Computer Integrated manufacturing:

The computer has had and continues to have a dramatic impact on the development of

production automation technologies. Nearly all modern production systems are implemented

today using computer systems. The term computer integrated manufacturing (CIM) has been

coined to denote the pervasive use of computers to design the products, plan the production,

control the operations, and perform the various business related functions needed in a

manufacturing firm. CAD/CAM (computer-aided design and computer-aided manufacturing) is

another term that is used almost synonymously with CIM.

In a manufacturing firm the physical activities related to production that take place in the

factory can be distinguished from the information-processing activities, such as product design

and production planning, that usually occur in the office environment. The physical activities

include all the manufacturing processing, assembly, material handling, and inspections that are

performed on the product. These operations come in direct contact with the product during

manufacture. They touch the product. The relationship between the physical activities and the

information processing activities. Raw material flow in one end of the factory and finished

products follow out the other end. The physical activities (processing, handling, etc) take place

inside the factory. The information –processing functions form a ring that surround the factory,

providing the data and knowledge required to product the product successfully. These

information-processing functions include (1) certain business activities (e.g marketing and sales,

order entry, customer billing etc) (2) product design, (3) manufacturing planning, and (4)

manufacturing control. These four functions form a cycle of event that must accompany the

physical production activities but which do not directly touch the product.

Now considering the difference between automation and CIM. Automation is concerned with

the physical activities in manufacturing. Automated production systems are designed to

accomplish the processing, assembly, material handling, and inspecting activities with little or no

human participation. By comparison, computer integrated manufacturing is concerned more with

the information-processing functions that are required to support the production operations. CIM

involves the use of computer systems to perform the four types of information-processing

functions. Just as automation deals with the physical activities, CIM deals with automating the

information-processing activities in manufacturing. The growing applications of computer

systems in manufacturing are leading us toward the computer-automated factory of the future.

REASONS FOR AUTOMATING

1. Increased productivity: Automation of manufacturing operations holds the promise of

increasing the productivity of labour. This means greater output per hour of labour input.

Higher production rated (output per hour) are achieved with automation than with the

corresponding manual operations.

2. High cost of labour: the trend in the industrialised societies of the world has been towards

ever-increasing labour costs. As a result, higher investment in automated equipment has

become economically justifiable to replace manual operations. The higher cost of labour

is forcing business leaders to substitute machine for human labour. Because machines can

produce at higher rates of output, the use of automation results in a lower cost per unit of

product.

3. Labour shortage: In many advanced nations there has been a general shortage of labour.

West Germany, for example has been forced to import labour to augment its own labour

supply. Labour shortages also stimulate the development of automation as a substitute for

labour.

4. Trend of labour toward the service sector: This trend has been especially prevalent is the

United States. At this writing (1986), the proportion of the work force employed in

manufacturing stands at about 20%. In 1947, this percentage was 30%. By the year 2000,

some estimates put the figure as low as 2%. Certainly, automation of production jobs has

caused some of this shift. However, there are also social and institutional forces that are

responsible for the trend. The growth of government employment at the federal state, and

local levels has consumed a certain share of the labour market which might otherwise

have gone into manufacturing. Also, there has been a tendency for people to view factory

work as tedious, demeaning, and dirty. This view has caused them to seek employment in

the service sector of the economy (government, insurance, personal services, legal, sales

etc.).

5. Safety: By automating the operation and transferring the operator from an active

participation to a supervisory role, work is made sager. The safety and physical well-

being of the worker has become a notional objective with the enactment of the

Occupational Safety and Health Act of 1970 (OSHA). It has also provided an impetus for

automation.

6. High cost of raw materials: The high cost of raw material in manufacturing results in the

need for greater efficiency in using these materials. The reduction of scrap is one of the

benefits of automation.

7. Improved product quality: Automated operations not only produce parts at faster rates

than do their manual counterparts, but they produce parts with greater consistency and

conformity to quality specifications.

8. Reduced manufacturing lead time: Automation allows the manufacturer to reduce the

time between customer order and product delivery. This gives the manufacturer a

competitive advantage in promoting good customer service.

9. Reduction of in-process inventory: Holding large inventories of work-in-process

represents a significant cost to the manufacturer because it ties up capital. In-process

inventory is of no value. It serves none of the purposes of raw materials stock or finished

product inventory. Accordingly, it is to the manufacturer’s advantage to reduce work-in-

progress to a minimum. Automation tends to accomplish this goal by reducing the time a

work part spends in factory.

10. High cost of not automating. A significant competitive advantage is gained by

automating a manufacturing plant. The advantage cannot easily be demonstrated on a

company’s project authorisation form. The benefits of automation often show up in

intangible and unexpected ways, such as improved quality, higher sales, better labour

relations, and better company image. Companies that do not automate are likely to find

themselves at a competitive disadvantage with their customers, their employees, and the

general public.

ARGUMENTS AGAINST AUTOMATION:

1. Automation will result in the subjugation of the human being by a machine. This is really

an argument over whether workers’ jobs will be downgraded or upgraded by automation.

On the one hand, automation tends to transfer the skill required to perform work from

human operators to machines. In so doing, it reduces the need for skilled labour. The

manual worker left by automation requires lower skill levels and tend to involve rather

menial tasks (e.g. loading and unloading work parts, changing tools, removing chips,

etc.). in this sense, automation tends to downgrade factory work. On the other hand, the

routine monotonous tasks are the easiest to automate, and are therefore the first jobs to be

automated. Fewer workers are thus needed in these jobs. Tasks requiring judgement and

skill are more difficult to automate. The net result is that the overall level of

manufacturing labour will be upgraded, not downgraded.

2. There will be a reduction in the labour force, with resulting unemployment. It is logical to

argue that the immediate effect of automation will be to reduce the need for human

labour, thus displacing workers. Because automation will increase productivity by a

substantial margin, the creation of new jobs will not occur fast enough to take up the

slack of displaced workers. As a consequence, unemployment rates will accelerate.

3. Automation will reduce purchasing power. This follows from argument 2. As machines

replace workers and these workers join the unemployment ranks, they will not receive the

wages necessary to buy the products brought by automation. Markets will become

saturated with products that people cannot afford to purchase. Inventories will grow.

Production will stop. Unemployment will reach epidemic proportions. And the result will

be a massive economic depression.

Arguments in favour of automation:

1. Automation is the key to the shorter workweek. There has been and is a trend toward

fewer working hours and more leisure time. Around the turn of the century, the average

workweek was about 70 hours per week. The standard is currently 40 hours (although

many in the labour force work overtime). The argument holds that automation will allow

the average number of working hours per week to continue to decline, thereby allowing

greater leisure hours and a higher quality of life.

2. Automation brings safer working conditions for the worker. Since there is less direct

physical participation by the worker in the production process, there is less chance of

personal injury to the worker.

3. Automated production results in lower prices and better products. It has been estimated

that the cost to machine one unit of product by conventional general-purpose machine

tools requiring human operators may by 100 times the cost of manufacturing the same

unit using automated mass-production techniques. Example abound, the machining of an

automobile engine block by transfer line techniques may cost $25 to $35 . if conventional

techniques were used on reduced quantities (and the quantities would indeed be much

lower if conventional methods were used)the cost would increase to around $3000. The

electronics industry offers many examples of improvements in manufacturing technology

that have significantly reduced costs while increasing product value (e.g. colour TV sets,

stereo equipment, hand-held calculators, and computers)

4. The growth of the automation industry will itself provide employment opportunities. This

has been especially true in the computer industry. As the companies in this industry have

grown, new jobs have been created, these new jobs include not only workers directly

employed by these companies, but also computer programmers, systems engineers, and

others needed to the use and operate the computers.

5. Automation is the only means of increasing our standard of living. Only through

productivity increases brought about by new automated methods of production will we be

able to advance our standard of living. Granting wage increases without a commensurate

increase in productivity will result in inflation. In effect, this will reduce our standard of

living. To afford a better society, we must increase productivity faster than we increase

wages and salaries. Therefore, as this argument proposes, automation is a requirement to

achieve the desired increase in productivity.

NUMERICAL CONTROL:

What is numerical control???

Numerical control (NC) is a form of programmable automation in which the processing

equipment is control by means of numbers, letters, and other symbols. The numbers, letters, and

symbols are coded in a appropriate format to define a program of instructions for a particular

work part of job. When the job changes, the program of instructions is changed. The capability to

change the program is what makes NC suitable for low and medium volume production. It is

much easier to write a new program than to make major alterations of the processing equipment.

The applications of numerical control range over a wide variety of processes. We divide the

application into two categories: (1) machine tool applications such as drilling, milling, turning

and other metal working; and (2) non-machine tool application, such as assemble, drafting, and

inspection. The common operating principle of NC in all of these application is control of the

relative position of a tool or processing element with respect to the object (e.g. the work part)

being processed.

Basic components of NC

A numerical control system consists of the following three basic components:

1. Program of instructions

2. Machine control unit

3. Processing equipment

The general relationship among the three components is illustrated in Figure below. The program

is fed into the control unit, which directs the processing equipment accordingly.

The program of instructions is the detailed step by step commands that direct the processing

equipment. In its most common form, the commands refer to positions of a machine tool spindle

with respect to the worktable on which the part is fixture. More advanced instructions include

selection of spindle speeds, cutting tools, and other functions. The program is coded on a suitable

medium for submission to the machine control unit. The most common medium in use over the

last several decades has been 1inch wide punched tape. Because of the widespread use of the

punched tape, NC is sometimes called “tape control.” However, this is a misnomer in modern

usage of numerical control. Coming into use more recently have been magnetic tape cassettes

and floppy diskettes. The programming is done by a person called the programmer.

Program

Machine Control unit

Processing Equipment

The machine control unit (MCU) consists of the electronics and control hardware that read and

interpret the program of instruction and convert it into mechanical actions of the machine tool or

other processing equipment.

The processing equipment is the third basic component of an NC system. It is the component that

performs useful work. In the most common example of numerical control, one that performs

machining operations, the processing equipment consists of the worktable and spindle as well as

the motors and controls needed to drive them.

HISTORICAL PERSPECTIVE:

The development of numerical control owes much to the U.S. Air Force ant the early aerospace

industry. The first development work in the area numerical controls is attributed to John Parsons

and an associate named Frank Stulen at Parsons Corporation in Traverse City, Michigan. Parsons

was a contractor for the Air Force during the 1940s. The original NC concept involved the use of

coordinate positional data contained on punched cards to define the surface contours of

helicopter blades. After development work by Parsons and his colleagues, the idea was presented

to the Wright-Patterson Air Force Base in 1948. The initial Air Force contract was awarded to

Parsons in June 1949, and a contract was subsequently awarded to the Servomechanism

Laboratories at the Massachusetts Institute of Technology to develop the prototype NC machine

tool.

The first NC machine was developed by the retrofitting a conventional tracer mill with

rudimentary numerical controls. The prototype success full performed simultaneous control of

three axes of motion using punched binary tape. The machine was demonstrated at MIT in

March 1952

The machined tool builders gradually began initiating their own development projects to

introduce commercial NC products. Also, certain companies in the aerospace industry began t

devise numerical control machines to satisfy their own 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 for controlling NC machine tools. This

research resulted in the development of the APT (Automatically Programmed Tooling) language.

The objective of the APT research was to provide a means by which the part programmer could

communicate the machining instructions to the machine tool in simple English-like statements.

Although the APT language was sometimes criticised as being too large for many of the

computers of the time (early and mid – 1960), it nevertheless stands as a major accomplishment

in programmable automation. The language is still widely used in industry today, and most other

more recent part programming languages are based on APT concepts.

COORDINATE SYSTEM AND MACHINE MOTIONS

To program the NC processing equipment, it is necessary to establish a standard axis system by

which the relative position to the tool with respect to the work can be specified.

COORDINATE SYSTEM IN NC

Using an NC drill press as an example, the drill spindle is in a fixed horizontal position and the

table is moved relative to the spindle. However, to make things easier for the part programmer,

the view point is adopted that the work piece is stationary which the tool is moved relative to it.

Accordingly, the numerical control coordinate system is defined with respect to the machine tool

table.

Two axes, x and y, are defined in the plane of the table, as shown figure below. The z-axis is

perpendicular to this plane and movement in the z direction is controlled by the vertical motion

of the spindle. The positive and negative directions of motion of the tool relative to table along

these axes are shown in figure below. NC drill presses are classified as wither two axis or three

axis machines, depending on whether or not they have the capability to control the z-axis.

A numerical control milling machine and similar machine tools (boring mill, for example) use an

axis system similar to that of the drill press. However, in addition to the three linear axes, these

machines may possess the capacity to control one or more rotational axes. Three rotational axes

are defined in NC: the a, b, and c axes. These axes are used to specify angles about the x, y and z

axes respectively. To distinguish positive from negative angular motions, the “right-hand rule”

can be used. Using the right had with the thumb pointing in the positive linear axis direction (x, y

or z), the fingers of the hand are curled to point in the positive rotational direction. This is

illustrated in the figure above.

Machine tool coordinate system for NC

For turning operations, two axes are normal all that are required to command the movement of

the tool relative to the relating work piece. The z-axis is the axis of rotation of the work part, and

the x-axis define the radial location of the cutting tool. This arrangement is illustrated in Figure

below:

FIXED ZERO VERSUS FLOATING ZERO

The purpose of the coordinate system is to provide a means of locating the tool in relation to the

work piece. Depending on the type of NC machine, the part programmer may have several

options for specifying the location. One of these options depends on whether the machine has a

fixed zero or a floating zero.

In the case of fixed zero, the origin is always located at the same position on the machine table.

Usually, the position in the southwest corner (lower left-hand corner) of the table and all

locations must be defined by positive x and y coordinates relative to that fixed origin.

The second and more common feature on modern NC machines allows the machine operator to

set the zero point at any position on the machine table. This feature is called floating zero. The

part programmer is the one who decides where the zero point should be located. The decision is

based on the part programming convenience. For example, the work part may be symmetrical

and the zero point should be established at the centre of symmetry. The location of the zero point

is communicated to the machine operator. At the beginning of the job, the operator moves the

tool under manual control to some “target point” on the table. The target point is some

convenient place on the work piece or table for the operator to position the tool. For example, it

might be a predrilled hole in the work piece. The target point has been referenced to the zero

point by the programmer. In fact, the programmer may have selected the target point as the zero

point for tool positioning. When the tool has been positioned at the target point, the machine

operator presses a “zero” button on the machine tool console, which tells the machine where the

origin is located for subsequent tool movements.

+ Z - Z

- X

+ X

With fixed-zero systems, the part programmer and machine operator must reference the job to

the machine’s permanent zero point. This is less convenient arrangement.

ABSOLUTE VERSUS INCREMENTAL POSITIONING

Another option sometimes available to the part programmer is to use either an absolute system of

tool positioning or an incremental system. Absolute positioning means that the tool locations are

always defined in relation to the zero point. If a hole is to be drilled at a spot that is 8 inch above

the x-axis and 6 inch to the right of the y-axis, the coordinate location of the hole would be

specified as x = +6.000 and y = 8.000. By contrast, incremental positioning means that the next

tool location must be defined with reference to the previous tool location. If in the previous

drilling example, the previous hole had been drilled at an absolute position of x = +4.000 and y =

+5.000, the incremental position instructions would be specified as x = +2.000 and y = +3.000 in

order to move the drill to the desired spot. Figure below illustrates the difference between

absolute and incremental positioning.

TYPES OF NC SYSTEMS

The NC system must possess a means of controlling the relative movement of the tool with

respect to the work. There are three types of motion control used in numerical control:

1. Point-to-point

2. Straight cut

3. Contouring

The three types are listed in order of increasing level of control sophistication.

POINT-TO POINT NC

Point-to-point (PTP) is also sometimes called a positioning system. In PTP, the objective of the

machine tool control system is to move the cutting tool to a predefined location. The speed or

Δy = 3

Δx = 2

(4,5)

(6,8) Specify x=6, y=8 for

Absolute positioning

Specify x=2, y=3 for

Incremental positioning

path by which this movement is accomplished is not important in point-to-point NC. Once the

tool reaches the desired location, the machining operation is performed at that position.

NC drill presses are a good example of PTP systems. The spindle must first be positioned at a

particular location on the work piece. This is done under PTP control. Then, the drilling of the

hole is performed at that location, the tool is moved to the next hole location, and so on. Since no

cutting is performed between holes there is no need for controlling the relative motion of the tool

and workpiece between hole locations. On positioning systems, the speeds and feeds used by the

machine tool are often controlled by the operator rather than by the NC tape. Figure below

illustrates the point-to-point type of control.

Positioning systems are the simplest machine tool control systems and are therefore the least

expensive of the three types. However, for certain processes such as drilling operations and spot

welding, PTP is perfectly suited to the task and any higher level of control would be

unnecessary.

STRAIGHT-CUT NC

Straight-cut control systems are capable of moving the cutting tool parallel to one of the major

axes at a controlled rate suitable for machining. It is therefore appropriate for performing milling

operations to fabricate work pieces of rectangular configurations. With this type of NC system it

is not possible to combine movements in more than a single axis direction. Therefore, angular

cuts on the work piece would not be possible. An example of a straight-cut operation is shown in

figure below.

Point-to-point (positioning) control in NC

Workpiece

Starting point

Cutting tool

Tool path- operations Performed during tool motion parallel to x or y axes

Straight-cut control in NC

An NC machine tool capable of performing straight-cut movements is also capable of point-to-

point movements.

CONTOURING NC

Contouring is the most complex, the most flexible, and the most expensive type of machine tool

control. It is capable of performing both PTP and straight-cut operations. In addition, the

distinguishing feature of contouring NC systems is their capacity for simulations control for

more than one axis movement of the machine tool. The path of the cutter is continuously

controlled to generate the desired geometry of the work piece. For this reason, contouring

systems are also called continuous-path NC systems. Straight of plane surfaces at any

orientation, circular paths, conical shapes, or most any other mathematically definable form are

possible under contouring control. Figure below illustrated the versatility of continuous-path NC.

Milling and turning operations are common examples at the use of contouring control.

For the mathematically oriented reader, it might be useful to distinguish between PTP, straight-

cut and contouring in the following way. Consider a two-axis control system, where the table is

moved in the xy plane. With point-to-point system, control is achieved over the x and y

coordinates. With straight-cut systems, control is provided for either dx/dt and dy/dt, but only

one at a time. With contouring systems, both of the rates dx/dt and dy/dt can be controlled

simultaneously. On order to cut a straight path at some angle, the relative values of dy/dt and

dx/dt must be maintained in proportion to the tangent of the angle. In order to machine along a

curved path, the values of dy/dt and dx/dt must continually be changed so as fo follow the path.

This is accomplished by breaking the curved path into very short straight-line segments that

approximate the curve. Then, the tool is commanded to machine each segment in succession.

What results is a machined outline that closely approaches the desired shape. The maximum

error between the two can be controlled by the length of the individual line segments as

illustrated in figure below.

The reader can easily imagine the complexity involved when more than two axes movements

must be controlled by a contouring system. Some NC machine tools possess the capability to

simultaneously control five axes to achieve the desired machined surface.

THE MCU AND OTHER COMPONENTS OF THE NC SYSTEM

The machine control unit is the NC controller that reads the program and runs the processing

equipment (e.g. the machine tool). In this section some to the operating details of the MCU and

other components of an NC are discussed. It should be noted that nearly all modern controls for

NC system are designed around microprocessors. The term computer numerical control (CNC) is

used to identify these systems, as opposed to conventional numerical control (NC).

TAPE READER:

The tape reader is an electrical-mechanical device for winding and reading the punched tape

containing the program of instructions. The punched tape format consists of eight parallel tracks

of holes along its length. The presence or absence of a hole in a certain position represents bit

information, and the entire collection of holes constitutes the NC program.

Photoelectric cells:

Electrical contact fingers:

Vacuum method:

POSITION AND MOTION CONTROL IN AN NC SYSTEM

The data read into the MCU through the tape reader define machine table positions

corresponding to the axes of the machine tool. Each axis is equipped with a drive unit such as a

dc servomotor, stepping motor or hydraulic actuator. Using either the dc motor or stepping motor

to illustrate, the drive unit is connected to the table by means of a leadscrew, as shown in figure

below. Rotation of the motor causes the leadscrew to turn, which results in linear movement of

the table. The pitch of the leadscrew (i.e. distance between successive threads) determines the

distance travelled by the table on each revolution of the motor.

The axis positioning system may be designed as either an open-loop or a closed loop system. The

difference between the two is the absence or presence of feedback measurements to verify the

axis positions of the machine tool table. Schematic diagram of the two types are illustrated in

Figures below.

As shown in the figure, an open-loop NC system is one that does not used feedback signals to

indicate the table position to the controller unit. Open-loop NC systems typically make use of

stepping motors. The stepping motor is a motor that is driven and controlled by an electrical

pulse train generated by the MCU (or other digital device). Each pulse drives the stepping motor

by a fraction of one revolution, is called the step angle. The allowable step angles on a stepping

motor are determined by the relation:

α =

Where the number of step angles ns on a stepping motor must e an integer value. The angle of

rotation of the stepping motor in response to a pulse train is equal to the number of pulses

multiplied by the step angle.