Cnc machining nm09/2


Numerical control (NC) advantages



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1.5 Numerical control (NC) advantages


The great variety of numerical control applications were introduced in the preceding pages. We also examined the general characteristics of production jobs for which NC seems to be particularly well suited. When properly applied, numerical control provides the user with a significant number of economic advantages.

In this section the advantages and disadvantages of NC are discussed and compared with conventional manual methods of production.



  1. Reduced non productive time. Numerical control has little or no effect on the basic metal cutting process. However, NC can increase the proportion of time the machine is engaged in the actual metal cutting (or other manufacturing) process. It accomplishes this by means of fewer setups, less time setting up, reduced workpiece handling time, automatic tool changes on some machines and so on.

  2. Reduced fixture cost. Numerical control requires fixtures that are simpler and less costly to fabricate because the positioning is done by the NC program rather than the jig or fixture.

  3. Reduced manufacturing lead time. Because jobs can be set up more quickly with NC and fewer setups are generally required with NC, the lead time to deliver a job to the customer is reduced.

  4. Greater manufacturing flexibility. With numerical control it is easier to adapt to engineering design changes, alterations of the production schedule, changeovers in jobs for rush orders and so on.

  5. Improved quality control. NC is ideal for complicated workpieces where the chances of human mistakes are high. Numerical control produces parts with greater accuracy, reduced scrap and lower inspection requirements.

1.6 Numerical control (NC) disadvantages


Along with the advantages of NC, there are several features about NC which must be considered as disadvantages.

  1. Higher investment cost. Numerical control machine tools represent a more sophisticated and complex technology. This technology costs more to buy than its non-NC counterpart. The higher cost requires manufacturing managements to use these machines more aggressively than ordinary equipment. Machine shops must ideally operate their NC machines two or three shifts per day to achieve this high machine utilisation.

  2. Higher maintenance cost. Because NC machines are more complex technology and because NC machines are used harder, the maintenance problem becomes more acute. Although the reliability of NC machines will generally be higher than conventional machines the overall cost of maintaining them is greater.

  3. Finding and/or training NC personnel. Certain aspects of NC shop operations require a higher skill level than conventional operations. Part programmers and NC maintenance personnel are two skill areas where available personnel are in short supply. The problems of finding, hiring and training these people must be considered a disadvantage to the NC shop.

1.7 Features of numerical control machines


The term Numerical Control refers to the ‘encoding’ of information in a way so as to drive machine tools and slides. You will understand that, regardless oft heir application, most NC machines have three basic sub units:

  • The machine tool itself

  • The control unit

  • The machine positioning system.

On a conventional machine an operator controls these functions and sets or alters them when the operator considers it necessary, the decision resulting from his/her training, skill and experience.

Obviously, the machine settings may differ between operators as will the time taken to read scales, set positions, change tools, alter speeds and feeds, engage drives and set up the work piece etc.

CNC automatic control can be applied to these functions and so result in consistent and reduced machining times through optimised cutting data, fast accurate positioning between cuts and fast automatic tool changing.

1.7.1 CNC Machining centre

Axis designation


Like conventional milling machines a CNC machining centre has three basic axis of motion which are driven by the part program in either a pre-selected feed rate or in rapid traverse which is generally in the range of between 10 to 100 metres/minute. The tool slides can be programmed to move independently or as a combined movement of any two or three axis.



Figure 1.3 Machining centre axes

1.8 CNC machine features


CNC Machines differ in construction to conventional machines in many areas other than their method of control.

It is essential to accelerate CNC machine slides quickly and also to bring them to rest quickly, so the design must offer as little friction as possible and be rigid without excess weight. If weight and friction can be reduced then so can the size and weight of the drive motor and gearing thereby improving performance.

Slide friction can be reduced with the use of materials offering low co-efficient of friction as well as attention to details such as surface finish and lubrication.

Flat slides also can be 'floated' on a high pressure film of lubrication oil to virtually eliminate normal slide friction. This design is known as hydrostatic sideways, and is complex and costly compared to other systems. Roller or linear ball bearing slides offer low friction also but usually have a trade-off in load capacity.

The main structure of the machine, on which the slides are positioned, must be rigid and stable under conditions such as:


  • Heavy loads (static load)

  • Heavy cutting conditions (dynamic load)

  • Reaction forces from rapid acceleration of slides (dynamic load)

  • When slides over hang at the extremes of their travel (static load)

  • Heat build up after prolonged use (thermal source).

Because component sizes are produced by machine motions that are controlled by unchanging numeric data, it is important that the last part produced at the end of the day will not be altered from the first by thermal instabilities.

On conventional machines, sizes and positions are controlled manually, and it is quite usual for an operator to be constantly altering positions during the machining operations or throughout the day, so thermal stability is not as critical as for CNC machines.

Thermal instability falls into three basic groups:


  • Machine design

  • Machining processes

  • Machine siting

  • Machine design.

In the first, the greatest source of heat is from the spindle or geared head, but localised heating of slides and lead screws as well as heat transmitted from drive motors can also affect accuracy. Therefore, the machine tool manufacturer must take thermal effects into consideration at the design stage.

Machining processes can result in a great deal of heat. For example, heavy cutting of large work pieces on milling machines can result in heat being conducted readily into the machine table and slides.

Machines must be sited away from or screened from sources of heat such as afternoon sun through a window, heaters, hydraulic power packs, ovens etc.

Remember a temperature difference of only 1°C over 1000 mm can cause an error of 0.01 mm which may be within the required machine accuracy, but outside the tolerance for the job.

It is usual to build conventional machines from cast iron -a material that offers rigidity and vibration damping, however for a given weight a fabricated (welded) steel structure offers greater rigidity and strength, and it is this construction method that is commonly used for CNC machines.

CNC machines using chip producing machining methods also commonly have tool holding and automatic changing devices in order to maximise production by reducing tool changing times to a few seconds at most.


1.8.1 The control unit


The CNC Machine Control Unit (MCU) has to read and decode the part program, and to provide the decoded instructions to the control loops of the machine axes of motion, and to control the machine tool operations.

The main grouping of parts of a control could be considered as:



  • The control panel

  • The tape reader

  • The processors.

1.8.2 Control panel


This is the human interface that allows various modes of machine or control operation to be initiated, from switching on and homing, to program loading and editing, to setting work positions and tool offsets, manually controlled movements and commencing the automatic cycling of a program. Information about machine status and condition is available to the operator via VDU screens, gauges, meters, indicator lights and readouts.

A typical control panel is shown on the Sinumerik 820 Machine Control Unit. This panel divides into two broad functional areas.



  1. An interface which relates to loading editing and validating the program and,

  2. An interface which relates to the manual control of the machine including program over-ride.

1.8.3 Program interface


This part of the control panel allows the operator to communicate with the program and any supporting software which is part of the Read Only Memory (ROM). This interface also has ‘Keyboard’ facilities which allows for Manual Data Input as well as editing and validation of programs. The units video monitor provides a visual display of the programs either as readable data or animated graphics.



Figure 1.4 CNC machine control panel

A. Graphics display with softkey input

B. Display panel

C. Address keys

D. Symbol keys

E. Calculation keys

F. Numerical keys

H. Control keys

I. User defined keys

1.8.4 Machine control interface


Apart from such basic controls as stop and start, this aspect of the control panel also provides the means of manual control and program over-ride. This manual control is needed for tasks such as setting zero and tool offsets.



Figure 1.5 CNC control interface

1. Emergency stop button

2. Mode selection switch

3. Single block switch

4. Spindle over-ride switch

5. Feed rate over-ride switch

6. Machine ON switch

7. Key locked switch

8. RESET key

9. NC stop key

10. NC start key

11. Spindle stop key

12. Spindle start key

13. Feed stop key

14. Feed start key

15. Axis selector switch

16. Direction key

17. Aux. axis key

18. Serial interface

1.8.5 Tape reader


The tape reader, where fitted, is used to transfer the program information contained on a program tape into the control unit. Most tape readers are of the photo-electric type which offer high speed reading with reliability and accuracy providing the tape is in good condition and the reader is kept clean and free of paper dust particles.



Figure 1.6 Tape reader

The tape reader reads NC tape coding and passes the information to registers within the machine control unit (MCU). The coded information then passes electronically to the machine tool where appropriate action or movement occurs.

Punched tape may be read:


  1. Mechanically, via sensors that pass through the holes and operate electrically.

  2. Photo-optically, using light beams and photo cells to provide electrical signals.

  3. Pneumatically.

1.8.6 The data processing unit and the axis control processor

The processor


The processes within a control are the electronic circuits that permit conversion ofpart program data into the machine motions and they may be classified into two main sections:
The data processing unit

The prime function of a data processing unit is to receive and decode the commands detailed in a part program. Additional functions include:

  • The input device, such as a tape reader, keyboard or memory.

  • Reading circuits and parity checking logic.

  • Decoding circuits for distributing data to the controlled axes.

  • An interpolator to supply velocity commands to the axis, either singly or in combination.
The axis control processor

This part of the control unit circuitry receives the decoded signals from the data processing unit and in tum operates the slide drives. The axis control processor also receives and interprets feed back signals on the actual position and velocity of each action.

The axis control processor consists of the following circuits:



  • Position control loops for each and all axis.

  • Velocity control loops

  • Deceleration and backlash take up circuits.

An MCU is adaptable to virtually any machine, the differing control motions and codes being a result of the way the control has been programmed. This permanent resident program is known as an executive program and resides in the read only memory (ROM) of the control, whereas the N.C program resides in the Random Access Memory (RAM). RAM allows external access and alteration if necessary, while ROM is programmed by the manufacturer and cannot be accessed through the control keyboard.


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