Re: 3 axis controller

[john s finnila <jsfinnila-at-juno.com>]

Ray:

Your comments are very good.  What I mean by the feed hold input to the
emc is a remote push button with an input via I/O that instantly pauses
all axis travels.  This I believe is necessary to prevent crashes and
also to allow pausing the machine cut in the event of a broken or dulled
cutting tool.  With the program paused, you can escape the program, jog
away from the cut, replace the tool, and then return to a safe start
point to begin machining again.  You can also keep your finger very near
the feed hold button on a untested program ready to pause the feed in the
event of an obvious program error.  Now lets assume that you have a feed
hold input installed in your control, it is a very basic change to
incorporate this same feed hold in the tool changer control.  The PLC
tool changer control which controls all of the varied and sometimes
complex functions necessary for a tool changer outputs this feed hold
signal via I/O to the emc.  The remote feed hold push button mentioned
triggers an input to the PLC which in turn signals the emc to pause all
axis.  Now to the tool change.  The part program commands a move to the
tool change position. The PLC recognizes that it is at the tool change
position via  switches or sensors.  In the next block (or line) of part
program the T command along with  the tool number is outputted from the
emc to the PLC.  The PLC now knows that it is at the tool change position
and it also knows what tool to change to.  The PLC therefore instantly
outputs the feed hold signal to the emc thereby pausing  all axis and
preventing the part program from advancing to the next block.  The PLC
then completes the tool change to the desired tool.  Once this is
accomplished, the feed hold is released, and the emc advances to the next
and subsequent blocks until the procedure is started again for the next
tool.  I have successfully retrofitted a tool changer control using a
Mektronix PC based control in the manner I just outlined.  I realize that
the same function can be done internally with a soft PLC but there is
very little difference in cost because the I/O from the PC to the PLC is
very reduced if you use an external PLC.  The PLC of course comes with
I/O.  The tool change command M6  is not required for the above
procedure.  However if the M6 command results in a program pause and
therefore feed hold,  the M6 command could also be used to accomplish the
same objective.
The reason I outlined the above procedure is that it allows you to use
the exact same emc control to control a variety of differing machine
tools with the only change occurring in the external PLC.   Otherwise the
emc has to be re configured for each differing machine tool.  I hope I
haven't confused things.

John Finnila