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UNIT II

1. Define Numerical Control

The definition of numerical control (NC) given by Electronic Industries Association

(EIA) is A system in which the actions are controlled by direct insertion of numerical

data at some point. The system must automatically interpret at least some potion of this

data.

2. Explain Part program

The data for preparing the coded instruction, called part program is taken form the

finished components drawing. In the case of manufacture of complicated parts, the system

calculates the additional data points automatically.

3. What is a CNC machine?

In case of computer numerical control (CNC) machine tools, a dedicated computer is

used to perform all the basic NC functions. The complete part program to produce a

component is input and stored in the computer memory and the information for each

operation is fed to the machine tool i.e., motors etc., the part program can be stored in thememory of the computer and used in future.

4. What are the control features available in CNC machine tools?

The part program can be input to the controller unit through keyboard

The part program once entered into the computer memory can be used

again and again

The part program can be edited and optimized at the machine tool

itself.

The input information can be reduced with the use of special sub

programs

The CNC machines have the facility of proving the part without

actually running it on the machine tool

CNC control unit allows compensation for any change in dimensions

of cutting tool.

5. List out the advantages of CNC machines

Reduce lead time

Elimination of operation cost

Lower labor cost

Smaller batch sizes

Longer tool life Elimination of special jigs and fixtures

Reduced inspection and scrap

Accurate costing and scheduling.

6. Explain Binary Coded Decimal (BCD) system

The actual system used in numerical control is BCD system. In BCD, each digit is

represented by four binary bits. Thus 2 represented by 0010, 3 is represented by 0011 and

23 is represented by 0010 0011, similarity there are codes for all the characters A to Z, a

to z and special characters like, etc.

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7. Explain ISO numerical code system

The ISO 7 bit code for numerical control suggested by International Standard

Organization. The ISO code is 7 bit code and uses 7 tracks for coding and the 8 th track

(left most tracks) is resumed for error checking. In ISO numerical reserved for error

checking. In ISO numerical code the digit 0-9 have punched in tracks 5 & 6 and

characters A Z, a hole is punched in track 7.

8. Explain EIA numerical control code

The EIA code is also 7 bit code in an 8 track format with track 5 reserved for parity

check. The digits 1 to 9 is punched in track 6. The alphabetic characters are divided into

three groups. A to I holes punched in track 6 and 7, J to R all have a hole punched in track

7 and S to Z a hole punched in track 6.

9. Specify the major elements in CNC system

A CNC system may be characterized in terms of three major elements:

1. Hardware 2. Software 3. Information

10. What is parity checking?

In ISO and EIA coding systems, a track is reserved for parity checking. Parity track is

a device for error checking and to ensure that the tape has been punched correctly. The

ISO code specifies even parity which means that the number of holes in any row should

be even. If a character code results in odd number of hole, then a hole is punched in parity

(track no:8) to make the number of holes even. When the tape reader reads the data from

the tape, a parity check is made on incoming data. If the umber of holes in a row is odd,

the machine will stop and error will be conveyed to the operator. The EIA code specifies

odd parity and uses the track 5 as parity track.

11. Give the fundamental aspects of machine control.

The primary objective of CNC is automatic control of machine tools operation. The

three major objectives of a CNC machine are,

1. Motion 2. Positioning 3. Switching function

12. List the Numerical Control procedure in manufacturing

To utilize numerical control in manufacturing the following steps must be

accomplished

i. Process planning

ii. Part programming

iii. Tape preparation

iv. Tape verification

v. Production

13. Explain the control systems in CNC machine

i. Point to point control system

In point to point control, movement is purely for positioning of the tool at

programmed points. This type is usually encountered in drilling, tapping or

boring operations.

ii. Continuous path control system

In continuous path control, there is a functional relationship betweenindividual coordinate axes. This is the most universal type of control.

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Depending upon the number of simultaneous controllable axes, there are 2,3,4

and 5 axis control systems.

14. Explain interpolation and classify the same

To carryout continuous path controls and definite cycles of movements in a plane on a

3-D space, an interpolation is needed to achieve the necessary relationship among

velocities in the axis movements in the CNC control.

Various types of interpolations are as follows:

i. Linear interpolation

ii. Circular interpolation

iii. Helical interpolation

15. Explain the horizontal spindle machining centre and describe the various axis.

These are generally single spindle machines with automatic tool changers. In this type

of machines X-axis transverse is provided by tape on column and Y axis transverse is

provided by spindle head. The Z axis transverse is provided by the saddle or column or

head stock or spindle head.

16. Explain universal machining centers and describe the various axes.These are similar to horizontal machining centers with the spindle axis capable of

tilting from horizontal to the vertical position continuously under computer control. This

constitutes the fifth axis of the machine. In some cases this movement is provided by

tilting of the table instead of the spindle. In this type of machine multiple side faces can

be machines in a single setup. In order to save setting up time, machining centers are now

available with two or more pallets. It is also possible to load and machine different

components depending upon the need.

17. Explain a turning center

Some lathes have rotating tools in the turret to facilitate off-axis machining such as

drilling, milling, reaming, tapping, boring etc., these machines have in addition to theconventional X and Z axes, CNC control of the spindle rotation (i.e C-axes), such

machines are known as turning centers.

18. What is DNC? Explain Briefly.

Direct numerical control can be defined as a manufacturing system in which a number

of machines are controlled by a computer through direct connections. The DNC computer

designed to provide instruction to each machine tool on demand.

19. Give the advantages of DNC.

Time sharing the control of more than one machine

Greater computational capability

Elimination of tapes and tape reader

Programs stored as cutter location data can be post processed for

whatever suitable machine assigned to process the job.

20. Define adoptive control

For a machining operation, the term adoptive control denotes a control systems that

measure certain output process variables and used these to control speed and/or feed.

Some of the process variables that have been in adoptive control machining systems

include spindle deflection or force, torque, cutting temperature, vibration amplitude and

horse power.

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21. What is Machine Control Unit (MCU)?

MCU is a hardware system which reads, interprets and translates the program of the

instructions into mechanical actions of machine tool.

22. List out any four advantages of using Numerical Control (NC)

i. Increased flexibility

ii. Production of complex parts

iii. Reduced setup time

iv. Elimination of special jigs and fixtures

23. Write the main functions of CNC?

i. Machine tool control

ii. In-process compensation

iii. Improved programming and operating features

iv. Diagnostics

24. What are the functions of diagnostics system in NC machine tools?

To identify the reason for a down time occurrence so that maintenance

personal should make repairs fastly

To alert to signs that indicate the imminent failure of certain

component

To obtain certain amount of redundancy of components which are

considered unreliable

25. Define open loop control system and closed loop control system

A control system in which the output value is not checked against the

desired input value is known as open loop control system

A closed loop control system is the one in which the output variable is

compared with an input parameter. If there is any difference between the

input and output, the difference is used to drive the output into agreement

with an input.

26. What are the components of Machine Control Unit (MCU)?

Central processing unit

Memory

I/O interface

Controls for machine tool axes and spindle speed Sequence controls for other machine tool functions

27. What are the sections available in Central Processing Unit (CPU)?

Control section

Arithmetic-logic Unit (ALU)

Immediate access memory

28. What is control section in CPU?

The control section retrieves commands and data from memory and generates signals

to activate other components in the MCU. It sequences, coordinates and regulates all of

the activities of the MCU computer.

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29. What is ALU in central processing unit?

ALU consists of a circuitry to perform various calculations (addition, subtraction,

multiplication), counting and logical functions required by software residing in memory.

30. What are the components of DNC?

Central computer

Bulk memory

Telecommunication lines

Machine tools

31. What are the types of software programs used in CNC systems?

Operating system software

Machine interface software

Application software

32. What is the function of Operating system software in CNC systems?

It function of operating system software is to interpret the NC part programs and

generate the corresponding control signals to drive the machine tool axes. It is installedby the controller manufacturer and is stored in ROM in the MCU.

33. What is the function of Machine interface software in CNC systems?

The Machine interface software is used to operate the communication link between

CPU and the machine tool to accomplish the CNC auxiliary function. The machine

interface software is often written in the form of ladder logic diagrams.

34. What is the function of Operating system software in CNC systems?

The application software consists of the NC part programs that are written for

machining applications in the users plant.

35. What is the meaning of diagnostics in CNC system?

Many modern CNC systems possess an online diagnostics capability that monitors

certain aspects of the machine tool to detect malfunctions or signs of impending

malfunctions or to diagnose system breakdowns.

36. What are the common features of a CNC diagnostics system?

Control start-up diagnostics

Malfunction and failure analysis

Extended diagnostics for individual components

Tool life monitoring Programming and preventive maintenance notices

37. Write any four features of CNC

Storage of more than one part program

Various forms of program input

Program editing at the machine tool

Fixed cycles and subroutines

38. What are the machine tool movements in a two axis CNC machine?Machine tool movements are taking place in two orthogonal directions in a plane.

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Examples for Two axis machines:

Lathe Punch press

Flame and plasma arc machines Clothe cutting

machines

39. Draw the block diagram for configuration of CNC machine control unit

40. What are the machine tool movements in a three axis CNC machine?

Machine tool movements are taking place along the three principal directions (x,y

and z) of the Cartesian coordinate system. Examples for Three axis machines:

Milling

Boring CMM

Drilling

41. What are the machine tool movements in a four axis CNC machine?

Machine tool movements involve three linear and one rotary axis or perhaps two x-y

motions.

Examples for Four axis machines:

Lathe with supplementary milling heads.

42. What are the machine tool movements in a five axis CNC machine?Machine tool movements involve three linear and one rotary axis with rotation about

two of these normally x and y and are generally milling machines.

43. What are the problems with conventional NC system?

Part programming mistakes

Non optimal speed and feeds

Fragile nature of punched tape material

Lesser reliability of tape reader

Inflexibility in Controller

Insufficient management information like counts, tool changes etc.,

Memory

ROM-Operating systemRAM-Part programs

CentralProcessing

Unit

(CPU)

Input / Outputinterface

Operating panelTape Reader

Machine tool Controls

Position controlSpindle speed control

Sequence control

CoolantFixture clamping

Tool changer

System

bus

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44. What is 2 axis contouring?

In 2-axis contouring it is possible to combine the movements of tool in both X and Y

axis simultaneously. Therefore it is possible to make angular cuts, circular paths and other

mathematically defined shapes.

45. What is control system in CNC system and explain its functions

A control system can be defined as an automated system which executes the program

of instructions.

Functions of control system are:

To complete the processs defined function by carrying out some

manufacturing operating.

To control the position / velocity of a machine slide, by using a group of

electromechanical, pneumatic or hydraulic components which are known as

servo mechanism

46. What is 3 axis contouring?

In 3-axis contouring the movements of tool is controlled in all axis (X, Y and Z) axissimultaneously. So it is possible to make 3D shapes.

47. What are the applications of contouring control system?

The most common types of contouring operations are: milling and lather operations.

Contouring systems are best suited for fusion welding, flame cutting and fabrication of

intricated die templates.

48. What is linear interpolation and write the parameters required for linear

interpolation?

Linear interpolation is used when a straight line path is to be generated in

continuous path NC.Parameters required to program Linear interpolation are:

Beginning point of the straight line End point of the straight line Feed rate

49. What is circular interpolation and write the parameters required for circular

interpolation?

Circular interpolation is used to simplify the programming of arcs and circles.

Parameters required to program circular interpolation are:

The coordinates of the starting point and end point

The center or radius of the arc

The direction through which the cutter has to travel

50. What is helical interpolation?

This method combines the circular and linear interpolation. Circular

interpolations used scheme for two axes described above with linear movement of

the third axis. This permits the definition of a helical path in three dimensional

space.

These types of interpolations are used in the machining of large internal

threads either straight or tapered.

51. What are parabolic and cubic interpolations?

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These routines provide approximations of free form curves using higher order

equations. They generally require considerable computational power. So these

routines are not as common as linear and circular interpolation.

Most applications are in the aerospace and automotive industries for free form

designs that cannot accurately and conveniently be approximated by combining

linear and circular interpolations.

This is achieved by describing the coordinates of end points of parabolicsegments along with initial, end and intersection points of tangents.

UNIT III

1. What are the major areas to be considered in the design of NC machine tools?

i. Machine structure and frame

ii. Location of transducer

iii. Slideways

iv. Elements of transmission and positioning of sliders

v. Spindles

vi. Tool holding arrangements

2. Explain the term stick-up

Conventional sliders operating under sliding friction do not have a constant

coefficient of friction and the highest value of co-efficient tends to be at the lowest rates

of slide velocity. This phenomenon given to the familiar stricking of oil lubricated

slideing surfaces when the fine adjustment is needed, a jerky action when movement

takes place at low velocities. The term stick-slip is used to describe these situations.

3. How the heat effect on the machine bed, tool holder can be taken care?

i. Providing correctly designed mild steel structure with higher stiffness

ii. Use of ribs, braces, angle plates to increase stiffness

iii. Normal weight distribution over the entire frameiv. The hollow cross section for beds, bases and columns with a number of ribs

welded with the walls cater for the rigidity as well as opening for inspection,

lubrication and collection of chip coolants.

v. Thermo symmetrical design of all parts.

vi. Providing large heat removing surfaces

vii. Use of excellent coolants

viii. Avoiding direct as well as local sources of heat such as sunlight and electrical

motors.oil pumps respectively.

ix. Reduction of ambient temperature by using air conditioning units

x. Proper alignment of the machine elements relative to each other while in operation

as well as in stationary conditions

4. Explain Slide and Slideways

In general machine tools are provided with tables, slides, carriages etc., to carry the

work pieces or cutting tools etc., These parts are sliding in nature and mounted on the

ways that are fixed on the other parts (column, housing, bed or knee) of the machines

known as sliding ways.

5. Give the four methods of achieving rolling action in slideways

1. Vee & Flat roller 2. Linear ball bushing

3. Rotax Tychoway 4. Dexter ball slide

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6. Give the requirements of a good slideway system

A good slide way mush possess

Low coefficient of friction at varying slide velocities

Minimum difference between static and dynamic friction coefficient

Positive slope for friction velocity characteristics

Low rate of wear

High stiffness at the sliding points

Sufficient damping.

7. Give the advantages of re-circulating ball screw and nut assembly.[AU-APRIL 2004]

High efficiency (over 90%)

Reversibility

Reduces wear and has longer life without loss of accuracy

No stick slip in the sliding metal to metal contact is substituted by rolling contact

8. What is the importance of control elements?

The control unit should be situated so that it is convenient for the machine operator to

operate the machine from the central place. A control unit should:

i. Indicate the current status and position of various machine tool features

and give feedback

ii. Allow manual on semi manual control of machine elements

iii. Enable machine tool to be programmed

9. What are the various types of DC motors used in a CNC machine tool?

i. DC permanent magnet motors

ii. Separately excited DC motor with analog thyristor derives

10. What are the advantages of a permanent magnet DC motor?In general the advantages of permanent magnet DC motor over winded field motors

are:

i. Indicate the current status and position of various machine tool features

and give feedback

ii. High acceleration torque

iii. No need for electric power to generate magnetic flux

iv. Smaller frame size

11. What are the advantages of AC spindle drives over DC drives?

AC spindle motors are now preferred for the main drive by CNC machine tool

designers due to a variety of reasons as follows:i. AC motors are reliable than DC motors under several operating conditions

ii. AC motors are being free of brushes and other wearing parts do not require

frequent maintenance

iii. The unique stator cooling system in AC motors result in high speed output

characteristics with compact size

iv. AC drive units provide stable and smooth operation with reduced vibration

and noise from low speed to high speed.

12. Describe a typical slide way design incorporating plastic inserts.

In slide way system plastic or non metallic inserts are used. These insets are bonded to

the underside of the sliding members. They can be of thermoplastics (Turcute-B) or

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thermosetting (SKC-3, Moglice) types. It is found that for these coated slide ways the

static coefficient is less that the dynamic coefficient of friction.

13. Discuss the advantages of plastic inserts for slide ways of machine tools?

Plastic inserts / composites are made of two or more materials in which one reduces

coefficient of friction. The other increases strength, wear resistance and load bearing

capacity. They also have self lubricating capacity. They can take up dirt, dust particles

and eliminate scoring. Another advantage is the case with a worn out strip can be replaced

without the need of any scraping or machining of bedways.

14. What are the common types of tool magazines?

The concept of the ATC is that the range of tools for a specified job shall be made

available for automatic selection and positioning . ATC can be :

i. Drum type for holding small number of tools store in periphery of drum

ii. Chain type for more number of tools (30 40 or more tools can be used)

15. Why are the balls re-circulating screws used in CNC machines?

Ball screws are primarily employed in feed mechanism in machine tools. The

advantages are as follows:i. In the ball screw the lead between the threads of the screw and the nut is not

transmitted by direct contract, but through spherical balls. Balls rotate between

the helical grooves of the screw and nut in a manner that function in a ball

bearing. An essential feature is the provision of recirculation of balls.

ii. Low coefficient of friction in the order of 0.004

iii. Higher transmission efficiency allows larger thrust loads to be carried with

less torque

iv. By preloading the assembly, clearances and consequent backlash can be

eliminated. The accuracy of ball screw is high.

16. What are the desirable characteristics of tool magazines?i. Tool magazines must be capable of holding enough tools needed for performing

complete operation for work pieces on the machining centers

ii. As the magazine gets larger, the space needed is more and more expensive and

hence the magazine has to be compact and as simple as possible

iii. Interchange of tools should not interfere with the workpiece space and tool space.

This means that the tool magazine should be located outside the working space of

the machine tool

iv. Easier and safer manual exchange of tools in the tool magazine during loading and

unloading should be facilitated

v. All preparatory works for tool exchange should be made during machining

17. How do you classify Automatic Tool Changer (ATC)?

i. According to the kind of cutting tools

For single tool

For multi tool arm

Special tools such as micro bores

ii. According to system tool change

Without tool change arm

With tool change arm

With tool parking position

iii. According to position of axes of tool With parallel axis

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With intersecting axis

iv. According to tool position

With horizontal tool position

With vertical tool position

18. What are the types of spindle heads?

Robot-like head

Horizontal / Vertical (universal head)

Inclined heads

19. What is automatic pallet changer?

For machines with automatic pallet changer (ATC) the table is replaced by pallet. The

pallet when transferred from the pallet changer on the machine gets located and clamped

with heavy clamping force either hydraulically or mechanically. The function of pallet

changer is to interchange the pallet on the machine which has the finished component and

the other pallet with newly loaded component.

20. What are the design requirements of CNC machine spindles?

High stiffness Running accuracy

Axial load carrying capacity

Thermal stability

Axial freedom for thermal expansion

High speeds of operation

21. What are the different types of friction guideways?

Flat guide

Vee guide

Dovetail guide

Cylinder guide

22. What is hydrostatic guideways?

The guideways in which the surface contact between the sliding parts is separated by

a thin layer of fluid (oil or air) is known as hydrostatic guideways.

23. What is actuating system and explain the types

Actuation systems are used to convert the rotational movement into translational

movement.

Two types of actuation systems in CNC are Screw & Nut and Rack & pinion.

24. What is meant by feed drive? List the different types of feed drives

Feed drive is used to provide motion to the slide as per the motion commands

Feed drives are classified as follows:

i. Servo motors

AC servo motor DC servo motor Brushless DC

servo motor

ii. Stepper motors

iii. Linear motors

25. Write the salient features of tooling system

i. Rigidity to withstand higher cutting forces

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ii. Rigidity to transmit the spindle power at higher speed

iii. Perform variety of cutting operations

iv. To keep down time

26. What is machine structure?

Machine structure is the load carrying and supporting member of the CNC

machine tool

Machine structure consists of a bed, column and other supporting members like

head stock, tailstock of lathe. These supporting members and other functional

assemblies are rigidly fixed to the machine structure.

27. What are the types of forces transmitted to the machine structure?

Static load Dynamic load Thermal load

28. What is dynamic load on machine structure and give the origin of dynamic load?

Dynamic load is the term used for constantly varying forces i.e., inertial forces. Inertia

forces are exerted due to the rapid acceleration and deceleration of the cutting

tool/machine slide during machining.

29. What are the heat sources creating thermal load on machine structure?

Electric motors

Friction in bearing and slideways

Friction in the machine derives and gear boxes

Machining process

Temperature of the surroundings

30. What are the origins of static load on machine structure?

Static load is exerted due to the following: Weight of the slides and workpiece Forces during machining

31. What are the ways to reduce thermal deformation in machine structure?

Providing large heat removing surfaces

Using excellent coolants

Thermo symmetrical design of all parts

Avoiding direct heat source

Providing proper lubricating system

Usage of air conditioning units

By mounting gear boxes and motors externally

Properly aligning the machine elements relative to each other during

machining as well as in stationary conditions

32. What are the functions of slideways?

To control the line of action of the carriage/table on which the tool/workpiece is

held

To absorb the static and dynamic forces

33. What are the types of Guideways/slideways?

Friction guideways

Antifriction or linear motion guideways

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Hydrostatic guideways

34. How can we overcome stick-slip phenomenon?

The stick-slip phenomenon prevents the smooth starting of carriage/slide from rest. So

as to reduce the possibility of stick-slip, there should be minimum but constant coefficient

of friction between the surfaces of contact. This is achieved by providing stripes or lining

of low constant coefficient of friction material in between the contact surfaces. Trucite or

Poly Tetra Fluro Ethylene (PTFE) is normally used as matrial.

35. What are the types of linear motion guideways?

Linear bearing with balls and rollers

Recirculating ball bushings

36. Give the classification of hydrostatic slideways?

Oil lubricated slideways

Air bearing slideways

37. Give the classification of tool holding devices

Spindle tooling Flexible tooling

38. What is spindle and give the functions?

Spindle is the main part of the machine tool. It receives power from the drive unit and

delivers it to the job/workpiece.

The functions are as follows:

To deliver power to the job/tool

To hold the job/tool

The center the job/tool

39. What are the requirements of a spindle?

Spindle should have enough stiffness to minimize the torsional strain while

rotating

Spindle should be less in weight

Spindle should be located very nearer to the bearings to avoid overhung effect.

The accuracy of the spindle mostly depends on the quality of the bearings

The accuracy and quality of the workpiece depends on the geometric, running

and stiffness of the spindle assembly

40. What are the requirements of a CNC feed drive?

Constant torque to overcome frictional and work forces

Low mechanical and electrical time constants

High torque to weight ratio

Quick response to commands

Ability to position the tools with minute increments

Ability to provide variable speed range

Compactness

41. Give the types of stepper motors

Variable reluctance motor

Permanent magnet stepper motor

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Hybrid stepper motor

42. What are tool holding devices?

Tool holding devices are used to hold the tool. Since CNC machines are working at

faster rate, the tool holding devices should have the capability to withstand the forces

while machining.

43. State the requirements of tool holding device.

Rigidity

Security and quick setup

Adequate access of the cutting tool to the part

44. What are the factors to be considered in selection of tool magazine?

Storage capacity

Type and shape

Tool change procedure

45. What are the components of Automatic Tool Changer (ATC)?

Tool magazine

Tool adapter

Control function

Tool change procedures

46. What are the characteristics of work holding devices?

It should allow to perform a number of operations on different faces in a single

setting

It should have a provision for easy removal of chips, burrs, swarf etc., It should not interfere the cutting tools during operation

It should be fool-proofing to avoid incorrect loading of the job

It should be strong and rigid in construction against distortion and deflection

caused due to cutting forces in the operation

There must be an arrangement for quick loading and unloading of the

component

It should be safe to use

The component should not change its position while machining

It should be capable to adopt the automated loading or unloading of job

It should be simple in construction

47. What is tool changing cycle and give the stages in tool changing cycle?

During machining the tool is transferred from magazine to the spindle and comes

back to the magazine after completing the machining operation. This completes a cycle

known as tool change cycle. Stages in tool change cycle:

Tool selection

Tool transfer

48. What are the functions of feedback device or transducer?

It converts one form of physical quantity like speed, displacement, etc., into

electrical signal

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This electrical signal is then compared with the reference signal to check

whether the desired motion of tool/work table is achieved or not.

49. What are the types of feedback devices or transducers?

Velocity feedback devices

Position feedback devices

50. What is tachogenerator?

The most commonly used velocity feedback device producing analog type signal

output is the tacho generator. It is used to measure the angular velocity of rotating shaft.

51. What are qualified tools?

The tool which fits into a location on the machine, where its cutting edge is accurately

positioned within close limits relative to a specified datum on the tool holder or slide.

52. What is a rotary type encoder? Where its used? [AU-NOV 2004]

An encoder is a rotary transducer that provides continuous or discrete signal value

of an angular or linear movement. It is used in the CNC at the end of the slide screw directly or through gearing and

timer belt.

UNIT - IV

1. What are the techniques used for generating CNC instructions?

i. Manual part programming

ii. Computer assisted part programming

iii. Graphic numeric control programming (CAD/CAM based programming)

iv. CNC programming based on solid modeling

2. What is meant by Machine datum?

It is the origin of the co-ordinate system with the lathe. It is on the mounting flange of

the main spindle and the turning axis. It cannot be changed by the user of the machine. It

is fixed by the manufacturer and programmed into the computer. The point generally has

the coordinates X=0, Z=0.

3. Explain the machine reference point and tool reference point.

Machine reference point:

The position of the reference point R is determined by the manufacturer. The value of

the machine reference coordinates X and Z are fixed and cannot changed by the user.Tool reference point (T):

Point T is exactly the point of rotation of the tool post.

4. Describe the general form of a program line and explain each term.

The tool path of the CNC machine is described in machine tools. The general form is

as follows: (some of the terms are optional).

N.. G.. X.. Y.. Z.. A.. B.. C.. F.. S.. T.. M..

Where, N - Sequence number of the instruction

G - Preparatory function referring to a particular machine variety

X,Y,Z - Coordinates

A,B,C - Angular dataS - Spindle speed

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T - Tool code to select the tool

M - Miscellaneous function or non-machining operations

Sample line of program given below: N 001 G 01 X 120,5 Z-43 F 100.0 M08

5. What is meant by work piece zero point?

During different machining process, various jaw positions of chuck are used and these

are mounted on the main spindle. The distance of the face side of the work piece from the

machine datum differs depending on the jaw surface, which is used for chucking. This has

to be considered while programming. It is desirable to offset the origin of the coordinate

system into the work piece zero point W instead of the machine. Reference point, which

is the useful origin of the machine.

6. Describe G functions or Preparatory functions:

These are the commands which prepare the machine for the different modes of

movement like positioning, contouring, thread cutting etc., the preparatory functionsalways precede the dimension word.

Function Meaning

G00 Point to point, Rapid positioning

G01 Linear interpolation

G02 Circular interpolation Clockwise

G03 Circular interpolation Anti-Clockwise

G04 Dwell

7. What is a canned cycle?

A canned cycle is a combination of machine moves that performs any one particularmachining function such as drilling, milling, boring, tapping, dwell etc.,

8. What is zero suspension?

All CNC machines are equipped with the decimal point input. In order to simplify

program input the leading zero can be omitted. Ex. 12.145 instead of 0012.145

Consecutive zeros after decimal point need not be entered 12 instead of 12.000 and

10.4 instead of 10.4000

9. Explain subroutines.

Program sections required more than one can be defined as subroutines and stored in

the control. They can be called several times in part programs. Geometric values, feedrates, spindle speeds etc., are defined in subroutines as parameters. These parameters are

assigned actual values before the subroutines are called making it possible to adopt

dimensions and technological data to the actual work piece. A typical application for

subroutines is the machining of similar parts with different dimensions.

10. What is tool length offset? Where do you store tool length offsets?

i. In addition to the other data, tool length information as to be input to enable the

computer to calculate the toll path. In case of the machining centers tools are

preset using a pre setting device. With preset tools the tool tip position has to be

programmed in the tool reference point. Some machines have an automatic tools

presetting device which can be mounted on the machine tool itself. This slide

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position information will be used by the computer to determine the slide

displacements for particular tool path.

ii. Another method is to hold the blank in the chuck and jog the tool to touch the

diameter and a face of the work piece. The diameter and length information can

then be input into the memory of the machine.

11. Why is the tool nose compensation important?

The carbide insert used in turning operation has a certain corner radius and the carbide

tip tool will break or melt when working. These radii are standardized according to ISO.

12. What do you mean by machining cycles?

Machining cycles are technology based subroutines for often needed machining

operations such as deep hole drilling, pocket milling etc., in modern CNC systems

standard cycles are available for various types of drilling/boring and milling patterns. The

machining cycles are programmed in the usual NC programming language.

13. What is the importance of presetting device?

A presetting device is used to preset axial and radial positions of the tip on the toolholder. Once this is done the tool holder is ready to be mounted on the machine or tool

magazine and produce a known dimension. The amount of compensation can be quickly

decided with the presetting device. Accuracies in the order of 0.0002 mm can be quickly

and easily maintained.

14. Explain Preset tools

The setting of tools in advance at a place away from the machine tool on offline in

special holders is known as preset tools. The tools are carefully positioned in the holder

so that when the holders are clamped in the machine tool, their effective cutting positions

will correspond to these cutting positions assumed while preparation of part programmed.

15. Write brief notes on Modular fixtures.The multi hole plug board concept has proved very successful in this type of

fixtures. Its special advantage is that a work holding fixture can be built with absolute

positional accuracy by simple pegging of a few fixture elements. The multi hole grid

system enables different jig and fixture combination to be assembled for a wide range of

work pieces.

16. What are the different types of dimensioning systems used?

Absolute dimensioning or Baseline dimensioning system

Incremental dimensioning or Delta dimensioning system

17. What is a block? Write the significance of it.Block is the basic unit of a part program input to the control. It contains adequate

information for the machine to perform a movement and for functions.

18. List the differences between absolute and incremental dimensioning.

Sl.No Absolute dimensioning Incremental dimensioning

1 All measurements are taken

from a fixed origin

All the measurements for the next position

are calculated from the point at where the

slide was resting previously

2 In these all the dimensionsshould be positive

Both positive negative dimensions maycome

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19. Write the different types of tape programming format

Word address format

Table sequential format

Fined block format

20. What are the steps involved in part programming? Selecting the coordinate values

i.e., Absolute or incremental mode

Selecting units of dimensions

i.e., inches or metric, spindle speed and tool number if necessary

Preparing a program sheet where each block will contain the sequence

number, details of operation and basic information such as preparatory feed codes,

feed rate, miscellaneous M code

21. List some of the canned cycle and mention its use

G81 Drill cycle

G82 Dwell cycle

G79 Basic mill cycle

G83 Peck drill cycle

G85 Bore cycle

G80 Cancel Cycle

22. What is the roll of computer in computer in computer aided part programming

Input translation

Arithmetic calculations

Cutter offset compensators

Post processors23. List out some of the important NC languages [AU-APRIL 2004]

APT Compact II

ADAPT PROMPT

EXAPT CINTURN II

24. What are the four statements used in APT language?

1. Geometric statements 2. Motion statements

3. Post processor statements 4. Auxiliary statements

25. Differentiate 2 and 3 axis machine

2 axis machine implies that machine movements are in planes parallel to the X-

Y plane

3 axes machine implies that machine movements are in all the 3 axes (X, Y and Z)

26. What is a punched tape?

The punched tape is the widely used input medium, because of its low manufacturing

cost. Punched tapes are made up of materials like paper, mylar and foil.

27. What is part program?

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Part program is the planned and documented procedure by which the sequence of

processing steps to be performed on the NC machine.

28. What is Bit?

Bit is an abbreviation of Binary digit, which can be either 0 or 1. It represents

the absence or presence of a hole punched on the tape.

29. What is Block number/sequence number (N words) in part program?

Block/sequence number is the first word in the instruction block. It is used to identify

the sequence of a block of data in it. It consists of a character N H or O followed by

a three digit number raising from 0 to 999. i.e., N001 to N999

30. What is Preparatory function (G codes) in part program?

Preparatory function is used to initiate the control commands such as cutter motion,

etc., it consists of the address character G followed by a two digit number starting from

00 to 99

31. What are the types of G codes?

Modal codes Non modal codes

32. What are modal and non modal codes?

Modal codes remain active until it is cancelled by a contradictory G code of same

class. For example, G70 which defines the dimensional units are metric will remain

active until it is cancelled by G71

Non modal codes are active within the block. For example, G02 which defines

circular interpolation.

33. What are linear dimension words in part program?

X, Y, Z used to represent primary or main motion

U, V, W are used for secondary motion parallel to X, Y, Z axes respectively

p, q, r for another third type of motion parallel to X, Y, Z axes respectively

34. What are angular dimension words in part program?

a, b, c are used for angular motion around X, Y, Z axes respectively

I, J, K are used for positioning of arc center in case of thread cutting operation i.e.,

thread parallel to X, Y, Z axes

These words are represented by an alphabet representing the axes followed by five or

six digits depending upon the input.

35. What is Feed Rate word (F-word) in part program? It is used to program the feed rate in mm/rev or mm/min as determined by the prior G

code selection. G94 and G95 are used to represent feed rate in mm/min and mm/rev

respectively.

It is represented by the letter F followed by a three digit number raising from 001 to

999. For example, F100 represents a feed rate of 100 mm/min. This word is

applicable to straight line or contouring machines. In point to point systems a constant

feed rate is used in moving from point to point and in contouring systems the feed rate

is a variable one.

36. What is Spindle speed/cutting speed word (S-word) in part program?

It specifies the cutting speed of the process or the rpm of spindle

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It is represented by the letter S followed by the three digit number

If the speed is given in meter per inch, then the speed is converted in rpm rounded to

two digit accuracy. e.g., S-800 represents the 800rpm of spindle.

37. What is Tool selection word (T-word) in part program?

It specifies the type of tool to be used. It consists of T followed by the tool number.

Different numbers are used for each cutting tools. When the T number is read from the tape, the appropriate tool is automatically

selected by ATC. Hence this word is used only for machines with ATC. e.g., T01,

T02 represents the tool selection word.

38. What are Miscellaneous words (M-codes) in part program?

It consists of character M followed by two digit number (M00 to M99) representing

an auxiliary function such as switching ON/OFF spindle, coolant ON/OFF or

rewinding the tape

These functions do not relate the two dimensional movement of the machine.

39. What is End of Block (EOB) in part program? It identifies the end of instruction block

40. What is fixed zero system in part program?

A fixed zero is a location on the machine table, at the centre or lower left hand corner

as the operator faces the machine. In absolute system, all the dimensions used to carry out

the machining operations are based on this fixed zero. In case of incremental

dimensioning system, the first dimension is located from the fixed zero.

41. What is floating zero in part program?

In this system the zero point can be located anywhere on the machine table. So that

the operator sets the job to be machined in any location on the table and locates the tool to

begin the program.

42. What are the machine tool movements in lathe?

X axes is used to represent the axis of movement of the tool

Z axes is used to represent the axis of spindle

43. What are the machine tool movements in milling?

In milling operations, all the three axes (X, Y and Z) are used to control the

movement of work table and the cutting tool.

44. Name some of the non-standardized cycles

Do loops Parametric subtracting Macros

45. What is Do loop in non-standardized cycle?

Do-loop is the number of steps or operations to be repeated over a number of equal

steps for a previously defined number of times.

46. What are Geometry statements in APT?

These statements define the geometric elements of the workpart. The tool is then

directed to move to the various locations and surfaces of the workpart defined by the

geometry statements. These geometry statements include point, line, circle, plane etc.,

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47. What are post processor statements in APT?

The post processor statements are used to control the operation of the coolant, feed

rate, spindle speed and other features of the machine tool.

Some of the post processor statements are as follows:

COOLNT/for coolant control - ON, OFF, MIST

RAPID/to select rapid cutter motion END/to indicate the end of program

FROM is used to restart

TURRET/ to select cutter number from the turret or from an automatic tool changer

MACHIN/ to specify the machine type, number and post processor

48 What are motion statements in APT?

These statements are used to describe the path to be followed by the cutting tool.

Types of motion statements:

Setup commands Point to point commands Continuous path commands

49. What are Auxiliary statements in APT?

These statements are used for specifying cutter size, part identification, etc.,Some of the auxiliary statements are as follows:

CLPRNT - to obtain a computer print out of the cutter location sequence on the NC tape

INTOL/ defines the inside tolerance the allowable deviation from the inside of a curve

OUTTOL/outside tolerance the allowable deviation outside the curve

CUTTER/ defines the cutter diameter to be used

PARTNO/used to identify the part program at the beginning of the program

FINI/the last statement in the APT program

50. Explain the approach to part programming using CAD/CAM approach.

Identify the aspects of part geometry that are important for machining purposes andmay be edited or additional parameters are added

Defining the tool geometry perhaps by selecting the tools from the tool library

Identifying the desired sequence of operations and tool paths.

Displaying the tool motion and the tool motion my be edited or refine the tool motion

Cutter location data (CLDATA) file is produced from edited paths

The CLDATA file is post processed to machine control data which then transmitted to

the machine tool.

1. Explain the technology of Numerical Control (NC) and the coordinate system for NC

machine tools with neat sketch

2. Discuss the functions of CNC control in machine tools

3. Explain the types of CNC controls

4. Explain the types of interpolators in CNC machine

5. Briefly explain about Machine control unit and its elements.

6. Explain about Direct Numerical Control (DNC)

1. Explain the design process applied to the CNC machines

2. Explain the types of slideways in CNC machines3. Discuss the types of spindle drives

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4. Discuss the work holding devices in a CNC machine

5. Explain the tool holding devices in a CNC machine

6. Explain the working principle of Automatic Tool Changer (ATC)

7. Explain the feedback devices in a CNC machine

8. Explain the tooling for CNC machines

1. Explain the procedure for developing manual part program with example

2. Write a part program for the following part with plate thickness of 20mm.

3. Write a part program for the following part with plate thickness of 5mm.

+x

-z

+y

1

2

3

4

A

+z

2

5

60

9

0

1

5

2

0

2

55

08

0

95

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Take spindle speed = 1500rpm; feed rate = 100 mm/min.

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4. Explain Standardized fixed cycle or canned cycle

Write the APT program for drilling holes of dia 10mm in the 10mm thick plate

5. Explain the CAD/CAM approach to part programming