bhel - cnc machines
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INDUSTRIAL TRAINING REPORT
COMPUTER NUMERICALLY CONTROLLED MACHINES undertaken at
BHARAT HEAVY ELECTRICALS LTD.
(GOINDWAL,TARN TARAN)
Submitted by
SARAVPREET SINGH DHILLON
UE 95083
Under the Guidance of
Mr.Sanjeev Kumar Mr.Tejinder Singh
HOD
Department of Electronics & Communication Engineering
UNIVERSITY INSTITUTE OF ENGINEERING & TECHNOLOGY
PANJAB UNIVERSITY, CHANDIGARH
JUNE -2013
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DECLARATION
I hereby declare that the project work entitled COMPUTER NUMERICALLY CONTROLLED
MACHINES is an authentic record of my own work carried out at BHEL (GO+NDWAL) as
requirements of six months Industrial Training for the award of the degree of B.E. at University
Institute of Engineering & Technology, Panjab University, Chandigarh under the guidance of
Mr.Tejinder Singh and Mr.Sanjeev, during January 4,2013 to July 3, 2013
SARAVPREET SINGH DHILLON
UE 95083
Date : June 12, 2013
Certified that the above statement made by the student is correct to the best of our knowledge
and belief.
Mr.Sanjeev Kumar Mr.Tejinder Singh
HOD
+
+
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INDEX
OVERVIEW OF THE ORGANISATION 1 INTRODUCTION 1
POWER TRANSMISSION & DISTRIBUTION (T & D) 3
INDUSTRIES 4
TRANSPORTATION 4
RENEWABLE ENERGY 5
MISSION 5
INTERNATIONAL OPERATIONS 6
TECHNOLOGY UPGRADATION AND R&D 6
PROJECT REPORT
Introduction 9
Configuration of CNC Systems
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Input Device 13
Machine Control Unit 15
Machine Tool 15
Driving System 16
Feedback Systems 17
Display Unit 21
CNC PART PROGRAMMING 23
PROGRAMMING SYSTEMS 24
CNC POSITIONING SYSTEM 27
INTERPOLATION 29
PROGRAMMING FORMAT 31
STEPS FOR CNC PROGRAMMING 37
ELECTRONICS OF A CNC MACHINE 41
CNC CONTROLLER COMPONENTS SETUP 42
PROGRAMMABLE LOGIC CONTROLLER 44
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ANALYSIS AND PROGRAMMING OF A VERTICAL MACHINING
CENTER 48
MACHINE SPECIFICATIONS 51
PROGRAMM USED IN MANUFACTURING OF TURBINE COUPLINGS 53
STEPS IN USING THE MACHINE 55
ADVANTAGES OF CNC MACHINES 56
REFERENCE 57
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BHARAT HEAVY ELECTRICALS LTD.
BHEL was established more than 50 years ago when its first plant was setup in Bhopal ushering
in the indigenous Heavy Electrical Equipment Industry in India. A dream which has been more
than realized with a well recognized track record of performance it has been earning profits
continuously since 1971-72 and achieved a turnover of Rs 2,658 crore for the year 2007-08,
showing a growth of 17 per cent over the previous year. Bharat Heavy Electricals Limited is
country’s ‘Navratna’ company and has earned its place among very prestigious national and
international companies. It finds place among the top class companies of the world for
manufacture of electrical equipments.
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BHEL caters to core sectors of the Indian Economy viz., Power Generation's & Transmission,
Industry, Transportation, Telecommunication, Renewable Energy, Defense, etc. BHEL has
already attained ISO 9000 certification for quality management, and ISO 14001 certification for
environment management and OHSAS – 18001 certification for Occupational Health and Safety
Management Systems. The Company today enjoys national and international presence featuring
in the “Fortune International -500” and is ranked among the top 10 companies in the world,
manufacturing power generation equipment. BHEL is the only PSU among the 12 Indian
companies to figure in “Forbes Asia Fabulous 50” list.
BHARAT HEAVY ELECTRICALS LTD.
An Overview:
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BHEL today is the largest Engineering Enterprise of its kind in India with excellent track record
of performance, making profits continuously since 1971-72.
BHEL's vision is to become a world-class engineering enterprise, committed to enhancing
stakeholder value. The company is striving to give shape to its aspirations and fulfill the
expectations of the country to become a global player.
BHEL business operations cater to core sectors of Indian Economy like.
Power
Industry
Transportation
Transmission
Defenses etc.
The greatest strength of BHEL is its highly skilled and committed employees. Every
participative style of management all these have engendered development of a committed and
motivated workforce setting new benchmarks in terms of productivity, quality and
responsiveness. employee is given an equal opportunity to develop himself and grow in his
career. Continuous training and retraining, career planning, a positive work culture.
POWER TRANSMISSION & DISTRIBUTION (T & D)
BHEL offer wide ranging products and systems for T & D applications. Products
manufactured include power transformers, instrument transformers, dry type transformers, series
– and stunt reactor, capacitor tanks, vacuum – and SF circuit breakers gas insulated switch gears
and insulators.
A strong engineering base enables the Company to undertake turnkey delivery of electric
substances up to 400 kV level series compensation systems (for increasing power transfer
capacity of transmission lines and improving system stability and voltage regulation), shunt
compensation systems (for power factor and voltage improvement) and HVDC systems (for
economic transfer of bulk power). BHEL has indigenously developed the state-of-the-art
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controlled shunt reactor (for reactive power management on long transmission lines). Presently a
400 kV Facts (Flexible AC Transmission System) project under execution.
INDUSTRIES
BHEL is a major contributor of equipment and systems to industries, cement, sugar, fertilizer,
refinances, petrochemicals, paper, oil and gas, metallurgical and other process industries. The
range of system & equipment supplied includes: captive power plants, co-generation plants DG
power plants, industrial steam turbines, industrial boilers and auxiliaries. Water heat recovery
boilers, gas turbines, heat exchangers and pressure vessels, centrifugal compressors, electrical
machines, pumps, valves, seamless steel tubes, electrostatic precipitators, fabric filters, reactors,
fluidized bed combustion boilers, chemical recovery boilers and process controls.
TRANSPORTATION
BHEL is involved in the development design, engineering, marketing, production, installation,
and maintenance and after-sales service of Rolling Stock and traction propulsion systems. In the
area of rolling stock, BHEL manufactures electric locomotives up to 5000 HP, diesel-electric
locomotives from 350 HP to 3100 HP, both for mainline and shunting duly applications. BHEL
is also producing rolling stock for special applications viz., overhead equipment cars, Special
well wagons, Rail-cum-road vehicle etc., Besides traction propulsion systems for in-house use,
BHEL manufactures traction propulsion systems for other rolling stock producers of electric
locomotives, diesel-electric locomotives, electrical multiple units and metro cars. The electric
and diesel traction equipment on India Railways are largely powered by electrical propulsion
systems produced by BHEL. The company also undertakes retooling and overhauling of rolling
stock in the area of urban transportation systems. BHEL is geared up to turnkey execution of
electric trolley bus systems, light rail systems etc. BHEL is also diversifying in the area of port
handing equipment and pipelines transportation system.
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RENEWABLE ENERGY
Technologies that can be offered by BHEL for exploiting non-conventional and renewable
sources of energy include: wind electric generators, solar photovoltaic systems, solar lanterns
and battery-powered road vehicles. The Company has taken up R&D efforts for development of
multi-junction amorphous silicon solar cells and fuel based systems.
MISSION
The leading Indian engineering enterprise providing quality products systems and
services in the fields of energy, transportation, infrastructure and other potential areas.
The design and construction of Computer Numerically Controlled(CNC) machines differs
greatly from that of conventional machine tools.This difference arises from the requirements of
higher performance levels.The CNC machines often employ the various mechatronics elements
thathave been developed over the years. However, the quality and reliability of these machines
depends on the various machine elements and subsystems of the machines. There are some of the
important constituents parts and aspectsof CNC machines to be considered in their designing, for
example Machinestructure, Guideways, Feed drives, Spindle and Spindle bearings,
Measuringsystems, Controls, Software and Operator interface, Gauging, Toolmonitoring.The
control of a machine tool by means of stored information
throughthe computer is known as Computer Numerically Controlled. Theinformation stored in
the computer can be read by automatic means andconverted into electrical signals, which operate
the electrically controlledservo systems. Electrically controlled servo systems permits the slides
of amachine tool to be driven simultaneously and at the apporopriate feeds anddirection so that
complex shapes can be cut, often with a single operationand without the need to reorient the
workpiece.Computer Numerically Control can be applied to milling
machines,Lathe machines, Grinding machines, Boring machines, Flame cutters,Drilling
machines etc.
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International Operations
BHEL has, over the years, established its references in around 60 countries of the world,
ranging for the United States in the West to New Zealand in the Far East. These references
encompass almost the entire product range of BHEL, covering turnkey power projects of
thermal, hydro and gas-based types, substation projects, rehabilitation projects, besides a wide
variety of products, like transformers, insulators, switchgears, heat exchangers, castings and
forgings, valves, well-head equipment, centrifugal compressors, photo-voltaic equipment etc.
Apart from over 1110MW of boiler capacity contributed in Malaysia, and execution of four
prestigious power projects in Oman, Some of the other major successes achieved by the
Company have been in Australia, Saudi Arabia, Libya, Greece, Cyprus, Malta, Egypt,
Bangladesh, Azerbaijan, Sri Lanka, Iraq etc.
The Company has been successful in meeting demanding customer's requirements in terms
of complexity of the works as well as technological, quality and other requirements viz extended
warrantees, associated O&M, financing packages etc. BHEL has proved its capability to
undertake projects on fast-track basis. The company has been successful in meeting varying
needs of the industry, be it captive power plants, utility power generation or for the oil sector
requirements. Executing of Overseas projects has also provided BHEL the experience of
working with world renowned Consulting Organisations and inspection Agencies.
In addition to demonstrated capability to undertake turnkey projects on its own, BHEL
possesses the requisite flexibility to interface and complement with International companies for
large projects by supplying complementary equipment and meeting their production needs for
intermediate as well as finished products.
Technology Upgradation and Research & Development
To remain competitive and meet customers' expectations, BHEL lays great emphasis on
the continuous upgradation of products and related technologies, and development of new
products. The Company has upgraded its products to contemporary levels through continuous in
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house efforts as well as through acquisition of new technologies from leading engineering
organizations of the world. The Corporate R&D Division at Hyderabad, spread over a 140 acre
complex, leads BHEL's research efforts in a number of areas of importance to BHEL's product
range. Research and product development centers at each of the manufacturing divisions play a
complementary role.
BHEL's Investment in R&D is amongst the largest in the corporate sector in India.
Products developed in-house during the last five years contributed about 8.6% to the revenues in
2000-2001.
BHEL has introduced, in the recent past, several state-of-the-art products developed in-
house: low-NQx oil / gas burners, circulating fluidized bed combustion boilers, high-efficiency
Pelton hydro turbines, petroleum depot automation systems, 36 kV gas-insulated sub-stations,
etc. The Company has also transferred a few technologies developed in-house to other Indian
companies for commercialization.
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PROJECT REPORT
CNC SYSTEMS
The definition of CNC given by Electronic Industry Association (EIA) is as follows:
“A system in which actions are controlled by the direct insertion of numerical
data at some point. The system must automatically interpret at least some
portion of this data.”
In a simple word, a CNC system receieves numerical data, interpret the data and then control the
action accordingly.
A Vertical Machining Center
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INTRODUCTION
Numerical control (NC) is a method employed for controlling the motions of a machine tool
slide and its auxiliary functions with input in the form of numerical data. A computer numerical
control (CNC) is a microprocessor-based system to store and process the data for the control of
slide motions and auxiliary functions of the machine tools. The CNC system is the heart and
brain of a CNC machine which enables the operation of various machine members such as slides,
spindles, etc. as per the sequence programmed into it, depending on the machining operations.
The main advantage of a CNC system lies in the fact that the skills of the operator hitherto
required in the operation of a conventional machine is removed and the part production is made
automatic.
The CNC systems are constructed with a NC unit integrated with a programmable logic
controller (PLC) and some times with an additional external PLC (non-integrated). The NC
controls the spindle movement and the speeds and feeds in machining. It calculates the traversing
path of the axes as defined by the inputs. The PLC controls the peripheral actuating elements of
the machine such as solenoids, relay coils, etc. Working together, the NC and PLC enable the
machine tool to operate automatically. Positioning and part accuracy depend on the CNC
system's computer control algorithms, the system resolution and the basic mechanical machine
accuracy. Control algorithm may cause errors while computing, which will reflect during
contouring, but they are very negligible. Though this does not cause point to point positioning
error, but when mechanical machine inaccuracy is present, it will result in poorer part accuracy.
Computer Numerical Control (CNC) is a specialized and versatile form of Soft
Automation and its applications cover many kinds, although it was initially developed to
control the motion and operation of machine tools.
Computer Numerical Control may be considered to be a means of operating a machine through
the use of discrete numerical values fed into the machine, where the required 'input' technical
information is stored on a kind of input media such as floppy disk, hard disk, CD ROM, DVD,
USB flash drive, or RAM card etc. The machine follows a predetermined sequence of
machining operations at the predetermined speeds necessary to produce a work piece of the
right shape and size and thus according to completely predictable results. A different product can
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be produced through reprogramming and a low-quantity production run of different products is
justified.
Hitachi Seiki VA35 CNC Machining Center
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Hitachi Seiki VA35 CNC Machining Center
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CONFIGURATION OF THE CNC SYSTEM
Fig. shows a schematic diagram of the working principle of a NC axis of a CNC
machine and the interface of a CNC control.
CNC system
Fig.1 Schematic diagram of a CNC machine tool
A CNC system consists of the following 6 major elements:
a. Input Device
b. Machine Control Unit
c. Machine Tool
d. Driving System
e. Feedback Devices
f. Display Unit
N
C
P
L
C
Servo
Drive
Servo
Motor
Spindle
Head
Work
piece
Table
Encoder
Position Feedback
Tacho
Generator Velocity
Feedbac
k
Tape Reader
Tape Punch
Other Devices
Machine
Elements
Inputs
Output
s
Lead
Screw
Command
value
Proximity
switches
Limit switches
Relay coils
Pressure switches
Float switches
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Input Devices
a. Floppy Disk Drive
Floppy disk is a small magnetic storage device for CNC data input. It has been the most common
storage media up to the 1970s, in terms of data transfer speed, reliability, storage size, data
handling and the ability to read and write. Furthermore, the data within a floppy could be
easily edited at any point as long as you have the proper program to read it.
However,this method has proven to be quite problematic in the long run as floppies have a
tendency to degrade alarmingly fast and are sensitive to large magnetic fields and as well as the
dust and scratches that usually existed on the shop floor.
b. USB Flash Drive
A USB flash drive is a removable and rewritable portable hard drive with compact
size and bigger storage size than a floppy disk. Data stored inside the flash drive are impervious
to dust and scratches that enable flash drives to transfer data from
place to place. In recent years, all computers support USB flash drives to read and
write data that make it become more and more popular in CNC machine control unit.
c. Serial communication
The data transfer between a computer and a CNC machine tool is often accomplished
through a serial communication port. International standards for serial communications are
established so that information can be exchanged in an orderly way. The most common interface
between computers and CNC machine tools is referred to the EIA Standard RS-232. Most of
the personal computers and CNC machine tools have built in RS232 port and a standard RS-232
cable is used to connect a CNC machine to a computer which enables the data transfer in
reliable way. Part programs can be downloaded into the memory of a machine tool or
USB Flash Drive
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uploaded to the computer for temporary storage by running a communication program on
the computer and setting up the machine control to interact with the communication software.
SerialCommunication in a Distributed Numerical Control System
Direct Numerical Control is referred to a system connecting a set of numerically
controlled machines to a common memory for part program or machine program
storage with provision for on-demand distribution of data to the machines. (ISO
2806:1980) The NC part program is downloaded a block or a section at a time into the
controller. Once the downloaded section is executed, the section will be discarded to
leave room for other sections. This method is commonly used for machine tools that do not have
enough memory or storage buffer for large NC part programs.
Distributed Numerical Control is a hierarchical system for distributing data between a
production management computer and NC systems. (ISO 2806:1994) The host computer is
linked with a number of CNC machines or computers connecting to the CNC machines
for downloading part programs. The communication program in the host computer can
utilize two-way data transfer features for production data communication including: production
schedule, parts produced and machine utilization etc.
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Machine Control Unit (MCU)
The machine control unit is the heart of the CNC system. There are two sub-units in the machine
control unit: the Data Processing Unit (DPU) and the Control Loop Unit (CLU).
a. Data Processing Unit
On receiving a part programme, the DPU firstly interprets and encodes the part programme into
internal machine codes. The interpolator of the DPU then calculate the intermediate
positions of the motion in terms of BLU (basic length unit) which is the smallest unit length that
can be handled by the controller. The calculated data are passed to CLU for further action.
b. Control Loop Unit
The data from the DPU are converted into electrical signals in the CLU to control the driving
system to perform the required motions. Other functions such as machine spindle ON/OFF,
coolant ON/OFF, tool clamp ON/OFF are also controlled by this unit according to the internal
machine codes.
Machine Tool
This can be any type of machine tool or equipment. In order to obtain high accuracy and
repeatability, the design and make of the machine slide and the driving lead screw of a CNC
machine is of vital importance. The slides are usually machined to high accuracy and coated with
anti-friction material such as PTFE and Turcite in order to reduce the stick and slip phenomenon.
Large diameter recirculating ball screws are employed to eliminate the backlash and lost motion.
Other design features such as rigid and heavy machine structure; short machine table overhang,
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quick change tooling system, etc also contribute to the high accuracy and high repeatability
of CNC machines.
Different Machine Tools
Driving System
The driving system is an important component of a CNC machine as the accuracy and
repeatability depend very much on the characteristics and performance of the driving system.
The requirement is that the driving system has to response accurately according to the
programmed instructions. This system usually uses electric motors although hydraulic motors are
sometimes used for large machine tools. The motor is coupled either directly or through a gear
box to the machine lead screw to moves the machine slide or the spindle. Three types of
electrical motors are commonly used.
Stepping Motor
A stepping motor is a device that converts the electrical pulses into discrete mechanical
rotational motions of the motor shaft. This is the simplest device that can be applied to CNC
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machines since it can convert digital data into actual mechanical displacement. It is not
necessary to have any analog-to-digital converter nor feedback device for the control system.
They are ideally suited to open loop systems.
However, stepping motors are not commonly used in machine tools due to the following
drawbacks: slow speed, low torque, low resolution and easy to slip in case of overload. Examples
of stepping motor application are the magnetic head of floppy-disc drive and hard disc drive of
computer, daisy-wheel type printer, X-Y tape control, and CNC EDM Wire-cut machine.
Stepping Motor
FEEDBACK SYSTEMS
Following are the two types of control systems used in the CNCmachines
1. Open loop control system.
2. Closed loop control system.
1. Open loop control system:-
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In an open loop system the machine slides are displacedaccording to the information loaded from
the part program into thecontrol system. Hence there is no measurement of slide position and
nofeedback signals for comparison with the input signal. The correctmovement of slide entirely d
epends upon the ability of the drivesystems to move the slide through the required exact
distance.The most common method of driving the lead screw is by a stepper motor. The stepper
motors are the simplest way for converting detailelectrical signals into proportional movement.
As there is no check onthe slide position, the system accuracy depends upon the motors abilityto
step through the exact number of steps provided at the input asshown in fig.
2. Closed loop control system:-
A closed loop system is as shown in fig. sends back a signal to
thecontrol unit from a measuring device called as transducer. Thetransducer is attached to the
slide ways. The signal indicates the actualmovement and position of the slides.The control unit
continues to adjust the position of the slide untilit arrives it’s destination, this system has
feedback. Although morecostly and complex than open loop system, these system gives
moreaccurate positioning. For this type of system, servomotors are used
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Feedback Device
In order to have a CNC machine operating accurately, the positional values and
speed of the axes need to be constantly updated. Two types of feed back devices
are normally used, positional feed back device and velocity feed back device.
a. Positional Feed Back Devices
There are two types of positional feed back devices: linear transducer for direct
positional measurement and rotary encoder for angular or indirect linear
measurement.
Linear Transducers - A linear transducer is a device mounted on the
machine table to measure the actual displacement of the slide in such a
way that backlash of screws; motors, etc would not cause any error in the
feed back data. This device is considered to be of the highest accuracy and
also more expensive in comparison with other measuring devices mounted
on screws or motors.
Linear Transducer (Courtesy of Heidenhain)
Rotary Encoders - A rotary encoder is a device mounted at the end of the
motor shaft or screw to measure the angular displacement. This device
cannot measure linear displacement directly so that error may occur due to
the backlash of screw and motor etc. Generally, this error can be
compensated for by the machine builder in the machine calibration
process.
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b. Velocity Feedback Device
The actual speed of the motor can be measured in terms of voltage generated
from a tachometer mounted at the end of the motor shaft. DC tachometer is
essentially a small generator that produces an output voltage proportional to the
speed. The voltage generated is compared with the command voltage
corresponding to the desired speed. The difference of the voltages can is then
used to actuate the motor to eliminate the error.
Tachogenerator (Courtesy of Callan)
Display Unit
The Display Unit serves as an interactive device between the machine and the operator. When
the machine is running, the Display Unit displays the present status such as the position of
the machine slide, the spindle RPM, the feed rate, the part programmes, etc. In an advanced CNC
machine, the Display Unit can show the graphics simulation of the tool path so that part
programmes can be verified before the actually
machining. Much other important information about the CNC system can also displayed for
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maintenance and installation work such as machine parameters, logic diagram of the programmer
controller, error massages and diagnostic data.
A Display Unit
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CNC PART PROGRAMMING
Objectives:
To understand the Dimension Systems in CNC Part Programming.
To understand the structure of a CNC Part Programme.
To understand the G-codes and other functions of a CNC Part
Programme.
Axis of motion
In generally, all motions have 6 degrees of freedom. In other words, motion can
be resolved into 6 axes, namely, 3 linear axes (X, Y and Z axis) and 3 rotational
axes (A, B, and C axis).
Axis of Motion
x
Y
Z
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Programming Systems
Two types of programming modes, the incremental system and
the absolute system, are used for CNC. Both systems have
applications in CNC programming, and no system is either right or
wrong all the time.
Positioning reference point Systems:
1- Incremental
2- Absolute
Most controls on machine tools today are capable of handling either incremental or
absolute programming.
Incremental program locations are always given as the distance
and direction from the immediately preceding point .
Command codes which tell the machine to move the table, spindle,
and knee are explained here using a vertical milling machine as
an example:
A workpiece dimensioned in the incremental system mode
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• A “X plus” (X+) command will cause the cutting tool to be
located to the right of the last point.
• A “X minus” (X-) command will cause the cutting tool to be located
to the left of the last point.
• A “Y plus” (Y+) command will cause the cutting tool to be
located toward the column.
• A “Y minus” (Y-) will cause the cutting tool to be located away
from the column.
• A “Z plus” (Z+) command will cause the cutting tool or spindle
to move up or away from the workpiece.
• A “Z minus” (Z-) moves the cutting tool down or into the workpiece.
In incremental programming, the G91 command indicates to the
computer and MCU (Machine Control Unit) that programming is in
the incremental mode.
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Absolute program locations are always given from a single fixed
zero or origin point (Fig. 7). The zero or origin point may be a
position on the machine table, such as the corner of the worktable
or at any specific point on the workpiece. In absolute dimensioning
and programming, each point or location on the workpiece is given
as a certain distance from the zero or reference point.
A workpiece dimensioned in the absolute system mode. Note: All dimensions are given
from a known point of referenc
• A “X plus” (X+) command will cause the cutting tool to be
located to the right of the zero or origin point.
• A “X minus” (X-) command will cause the cutting tool to be located
to the left of the zero or origin point.
• A “Y plus” (Y+) command will cause the cutting tool to be
located toward the column.
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• A “Y minus” (Y-) command will cause the cutting tool to be located away from the
column.
In absolute programming, the G90 command indicates to the
computer and MCU that the programming is in the absolute mode.
Point-to-Point or Continuous Path
CNC programming falls into two distinct categories . The
difference between the two categories was once very distinct.
Now, however, most control units are able to handle both point-topoint
and continuous path machining. A knowledge of both programming
methods is necessary to understand what applications
each has in CNC.
CNC POSITIONING SYSTEM
Point-To-Point OR Positioning
Continuous Path OR Contouring
Point-to-Point Positioning
Point-to-point positioning is used when it is necessary to accurately
locate the spindle, or the workpiece mounted on the machine
table, at one or more specific Iocations to perform such
operations as drilling, reaming, boring, tapping, and punching (Fig.
9). Point-to-point positioning is the process of positioning from one
coordinate (XY) position or location to another, performing the
machining operation, and continuing this pattern until all the
operations have been completed at all programmed locations.
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The path followed by point-to-point positioning to reach various programmed points
(machining locations) on the XY axis.
In Fig. above, point 1 to point 2 is a straight line, and the machine
moves only along the X axis; but points 2 and 3 require that
motion along both the X and Y axes takes place. As the distance
in the X direction is greater than in the Y direction, Y will reach its
15 position first, leaving X to travel in a straight line for the remaining
distance. A similar motion takes place between points 3 and 4.
Continuous Path (Contouring)
Contouring, or continuous path machining, involves work such as
that produced on a lathe or milling machine, where the cutting tool
is in contact with the workpiece as it travels from one programmed
point to the next. Continuous path positioning is the ability to
control motions on two or more machine axes simultaneously to
keep a constant cutter-workpiece relationship. The programmed
information in the CNC program must accurately position the
cutting tool from one point to the next and follow a predefined
accurate path at a programmed feed rate in order to produce the
form or contour required.(Fig. on next page)
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Fig. 10 Types of contour
machining
(A) Simple
contour;
(B) complex
contour
Interpolation
The method by which contouring machine tools move from one
programmed point to the next is called interpolation. This ability to
merge individual axis points into a predefined tool path is built into
most of today’s MCUs. There are five methods of interpolation:
linear, circular, helical, parabolic, and cubic. All contouring controls
provide linear interpolation, and most controls are capable of both
linear and circular interpolation. Helical, parabolic, and cubic
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interpolation are used by industries that manufacture parts which
have complex shapes, such as aerospace parts and dies for car
bodies.
Linear Interpolation
Linear Interpolation consists of any programmed points linked
together by straight lines, whether the points are close together or
far apart . Curves can be produced with linear interpolation
by breaking them into short, straight-line segments. This
method has limitations, because a very large number of points
would have to be programmed to describe the curve in order toproduce a contour
shape.
A contour programmed in linear interpolation requires the coordinate
positions (XY positions in two-axis work) for the start and
finish of each line segment. Therefore, the end point of one line or
segment becomes the start point for the next segment, and so on,
throughout the entire program.
An example of two-axis linear interpolation.
Circular Interpolation
The development of MCUs capable of circular interpolation has
greatly simplified the process of programming arcs and circles. To
program an arc (Fig. 12), the MCU requires only the coordinate
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positions (the XY axes) of the circle center, the radius of the circle,
the start point and end point of the arc being cut, and the direction
in which the arc is to be cut (clockwise or counterclockwise) See
Fig. . The information required may vary with different MCUs.
For two-dimensional circular interpolation the MCU must be supplied with the XY axis,
radius, start point, end point, and direction of cut.
Programming Format
Word address is the most common programming format used for
CNC programming systems. This format contains a large number
of different codes (preparatory and miscellaneous) that transfers
program information from the part print to machine servos, relays,
micro-switches, etc., to manufacture a part. These codes, which
conform to EIA (Electronic Industries Association) standards, are
in a logical sequence called a block of information. Each block
should contain enough information to perform one machining
operation.
36
Word Address Format
Every program for any part to be machined, must be put in a format that the machine
control unit can understand. The format
used on any CNC machine is built in by the machine tool builder
and is based on the type of control unit on the machine. A variable-
block format which uses words (letters) is most commonly
used. Each instruction word consists of an address character,
such as X, Y, Z, G, M, or S. Numerical data follows this address
character to identify a specific function such as the distance, feed
rate, or speed value.
The address code G90 in a program, tells the control that all
measurements are in the absolute mode. The code G91, tells the
control that measurements are in the incremental mode.
Codes
The most common codes used when programming CNC machines
tools are G-codes (preparatory functions), and M codes
(miscellaneous functions). Other codes such as F, S, D, and T are
used for machine functions such as feed, speed, cutter diameter
offset, tool number, etc.
G-codes are sometimes called cycle codes because they refer to
some action occurring on the X, Y, and/or Z axis of a machine tool,
Fig. 13.
The G-codes are grouped into categories such as Group 01,
containing codes G00, G01, G02, G03. which cause some movement
37
of the machine table or head. Group 03 includes either
absolute or incremental programming, while Group 09 deals with canned cycles.
A G00 code rapidly positions the cutting tool while it is above the
workpiece from one point to another point on a job. During the
rapid traverse movement, either the X or Y axis can be moved
individually or both axes can be moved at the same time. Although
the rate of rapid travel varies from machine to machine, it ranges
between 200 and 800 in./min (5 and 20 m/min).
The G01, G02, and G03 codes move the axes at a controlled
feedrate.
• G01 is used for straight-line movement (linear interpolation).
• G02 (clockwise) and G03 (counterclockwise) are used for arcs
and circles (circular interpolation)
38
G- Codes
Group Code Function
01 G00 Rapid positioning
01 G01 Linear interpolation
01 G02 Circular interpolation clockwise (CW)
01 G03 Circular interpolation
counterclockwise (CCW)
06 G20* Inch input (in.)
06 G21* Metric input (mm)
G24 Radius programming (**)
00 G28 Return to reference point
00 G29 Return from reference point
G32 Thread cutting (**)
07 G40 Cutter compensation cancel
07 G41 Cutter compensation left
07 G42 Cutter compensation right
08 G43 Tool length compensation positive
(+) direction
08 G44 Tool length compensation minus (-)
direction
08 G49 Tool length compensation cancel
39
G84 Canned turning cycle (**)
03 G90 Absolute programming
03 G91 Incremental programming
(*) - on some machines and controls, these may be G70 (inch) and
G71 (metric)
(**) - refers only to CNC lathes and turning centers
M Codes-
Code Function
M00 Program stop
M01 Optional program stop
M02 Program end
M03 Spindle on clockwise
M04 Spindle on counterclockwise
M05 Spindle stop
M06 Tool change
M08 Coolant on
M09 Coolant off
M10 Clamps on
40
M11 Clamps off
M30 Program stop, reset to start
Block of Information
CNC information is generally programmed in blocks of five words.
Each word conforms to the EIA standards and they are written on
a horizontal line. If five complete words are not included in each
block, the machine control unit (MCU) will not recognize the
information, therefore the control unit will not be activated.
Using the example shown in Fig. 17 , the five words are as follows:
N001 represents the sequence number of the operation.
G01 represents linear interpolation
X12345 will move the table 1.2345 in. in a positive direction
along the X axis.
Y06789 will move the table 0.6789 in. along the Y axis.
M03 Spindle on CW.
A complete block of information consists of five words.
41
Steps for CNC Programming and Machining
The following is the procedures to be followed in CNC programming and
machining. The most important point is to verify the programme by test run it on
the machine before the actual machining in order to ensure that the programme
is free of mistakes.
a. Study the part drawing carefully.
b. Unless the drawing dimensions are CNC adapted, select a suitable
programme zero point on the work piece. The tool will be
adjusted to this zero point during the machine set up.
c. Determine the machining operations and their sequence.
d. Determine the method of work clamping (vice, rotary table,
fixtures etc).
e. Select cutting tools and determine spindle speeds and feeds.
f. Write programme (translate machining steps into programme
blocks). If many solutions are possible, try the simplest solution
first. It is usually longer, but better to proceed in this way.
g. Prepare tool chart or diagram, measure tool geometry (lengths,
radii) and note.
h. Clamp work piece and set up machine.
i. Enter compensation value if necessary.
j. Check and test programme. It is a good practice to dry run the
programme (i) without the workpiece, (ii) without the cutting
tools, or (iii) by raising the tool to a safe height.
If necessary, correct and edit programme and check again.
k. Start machining.
42
Programming for Positioning
Before starting to program a job, it is important to become familiar
with the part to be produced. From the engineering drawings, the
programmer should be capable of planning the machining sequences
required to produce the part. Visual concepts must be
put into a written manuscript as the first step in developing a part
program, Fig. 18. It is the part program that will be sent to the
machine control unit by the computer, tape, diskette, or other input
media.
The programmer must first establish a reference point for aligning
the workpiece and the machine tool for programming purposes.
The manuscript must include this along with the types of cutting
tools and work-holding devices required, and where they are to be
located.
Dimensioning Guidelines
The system of rectangular coordinates is very important to the
successful operation of CNC machines. Certain guidelines should
be observed when dimensioning parts for CNC machining. The
following guidelines will insure that the dimensioning language
means exactly the same thing to the design engineer, the technician,
the programmer, and the machine operator.
1. Define part surfaces from three perpendicular reference
planes.
2. Establish reference planes along part surfaces which are
parallel to the machine axes.
3. Dimension from a specific point on the part surface.
43
25
4. Dimension the part clearly so that its shape can be understood
without making mathematical calculations or guesses.
5. Define the part so that a computer numerical control cutter
path can be easily programmed.
Machine Zero Point
The machine zero point can be set by three methods—by the
operator, manually by a programmed absolute zero shift, or by
work coordinates, to suit the holding fixture or the part to be
machined.
MANUAL SETTING - The operator can use the MCU controls to
locate the spindle over the desired part zero and then set the X
and Y coordinate registers on the console to zero.
Stored zero shifts (G54...G59)
Programmed zero shift (G92)
R = Reference point (maximum travel of machine)
M = Machine zero point (X0,Y0,Z0) of machine coordinate system.
W = Part zero point workpiece coordinate system.
The relationship between the part zero and the machine system of coordinate
44
Under G54 ... G59 the actual machine coordinates of part
zero are stored in the stored zero offsets memory and
activated in the part program.
Under G92 the actual machine coordinates are inserted and
used on the G92 line of the part program.
ABSOLUTE ZERO SHIFT - The absolute zero shift can change
the position of the coordinate system by a command in the CNC
program. The programmer first sends the machine spindle to
home zero position by a G28 command in the program. Then
another command (G92 for absolute zero shift) tells the MCU
how far from the home zero location, the coordinate system origin
is to be positioned, Fig. 19.
The sample commands may be as follows:
N1 G28 X0 Y0 Z0 (sends spindle to home zero position)
N2 G92 X4.000 Y5.000 Z6.000 (the position the machine will
reference as part zero)
45
ELECTRONICS OF A CNC MACHINE
The CNC controller components work together to interpret positioning signals created by a
computer and NC software into precise motor control. This page will explain the function of
each of the controller components and how they work together to make a controller system.
The Components
There are three primary CNC controller components that make up a CNC controller, the
power supply unit, the circuitry protection system, and the motor driver.
The Power Supply Unit
When you plug a small flash drive up to your USB port, the computer powers the device
through the port. When you plug a larger device, say a printer for example, you have to
use an external power source because the computer is not capable of supplying sufficient
power. This condition means that you have to plug that device into the computer as usual,
but you also have to plug a second line to an auxiliary power source, such as the 110V
outlet on your wall.
The same conditions are true for CNC devices. They require a low-voltage
communication line, through which the computer tells the machine what to do, and a power
source that provides the power for moving, cutting, and other such operations.
A power converter, usually referred to as the “power supply unit (PSU),” is often used to change
the form of the supplied power from alternating current (AC) from the power grid, to direct
current (DC) that is more easily used by the machine’s drive motors.
The power supply handles large voltages and currents that could be harmful to the NC circuitry.
Therefore, the power source, motor drivers, and motors are often separated from the computer
with a circuitry protection system that isolates surges in electrical power.
46
The Circuitry Protection System
The circuitry protection system contains a breakout board to isolate signals from the
computer, distribute the signals to the desired drivers, and also allows easy hook up of
peripherals such as limit switches that feed information back to the computer. Fuses are
also part of the circuitry protection system. Fuses could save the equipment in case of
electrical spikes, shorts, or faulty wiring.
A low-voltage communication signal passes from the computer through the breakout
board unchanged to the motor drivers. This isolated your computer from the CNC controller
circuit but allows the signals to carry through to your motor drivers.
The motor drivers
The motor drivers receives the communication signal and and then coordinates pulses of
the desired current and voltage to elicit the movement in the drive motors. The motor drivers
may communicate position information one way to the motor (open loop system), or send and
receive position information(closed loop system), depending on the user’s choice of drive
system. More on these systems may be found in the drivers sections.
CNC Controller Components Setup
The figure below shows the typical setup of the CNC controller components, such as that for a 3
axis CNC router. The system is composed of a computer with NC control software, the controller
box, and the drive motors. The controller box includes a breakout board, power source (not
shown), and motor drivers. The “breakout board” provides the circuitry protection and signal
distribution inside the controller box.
In the figure
above, you
47
can see the computer is connected to the breakout board, usually through the printer port (DB25).
There are many types of breakout boards; some of very high quality and great protection, and
lower budget options that do not offer much protection. The best models employ opto-isolators,
which use a light signals to transmit data across an air gap. These devices provide complete
conductive isolation between the controller circuit and your computer.
As a note, the computer is fully capable of connecting directly to the drivers and driving the
motors, but this setup puts your computer at risk.
The first figure is an idealized block diagram of your CNC electrical system. A little more detail
has been added in the figure below to show the conceptual layout of signal wires (light black)
and power wires (heavy black).
Although your computer will run on 110VAC, the CNC machine may run on 110VAC, or
220VAC, or 480VAC, and may be single phase, three phase, etc… This power enters the
controller box and will be distributed by the power supply. For most machines, the power supply
will convert the incoming power from alternating current (AC) to direct current (DC). This DC
supply will be of a lower voltage, such as 12V, 24V, 36V, or higher. The power supplied
depends on you CNC controller components, specifically the motor drivers.
48
PROGRAMMABLE LOGIC CONTROLLER
“A digitally operating electronic apparatus which uses a programmable memory for the internal
storage of instructions by implementing specific functions such as logic sequencing, timing,
counting, and arithmetic to control, through digital or analog input/output modules, various types
of machines or processes
Programmable Logic Controller (PLC)
A PLC matches the NC to the machine. PLCs were basically introduced as replacement for hard
wired relay control panels. They were developed to be reprogrammed without hardware changes
when requirements were altered and thus are reusable. PLCs are now available with increased
functions, more memory and large input/output capabilities. Fig.7 gives the generalized PLC
block diagram.
In the CPU, all the decisions are made relative to controlling a machine or a process. The CPU
receives input data, performs logical decisions based upon stored programs and drives the
outputs. Connections to a computer for hierarchical control are done via the CPU.
The I/O structure of the PLCs is one of their major strengths. The inputs can be push buttons,
limit switches, relay contacts, analog sensor, selector switches, proximity switches, float
switches, etc. The outputs can be motor starters, solenoid valves, position valves, relay coils,
indicator lights, LED displays, etc.
The field devices are typically selected, supplied and installed by the machine tool builder or the
end user. The voltage level of the field devices thus normally determines the type of I/O. So,
power to actuate these devices must also be supplied external to the PLC. The PLC power supply
is designated and rated only to operate the
internal portions of the I/O structures, and not the field devices. A wide variety of voltages,
current capacities and types of I/O modules are available.
49
The principle of operation of a PLC is determined essentially by the PLC program memory,
processor, inputs and outputs.
The program that determines PLC operation is stored in the internal PLC program memory. The
PLC operates cyclically, i.e. when a complete program has been scanned, it starts again at the
beginning of the program. At the beginning of each cycle, the processor examines the signal
status at all inputs as well as the external timers and counters and are stored in a process image
input (PII). During subsequent program scanning, the processor the accesses this process image.
To execute the program, the processor fetches one statement after another from the
programming memory and executes it. The results are constantly stored in the process image
output (PIO) during the cycle. At the end of a scanning cycle, i.e. program completion, the
processor transfers the contents of the process image output to the output modules and to the
external timers and counters. The processor then begins a new program scan.
Fig.7 Generalized PLC block diagram
Processor
Logic
memory
Storage
memory
Power
Supply
Inputs
Output
s
Power
Supply
Programmer
Field
Devices
50
The "Signals"
The signal lines coming from the computer
operate on 5V DC supplied by the computer
communication port, and is a square wave form
called a Transistor-to-Transistor Logic (TTL)
signal. This signal is essentially a series of
small pulses from 0V to +5V that represent 0’s
and 1’s in a binary computer language. This
signal is a form of a Pulse-Width Modulated
(PWM) signal where the length of the pulse is varied to indicate information. The width of the
pulse determines the binary code sent; either a “0” or a “1” as communicated by the computer
and interpreted by the motor driver. More on the signals may be found in the signals page.
The signal from the computer to the breakout board is the same as that from the breakout board
to the motor driver. Remember, the breakout board provides circuit protection and signal
distribution. Therefore, the signal coming out of the breakout board is also a 5V TTL signal of
the same form. However, as discussed previously, the signal after the driver has been
conditioned as needed to provide the large “move” voltage and current needed to drive the
machine.
51
Breakout Boards
Breakout boards are a common electrical components
that take a bundled cable and “breaks out” each
conductor to a terminal that can easily accept a hook-up
wire for distribution to another device. They are a
common item in electronic projects and enable easy,
clean installation of electronic devices. The image at
right shows a simple DB25 breakout circuit board
from Winford Engineering. The breakout board is
positioned between your computer or indexer and the
motor drivers and serves two purposes in the CNC control system: circuit protection and signal
distribution. Here we will describe the board’s function in the CNC control system and what you
need to know about how to select one to suit your need
52
Analysis And Programming Of A Vertical Machining System
CNC MACHINING CENTER
HITACHI SEIKI VA35 CNC MACHINING CENTER
The Hitachi Seiki VA35 CNC (Computer Numeric Controlled) machining center that
belongs
to the Department of Production Engineering is frequently used for accurate and
automated
machining of metals as well as for wood, plastic and other materials. The machining
accuracy
of the machine is 0.001mm. Figure 4-2 shows various parts of the machine tool
53
Hitachi Seiki VA35 CNC Machining Center
54
S Y
X
Z
Z
EEPROM OR
TAPE INPUT
Control Microprocessor ROM
Motor
Driver
Spindle
Motor
Spindle
Position
X,Y,Z Positions
X,Y,Z Spindle Motors
Numerical Control Of The Machine
55
MACHINE SPECIFICATIONS
GENERAL INFORMATION
� Manufacturer Hitachi Seiki Co. Ltd., Japan
� Model VA 35–II
� Control unit Fanuc System 6M–B
� Weight 4000 kg
TABLE
� Working area 1000*355 mm2
� Maximum carrying capacity 500 Kg
STROKES
� x-axis stroke in the crosswise
direction of the table 560 mm
� y-axis stroke in the longitudinal
direction of the table 350 mm
� z-axis stroke in the vertical
direction of the spindle head 400 mm
� Distance between the spindle nose
and top of the table 150-550 mm
SPINDLE HEAD
� Spindle nose contour NT 40
� Spindle speed 60-600 rpm
� Spindle speed change Stepless (s 4 digit)
� Spindle motor AC 5.5 kW (30 min)
56
FEED
� Least increment 0.001 mm
� Cutting feed rate 3600 mm/min
� Rapid traverse 13000 mm/min
AUTOMATIC TOOL CHANGE (ATC)
� No. of tools 30
� Shank type BT 40, CAT 40
� Maximum tool diameter 95 mm
� Maximum tool length 250 mm
� Maximum tool weight 10 kg
� Tool selection method Random shortest course
� Pull stud type MAS - 1
G CODE AND M CODE
The entire functioning of the machine is based on G Code and M Code specifications.
G Codes define the preparatory functions of the machine. In simple terms, they control
the
movement and machining related functions of the machine tool. For example, the code
"G76"
followed by some related arguments is used for fine boring. "G00" with X,Y,Z arguments
rapidly moves the bed and the spindle head to the position specified by the arguments.
M Codes are known as auxiliary functions. They control specific behaviors of the
machine.
For example "M08" turns on the coolant, M05 stops the spindle.
57
A Program Used In manufacturing Of Turbine Couplings
The listing given below is a program which was used to bore holes in couplings of two
turbines which were manufactured in the Engineering Workshops. It is written in G and
M
Codes.
Unless otherwise stated, all the dimensions are in mm.
1 G28 G91 Z0;
2 G28 X0 Y0;
3 G40 G49;
4 G90;
5 G92 X253.087 Y177.818 Z343.05; 21
6 G00 x131.25;
7 G00 Z5.0 F10;
8 M03 S150;
9 M98 P151;
10 M05;
11 M09;
12 G28 G91 Z0;
13 G28 X0 Y0;
14 M30;
15 %
The meaning of each line is given below.
1 Return to reference point, Incremental programming, Z=0 is the reference point (Z
movement only)
2 Return to reference point, X=0 and Y=0 (X and Y movements only)
3 Tool diameter compensation cancel, Tool length offset cancel
58
4 Absolute programming
5 Programming of absolute zero point, X=253.087, Y=177.818, Z=343.05
6 Positioning (rapid), X=131.25, Y=0
7 Positioning (rapid), Z=5.0, Feed rate set to 10 mm/min
8 Spindle rotation CW, speed=150 rpm
9 Sub program (o0151) call-out
10 Spindle stop
11 Mist/coolant off
12 Return to reference point, Z=0, Incremental programming (Z movement only)
13 Return to reference point, X=0 and Y=0
14 End of program, Control unit reset
15 Just display the end of current listing
Line 9 in the above program calls the sub program o0151. This sub program is
the actual part
of the program which bore holes and is listed below.
1 G76 G98 X131.25 Y0.0 Z-52.0 Q0.5 R2.0;
2 X119.903 Y53.384;
3 (some more x and y values)
4 M99;
The meaning of each line is as follows:
1 Fine boring; Return to initial level in canned cycle after finishing; Starting X,Y
coordinate: X=131.25, Y=0.0; Final Z coordinate = -52.0; Before boring tool is taken
out, move it 0.5 away from the bored wall of the workpiece; Radius of boring = 2.0
(This value does not have any effect on boring since the tool determines the actual
radius.).
2 Repeat boring for X=119.903 and Y=53.384.
3 Repeat the same in line 2.
4 End of sub program.
59
STEPS IN USING THE MACHINE
The distinct operations involved in using the CNC machine are listed below in sequence
they
are done.
1. Generating the program (in G & M Codes)
2. Sending it to the machine
3. Running the program
First a drawing of the machined workpiece is created using AutoCAD in a PC. Then
using a
special routine of AutoCAD, the contours of the cutting tool are generated. This is finally
stored as a text file in the hard drive of the PC.
Next, the CNC machine is set to retrieve this file. Through the coaxial cable which links
the
PC and CNC machine, it is then fed into the machine tool. A numeric name for the
program is
given at the beginning of the file retrieval to figure out the starting point (or the address
in the
memory) of the retrieving program from earlier read programs.
Using this numeric name of the program, it is taken to the front from other programs in
the
memory and it stays waiting to run. Pressing the "Start" button sequentially executes the
listing.
60
ADVANTAGES OF CNC MACHINES
CNC machines are widely used in the metal cutting industry and are best used to produce the
following types of product:
• Parts with complicated contours
• Parts requiring close tolerance and/or good repeatability
• Parts requiring expensive jigs and fixtures if produced on conventional machines
• Parts that may have several engineering changes, such as during the development stage of
a prototype
• In cases where human errors could be extremely costly
• Parts that are needed in a hurry
• Small batch lots or short production runs
Some common types of CNC machines and instruments used in industry are as following:
• Drilling Machine
• Lathe / Turning Centre
• Milling / Machining Centre
• Turret Press and Punching Machine
• Wirecut Electro Discharge Machine (EDM)
• Grinding Machine
• Laser Cutting Machine
• Water Jet Cutting Machine
And Most important For Electronics industry is that CNC Milling and Drilling is used for
Manufacturing Of PCB .( Printed Circutiry Board)..
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