Cnc machining nm09/2

Review questions — section 1

Industrial applications of CNC machines

1. 
   What does ‘numerical control’ mean?
   
2. 
   List the three basic components that make up and operational control system.
   
3. 
   What does M. C. U. stand for?
   
4. 
   What does CNC stand for?
   
5. 
   What are four functions of a CNC control?
   
6. 
   Give three examples of CNC metal cutting machines.
   
7. 
   Give two examples of where CNC is used in the component assembly area.
   
8. 
   Give three examples of other industries using NC systems.
   
9. 
   Why is a small batch ideal for manufacture on a NC machine?
   
10. 
    Give three advantages of utilising NC for production jobs.
    
11. 
    Give three disadvantages of utilising NC for production jobs.
    
12. 
    What are the three basic sub-units of a NC machine?
    
13. 
    What is meant by ‘thermal stability’?
    
14. 
    On a CNC machining centre, which is the X axis?
    
15. 
    What is one advantage and one disadvantage of linear ball bearing slides?
    
16. 
    What is the purpose of the control panel on the MCU?
    
17. 
    If a machine has a tape reader, what type of tape reader would it most likely be fitted?
    
18. 
    What does ROM stand for?
    
19. 
    What is meant by the term ‘tool ring’?
    
20. 
    List four advantages of CNC as opposed to NC.
    
21. 
    What does DNC mean?
    
22. 
    What letters are used to describe the rotary axes on a CNC machine?
    
23. 
    How can you determine what should be the positive direction of a rotary axis without accessing the machine?
    
24. 
    What are the two main axes on a CNC lathe?
    

Section 2: Basic operation of CNC machines

This section covers the basic operation of CNC machines with regard to axes movement, stored stroke limits and buffer storage, work handling methods and the role of CNC in flexible manufacturing systems.

Objectives

At the end of this section, you will be able to:

• 
  describe the methods of moving CNC axes
  
• 
  describe stored stroke limits
  
• 
  state the purpose of buffer storage
  
• 
  list methods of work holding
  
• 
  outline the role of CNC in flexible manufacturing systems.
  

Safety reminders

• 
  In the workshop, always wear safety glasses, safety boots, hair protection and suitable clothing.
  
• 
  Avoid back injuries -lift the correct way.
  
• 
  Do not use a machine fitted with a Danger Tag.
  
• 
  Know where the first aid station is.
  
• 
  Don't run or play in the workshop.
  
• 
  Use ear muffs or plugs to protect your hearing.
  

2.1 Machine positioning system

Once the Machine Control Unit has decoded the part program it can now send the signals required to drive and position the table and tool slides as required by the program. This chain of event normally involves three specific sub units.

1. 
   Slide driving motors
   
2. 
   Slide positioning devices
   
3. 
   Feed back controls.
   

The figure below shows the general flow path of signals which drive, position and control all aspects of slide motion.



Figure 2.1 Machine motion drives and controls

2.1.1 Maw spindles and slide drive motors

In general, the multi-change gear boxes with fixed ratios driven by constant speed electric motors, as commonly used on conventional machine tools, are not always entirely suitable for NC machines. Variable speed drives enable cutting speeds and feeds to be maintained at optimum values, thus utilising a machines cutting capacity to the full. The extra cost of providing variable speed drives and the somewhat lower efficiency of the units (disadvantages in relation to conventional machines) are less significant on NC machine tools because the higher cost and depreciation rates of NC machines make it essential to fully exploit their potential.

On NC machine tools, speed changes are carried out in response to instructions written into the tape input data according to the machining program. Selection of the most suitable ratio is readily carried out ifvariab1e speed systems are used in the transmission. Some of the recognised methods of obtaining speed ratios are considered in the following paragraphs.

Mechanical drives

For reasons mentioned above, the conventional fixed ratio gearbox is used less often in NC applications, but when the system is used, the gears providing the various ratios are usually in constant mesh and remotely controlled electro-magnetic or hydraulic clutches are employed to engage the selected ratio. Variable speed drives based on mechanical principles have limitations in respect of the rate at which a change can be carried out, and of the relatively high forces which are needed to operate the change.

Electrical drives

Drives in which speed variation is obtained entirely by electrical means are of two main types, ie. systems utilising either an AC or a DC final drive motor. The AC motor achieves speed variation by changing the frequency of the supply through a frequency controller. The DC motor speed variation is obtained by altering the DC supply voltage, which in tum has been converted from normal AC supply. Both the AC and DC motors offer similar speed/power performance, with the AC motor being smaller and lighter and therefore giving superior acceleration and deceleration. Because the AC motor is an induction motor, it does not have brushes to wear and to replace as does the DC motor, but the DC motor is less expensive to produce and fit to the machines.

Hydraulic drives

Hydrostatic hydraulic drives are being used increasingly for NC applications. The usual system spindle drives is based on a constant speed electric motor driving and hydraulic pump which then supplies hydraulic oil under pressure to drive an hydraulic motor. The pumps and motors may be of either the fixed or the variable displacement type, and the choice will often depend on the characteristics required from the output, ie. high efficiency, rapid response, constant torque, constant power, wide speed range, etc.

Advantages of hydraulic systems, especially those incorporating both variable delivery pumps and variable displacement motors, are:

1. 
   Wide range of stepless variable speeds
   
2. 
   High torsional stiffness
   
3. 
   Relief valves ensure machine can stall without damage if an overload occurs
   
4. 
   No backlash.
   

Disadvantages

1. 
   There is a limitation on minimum motor speeds needed for smooth operation
   
2. 
   Heat generated in the system may have to be dissipated by a heat exchanger
   
3. 
   Oil leakage may occur at higher loads (arrangement to compensate for oil losses can be made)
   
4. 
   Special oils are needed to reduce hazards due to fire risks.
   

2.1.2 Slide positioning devices

Many of the foregoing comments relating to spindle drives will also apply to drives operating feed mechanisms, ego drives to tables and slides, whereas rotary drives are required for spindles, linear motion is usually required for feed mechanisms. The choice of mechanism is largely confined to:

1. 
   Hydraulic ram
   
2. 
   Rack and pinion, or
   
3. 
   Leadscrew and nut
   

Each of these mechanisms have their place, and all can be readily controlled for NC purposes. The choice is usually influenced by two factors, the length of stroke and the mass to be displaced.

Hydraulic rams are suitable for relatively short strokes and are particularly attractive for low and medium power ranges. Hydraulic actuators are economic and provide a smooth, still transmission in the smaller range, but as size increases, compliance becomes something of a problem. The column of oil in a cylinder is subjected to slight compression and acts as a liquid spring. In addition, slight elastic deflection of mechanical elements, ego cylinder walls, may occur. When such considerations become noticeable, a change is frequently made to a leadscrew drive.

Rack and pinion drives are particularly suitable for moving the slides of very large machines mainly because the range of stroke is not limited as is the case for machines relying on a leadscrew. Very long leadscrews need to be well supported and are generally of large diameter to minimise linear and torsional deflection over their length. Machine slides operates by rack and pinion have the advantage of high rigidity regardless of stroke length although the positional accuracy of rack and pinion systems is not as high as other methods.

Screw thread drives used on conventional machine tools, usually employ trapezoidal threads, ego Acme. These thread have several disadvantages. They are very inefficient (often less than 25% efficiency is obtained form Acme thread), due to high frictional resistance between the flanks of the screw and nut -and with the increased diameter of leadscrews used on NC machines this friction increases the torque requirements.

The friction gives rise to local heat, and inaccuracy results from this cause. Backlash of the magnitude met with in normal screw drives is quite unacceptable for many NC applications; it can be removed by using a backlash eliminator, but this device introduces further frictional resistance and accentuates the problems referred to above.

The use of recirculating ball leadscrew and nut, ensures which the connection between the screw and nut is achieved by an endless stream of recirculating steel balls.



Figure 2.2 Recirculating ball leadscrew

The efficiency of the recirculating ball screw is very high, often 90%, and even when subject to pre-loading to eliminate backlash, the frictional resistance is still not objectionable and the efficiency remains remarkably high.

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