Lecture No.

BME - CNC

Unit VI

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 switch ability.

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.

3. Direct Numerical Control

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

simultaneous operations of a number of NC machines. 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.

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 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 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.

4. Based on the control loops 'Open loop & Closed loop’ systems

4.1 Open loop systems:

Programmed instructions are fed into the controller 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.

4.2. 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.

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

5.1 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.

5.2 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.

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.

5.3 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. Chips fall freely because of slant bed configuration

which is more ergonomically acceptable from operator's point of view.

5.4 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.

5.4.1 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.

5.4.2 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 servo systems which require having a

rapid response.

6.Different components related to CNC machine tools

Any CNC machine tool essentially consists of the following parts:

6.1 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.

6.2 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.

6.3 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.

6.4 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.

6.5 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.

6.6 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.

7. 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. Following Figure shows the right-hand rule

applied to vertical and horizontal machines.

Right hand rule for vertical and horizontal machine

8. CNC SYSTEMS - ELECTRICAL COMPONENTS

8.1 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.

8.2 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.

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.

i. 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.

Stepper motors are inherently digital.

they are inexpensive, easily and accurately controlled, and there are no brushes to maintain.

they offer excellent heat dissipation, and they are very stiff motors with high holding torques

for their size.

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.

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

shaft oscillation at resonance points.

Stepper motors may perform poorly in high-speed applications.

8.4 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.

9 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.

9.1 Incremental encoders

With J 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 J "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.

9.2 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.

9.3 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.

10. CNC Controller

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

10.1 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.

11.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.

(a) Recirculating ball screw

The recirculating ball screw assembly shown in following figure, 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%.

Recirculating ball screw assembly 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.

(b) 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.

(c) Planetary roller screws:

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.

(d) 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.

(12) 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.

(12.1) 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.

(12.2) 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. 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.

( 13) 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. The entire

turret can be removed from the machine for setting up of tools.

( 14 ) 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.

15. 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.

Rotating arm mechanism

( 16 ) 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.

17. 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.

18. 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 work piece 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

caused due to clamping force

Limited to a range of flat parts with

little overhang. Bar-feeders cannot be

incorporated. Parts on magnetic

chucks 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.

Indexing chucks Mandrels Vise Pallets

(19) Programming fundamentals

Machining involves an important aspect of relative movement between cutting tool and work

piece. In machine tools this is accomplished by either moving the tool with respect to work

piece 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 / work piece 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.

(19.1) 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.

Reference points and axis on a lathe

Reference points and axis on a Milling Machine

(19.2 ) 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.

(19.3 ) 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 work piece and setting that surfaces as X=x, Y=y and Z=z.

(19.4 ) 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.

(20) 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.

(21) Types of CNC codes

(21.1) 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.

(21.2) 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.