E-Stop and Overtravel Switches...

[Gene Warren <gwarren-at-execpc.com>]

Ray Henry wrote:
> 
> 
> I set the home switches so the control thinks they are closed.  I set the
> overtravel so the machine thinks they are open.  Not having them connected
> should not make a difference.  I've read about folk on the list who never
> hook some of these things up.
> 
> Get back to me cause I love a good fight. (read problem)
> 


     Hi Ray,

      I would like to point out that the industrial machine tools all
(as far as I know) have the home switches, and especially the overtravel
switches, set for constant voltage sensing.  In other words, they have
to have a complete circuit when NOT actuated (closed, open on
actuation).  I repair these machines on a daily basis.  The reason for
wiring them like this is vibration of the machines sometimes loosens
wires, and a loose wire is an open circuit at times.  The theory is that
if the wire is loose, the machine will go into overtravel no matter
where it is physically and shut down machine operation until fixed.  A
loose wire will become a nuisance, I will be called, and I will have to
find the loose wire.  It saves a lot of ballscrews, blown fuses, and
overheated motors.  Home switches and overtravel switches wired not to
constantly detect voltage (open, closed on actuation) can loosen up and
the machine won't stop, no matter if you want it to or not.  I only
point this out for people who might not realize the difference.

     You mention E-Stop notification to the computer.  This is good, but
I would also like to point out (as you know) that the E-Stop switch
should not rely on the computer to shut down the axis drives.  Those
drives should be wired seperately to the E-Stop switch so no matter what
the computer is doing, the axis will stop.  Open up the relay
(contactor) that supplies power to the motors.  The computer can
energize this relay through the normally closed (non-actuated) E-Stop
switch.  When you are tossing parts all over the shop, you want that
sucker to stop.

     Don't really want to start a fight, but just want to point out
safety considerations to people without a lot of experience.

Gene