1、14. CNC MACHINES Computer Numerical Control machines use a computer to guide a process that might otherwise be done manually. 14.1 MACHINE AXES14.2 NUMERICAL CONTROL (NC) The use of numerical data to drive a machine for processes such as,- milling- turning- drilling- grinding- shot peening- tube bending- flame cutting- automated knitting machines- automatic riveting- etc. Basic components of NC systems,- program- controller unit- machine tool Most suited to,- parts are pr ocessed frequently in s
2、mall lot sizes- complex part geometry- close tolerances on workpart- many oper ations 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 gener ated programs The manual and computer aided methods use various NC programming languages,- APT (Automatically Programmed Tools)- AUTOSPOT (Automa
3、tic System for Positioning Tools)- SPLIT (Sundstrand Processing Language Internally Translated)- COMPACT II- ADAPT (ADaptation of APT)- EXAPT (Extended Subset of APT)- UNIAPT 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,- punched paper tape- Computer Numerical Control (CNC)- Direct Numerical Control (DNC) The automatic methods work with geometry created in a
4、 CAD program.14.2.1 NC Tapes NC Progr ams 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 mechanical parts in the readers reduced reliability- testing had to be done on the NC machine- no program editing abilities (increased le
5、ad 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 cur rently program transfer media are not required. 14.2.2 Computer Numerical Control (CNC) A computer controll
6、er 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,- progr am 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 a
7、pproached 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,- local programming,- communication over interfaces,- hard disk storage,- program simulation- etc. ASIDE: Direct Numerical Control is similar to CNC, except a r
8、emote 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 rem
9、oval rate varies- tool wear detection 14.2.3 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 impr oved it became practical to place a computer beside the NC machine, and DNC changed in form. Characteristics of mode
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