43.2 NUMERICAL CONTROL (NC)
The use of numerical data to drive a machine for processes such as,
Basic components of NC systems,
The methods for developing NC programs include,
The manual and computer aided methods use various NC programming languages,
These languages are used by a parts programmer to define the motion of the cutting tool.
The languages may be preprocessed, and then used for a number of various control types, such as,
The automatic methods work with geometry created in a CAD program.
43.2.1 NC Tapes
NC Programs are preprocessed on computers, and punched onto paper or mylar tapes.
Simple NC machines can use a tape reader to direct the machine.
The end of tapes was the result of two competing developments
- CNC allowed the use of a local computer to overcome problems with tapes, and the problems with distant computers. While CNC was used to enhance tapes for a while, they eventually allowed the use of other storage media, and currently program transfer media are not required.
43.2.2 Computer Numerical Control (CNC)
A computer controller is used to drive an NC machine directly.
- the communication problems with DNC systems became obvious, and local computers were added to act as tape readers which would read tapes once, and play them back to the NC machine indefinitely
ASIDE: Direct Numerical Control is similar to CNC, except a remote computer is used to control a number of machines. This gives the advantage of more computer power. This approach is no longer popular, as the dropping cost of computers offsets any advantages.
Some companies use proprietary NC Languages, such as the example of DYNA Mill NC code shown later
These machines are often programmed by downloading NC code from a computer, or manually programming the controller computer.
43.2.3 Direct/Distributed Numerical Control (DNC)
- the oldest methods used modems, and a mainframe which emulated a tape reader, to control the NC machine (no storage)
- a more recent advance used a local computer which acts as a storage buffer. Programs are downloaded from the main DNC computer, and then the local controller feeds instructions to the hardwired NC machine, as if they have been read from tape.
- the newer methods use a central computer which communicates with local CNC computers (also called Direct Numerical Control)
DNC controllers came before CNC machines, but as computer technology improved it became practical to place a computer beside the NC machine, and DNC changed in form.
Characteristics of modern DNC systems are,
- simplifies implementation of group technology, computer aided process planning, and other CIM concepts
- telephone links used to send instructions from large computers to hard wired NC machines. Basically replaced a tape reader.
- Aerospace companies used DNC because of the large number of distributed machines in their facilities.
Initial resistance to DNC technology was (previously) based on,
when downloading programs there are two popular opinions,
- a program should only be downloaded in part, this accommodates easy engineering changes in a real-time environment.
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