19.2 NUMERICAL CONTROL (NC)
The use of numerical data to drive a machine for processes such as,
- automated knitting machines
Basic components of NC systems,
Most suited to,
- parts are processed frequently in small lot sizes
- close tolerances on workpart
- many operations on part in processing
- large amounts of metal to be removed
- engineering design will possibly change
- parts that are too expensive for mistakes
The methods for developing NC programs include,
- manual part programming
- computer-assisted part programming
- computer generated programs
The manual and computer aided methods use various NC programming languages,
- APT (Automatically Programmed Tools)
- AUTOSPOT (Automatic System for Positioning Tools)
- SPLIT (Sundstrand Processing Language Internally Translated)
- ADAPT (ADaptation of APT)
- EXAPT (Extended Subset of APT)
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,
- Computer Numerical Control (CNC)
- Direct Numerical Control (DNC)
The automatic methods work with geometry created in a CAD program.
19.2.1 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.
Problems,
- required storage, transportation, and manual loading of NC tapes
- has to reread the tape for each new part
- tapes tend to wear, and become dirty, thus causing misreadings
- the readers are slow, and can cause `dwell marks' on complex pieces
- the mechanical parts in the readers reduced reliability
- testing had to be done on the NC machine
- no program editing abilities (increased lead time)
The end of tapes was the result of two competing developments
- DNC used remote computers to replace tape readers, these were displaced in most cases by CNC
- 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.
19.2.2 Computer Numerical Control (CNC)
A computer controller is used to drive an NC machine directly.
Characteristics are,
- controls a single machine
- located very close to machine tool
- allows storage/retrieval/entry of NC programs without preprocessing of NC code
Advantages of CNC,
- program is only entered into memory once, so it is more reliable
- the programs can be tested and altered at the machine
- increased flexibility and control options on the local computer
- easy to integrate into FMS systems
The Background,
- the problems with NC tapes were approached using DNC networks
- 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
- CNC controllers began using other storage media like magnetic tapes, and floppy disks
- CNC now offers features like,
- communication over interfaces,
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.
Future trends involve,
- adaptive feed rates to increase speeds as the metal removal rate varies
19.2.3 Direct/Distributed Numerical Control (DNC)
Uses a few methods,
- 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,
- uses a server (with large storage capacity) to store a large number of part programs
- the server will download part programs on demand to local machines
- display and edit part programs
- transmit operator instructions and other data needed at the machines
- collect and process machine status information for management purposes
Advantages are,
- eliminates the need for NC tapes (the advantages are obvious)
- design changes are immediate
- NC programs may be edited quickly
- can be used to support an FMS system
- increase efficiency of individual machine tools
- more shop up-time than with stand alone machines
- simplifies implementation of group technology, computer aided process planning, and other CIM concepts
- reduces peripheral costs with NC tapes
A Brief History,
- concept proved by Cincinnati Milacron and G.E.
- telephone links used to send instructions from large computers to hard wired NC machines. Basically replaced a tape reader.
- several commercial DNC systems announced.
- Aerospace companies used DNC because of the large number of distributed machines in their facilities.
Initial resistance to DNC technology was (previously) based on,
- high cost of computer hardware
- the number of machines which could be controlled by one computer was limited
- computer software was limited for maintenance, scheduling, control, and data collection
- a backup computer was usually required
- was hard to justify on the basis of downloading parts programs
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.
- many programs should be downloaded to the local controller to provide protection against system failure, and eliminating the cost of real-time response in the DNC central computer.
