Cnc machining nm09/2

1.1 Numerical control

The numeric data is, in the beginning, written words in an easily understood code of letters and numbers (alphanumeric characters) known as a program, which in tum is converted by the machine control unit (MCU) into the electrical signals used to control the machine movements.

The relationship between the words 'numerical' and 'control’ is shown below.

Numerical: An instructional expression, in a language of numbers, which represents a series of commands for specific machine tool movements.

Control: To control such machine actions as:

• 
  Directing
  
• 
  Commanding
  
• 
  Prescribing
  
• 
  Sequencing
  
• 
  Initiating
  
• 
  Altering
  
• 
  Timing
  
• 
  Ceasing
  
• 
  Guiding.
  

1. 
   Program of instructions.
   
2. 
   Controller unit, also called a Machine Control Unit (MCU).
   
3. 
   Machine tool or other controlled process.
   

When considering the applications and general characteristics of NC machines it is important that two points be kept in mind.

Point 1

Point 2

1.2 CNC operation

CNC stands for Computer Numerical Control. It is a N.C system in which a dedicated stored program computer is used to perform basic control functions.

The functions of CNC controllers are:

1. 
   To read and store program information
   
2. 
   To interpret the information in a logical command sequence.
   
3. 
   To control the motion of the machines mechanical members.
   
4. 
   To monitor the status of the machine.
   

The interpretation of program commands by a machine control unit and its conversion of those commands into machine motion is complex.

Some of the features and functions are discussed later in this unit, but a simplified diagram of the basic elements of a CNC machine are shown in the schematic below.

Figure 1.2 Basic elements of an NC system

1.3 Computer numerical control (CNC) applications

Most trades persons are usually aware of numerical control metal cutting machines such as CNC mills and CNC lathes. However the scope for NC application extends much further than these applications. Generally NC applications may be considered under the following headings.

1.3.1 Metal machining

• 
  lathes
  
• 
  milling machines and machining centres
  
• 
  drilling machines
  
• 
  electric discharge machines(EDM)
  
• 
  tool and cutter grinders
  
• 
  surface and cylindrical grinders.
  

1.3.2 Metal forming

• 
  punches and nibblers
  
• 
  guillotines
  
• 
  pipe benders
  
• 
  flame cutters.
  

1.3.3 Metal finishing

• 
  painting
  
• 
  plating.
  

1.3.4 Component assembly/inspection

• 
  pick and place robots
  
• 
  spot and seam welding machines and robots
  
• 
  assembly of components into printed circuit boards.
  

1.3.5 Design

• 
  computer aided drafting machines
  
• 
  computer controlled plotting machines.
  

Other industries use NC systems for a wide range of activities such as:

• 
  composite materials laying such as kevlar, carbon fibre etc.
  
• 
  carpet dyeing and weaving
  
• 
  wood machining and furniture manufacture
  
• 
  cloth cutting in the clothing trade
  
• 
  warehouse control and materials handling
  
• 
  control of testing and measuring machines.
  

1.4 Economics of numerical control

Within the machining category, NC machine tools are appropriate for certain jobs and inappropriate for others. Following are the general characteristics in metal machining for which numerical control would be most appropriate.

• 
  parts are processed frequently and in small batches
  
• 
  the part geometry is complex
  
• 
  many operations must be performed on the part in its processing
  
• 
  much metal needs to be removed
  
• 
  engineering design changes are likely
  
• 
  close tolerances must be held on the workpiece
  
• 
  high cost part where mistakes in processing would be costly.
  

It has been estimated that a large proportion of manufactured parts are produced in lot sizes of 50 or fewer. Small lot and batch production jobs represent the ideal situations for the application of NC. This is made possible by the capability to program the NC machine and to save that program for subsequent use in future orders. If the NC programs are long and complicated (complex part geometry, many operations, much metal removed), this makes NC all the more appropriate when compared to manual methods of production. If engineering design changes or shifts in the production schedule are likely, the use of tape control provides the flexibility needed to adapt to these changes. Finally, if quality and inspection are important issues (close tolerances, high part cost, 100% inspection required), NC would be most suitable, owing to its high accuracy and repeatability.

In order to justify that a job be processed by numerical control methods, it is not necessary that the job possess every one of these attributes. However, the more of these attributes that are present, the more likely it is that the part is a good candidate for NC.

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