Two mark questions and answers

TWO MARK QUESTIONS AND ANSWERS

1. Brief on the term Numerical control.

[Anna Univ. Nov’19]

Numerical control (NC) refers to a form of programmable automation in which the mechanical actions of a machine tool or other equipment are controlled by a program containing coded alphanumeric data encoded on a storage medium. NC machine tools are the machine tools operated by programmed commands in contrast to the manual control through hand wheels or levers, or mechanically automated through cams alone.

2. Distinguish Mechanisation and Automation.

3. What is meant by numeric control? State their advantages.

[Anna Univ. June 09 & Nov'13]

Controlling a machine tool by means of a prepared program is known as Numerical Control or NC.

4. State the advantages of integrating computer with NC machine tools.

1. The memory for part program processing is increased. 

2. The capacity for storing large part programs is increased. 

3. Editing of part programs on the control console made easy. 

4. Substantial improvements made in controls for NC systems.

5. Mention the various problems with conventional NC machines.

1. Wear and tear of punched tape

2. Least reliable tape reader

3. Flexibility of controller

4. Less management information

5. Part programming mistakes

6. Non-optimal speed and feed.

6. What are the elements of NC system?

1. Part program or software

2. Machine Control Unit (MCU)

3. Machine tool.

7. Classify NC machines based on the machine control.

(a) Traditional Numerical Control (NC)

(b) Computer Numerical Control (CNC)

(c) Distributed Numerical Control (DNC).

8. Define CNC.

[Anna Univ. May'15]

Computer Numerical Control (CNC) is a machine which controls the functions and motions of a machine tool by means of a prepared program containing coded alphanumeric data.

9. What is the role of computer for NC machine tool?

[Anna Univ. Dec'07]

Computer Numerical Control is an NC system that utilizes the stored program to perform the basic numerical control functions. Mini or microcomputer based controller unit is used

10. Draw the simple configuration of CNC.

11. List down the features of CNC systems.

1. Storage of more than one part program

2. Various forms of program input

3. Program editing at the machine tool

4. Fixed cycles and programming subroutines 

5. Interpolation

6. Positioning features for setup

7. Tool length compensation

8. Acceleration and deceleration calculations

9. Communications interface

10. Diagnostics.

12. Classify CNC systems.

(a) According to the structure of the control system used

1. Analog system

2. Digital system.

(b) According to the type of control loop used

1. Closed-loop system

2. Open-loop system.

(c) According to the type of tool motion control system used

1. Point-to-point or positioning system

2. Continuous path system.

a) Straight-cut system

b) Contouring system

(d) According to the programming mode used

1. Absolute mode

2. Incremental mode.

(e) According to the controller design 

1. Hybrid CNC

2. Straight CNC.

13. What are the functions of CNC systems in machine tools?

1. Machine tool control

2. In-process compensation

3. Diagnostics.

14. Classify CNC machines based on the type of control system.

(a) Open-loop system

(b) Closed-loop system.

15. Compare a closed loop CNC system with open loop system.

16. What are the classifications of CNC machines based on motion control?

[Anna Univ. Dec'08 & Dec'10]

1. Point to point NC system

2. Straight cut NC system

3. Contouring NC system.

17. What is point to point (PTP) system?

[Anna Univ. Dec'06]

The objective of the machine tool control is to move the cutting tool to a predefined location. The speed or path is not important in this system. It is also called positioning system.

18. How do continuous path machines work?

In continuous path machines, the tool performs the process while the worktable is moving.

19. Distinguish between point to point and continuous path systems. 

[Anna Univ. Dec'11]

20. Comment on the sensing requirements for PTP and Contouring axes.

 [Anna Univ. Apr'18]

PTP systems need only feedback of position but contouring axes need feedback of both position and velocity. In PTP systems, velocity is adjusted in open loop to achieve rapid traversal of the table with preprogrammed acceleration, deceleration patterns. But the hardware capabilities of the axis confirm often to contouring.

21. How are various functions timed in CNC machines?

1. Preparatory function

2. Miscellaneous function.

22. State any four advantages and disadvantages of CNC machines. 

Advantages of NC machines:

1. Reduced nonproductive time

2. Provides greater accuracy and repeatability

3. Lower labour cost

4. High production rates.

Disadvantages of NC machines:

1. Higher investment cost

2. Higher labour cost

3. Higher maintenance effort

4. Higher running cost.

23. Name the various elements of CNC machines. 

[Anna Univ. Dec'08, June'09 & May'14]

(a) Part program

(b) Program input device

(c) Machine control unit

(d) Machine tool

(e) Driving system (f) Feedback devices

(g) Display unit.

24. Write down the requirement of a good slideway system.

1. Low coefficient of friction at varying slide velocities.

2. Minimum difference between static and dynamic friction coefficient - positive slope for friction - velocity characteristics.

3. Low rate of wear.

4. High stiffness at the sliding joints.

5. Sufficient damping.

25. What are the requirements of spindles for CNC machines?

(i) High stiffness - both static and dynamic

(ii) Running accuracy

(iii) Axial load carrying capacity

(iv) Thermal stability

(v) Axis freedom for thermal expansion

(vi) High speeds of operation.

26. List down the elements of MCU.

(a) Central processing unit

(b) Memory

(c) Input/Output interface

(d) Controls for machine tool axes and spindle speed, and

(e) Sequence controls for other machine tool functions.

27. Mention the modes of operation in CNC machines.

(a) Manual mode

(b) Manual data input (MDI) mode

(c) Automatic mode

(d) Reference mode

(e) Input mode and Output mode, etc.

28. What are feed drives?

[Anna Univ. May’11]

Feed drives are used to drive the axis as per the program feed in CNC machine.

29. Mention the advantages of stepping motor.

[Anna Univ. May'13]

(i) Stepping motors can be used in open loop NC system.

(ii) The system is cheaper.

(iii) More accuracy is achievable.

30. What is the function of servovalve?

The electro-hydraulic servovalve controls the flow of the high-pressure oil to the hydraulic motor. The servo valve receives a voltage-actuating signal and it drives a solenoid device to move the valve spool.

31. What are linear bearings?

[Anna Univ. May’11]

A linear-motion bearing or linear slide is a bearing designed to provide free motion in one dimension. Linear motion bearings are widely used to guide, support, locate and accurately move machinery components and products in a wide range of automation applications.

32. What are compensation CNC systems offering for the various machines accuracy?

(i) Lead screw pitch error compensation

(ii) Backlash compensation

(iii) Sag compensation

(iv) Tool nose compensation

(v) Cutter diameter compensation.

33. Mention the points to be considered while designing of CNC tooling.

(i) High accuracy

(ii) Variety of operations

(iii) Interchangeability to produce the same accuracy

(iv) Flexibility

(v) Rigidity of tooling to withstand cutting forces

(vi) Rigidity to transmit the power at higher speeds

(vii) Quick changing of tools to keep the downtime minimum.

34. Define preset tooling.

The setting of fools in advance at a place away from the machine tool or offline in special holders is known as preset tooling.

35. What is meant by 'tool magazine' in a CNC machine?

[Anna Univ. June'12]

In CNC machines, the tool magazine is an indexable tool-holding device which accommodates several tools in the form of packets along with the specified packet number in such a way to easily identify it. This arrangement helps to obtain an automatic tool changing.

36. What is a preparatory function? How is it important in CNC Programming?

[Anna Univ. May’12]

Preparatory commands which prepare the machine or tool for the different modes of movement such as positioning contouring, thread cutting and also precede the dimension word. They are grouped. The group cannot affect each other. Only one function from the same group can be at the same time.

37. What are G-codes and M-codes? Give examples.

[Anna Univ. Dec'06]

G-codes are preparatory function codes which prepare the machine or tool for different modes of movement such as positioning, contouring, thread cutting etc.

Eg. 

G00-Point to point positioning

G01 - Linear interpolation.

M-codes are miscellaneous function codes which denote the auxiliary or switching information coolant on/off, spindle speed etc.

Eg. 

M00-Pregame stop

M01 - Optional stop.

38. Mention the types of ball screws.

(i) By ball circulation method

a) Return-pipe type

b) Deflector type

c) End-cap type.

(ii) By preloading method

a) Fixed-point preloading type

b) Constant pressure preloading type

(iii) By screw shaft

a) Precision ball screws

b) Rolled ball screws

39. Classify CNC machines based on type of machining operation.

1. CNC Machining centre

2. CNC Turning centre

3. CNC Lathes

4. CNC milling/drilling machines

5. CNC special purpose machines.

40. What are the elements of ATC?

(a) Rotary tool storage magazine for numerous tools.

(b) Automatic tool changer to remove tool holders from the machine spindle and replace them with tape programmed tools.

(c) Basic tool holders adaptable to a multiplicity of cutting tool types and work specifications.

(d) Tool coding rings and system for selection of proper tools in accordance with tape signals.

41. Define machining centre.

A machining centre is a highly automated machine tool capable of performing multiple machining operations under computer numerical control in one setup and it can work on more than one face of a component with minimal human attention.

42. What do you mean by machining centre with respect to NC machines?

[Anna Univ. Dec'11]

Machine centre is a machine tool capable of performing several different machining operations on a workpiece under one control program in a single setting.

43. List out the capabilities of MCU.

1. Starting and stopping the machine tool spindle.

2. Varying the spindle speed.

3. Changing the direction of rotation of the spindle.

4. Starting and stopping the coolant supply.

5. Changing the desired tool.

6. Changing to the desired workpiece.

7. Locking and unlocking the fixtures and workpieces.

8. Guiding the cutting tool tip along the desired path.

44. State the features of CNC machining centres.

(i) Automatic tool changer

(ii) Automatic component positioner

(iii) Automatic pallet changer

(iv) Multiple operations in one setup.

45. Classify machining centres.

1. Horizontal spindle machining centre

2. Vertical spindle machining centre

3. Universal machining centre.

46. State the advantages of CNC.

(i) It increases the flexibility.

(ii) An improvement in the possibilities is for correcting errors in part programming. 

(iii) There is a possibility of using the computer's peripheral equipment.

(iv) Tape and tape reader are used only once for resulting the improved reliability. 

47. State the limitations of CNC machine tools. 

[Anna Univ. May'10 & Nov'17]

(i) CNC machines are more expensive than manually operated machines, although costs are slowly coming down.

(ii) A CNC machine operator only needs basic training and skills which are enough to supervise several machines. Engineers are needed years of training to operate centre lathes, milling machines and other manually operated machines. It means many of the old skills are been lost.

(iii) Less workers are required to operate CNC machines compared to manually operated machines. The investment for CNC machines can lead to unemployment.

48. State any four applications of CNC.

1. Welding machines

2. Press working machine tools

3. Assembly machines

4. Inspection machines.

49. List the differences between NC and CNC.

50. Compare NC and CNC system with conventional systems.

51. Define DNC.

Direct numerical control is also called Distributive numerical control (DNC). It is a technology that permits a single computer to be networked with one or more machines that use Computer Numerical Control (CNC).

52. List the various components of DNC.

(i) Central computer

(ii) Bulk memory which stores NC part programs.

(iii) Telecommunication lines

(iv) Machine tools.

53. State the differences between CNC and DNC.

Or

Compare CNC with DNC.

54. State the advantages of DNC.

1. It eliminates the use of punched tapes and tape readers.

2. The computer can be remotely located even a thousand miles away. 

3. The computer can simultaneously program for many NC machines. 

4. It provides a convenient storage of NC part programs in computer files.

5. It ensures greater computational capability and flexibility

6. It is more convenient to incorporate editing and diagnostic features.