell in the

next blocks, this code has to be reprogrammed. The control does not memorize the non-modal

code, so it is called as one shot codes. One-shot commands are non-modal. Commands known

as "canned cycles" (a controller's internal set of preprogrammed subroutines for generating

commonly machined features such as internal pockets and drilled holes) are non-modal and only

function during the call.

On some older controllers, cutter positioning (axis) commands (e.g., G00, G01, G02, G03, &

G04) are non-modal requiring a new positioning command to be entered each time the cutter (or

axis) is moved to another location.

Command group G-

code Function and Command Statement Illustration

Tool motion

G00 Rapid traverse

G00 Xx Yy Zz

G01 Linear interpolation

G01 Xx Yy Zz Ff

G02

Circular Interpolation in

clock-wise direction

G02 Xx Yy Ii Jj

G02 Xx Zz Ii Kk

G02 Yy Zz Jj Kk

G03

Circular interpolation in

counter- clockwise direction

G03 Xx Yy Ii Jj

G03 Xx Zz Ii Kk

G03 Yy Zz Jj Kk

Command group G-code Function and Command Statement Illustration

Plane Selection

G17

XY - Plane selection

G18 ZX - Plane selection

G19

YZ - plane selection

Command group G-code Function and Command Statement Illustration

Unit Selection

G20 or

G70

Inch unit selection

G21 or

G71

Metric unit selection

Command

group

G-

cod

e

Function

and

Command

Illustration

Statement

Offset and

compensati

on

G4

0

Cutter

diameter

compensati

on cancel

G4

1

G4

2

Cutter

diameter

cancellatio

n left

Cutter

diameter

compensati

on righ

Command

group G-code

Function and Command

Statement Illustration

Tool

motion

G00 Rapid traverse

G00 Xx Zz

G01 Linear interpolation

G01 Xx Zz

G02

Circular Interpolation in

clock-wise direction

G02 Xx Zz Ii Kk

(or)

G02 Xx Zz Rr

G03

Circular interpolation in

counter- clockwise

direction

G03 Xx Zz Ii Kk

(or)

G03 Yy Zz Rr

Illustrative Example Program

A contour illustrated in figure 29.3 is to be machined using a CNC milling machine. The details

of the codes and programs used are given below.

Example:

Figure 29.3 An illustrative example

O5678 Program number

N02 G21 Metric programming

N03 M03 S1000 Spindle start clockwise with 1000rpm

N04 G00 X0 Y0 Rapid motion towards (0,0)

N05 G00 Z-10.0 Rapid motion towards Z=-10 plane

N06 G01 X50.0 Linear interpolation

N07 G01 Y20.0 Linear interpolation

N08 G02 X25.0 Y45.0

R25.0 Circular interpolation clockwise(cw)

N09 G03 X-25.0

Y45.0 R25.0 Circular interpolation counter clockwise(ccw)

N10 G02 X-50.0

Y20.0 R25.0 Circular interpolation clockwise(cw)

N11 G01 Y0.0 Linear interpolation

N12 G01 X0.0 Linear interpolation

N13 G00 Z10.0 Rapid motion towards Z=10 plane

N14 M05 M09 Spindle stop and program end

30. CNC Part Programming II

In the previous section, fundamentals of programming as well basic motion commands for

milling and turning have been discussed. This section gives an overview of G codes used for

changing the programming mode, applying transformations etc.,

30.1 Programming modes

Programming mode should be specified when it needs to be changed from absolute to

incremental and vice versa. There are two programming modes, absolute and incremental and is

discussed below.

30.1.1 Absolute programming (G90)

In absolute programming, all measurements are made from the part origin established by the

programmer and set up by the operator. Any programmed coordinate has the absolute value in

respect to the absolute coordinate system zero point. The machine control uses the part origin as

the reference point in order to position the tool during program execution (Figure 30.1).

30.1.2 Relative programming (G91)

In incremental programming, the tool movement is measured from the last tool position. The

programmed movement is based on the change in position between two successive points. The

coordinate value is always incremented according to the preceding tool location. The

programmer enters the relative distance between current location and the next point ( Figure

30.2).

30.2 Spindle control

The spindle speed is programmed by the letter 'S' followed by four digit number, such as S1000. There are two

ways to define speed.

1. Revolutions per minute (RPM)

2. Constant surface speed

The spindle speed in revolutions per minute is also known as constant rpm or direct rpm. The change in tool

position does not affect the rpm commanded. It means that the spindle RPM will remain constant until another

RPM is programmed. Constant surface speed is almost exclusively used on lathes. The RPM changes according to

diameter being cut. The smaller the diameter, the more RPM is achieved; the bigger the diameter, the less RPM is

commanded. This is changed automatically by the machine speed control unit while the tool is changing positions.

This is the reason that, this spindle speed mode is known as diameter speed.

30.3 Loops and Unconditional jump (G25)

The unconditional jump is used to repeat a set of statements a number of times.

Example: N10

In the above example, the program statements from N70 to N100 are repeated once when the statement N160 is

executed. Usually the G25 is used after a mirror statement. Illustrative example geometry and its program are given

below (Figure 30.3).

Example:

Fig. 30.3 Illustrative example for programming loops

30.4 Mirroring

The mirroring command is used when features of components shares symmetry about one or more axes and are also

dimensionally identical. By using this code components can be machined using a single set of data and length of

programs can be reduced.

G10 cancellation of mirroring image

G11 Mirror image on X axis

G12 Mirror image on Y axis

G13 Mirror image on Z axis

Example:

Fig. 30.4 Illustrative Example for mirroring

30.5 Shifting origin

G92 code is used to temporarily shift the origin to the reference point specified.

Example: G92 X-100 Y-80

In the above statement the x and y values gives the present values of original origin after shifting it. This is

illustrated through an example (Figure 30.5).

Example:

Fig. 30.5 Illustrative Example for shifting origin

30.6 Scaling

Scaling function is used to program geometrically similar components with varying sizes.

Syntax: G72 Kk, where k is the scaling factor.

The scaling command can be cancelled by using the statement G72 K1.0.

Example:

Fig. 30.6 Illustrative Example for scaling

30.7 Pattern rotation

Pattern rotation is used to obtain a pattern of similar features. G73 code is used to rotate the

feature to form a pattern.

Syntax G73 Aa, where 'a' is the angle of rotation. This command is cumulative, and the angle

gets added up on time the program is executed. So all the rotational angle parameters should be

cancelled using the code G73.

The unconditional jump code G25 is used in conjunction with this code to achieve the desired

rotation.

The following example (Figure 30.7) depicts the case of a pattern which needs to be programmed

through G73.

Example:

Fig. 30.7 Illustrative Example for Pattern rotation

30.8 Tool selection

Tool selection is accomplished using 'T' function followed by a four digit number where, first

two digits are used to call the particular tool and last two digits are used to represent tool offset

in the program. The tool offset is used to correct the values entered in the coordinate system

preset block. This can be done quickly on the machine without actually changing the values in

the program.

Using the tool offsets, it is easy to set up the tools and to make adjustments

Feed rate control

Cutting operations may be programmed using two basic feed rate modes:

1. Feed rate per spindle revolution

2. Feed rate per time

The feed rate per spindle revolution depends on the RPM programmed.

31. CNC Part Programming III

31.1 Tool radius Compensation

The programmed point on the part is the command point. It is the destination point of the tool. The point on the tool

that is used for programming is the tool reference point. These points may or may not coincide, depending on the

type of tool used and machining operation being performed. When drilling, tapping, reaming, countersinking or

boring on the machining center, the tool is programmed to the position of the hole or bore center - this is the

command point.

When milling a contour, the tool radius center is used as the reference point on the tool while writing the program,

but the part is actually cut by the point on the cutter periphery. This point is at 'r' distance from the tool center. This

means that the programmer should shift the tool center away from the part in order to perform the cutting by the tool

cutting edge. The shift amount depends upon the part geometry and tool radius. This technique is known as tool

radius compensation or cutter radius compensation.

In case of machining with a single point cutting tool, the nose radius of the tool tip is required to be accounted for, as

programs are being written assuming zero nose radius. The tool nose radius center is not only the reference point

that can be used for programming contours. On the tool there is a point known as imaginary tool tip, which is at the

intersection of the lines tangent to the tool nose radius.

Cutter compensation allows programming the geometry and not the toolpath. It also allows adjusting the size of the

part, based on the tool radius used to cut part. This is useful when cutter of the proper diameter is not found. This is

best explained in the Figure 31.1.

Figure 31.1. Cutter diameter compensation

The information on the diameter of the tool, which the control system uses to calculate the required compensation,

must be input into the control unit's memory before the operation. Tool diameter compensation is activated by the

relevant preparatory functions (G codes) as shown in Figure 31.2.

Compensation for tool radius can be of either right or left side compensation. This can be determined by direction of

tool motion. If you are on the tool path facing direction of tool path and if tool is on your left and workpiece is on

your right side then use G41 (left side compensation). For, reverse use other code G42 (Right side compensation).

Both the codes are modal in nature and remain active in the program until it is cancelled by using another code,

G40.

Offset Direction = Left (G41)

Offset Direction = Right (G42)

Offset Direction = Off (G40)

31.2 Ramp on/off motion

When activating cutter radius compensation, it must be ensured that the slides will first make a non-cutting move to

enable the correct tool and workpiece relation to be established. A similar move is necessary prior to cancellation of

the radius compensation. These non-cutting moves are referred as "ramp on" and "ramp off" respectively. Figure

31.3 shows the ramp on motion for different angles of approach.

Figure 31.3. Ramp up motion

31.3 Subroutines

Any frequently programmed order of instruction or unchanging sequences can benefit by becoming a subprogram. Typical applications for

subprogram applications in CNC programming are

Repetitive machining motions

Functions relating to tool change

Hole patterns

Grooves and threads

Machine warm-up routines

Pallet changing

Special functions and others

Structurally, subprograms are similar to standard programs. They use the same syntax rules. The benefits of subroutines involve the reduction

in length of program, and reduction in program errors. There is a definition statement and subroutine call function.

Standard sub-routine

N10

N20

N30

….

N70 G22 N5

N80

N90

….

N100 G24

….

N160 G20 N5

In the above example G22 statement defines the start block of the sub-routine and G24 marks the end of the sub-routine statement. The

subroutine is called by another code G20 identified by the label N5.

Parametric subroutine

..

..

..

G23 N18

G01 X P0 Y P1

..

..

G21 N18 P0=k10 P1=k20

In the above example G23 starts the subprogram label and starts the definition, and the parameters P0 , P1 are defined for values of x and y.

The G21 statement is used to call the subroutine and to assign the values to the parameters.

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31.4 Canned Cycles

A canned cycle is a preprogrammed sequence of events / motions of tool / spindle stored in memory of controller.

Every canned cycle has a format. Canned cycle is modal in nature and remains activated until cancelled. Canned

cycles are a great resource to make manual programming easier. Often underutilized, canned cycles save time and

effort.

31.4.1 Machining a Rectangular pocket

This cycle assumes the cutter is initially placed over the center of the pocket and at some clearance distance

(typically 0.100 inch) above the top of the pocket. Then the cycle will take over from that point, plunging the cutter

down to the "peck depth" and feeding the cutter around the pocket in ever increasing increments until the final size

is attained. The process is repeated until the desired total depth is attained. Then the cutter is returned to the center of

the pocket at the clearance height as shown in figure 31.4

Figure 31.4. Pocket machining

The overall length and width of the pocket, rather than the distance of cutter motion, are programmed into this cycle.

The syntax is : G87 Xx Yy Zz Ii Jj Kk Bb Cc Dd Hh Ll Ss (This g code is entirely controller specific and the syntax

may vary between controller to controller).

Description:

x,y - Center of the part

z - Distance of the reference plane from top of part

i - Pocket depth

j,k - Half dimensions of the target geometry (pocket)

b - Step depth

c - Step over

d - Distance of the reference plane from top of part

h - Feed for finish pass

l - Finishing allowance

s - Speed

For machining a circular pocket, the same syntax with code G88 is used.

31.4.2 Turning Cycles The G80 command will make the tool move in a series of rectangular paths cutting material axially until the tool tip

reaches target point P1 where the cycle ends as shown in figure 31.5. Cutting movements will be at the cutting feed

rate. All other movements will be at rapid traverse rate.

Figure 31.5 Turning cycle (Straight cutting)

The syntax is G80 Xx Zz Ff

31.4.3 Roughing Cycle In roughing cycle, the final finishing cycle profile is used to perform the roughing operation for the higher material

removal rate. The syntax for the roughing cycle is given below.

G81 Pp Qq Uu Ww Dd Ff Ss

31.5 The APT Programming Language The APT (Automatically Programmed Tool) programming language was

developed in early 1960s to assist engineers in defining the proper instructions and calculations for NC part

programming. A great strength of APT is its ability to perform precise calculations for complicated tool paths when

contouring on a three dimensional surface in a multi- axis programming mode. Now APT has become obsolete.

Please click here to know more about APT. Automatic generation of NC code is dealt in this page

Part Programming for machining curved surfaces

With increased demand for aesthetic form and functional surface shape in many industrial

products, the need for a method of CNC machining of curved and intricate surfaces is required.

One of the most important features in determining CNC machining efficiency and productivity is

cutter path motion. This cutter motion on compound-curvature surfaces determines the time for

machining as well as determines the surface roughness (or cusp height or scellop height). Surface

roughness always exists because of the lack of geometry matching between cutter and surface.

( 1 ) Representation of Curves

Description of geometry of curves and surfaces may be done in several ways. It can be both

analytical and parametric. It can be described mathematically by nonparametric or parametric

equations. Nonparametric equations can be explicit or implicit. For a nonparametric curve, the

coordinates y and z of a point on the curve are expressed as two separate functions of third

coordinate x as the independent variable.

( 2 ) Advantages of Parametric representation

Independent control of x and y

Segments can be represented easily

Piecewise linear approximation is easier

It can be easily extended to higher dimension

Transformations are easier to apply

Helps mathematical computation

Parametric representation of geometry for NC tool path generation

Curve Surface

x (u) x(u,w)

y (u) y(u,w)

z (u) z(u,w)

A Surface patch as shown in figure 32.1 is represented parametrically along two directions. The

two parameters are u and w. The parameters u and w can take values between 0 and 1.

32.1 A surface patch

The representation of surface patch can be any one of the following forms

In generalized form

x(u,w)=f(u,w)+rnx(u,w)

y(u,w)=f(u,w)+rny(u,w)

z(u,w)=h(u,w)+rnz(u,w)

The representation of ruled and bilinear surfaces is given below.

Ruled surface X(u,w) = x(u) + [x'(u) - x(u)]w

Y(u,w) = y(u) + [y'(u) - y(u)]w

Z(u,w) = z(u) + [z'(u) - z(u)]w

Bilinear Surface X(u,w) = x(0,0)(1 - u)(1 - w) + x(0,1)(1-u)w + x(1,0)u(1-w) + x(1,1)uw

Y(u,w) = y(0,0)(1 - u)(1 - w) + y(0,1)(1-u)w + y(1,0)u(1-w) + y(1,1)uw

Z(u,w) = z(0,0)(1 - u)(1 - w) + z(0,1)(1-u)w + z(1,0)u(1-w) + z(1,1)uw

For, generation of optimum path for machining using parametric representation various

properties of surfaces is used. Some of the properties are shown below.

The unit normal is

Curvature can be expressed as k=1/r

Where

k ->curvature

r ->radius of curvature

In order to find the maximum diameter of the ball mill, mean and Gaussian curvature is used and

is given below

Mean Curvature H :

The product of the principal curvatures at a point on a surface is called the total or Gaussian

curvature at that point

Gaussian curvature K :

Where kmax and kmin are the principle curvatures, and

From the values of mean and Gaussian curvature, principle curvature can be determined. The

maximum curvature defines maximum diameter of the cutter that can be used to machine

surfaces.

Maximum Radius of ball mill = 2/kmax

( 3 ) Generation of part program using parametric representation

Generation of cutter paths may be classified, according to the methodology for calculation of

cutter location data, into the following three approaches:

(3.1 ) Isoparametric method:

Cutter is moved along contact points at the intersection of constant parameter lines on the surface

(modeled with parametric surface patches). The number of line segments used to approximate

curve depends on required accuracy. More line segments give better approximation of surface.

For better approximation circular interpolation can be used instead of liner interpolation between

two consecutive points. Figure 31.2 (b) shows a surface obtained by approximating isoparametric

curves of a free form surface as shown in 31.2(a).

31.2 (a) Original Surface

31.2 (b) Approximated surface

( 3.2 ) CC-Cartesian method:

Cutter is moved along contact points on the intersection lines formed between the surface and

intersecting drive planes (normally located parallel to the principal planes of the coordinate

system). Instead of using intersection of isoparametric curves here, intersection of two principle

planes and surface is used to approximate suface.

( 3.3 ) CL-Cartesian method or APT-style method:

Cutter is positioned tangent or parallel to drive planes and in contact with appropriate surface

points In the CC-Cartesian and CL-Cartesian methods, concentric cylinders or spirals are

sometimes used as the drive surfaces.

33. CNC Part Program Verification

Programs prepared for any kind of CNC machine should be cautiously verified. Though there are

exceptions to this rule, manually prepared programs are more prone to having mistakes than

CAM system generated programs. However, even the best CAM system generated CNC

programs could still include disastrous problems.

Syntax Mistakes-These are "silly" mistakes on the programmer's part that cause the program to

be unacceptable to the control.

Motion Mistakes-This kind of mistake is usually harder to find and correct.

Setup Mistakes-Even a perfectly prepared program will behave poorly if setup mistakes are

made.

Cutting Condition Mistakes-Though the program's motions may be correct, the operator must

be on guard for cutting condition problems. Feeds and speeds must be properly applied. While

machining the first workpiece with any program, the operator must be very cautious, watching

for possible machining problems.

Recommended Procedures

The basic procedures that will help insure safety during the verification of a CNC program are discussed below

1.1 Machine Dry Run

Prior to letting a program cause motion, it is wise to let the control check the program for syntax mistakes. With Machine Lock and Dry Run

turned on, the operator can rest assured that the axes of the machine will not move. When the program is executed, the control will scan the

program for basic mistakes. If the control determines a problem, it will go into alarm state. While there could still be serious problems if the

control completes the program, the operator can be rest assured that at least the program is acceptable to the control. "Free Flowing" Dry Run

Once the Machine Lock Dry Run can be executed without generating alarms, the operator is ready to let the program generate motion. The

Free Flowing Dry Run will allow the operator to see the motion the program will generate, and also allow the operator to control how fast

motion will be. With Dry Run in the on condition, a multi position switch (usually Feed Rate Override or Jog Feed Rate) acts like a rheostat,

allowing the operator to manipulate how fast axis motion will be. If the operator senses a problem, Feed Hold is pressed. With no part loaded

into the setup, the operator can allow the motion generated by the program to take place and will be able to tell if the basic motions are

correct. The first time the Free Flowing Dry Run is executed for a program, the operator will be most concerned with dangerous situations

like interference with obstructions and spindle direction. For this reason, it may be necessary to repeat this procedure several times before the

operator becomes comfortable with the cycle.

1.2 On-line program verification

Any time spent between production runs verifying CNC programs must be considered as part of setup time - and anything that can be done to

reduce on-line program verification time effectively reduces setup time. Since new programs will contain more potential for mistakes than

proven programs, companies that perform little repeat business should be highly interested in minimizing the time spent verifying the CNC

program. One way to achieve this goal is to move the task of program verification off line. This means performing as much of the program

verification procedures as is possible while the machine is still running workpieces in a previous production run. Following are several ways

to verify CNC programs.

The CNC machine makes a poor verification tool. If the quality of the CNC program is unknown before it is loaded into the machine, the

setup person must be very careful at every step of the verification process. A series of dry runs must be performed just to confirm that the

basic motions of the program will not cause interference. And since the machine is down between production runs, the entire task of program

verification must be done on line.

More CNC controls are coming with graphic capabilities that allow you to plot a program's movement's right on the display screen. While

this is an excellent feature, and one we would recommend you purchase if it is available, keep in mind that many controls do not allow a tool

path to be shown for one program while the machine is running another program. In such a case, program verification is still an on-line task.

If the tool path display exposes mistakes in the program, corrections must be made during setup, meaning corrections must also be done on

line. This can waste a great deal of precious machine time.

Almost all current computer-aided manufacturing (CAM) systems allow some form of tool path display aimed at helping a programmer

locate motion mistakes in a program. More and more software companies are developing attractively priced products aimed at helping their

users verify CNC programs off line. Several companies offer PC based tool path display capabilities, similar to those found in many current

CNC controls.

4. CNC Part Program generation from CAD Model

Generation of part program for machining a complex shapes is very difficult without use of

available CAD/CAM software packages. Use of this leads to decrease in time required for part

programming and increases the accuracy of programming.

Software packages like PRO/engineer, ideas, MasterCAM allows the user to create machining

tool paths. Within the package you can create milling and turning (lathe) machining simulations.

After creating a series of these simulations, the user can machine the 3D model created and at a

later stage transfer the information to a NC lathe or miller.

Generation of part program requires some basic information like final geometry, workpiece

geometry, orientation of principle axis and location of origin on the raw material geometry,

machining parameters, tool geometry, machine which is to be used etc. Other than this some

information like which type of operation has to be done, volume which is to be removed, etc also

has to be given.

Module G (1) : Computer Aided Assembly Planning

INTRODUCTION

(1) Introduction

An assembly is a collection of independent parts. It is important to understand the dependencies between various parts

in an assembly to assemble the parts properly. Assembly model includes the spatial positions and hierarchical

relationships among the parts and mating conditions between the parts. One of the obvious ways to facilitate the

assembly process at the design phase is to simplify the product by reducing the number of different parts to a

minimum. In addition to product simplification, the assembly process can be greatly facilitated by introducing guides

and tapers into the design of various parts. Sharp corners usually hinder guiding parts into their correct positions

during assembly. Assembly planning is the checking of both hard and soft clashes, of generating assembly sequences

in order to check the feasibility of the removal path and verifying the correct sequence and the correct fit both

dynamically and statically during the disassembly process.

Figure 35.1 (a) Collection of pieces used to define an assembly

planning problem

Figure 35.2 (b) Assembly planning determining a sequence of

motions for assembly

The use of computers for planning the assembly of mechanical products originated in the research on planning with

artificial intelligence. There are many reasons for systematization and the computerization of assembly planning,

some of which are as follows;

Industrial designers will benefit from having a tool with which they can quickly assess their designs for ease of assembly

Although many experienced personals have a skill for devising an efficient ways to assemble a given product, systematic procedures are necessary to

guarantee that no good assembly plan has been over looked. Sometimes, the number of different assembly alternatives is so large that even skillful engineer

may fail to notice many possibilities.

An automatic generator of assembly sequence can be an efficient aid to designer. Whenever one modifies the feature

of the product, the influence of these modifications can immediately be checked on the sequences. For small batch

production, the automatic generation of assembly sequences is faster, more reliable and more cost-effective than

manual generation.

(2) Computer Aided Assembly Planning

Computer aided assembly planning involves automatically determining a sequence of motions to

assemble a product from its individual parts. The motions can include part motions, grasping

locations, tool access, fixture planning, factory layout, and many other issues, all of which have

complex geometric components that use computational geometry techniques. A number of

technical issues that must be addressed for assembly automation includes following;

(2.1) Representation of assemblies and assembly plans

A computer representation of mechanical assemblies is necessary in order to automate the

generation of assembly plans. The main issue in this stage is to decide what information about

assemblies is required, and how this information is represented in the computer. An assembly of

parts can be represented by the description of its individual components and their relationships in

the assembly. Assembly data base stores the geometric models of individual parts, the spatial

positions and orientations of the parts in the assembly, and the assembly or attachment

relationships between parts. One of the widely used methods for representation of assemblies is

based on graph structures. In this scheme, an assembly model is represented by a graph structure

in which each node represents an individual part or a sub assembly. The branches of the graph

represent relationship among parts. Four kinds of relationships exist: part-of (P), attachment (A),

constraint (C) and sub assembly (SA).

The "part-of" relation represents the logical containment of one object in another. For example, the head and shaft of a screw

are "part-of" the screw itself.

There are three types of attachment relationships: rigid, non rigid, and conditional.

o Rigid attachment occurs when no relative motion is possible between two parts.

o Non rigid attachment occurs when parts cannot be separated by an arbitrarily large distance but relative motion

between the two parts is possible.

o Conditional attachment is related to parts supported by gravity, but not strictly attached.

Constraint relationships represent physical constraint of one part on another.

Subassembly relationship indicates that an assembly is merged into a higher assembly.

The graph structure of electric clutch assembly is shown in figure 35.2

Figure 35.2 Graph structure of electric clutch assembly

Another method for representation of assembly is location graph of the part which is a relative

property. In this method, a co-ordinate system is the used to specify location of one part relative

to another. A location in one coordinate system also defines a new coordinate system for the

located part, with its origin and axes. Other locations can be defined in terms of this second one.

Thus, a chain of locations can be defined such that each location is defined in terms of another

part's coordinate system. A set of these chains results in a graph referred to here as a location

graph.

Figure 35.3 Location graph of electric clutch assembly

(2.2) Generation of Assembly Sequences and Assembly plans

For usefulness, an assembly planning system must generate correct assembly plans. Further, to

solve problems that require optimization, such as selection of best assembly alternative, one must

be able to traverse the space of all candidate solutions. The number of distinct feasible assembly

plans can be large even for assemblies made of a small number of parts therefore complete

enumeration is not possible in most cases real applications. Finding systematic ways to narrow

down alternatives is crucial for the automatic planning of assembly. Some of the widely used

techniques in evaluation of assembly sequence are discussed below.

Precedence Diagram

The precedence diagram is designed to show all the possible assembly sequences of a given

product. To develop the precedence diagram for a product, each individual assembly operation is

assigned a unique number and it is represented by an appropriate circle with the number

inscribed. The circles are connected by arrows showing the precedence relations. The precedence

diagram is usually organized into columns. All the operations that can be carried out first are

placed in the first column, and so on. Usually, one operation appears in the first column: the

placing the base part on the work carrier where whole assembly process occurs.

Figure 35.3 Precedence diagram of electric clutch assembly

Liaison-Sequence Analysis

The liaison method develops all possible assembly sequences in two steps. First it characterizes

the assembly by a network wherein nodes represent parts and lines between nodes represent any

mating conditions between parts. These mating conditions referred to in this method as liaisons.

The network itself is known as liaison diagram.

Figure 35.4 Liaison-Sequence of electric clutch assembly

When developing a liaison diagram, to note that the liaison count (number of liaisons) l is related

to the part count (number of parts) n by the following inequality:

Precedence Graph

Unlike the previous two methods, this method is fully automatic. It is based on the virtual link

data structure and requires the mating conditions as input to automatically generate assembly

sequences for various assemblies. Once the mating conditions are provided, they are organized

in the form of a mating graph. The parts in an assembly are then structured in a hierarchal

assembly tree. Then assembly sequence is generated with the aid of interference checking. In

this method, the assembly sequence is referred to as a precedence graph.

Figure 35.4 Precedence graph of electric clutch assembly

(2.3) Integration with CAD programs

A mechanical assembly is a composition of interconnected parts. Frequently, the parts are being

designed using CAD programs therefore the shape of each part and geometric information are

already available in computer database. The assembly planning will be more efficient if these

CAD databases can be directly integrated with programs that generate assembly models.

(2.4) Integration with task and motion planners

With the progress of digital electronics, the programmable robots are introduced in

manufacturing. These robots can be adapted to execute different operations by changing their

internal programs. Task and motion planners that will facilitate robot programming are

constantly getting developed. With a view toward future integration, the output of assembly

planners should compatible with what is required by task and motion planners. It is also desirable

that assembly planners also take into account the capabilities and limitations of task and motion

planners.

(3) Benefits of Computer Aided Assembly Planning

Accelerate new product introductions

Shorten time-to-production

Optimize production management

Decrease operating costs

Ensure overall product and process quality

Allow engineers, designers and shop floor personnel to collaborate interactively

( 1 ) Introduction

Inspection or testing is an act of checking materials, parts, components or products at various

stages of manufacturing detecting poor quality manufactured products for taking corrective

action. Inspection is performed before, during and after manufacturing to ensure that the quality

of the product is consistent with the accepted design standard. The design standards are defined

by the product designer, and for mechanical components they relate to factors such as dimension,

surface finish and appearance. The objective of any inspection process is either to take actual

measurements of the values of the specified product characteristics or to check whether specific

characteristics meet design standards.

When inspection and testing is carried out manually, the sample size is often small compared to

size of the population. In high production runs, this size may be very small which may result in

slipping of defective parts. In principle, the only way to achieve 100% good quality is to use

100% inspection using which only good quality parts will pass through the inspection procedure.

But when this is done manually, the problem of expenses involved and error associated with the

procedure is of major concern. Automation of inspection offers an opportunity to overcome these

problems. Automated inspection procedures are carried out by sensors that are controlled by

computers.

( 2 ) Computer Aided Inspection

Computer Aided Inspection (CAI) is a new technology that enables one to develop a comparison

of a physical part to a 3D CAD model. This process is faster, more complete, and more accurate

than using a Coordinate Measuring Machine (CMM) or other more traditional methods. An

automatic inspection method and apparatus using structured light and machine vision camera is

used to inspect an object in conjunction with the geometric model of the object. Camera images

of the object are analyzed by computer to produce the location of points on the object's surfaces

in three dimensions. Point-cloud data is taken from a laser scanner or other 3-D scanning device.

During a setup phase before object inspection, the points are analyzed with respect to the

geometric model of the object. The software provides a graphical comparison of the

manufactured part compared to the CAD model. Many points are eliminated to reduce data-

taking and analysis time to a minimum and prevent extraneous reflections from producing errors.

When similar objects are subsequently inspected, points from each surface of interest are

spatially averaged to give high accuracy measurements of object dimensions. The inspection

device uses several multiplexed sensors, each composed of a camera and a structured light

source, to measure all sides of the object in a single pass.

Computers are used in many ways in inspection planning and execution also.

( 2.1 ) Computer controlled inspection equipment

Coordinate Measuring Machine (CMM) is a 3-dimensional measuring device that uses a contact

probe to detect the surface of the object. The probe is generally a highly sensitive pressure

sensing device that is triggered by any contact with a surface. The linear distances moved along

the 3 axes are recorded, thus providing the x, y and z coordinates of the point. CMMs are

classified as either vertical or horizontal, according to the orientation of the probe with respect to

the measuring table.

Figure 36.1 Coordinate Measuring Machine (CMM)

( 2.2 ) Computer aided inspection setup planning

Computer-Aided Inspection Planning (CAIP) is the integration bridge between CAD/CAM and

Computer Aided Inspection (CAI). A CAIP system for On-Machine Measurement (OMM) is

proposed to inspect the complicated mechanical parts efficiently during machining or after

machining. The inspection planning consists of Global Inspection Planning (GIP) and Local

Inspection Planning (LIP). In the GIP, the system creates the optimal inspection sequence of

features in a given part by analyzing the various feature information. Feature groups are formed

for effective planning, and special feature groups are determined for sequencing. The integrated

process and inspection plan is generated based on the series of heuristic rules developed. The

integrated inspection planning is able to determine optimum manufacturing sequence for

inspection and machining processes. Finally, the results are simulated and analyzed to verify the

effectiveness of the proposed CAIP.

( 2.3 ) Computational metrology

Computational metrology deals with fitting and filtering discrete geometric data that are obtained

by measurements made on the parts. Some basic facts about manufacturing and measurement are

best captured using following two axioms.

Axiom of manufacturing imprecision: All manufacturing processes are inherently imprecise and produce parts that vary.

Axiom of measurement uncertainty: No measurement can be absolutely accurate and with every measurement there is some

finite uncertainty about the measured attribute or measured value.

Fitting:- Fitting is the task of associating ideal geometric forms to non-ideal forms (such as, for

example, discrete set of points sampled on a manufactured surface). Normally, Fitting is done

for the following reasons:

Datum establishment: Datum is a reference geometric object of ideal form established on one or more non-ideal geometric

forms of a manufactured part. Datums are used for relative positioning of geometric objects in parts and assemblies of parts.

Deviation assessment: It is often important to determine how far a manufactured surface deviates from its intended ideal

geometric form which can be quantified by fitting.

Fitting problems are broadly divided into two categories on the basis of the objective function

which is to be optimized. First one is Least squares fitting, which has the objective to find an

ideal geometric object (a smooth curve or surface) to minimize the sum of squared deviations of

data points from desired object. Second on is Chebyshev fitting, which has the objective to

minimize the maximum deviation.

Filtering - Filtering is the task of obtaining scale-dependent information from measured data. At

a more mundane level, filtering can be used to remove noise and other unwanted information

from the measured data. In the context of engineering metrology, engineers are interested in use

of filtering mainly for the two applications.

Surface roughness: Many engineering functions depend on roughness or smoothness of a surface on the piece. Designers

define bounds on certain roughness parameters to ensure functionality of parts. These small-scale variations are subtracted

from the surface measurement data before form and other deviations are assessed.

Manufacturing process diagnosis: Manufacturing processes leave tool marks on surfaces. By measuring surfaces at fine

scale, it is possible to detect tool erosion and its effect on the surface quality.

The computational scheme used for filtering is one of convolution. Two types of convolutions are in use;

Convolution of functions: Filtering is often implemented as discrete convolution of functions. In the most popular version,

the measured data is convolved with the Gaussian function. It has a smoothing effect on the surface data.

Convolution of sets: Morphological filters are implemented using Minkowski sums. These can be regarded as convolutions

where the input set is convolved with a circular or flat structuring element.

( 2.4 ) Computer aided part localization and Shape Matching

Many a times in inspection process comparison of shapes of two geometric entities is done.

Shape matching can be used for comparison of shapes of two geometric entities. For example

comparison of shapes between designed model and molded part. Shape localization is done with

the help of transformations. Transformations can be translation and rotations. Shape matching

can be broadly classified as

Curve-Curve Matching

Surface-Surface Matching

Solid-Solid Matching

Matching curve, surface or solid with point data set

Figure 36.2: Shape Localization and matching of two curves

Shape matching is used for many applications like character recognition, object recognition,

medical imaging, etc. The same concept can be extended in the inspection of mechanical parts.

In shape matching, the manufactured part is located with respect to designed part and error is

measured. In character reorganization, characters are fitted with smooth curves and then matched

with predefined templates.

Figure 36.3: Shape matching

( 3 ) Benefits of CAI

The CAI process saves both time and money.

The computer software processes data from a 3-D point cloud from a laser scanner, eliminating the need for slower and more

time-consuming CMM measurements.

Inspecting with CMMs requires that designers create a 2-D drawing in addition to the 3-D CAD model of a part. The drawing

is used to inspect the part at specific locations to verify that it matches the design. Point-cloud data is taken from a laser

scanner or other 3-D scanning device. The software provides a graphical comparison of the manufactured part compared to the

CAD model.

By producing point clouds of the entire part, measurement of the part can be done everywhere and not be limited to the

specific locations on a drawing.

Reverse Engineering

INTRODUCTION

(1) Introduction

Engineering is the profession involved in designing, manufacturing, and maintaining products, systems, and structures.

The whole engineering process can be broadly classified in two groups; forward engineering and reverse engineering.

Forward engineering is the traditional process of moving from high-level abstractions and logical designs to the

physical implementation of a system.

Figure 37.1 Forward Engineering

The process of duplicating an existing component, subassembly, or product, without the aid of drawings,

documentation, or computer model is known as reverse engineering.

Figure 37.2 Reverse Engineering

Reverse engineering can be mainly viewed as the process of analyzing a system to identify its components and their

interrelationships, to create representations of it in another form or a higher level of abstraction. An important reason

for application of reverse engineering is reduction of product development times. In the intensely competitive global

market, manufacturers are constantly seeking new ways to shorten lead-times to market a new product. For example,

injection-molding companies must drastically reduce the tool and die development times. By using reverse

engineering, a three-dimensional product or model can be quickly captured in digital form, re-modeled, and exported

for rapid prototyping/tooling or rapid manufacturing.

Some of the important reasons for the for reverse engineering of a product or part are;

The original manufacturer of a product no longer produces a product

There is inadequate documentation of the original design

The original manufacturer no longer exists, but a customer needs the product

The original design documentation has been lost or never existed

Some bad features of a product need to be designed out. For example, excessive wear might indicate where a product should be improved

To strengthen the good features of a product based on long-term usage of the product

To analyze the good and bad features of competitors' product

To explore new avenues to improve product performance and features

To gain competitive benchmarking methods to understand competitor's products and develop better products

The original CAD model is not sufficient to support modifications or current manufacturing methods

To update obsolete materials or antiquated manufacturing processes with more current, less-expensive technologies

It can be said that reverse engineering begins with the product and works through the design process in the opposite

direction to arrive at a product definition statement. In doing so, it uncovers as much information as possible about the

design ideas that were used to produce a particular product.

(2) Reverse engineering methodology

The reverse engineering process can be divided into the following broad steps:

Figure 37.3 Reverse Engineering Methodology

(2.1) Digitizing or collecting data from physical part

One of the reverse engineering methods is construction of a CAD model of the physical parts

whose drawing is not available. This is done by digitizing an existing prototype which is mainly

creating a computer model and then using it to manufacture the component. The objective of this

method is to generate a 3D mapping of the product in form of a CAD file. This involves the

acquisition of the product surface data by either contact or non contact methods in form of X, Y

and Z coordinates of large number of points on the product surface. The methods of obtaining

the product surface data can be divided into two broad categories; Contact method and Non-

contact method. The contact method requires contact between the component surface and a

measuring tool that is usually a probe or a stylus. The non-contact method uses light as the main

tool in extracting the required information. The contact discretization method uses Co-ordinate

Measuring Machines (CMM) or electromagnetic digitizers or sonic digitizers to get the co-

ordinates of the desired points on the surface. The non-contact discretization technique uses

white light or laser scanners to scan the 3D object from which the CAD model is generated. The

choice of discretization method is based on the speed and performance during digitization and

avoidance of damage to the product.

A CMM is a 3-dimensional measuring device that uses a contact probe to detect the surface of

the object. The linear distances moved along the 3 axes are recorded, thus providing the X, Y and

Z co-ordinates of the point. The part to be discretized is placed on the measuring table, and the

co-ordinates of a number of points on the surface of the object are then read. These points are

input into a 'geometry data' file, which can be transferred to a CAD system to generate the model

of the part. In this way the shape of the object is captured in the form of a CAD drawing that can

be manipulated and modified as needed.

In electromagnetic digitizers, the product to be digitized is placed on a table which encloses

electronic equipment and a magnetic field source. It creates a magnetic field in the volume of

space above table. A hand held stylus is used to trace the surface of the part. This stylus houses a

magnetic field sensor that, in conjunction with the electronic unit, detects the position and

orientation of the stylus. The data can be transferred to a computer through a serial port.

In sonic digitizers, sound waves are used to calculate the position of a point relative to a

reference point. In this technique, the object is placed in front of a vertical rectangular board on

the corners of which are mounted four microphone sensors. A free hand held stylus is used to

trace the contours of the object. When a foot or a hand switch is pressed, the stylus emits an

ultrasonic impulse, and, simultaneously four clocks are activated. When the impulse is detected

by a microphone, the corresponding clock is stopped and the times taken to reach each of the

microphones recorded. These time recordings, called slant ranges, are processed by a computer

to calculate the x, y and z coordinates of the point.

(2.2) Manipulation of the collected data to obtain a CAD model

After obtaining the product surface data as a sea of points in space, the next important step is the

fitting of geometry to this point data. Various methods were developed for the fitting of surfaces

to the point data. The surface can be mathematically described as either algebraic or parametric

surfaces. Algebraic surfaces are represented by a polynomial equation of the type f(x, y, z) = 0,

and usually represent infinite surfaces. Parametric surfaces on the other hand, are finite surfaces

defined by certain basis functions and control points e.g. Beizer surfaces, NURBS surfaces.

Surface fitting techniques can be broadly classified in to two categories; interpolation techniques

and approximation techniques. In interpolation technique, the surface to be fitted passes through

all the data points and is normally used when the data points are accurately measured without

any errors. In approximation technique, the surface need not to pass through any of the data

points, but represents a generalized average or a best fit to the data points. This is usually used

when there are a large number of data points through which the surface has to be fitted, or when

there errors in the measurement are to be averaged out.

(2.3) Generation of functional parts from CAD model

Once the geometric model is obtained, it can be used as the basis for a variety of operations such

as automated process planning, automated manufacturing, automated dimensional inspection and

automated tolerance analysis. In automated manufacturing, these geometric models can be used

to generate the tool motion commands which can be made execute on any of the standard CNC

machines or input CAD model for rapid prototyping processes. These applications require

feature extraction from the geometric model, followed by a process plan for the object, which

involves definition of various manufacturing sequences required to manufacture the object.

Reverse Engineering of a computer mouse

Step 1: Point Cloud Data in Sub Regions

Step 2: Point Cloud Data after applying Maximum Error Method

Step 3: Surface fitting to Point Cloud Data

Step 4: Surface after Cleaning

Step 5: Computer mouse after Prototyping

Rapid Prototyping

INTRODUCTION

(1) Introduction

One of the important steps prior to the production of a functional product is building of a physical prototype. Prototype

is a working model created in order to test various aspects of a design, illustrate ideas or features and gather early user

feed-back. Traditional prototyping is typically done in a machine shop where most of parts are machined on lathes and

mills. This is a subtractive process, beginning with a solid piece of stock and the machinist carefully removes the

material until the desired geometry is achieved. For complex part geometries, this is an exhaustive, time consuming,

and expensive process. A host of new shaping techniques, usually put under the title Rapid Prototyping, are being

developed as an alternative to subtractive processes. These methods are unique in that they add and bond materials in

layers to form objects. These systems are also known by the names additive fabrication, three dimensional printing,

solid freeform fabrication (SFF), layered manufacturing etc. These additive technologies offer significant

advantages in many applications compared to classical subtractive fabrication methods like formation of an object

with any geometric complexity or intricacy without the need for elaborate machine setup or final assembly in very

short time. This has resulted in their wide use by engineers as a way to reduce time to market in manufacturing, to

better understand and communicate product designs, and to make rapid tooling to manufacture those products.

Surgeons, architects, artists and individuals from many other disciplines also routinely use this technology.

(2) Methodology of Rapid Prototyping (RP)

RP in its basic form can be described as the production of three dimensional (3D) parts from

computer aided design (CAD) data in a decreased time scale. The basic methodology of all RP

process can be summarized as shown in following figure.

Figure 38.1 Rapid prototyping process chain

(2.1) Development of a CAD model

The process begins with the generation CAD model of the desired object which can be done by

one of the following ways;

Conversion of an existing two dimensional (2D) drawing

Importing scanned point data into a CAD package

Creating a new part in CAD in various solid modeling packages

Altering an existing CAD model

RP has traditionally been associated with solid rather than surface modelling but the more recent

trends for organic shapes in product design is increasing the need for free flowing surfaces

generated better in surface modelling.

(2.2) Generation of Standard triangulation language (STL) file

The developed 3D CAD model is tessellated and converted into STL files that are required for

RP processes. Tessellation is piecewise approximation of surfaces of 3D CAD model using

series of triangles. Size of triangles depends on the chordal error or maximum fact deviation. For

better approximation of surface and smaller chordal error, small size triangle are used which

increase the STL file size. This tessellated CAD data generally carry defects like gaps, overlaps,

degenerate facets etc which may necessitate the repair software. These defects are shown in

figure below. The STL file connects the surface of the model in an array of triangles and consists

of the X, Y and Z coordinates of the three vertices of each surface triangle, as well as an index

that describes the orientation of the surface normal.

Figure 38.2 Tessellation defects

(2.3) Slicing the STL file

Slicing is defined as the creating contours of sections of the geometry at various heights in the

multiples of layer thickness. Once the STL file has been generated from the original CAD data

the next step is to slice the object to create a slice file (SLI). This necessitates the decision

regarding part deposition orientation and then the tessellated model is sliced. Part orientation

will be showing considerable effect on the surface as shown in the figures.

Figure 38.3 Effect of Part deposition Orientation

The thickness of slices is governed by layer thickness that the machine will be building in, the

thicker the layer the larger the steps on the surface of the model when it has been built. After

the STL file has been sliced to create the SLI files they are merged into a final build file. This

information is saved in standard formats like SLC or CLI (Common Layer Interface) etc.

(2.4) Support Structures

As the parts are going to be built in layers, and there may be areas that could float away or of

overhang which could distort. Therefore, some processes require a base and support structures

to be added to the file which are built as part of the model and later removed.

(2.5) Manufacturing

As discussed previously, the RP process is additive i.e. it builds the parts up in layers of

material from the bottom. Each layer is automatically bonded to the layer below and the

process is repeated until the part is built. This process of bonding is undertaken in different

ways for the various materials that are being used but includes the use of Ultraviolet (UV)

lasers, Carbon Dioxide lasers, heat sensitive glues and melting the material itself etc.

(2.6) Post processing

The parts are removed from the machine and post processing operations are performed

sometimes to add extra strength to the part by filling process voids or finish the curing of a part

or to hand finish the parts to the desired level. The level of post processing will depend greatly

on the final requirements of the parts produced, for example, metal tooling for injection

molding will require extensive finishing to eject the parts but a prototype part manufactured to

see if it will physically fit in a space will require little or no post processing.

(3) Various RP Processes

Several RP techniques are being developed and commercially available. The first commercial

process, StereoLithography (SL), came to the market in 1987. Nowadays, more than 30 different

processes (not all commercialized) with high accuracy and a large choice of materials exist.

These processes can be classified in different ways but the most popular way is according to the

form of material used as an input. This can be given as follows;

Liquid based processes

(i) Solidification of a liquid polymer

(ii)Solidification of an electro-stat fluid

(iii)Solidification of molten material

Discrete Particle based processes

(i) Fusing of particles by laser

(ii)Joining particles by binder

Solid Sheets based process

(i) Bonding of sheets with adhesive

(ii) Bonding of sheets with light

Some of the commercially popular RP processes are described below;

(3.1) Stereolithograpy

StereoLithography (SL) is the best known rapid prototyping system. The technique builds three-

dimensional models from liquid photosensitive polymers that solidify when exposed to laser

beam. The model is built upon a platform in a vat of photo sensitive liquid. A focused UV laser

traces out the first layer, solidifying the model cross section while leaving excess areas liquid. In

the next step, an elevator lowers the platform into the liquid polymer by an amount equal to layer

thickness. A sweeper recoats the solidified layer with liquid, and the laser traces the second layer

on the first. This process is repeated until the prototype is complete. Afterwards, the solid part is

removed from the vat and rinsed clean of excess liquid. Supports are broken off and the model is

then placed in an ultraviolet oven for complete curing.

Figure 38.5 Stereolithography

Application Range

Processing large variety of photo-sensitive polymers including clear, water resistant and flexible resins

Functional parts for tests

Tools for pre series production tests.

Manufacturing of medical models

Manufacturing of electro-forms for Electro Discharge Machining (EDM)

Form-fit functions for assembly tests.

Advantages

Possibility of manufacturing parts which are impossible to produce conventionally using a single process.

Continuous unattended operation for 24 hours.

High resolution.

Any geometrical shape can be made with virtually no limitation.

Disadvantages

Necessity to have support structures

Accuracy not in the range of mechanical part manufacturing.

Restricted areas of application due to given material properties.

Labour requirements for post processing, especially cleaning.

(3.2) Selective Laser Sintering

Selective Laser Sintering (SLS) is a 3-dimensional printing process based on sintering, using a

laser beam directed by a computer onto the surface of metallic or non-metallic powders

selectively to produce copies of solid or surface models. The process operates on the layer-by-

layer principle. At the beginning a very thin layer of heat fusible powder is deposited in the

working space container. The -laser sinters the powders. The sintering process uses the

laser to raise the temperature of the powder to a point of fusing without actually melting it. As

the process is repeated, layers of powder are deposited and sintered until the object is complete.

The powder is transferred from the powder cartridge feeding system to the part cylinder (the

working space container) via a counter rolling cylinder, a scraper blade or a slot feeder. In the

unsintered areas, powder remains loose and serves as natural support for the next layer of powder

and object under fabrication. No additional support structure is required.

Figure 38.6 Selective Laser Sintering (SLS)

Application Range

Visual representation models.

Functional and tough prototypes.

Cast metal parts (by use of wax).

Short run and soft tooling.

Advantages

Virtually any materials that have decreased viscosity upon heating can potentially be

used.

Do not require any post-curing except when ceramics are used.

No need to create a support structure, which saves time

Advanced softwares allowing concurrent slicing of the part geometry files while

processing the object

Disadvantages

Raw appearance on the part surface due to hardening of additional powder on the

borderline of the object

Necessity to provide the process chamber continuously with nitrogen to assure safe

material sintering

Careful handling of toxic gases emitted from the fusing process

(3.3) Laminated Object manufacturing (LOM)

In this technique, layers of adhesive-coated sheet material are bonded together to form a

prototype. The original material consists of paper laminated with heat-activated glue and rolled

up on spools. A feeder/collector mechanism advances the sheet over the build platform, where a

base has been constructed from paper and double-sided foam tape. In the next stage, a heated

roller applies pressure to bond the paper to the base. A focused laser cuts the outline of the first

layer into the paper and then cross-hatches the excess area (the negative space in the prototype).

Cross-hatching breaks up the extra material, making it easier to remove during post-processing.

During the build, the excess material provides excellent support for overhangs and thin-walled

sections. After the first layer is cut, the platform lowers out of the way and fresh material is

advanced. The platform rises slightly below the previous height, the roller bonds the second layer

to the first, and the laser cuts the second layer. This process is repeated as needed to build the

part, which will have a wood-like texture. Because the models are made of paper, they must be

sealed and finished with paint or varnish to prevent moisture damage.

Figure 38.7 Laminated Object Manufacturing (LOM)

Application Range

Considering the specified of the procedure, the main application range is the working area of conceptual design.

Used for large bulky models as sand casting patterns.

Advantages

Variety of organic and inorganic materials can be used such as paper, plastic, ceramic,

composite, etc.

Relatively low costs

Much faster process than competitive techniques

Virtually produces no internal stress and associated undesirable deformation.

Robust capacity of dealing with imperfect STL files, created with discontinuities,

Best suited for building large parts, as if the machine with the largest workspace on the

market today.

Disadvantages

Limited stability of the objects due to the bonding strength of the glued layers.

Not well suited for manufacturing parts with thin walls in the z-direction

Hollow parts, like bottles, can not be built.

(3.4) Fusion Deposition Modeling (FDM)

Fused Deposition Modeling (FDM) machine is basically a CNC-controlled robot carrying a

miniature extruder head. By feeding the head with a plastic wire, solid objects are built "string by

string". In this technique, filaments of heated thermoplastic are extruded from a tip that moves in

the x-y plane. Like a baker decorating a cake, the controlled extrusion head deposits very thin

beads of material onto the build platform to form the first layer. The platform is maintained at a

lower temperature, so that the thermoplastic quickly hardens. After the platform lowers, the

extrusion head deposits a second layer upon the first. Supports are built along the way, fastened

to the part either with a second, weaker material or with a perforated junction.

Figure 38.8 Fused Deposition Modeling (FDM)

Application Range

Conceptual modeling.

Fit, form and functional applications and models for further manufacturing procedures.

Investment casting and injection molding.

Advantages

Quick and cheap generation of models.

Easy and convenient date building.

No worry of possible exposure to toxic chemicals, lasers, or a liquid polymer bath.

No wastage of material during or after producing the model

No requirement of clean-up.

Quick change of materials

Disadvantages

Restricted accuracy due to the shape of the material used: wire of 1.27 mm diameter.

(3.5) Solid Ground Curing

Solid ground curing (SGC) is almost similar to stereolithography (SLA). In both one uses

ultraviolet light to selectively harden photosensitive polymers. Unlike SLA, SGC cures an entire

layer at a time. First, photosensitive resin is sprayed on the build platform. Secondly, the

machine develops a photomask (like a stencil) of the layer to be built. This photomask is printed

on a glass plate above the build platform using an electrostatic process similar to that found in

photocopiers. The mask is then exposed to UV light, which only passes through the transparent

portions of the mask to selectively harden the shape of the current layer. After the layer is cured,

the machine vacuums up the excess liquid resin and sprays wax in its place to support the model

during the build. The top surface is milled flat, and then the process repeats to build the next

layer. When the part is complete, it must be de-waxed by immersing it in a solvent bath.

Figure 38.9 Solid Ground Curing (SGC)

Advantages

Large parts, 500 × 500 × 350 mm, can be fabricated quickly.

High speed allows production-like fabrication of many parts or large parts.

Masks are created with laser printing-like process, then full layer exposed at once.

No post-cure required.

Milling step ensures flatness for subsequent layer.

Wax supports model: no extra supports needed.

Disadvantages

Creates a lot of waste.

Not as prevalent as SLA and SLS, but gaining ground because of the high throughput

and large parts

(3.6) 3D Printing

3D printing is very reminiscent of SLS, except that the laser is replaced by an inkjet head. The

multi-channel jetting head (A) deposits a liquid adhesive compound onto the top layer of a bed

of powder object material (B). The particles of the powder become bonded in the areas where

the adhesive is deposited.

Once a layer is completed the piston (C) moves down by the thickness of a layer. As in SLS, the

powder supply system (E) is similar in function to the build cylinder In this case the piston

moves upward incrementally to supply powder for the process and the roller (D) spreads and

compresses the powder on the top of the build cylinder. The process is repeated until the entire

object is completed within the powder bed.

After completion the object is elevated and the extra powder brushed away leaving a "green"

object. Parts must usually be infiltrated with a hardener before they can be handled without

much risk of damage.

Figure 38.10 3D Printing

Surfaces of the parts produced by layer manufacturing processes suffer from poor surface finish

and this is due to the inherent characteristics of the process itself like stair stepping effect,

shrinkage etc. Least build time in RP is generally preferred but stair stepping effect and poor

surface finish restricts it. This has been induced on to the surface of the parts during the various

stages that a particular part has to come across during a RP cycle, typically data preparation

stage, part orientation, part geometry, deciding layer thickness etc. Figure 38.4 shows these

effects on RP product.

(4) Some issues in RP

Because of layer by layer deposition of the material and due to the finite thickness of each layer,

situation similar to stair case will be resulting on the surface and this effect is known as stair

stepping effect. From the figures it can be seen that layer thickness will directly affect the

maximum cusp height attained and the stair case effect on the surface.

Figure 38.11 Staircase effect in RP Parts

Figure 38.12 Effect of layer thickness on stair stepping effect

To reduce the surface roughness, one may go with very fine layers, but this will be increasing the

overall build time and build cost considerably. If we choose maximum allowable layer thickness

then, this will be generating a part with high surface roughness. So an optimum layer thickness

must be decided. The solution for this is to go for adaptive slicing or local adaptive slicing.

Adaptive slicing is slicing the entire part with different thicknesses according to the local surface

geometry and maximum cusp height that can be reached as shown in the figure above. In local

adaptive slicing, maximum layer thickness will be considered and then checked for maximum

slice thickness and each layer is further divided accordingly if required.

Computer Aided Process Planning

Process Planning

(1) Process Planning

Products and their components are designed to perform certain specific functions. Every product has some design

specifications which ensure its functionality aspects. The task of manufacturing is to produce components such that

they meet design specifications. Process planning acts as a bridge between design and manufacturing by translating

design specifications into manufacturing process details. It refers to a set of instructions that are used to make a

component or a part so that the design specifications are met, therefore it is major determinant of manufacturing cost

and profitability of products. Process planning answers the questions regarding required information and activities

involved in transforming raw materials into a finished product. The process starts with the selection of raw material

and ends with the completion of part. The development of process plans involves mainly a set of following activities;

Analysis of part requirements

Selection of raw workpiece

Selection of manufacturing operations and their sequences

Selection of machine tools

Selection of tools, tool holding devices, work holding devices and inspection equipments

Selection of manufacturing conditions i.e. cutting speed, feed and depth of cut.

Determination of manufacturing times

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(2) The manual experience-based planning method

The manual experience-based process planning is most widely used. It is mainly based on a

manufacturing engineer's experience and knowledge of production facilities, equipment, their

capabilities, processes, and tooling. The major problem with this approach is that it is time

consuming and developed plans may not be consistent and optimum. The feasibility of

developed process plan is dependant on many factors such as availability of machine tools,

scheduling and machine allocation etc. Computer aided process planning is developed to

overcome this problems to some extent.

(3) Computer Aided Process Planning

As mentioned in article 39.1, the primary purpose of process planning is to translate the design

requirements into manufacturing process details. This suggests a system in which design

information is processed by the process planning system to generate manufacturing process

details. CAPP integrates and optimizes system performance into the inter-organizational flow.

For example, when one changes the design, it must be able to fall back on CAPP module to

generate manufacturing process and cost estimates for these design changes. Similarly, in case of

machine breakdown on the shop floor, CAPP must generate the alternative actions so that most

economical solution can be adopted in the given situation. A typical CAPP frame-work is shown

in figure 39.1.

Figure 39.1 A Computer Aided Process Planning (CAPP) frame-work

When comapred with manual experience-based process planning, CAPP offers following

advantages;

Systematic developemnt of accurate and consistent process plans

Reduction of cost and lead time of process planning

Reduced skill requirements of process planners

Increased productivity of process planners

Higher level application progams such as cost and manufacturing lead time estimation and work standards can be interfaced

Two major methods are used in computer aided process planning; the variant CAPP method and

the generative CAPP method

(3.1) The variant CAPP method

In variant CAPP approach, a process plan for a new part is created by recalling, identifying and

retrieving an existing plan for a similar part and making necessary modifications for the new

part. Sometimes, the process plans are developed for parts representing a fmily of parts called

'master parts'. The similiarities in design attributes and manufacturing methods are exploited for

the purpose of formation of part families. A number of methods have been developed for part

family formation using coding and classification schemes of group technology (GT), similiarity-

coefficient based algorithms and mathematical programming models.

The variant process planning approach can be realized as a four step process;

1. Definition of coding scheme

2. Grouping parts into part families

3. Development of a standard process plan

4. Retrieval and modification of standard process plan

A number of variant process planning schemes have been developed and are in use. One of the

most widely used CAPP system is CAM-I developed by McDonnell-Douglas Automation

Company. This system can be used to generate process plan for rotational, prismatic and sheet-

metal parts.

3.2 The generative CAPP method

The next stage of evolution is towards generative CAPP. In the generative CAPP, process plans

are generated by means of decision logic, formulas, technology algorithms and geometry based

data to perform uniquely many processing decisions for converting part from raw material to

finished state. There are two major components of generative CAPP; a geometry based coding

scheme and process knowledge in form of decision logic data. The geometry based coding

scheme defines all geometric features for process related surfaces together with feature

dimensions, locations, tolerances and the surface finish desired on the features. The level of

detail is much greater in a generative system than a variant system. For example, details such as

rough and finished states of the parts and process capability of machine tools to transform these

parts to the desired states are provided. Process knowledge in form of in the form of decision

logic and data matches the part geometry requirements with the manufacturing capabilities

using knowledge base. It includes selection of processes, machine tools, jigs or fixtures, tools,

inspection equipments and sequencing operations. Development of manufacturing knowledge

base is backbone of generative CAPP. The tools that are widely used in development of this

database are flow-charts, decision tables, decision trees, iterative algorithms, concept of unit

machined surfaces, pattern recognition techniques and artificial intelligence techniques such as

expert system shells.

(4) Advantages of CAPP and future trends

CAPP has some important advantages over manual process planning which includes;

Reduced process planning and production lead-times

Faster response to engineering changes in the product

Greater process plan accuracy and consistency

Inclusion of up-to-date information in a central database

Improved cost estimating procedures and fewer calculation errors

More complete and detailed process plans

Improved production scheduling and capacity utilization

Improved ability to introduce new manufacturing technology and rapidly update process plans to utilize the improved

technology

There are number of difficulties in achieving the goal of complete integration between

various functional areas such as design, manufacturing, process planning and

inspection. For example, each functional area has its own stand-alone relational

database and associated database management system. The software and hardware

capabilities among these systems pose difficulties in full integration. There is a need

to develop single database technology to address these difficulties. Other challenges

include automated translation of design dimensions and tolerances into manufacturing

dimensions and tolerances considering process capabilities and dimensional chains,

automatic recognition of features and making CAPP systems affordable to the small

and medium scale manufacturing companies.