Re: DNC

> From: TADGUNINC@...
>
> I am still researching things....what exactly is DNC?

DNC means Direct Numerical Control. NC (paper tape or card based) controls
had no memory to store part programs (G-codes files) they read the tape one
block (line) at a time, did that move, and continued. They might have
buffered a line or two in the control to prevent pausing the feed to read the
next block, but that is all the memory they had. Early CNC controls had a
limited amount of RAM that they could use to store a part program (or
several) as well as tool and work offsets. For example my old Bridgeport
BOSS8 control had 13k bytes of program memory. Control specs then (and now to
some degree) advertised the amount of program memory in terms of how many
feet of 10CPI paper tape was replaced by the internal memory (13k translates
to about 111 feet of paper tape). Many early controls also had paper tape
readers on them which could be used to store and run even longer programs.
Many CNC controls, early and late model, also support extensions of the EIA
standard RS-274 G-codes to perform operations such as rectangular pocket
milling in steps with finish passes, drilling entire bolt hole circles, etc.
all in one block. The purposes of this are to make manual programming easier
and to reduce the amount of memory needed to store programs. Another memory
saving feature was the use of macros and sub programs which allowed reuse of
code segments. The problem comes in when you want to run programs larger than
will fit in the internal memory of the control, or larger than one reel of
paper tape. Programs that finish the contours of molds for example are large,
consist of many short simple moves, and can't easily be broken up and run in
segments that fit available memory. DNC (or drip feeding as it is sometimes
called) involves using an auxiliary serial port on the CNC control to receive
programs a batch at a time from an external computer. Data flow is throttled
either by hardware handshaking lines, or more commonly by a proprietary
version of XON/XOFF. The DNC option on most controls costs about $1500, and
as you might guess isn't exactly rocket science. Modern controls (and I'll
put the EMC in this category), store programs on disk drives in their
computer and program size is limited by the amount of free space on the disk.
Also, most CAM programs don't output anything other than G0, G1, G2, and G3
for feed moves so extended G-codes are gradually disappearing along with
macro support. Technology marches on (however slowly in the machine tool
business), although there are still plenty of machines running off of paper
tape usually punched by hand on an ASR33 teletype.

So, to directly answer your question, the DNC program you were looking at is
what runs on your PC to drip feed large part programs into CNC controls via a
serial port and various communication protocols.

Matt

P.S. For an example of a program large enough to require DNC see:

ftp://ftp.isd.cme.nist.gov/pub/emc/emcsoft/racecar.zip