CNC MACHINES-Viva Voce Questions and Answers

1. Define NC system? NC is defined as a form of programmable automation in which the process is controlled by alphanumeric data.

2. What is MCU? MCU is a hardware system which reads, interprets and translates the program of instructions into mechanical action of machine tool.

3. List out any four advantage of using NC?

Increased flexibility.

Reduced setup time.

Production of complex part.

Elimination of special jigs and fixtures.

4. What are the limitations of using NC?

Relatively high price.

More complicated maintenance, a special crew is desirable.

Highly skilled and properly trained programmers are needed.

5. What are all the problems encountered with NC system?

Part programming mistakes.

Non-optional speeds and feeds.

Problem with punched tape and tape reader.

Problem with controller and management information.

6. Define CNC? CNC is defined as a NC system that utilizes a dedicated, stored computer program to perform some or the entire basic NC functions.

7. Write the main functions of CNC?

Machine tool control.

In-process compensation.

Improved programming and operating features.

Diagnostics

8. What are the functions of diagnostic system in NC machine tools?

To identify the reason for a down time occurrence so that maintenance personal should make repairs fast. To alert to signs that indicate the imminent failure of certain component which are considered unreliable

10. List the differences between NC and CNC system? NC system CNC system

1. Programming and punched tape is read every time

Only one time the tape is read and stored.

2. Moderate manual work. Less manual work.

3. Medium flexible. More flexible

4. Part programming is used Reprogramming is easy

11. List the differences between open loop and closed loop control system? Open loop control system Closed loop control system

1. Feedback device is not used Feedback device is used

2. Low accuracy. Higher accuracy can be achieved.

3. Low maintenance is required. More maintenance is needed

4. Less expensive. Cost of the equipment is high

12. List any two advantages and disadvantages of open loop control system? Advantages: Low cost and less manual work.

Disadvantages: Low accuracy.

Periodically adjustments are required to compensate backlash errors.

13. What is the function of a transducer?

Transducer is a device in which one form of physical quantity like speed, velocity is converted into another form of physical quantity.

14. What are all the basic elements of closed loop control system?

a. Input parameter

b. Output variable

c. Controller d. Process. e. Sensor.

f. Actuator

15. Write the various types of MCU used in CNC machines? Three types of MCU are a. Housed MCU

b. Swing around MCU

c. Stand alone MCU.

16. Define DNC? Direct numerical control system is defined as a manufacturing system in which a number of machine tools are controlled by a computer through direct connection and in real time.

17. List the main component of DNC? Main component are a. Central computer.

b. Bulk memory.

c. Telecommunications lines.

d. Machine tools.

18. Write the functions of DNC?

It will take some of the burden of the main computer.

Each satellite computer.

19. List out the types of CNC machine? CNC machine can be classified as: Machining centers

Lathe machines

Drilling machine

Turning centers

Milling machines

Gear shaping

CNC gear hobbling

20. Define the term “Basic unit length”? The minimum possible incremental movement is called the Basic unit length (BLU). The BLU might be on the order of 0.01 mm in typical machine tool system.

21. Write the functions of sideways. The functions of sideways: a. To control the line of action of the carriage/table on which the tool/work piece is held.

b. To absorb static and dynamic forces.

22. List out the characteristics of sideways used in CNC.

The sideways should have the following characteristics. a. Good rigidity and damping capability.

b. Good wear and friction resistance.

c. Geometric and kinematic accuracy.

d. Ensure higher velocity of slide.

e. Protect against damage and sward.

f. Provision for adjusting the play

g. Position in relation to work area.

23. What are the different types of friction guide ways?

The different types of friction guide ways are: a. Flat guide

b. Vee guide

c. Dovetail guide and

d. Cylindrical guide.

24. List the limitations of friction guide ways?

The limitations of friction guide ways ARE: a. Surface to surface contact between the sliding members, which leads to more amount of wear.

b. Large amount of friction and heat developed in the sliding parts.

25. Write the advantages of LM guide ways? The following are the advantages: a. To reduce the amount of wear

b. To reduce the friction. c. To reduce heat generation

d. To improve smoothness.

e. Zero stick slip

26. What is hydrostatic guide ways? State the classification of it? The guide ways in which the surface contact between the sliding parts is separated by a thin layer of fluid (oil or air) is known as hydrostatic guide ways.

27. What is actuating system? Write the various types of actuating system used in CNC? Actuation systems are used to convert the rotational movement into translation movement. The two types of actuation systems used in CNC are: a. Screw and nut, and

b. Rack and pinion.

28. What are the advantages of ball screws sideways over conventional sideways.

a. Ball screws have longer life.

b. Higher accuracy can be maintained.

c. Low frictional resistance and hence used for carrying heavier loads at faster rate.

d. Low power requirement for driving.

29. 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, a. AC servo motor,

b. DC servo motor,

c. Brushless DC servo motor II. Stepper motors, and

III. Linear motors.

30. What is the function of ATC?

The function of ATC is to change the tool once the particular operation is over and the next tool is automatically changed and positioned for machining based on the tool sequence given in part program.

31. What is machining center? Write the different types of machining centre. How it differs from conventional CNC milling machines? The centers which consists of multi function CNC machines equipped with ATC which are capable of carrying out milling, reaming, tapping, boring, counter boring and allied operations without operator intervention is called machining centre. Different types of machining centers are: a. Horizontal machining centre,

b. Vertical machining centre,

c. Universal machining centre.

32. What are the functions of feedback drives? The functions of feedback drives are: a. To convert one form of physical quantity like speed, displacement, etc., into electrical signals.

b. This electrical signal is then compared with the reference signal to check whether the desired motion of tool/worktable is achieved or not.

33. Enumerate the various feedback devices used in CNC system? There are two types of feedback devices used in CNC system. a. Position feedback devices (Example : Linear transducers, encoders)

b. Velocity feedback devices (Example : Tachogenerator)

34. What is the use of position feedback circuits and classify it. Position feedback circuits are used to measure the position of the tool edge with reference to the machine slide. They may be classified as: Rotary transducers Linear transducers

a. Rotary encoder b. Resolver

a. Glass scales with line gratin. b. Ferranti system. c. Binary coded system

35. What are the different forms of Inductosyn? Where it is used and why? Inductosyn are classified into: Rotary encoder and Linear encoder.Rotary form is used in precision servo systems on machine tools whereas linear forma are used in automatic machine tool slide controls.

Reason: Inductosyn is a precision feedback device for the accurate measurement and control of angles or distances with inductive coupling between conductors separated by a small space

36. What is a rotary type encoder? Where it is used? 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 timer belt.

37. What is the role of optical grating in CNC drive? An optical grating is a glass strip marked with series of equally spaced transparent opaque lines. They are used as transducers on machine tools. This is used to measure the position of the table.

38. What is Inductosyn? Inductosyn is a analogue type position back device which is used for the, measurement of control angle and/or linear distance.

39. Write the various measures employed in CNC machine to minimize the structural deformation. To avoid the structural deformation, following precautions have to be considered. a. Providing a proper design mild steel structure having higher stiffness.

b. Use of ribs, braces and plates to increase the stiffness of machine.

c. Providing large heat removing surfaces.

d. Use of excellent coolants.

e. Reduction of ambient temperature by using air conditioning units

40. Write the salient features of tooling system? The following features should be considered before the selection of tooling. a. Rigidity to withstand higher cutting forces

b. Rigidity to transmit the spindle power at higher speed.

c. Perform variety of cutting operations.

d. To keep down time

41. Define numerical control. Numerical control is a form of programmable automation in which the mechanical action of machine tool or other processing equipment are controlled by a program containing alphanumeric data numbers, letters and symbols. 42. How tapes are standardized? List some of them.

Tape standardization covers two important categories. a. Physical dimensions, and

b. Character coding.

43. What are the different types of dimensioning system used?

a. Absolute dimensioning or baseline dimensioning system.

b. Incremental dimensioning or delta dimensioning system.

44. What is BIT? What is the use of it?

„Bit‟ is an abbreviation of “Binary digit” which can be 1 or 0.Uses in binary coded decimal system. 45. What is 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. 46. What Is G code? How G codes are represented?

It is the series of combination of „1‟s and „0‟s.It represents a number or an alphabet or any symbol. 47. Write the different types of tape programming format?

The following are the types of tape programming format. a. Word address format,

b. Table sequential format,

c. Fined block format.

48. Explain the functions of codes G28 and M30.

G28 = Mirroring

M30 = End of data. 49. State the function of post processor in CAPP.

Post processor is separate computer program which has been used to prepare the punched tape for a specific machine tool. 50. List out some of the important NC languages.

a. APT

b. ADAPT

c. EXAPT

d. Compact II

e. PROMPT

f. CINTURNII

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

a. Geometric statements

b. Motion statements

c. Post processor statements

d. Auxiliary statements.

52. How can you represent an APT geometric statement?

Symbol = Geometry/Descriptive data Example: point, line, circle, plane 53. What are the different types of surfaces used for achieving continuous path commands?

a. Drive surface, which guides the tool.

b. Part surface, on which the bottom of the tool rides.

c. Check surface, which stops the movement of the tool.

54. List the motion command words used in achieving path commands.

a. GOLFT

b. GORGT

c. GOFWD

d. GOBACK

e. GOUP

f. GODOWN

55. What are the modifiers used with motion command words?

a. TO

b. ON

c. PAST

d. TANTO

56. List any four post processor statements.

a. COOLANT/ON,OFF,MIST

b. RAPID

c. TURRET

d. MACHIN

57. Write any four auxiliary statements.

a. INTOL

b. OUTTOL

c. CLPRNT

d. PARTNO

e. FINI

58. What are the identifications needed to store a tool library?

a. Tool dimension details and

b. Tool assembly details.

59. Difference between 21/2 and 3 axes machine.

a. 21/2 axis machining implies that the machine movements are in planes parallel to the X-Y plane.

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

60. What is the role of computer aided part programming? The role of computers in CAPP is as follows: a. Input translation

b. Arithmetic calculations

c. Cutter offset compensators

d. Post processor.