They're called fixture offsets was Re: 1 more "Hip Hip" for Lim "StepOffs"

Chris,

This is what fixture offsets are for...

A fixture offset is nothing more than a set distance from the
physical location of the home switches. Most current machines have at
least 6 (G54-59), and many have MANY more.

So when the machine is SENT to the home position (as opposed to
homing or homed, which you mentioned; and is a different thing) at
the end of your toolpath, it will go to the OFFSET zeroes for the
involved axes. And these WILL be away from the home switches (by
whatever amount was determined at setup(for toolpath), or
configuration(of the control)). Which of these two words fit best is
largely determined by the control topology, and really simply means
how easily the offsets are accessed, determined and changed.

So to sum up, any "responsible" control will have ONE, and one only:
machine HOME. It WILL be at the switches themselves.

AND this control WILL have the ability to "refer to" other positions
as toolpath or part zero for use with toolpaths. These are the
fixture offsets. So ALL of the things you mention ARE handled by the
fixture offsets, and it simply becomes a case of terminology
complicating things. Something for which CNC is well known!

Hope this helps,

Ballendo

--- In CAD_CAM_EDM_DRO@yahoogroups.com, CL <datac@l...> wrote:
> A while back there was a short discussion on Limit "Step Offs", An
> adjustable "distance" the machine moves in the positive direction
after
> finding the Limit Switches during a "homing" sequence. Some
controllers
> allow you to modify this distance, others do not. I personally find
it a
> necessary option. ALL controls being developed today should have
this
> option, or it will fall behind the curve.
>
> Properly set up, a controller should allow one to "step off" from
the
> switches a distance that is uniquely particular to the machines
design
> (read-adjustable). That final location then becomes in effect
Machine
> Zero ( this may be based on fence locations or fixture point(s)).
>
> I had many reasons why I liked the option based on simple head
changes
> and tool offsets alone, that are more simply handled in this
fashion
> than by any other means. But, one more important reason was in
regards
> open loop machines ( and there will continue to be many of those in
the
> years to come so please don't turn this into Servo is better than
> Stepper, BOTH have their place ).
>
> IF, and I say IF, the machine is NOT where it is supposed to be
after
> running a file, And, When using a Rewind command, or a command that
> tells the machine to Go to Machine Zero after the job, You have a
far
> better chance of a successful event IF there is some "clearance" or
> leeway, rather than having the machine smash into your limits at
Rapid
> rates. With the Proggy I use, I have a command in the last line of
the
> code that ALWAYS re-homes the machine after a Job. I am usually not
> standing there. When I do come back, I will know if it was within
my
> preset Guidelines or not in a "gentle fashion" rather than finding
out I
> had a clamp in the way, lost a few steps and Smashed my Limits,
losing
> position.
>
> As many find out as they are setting up a machine for the first
time, or
> as they are trying out a new "version" of someones release, or if
they
> are avid Beta testers..... Admit it, They have at one time or
another (
> I've done it quite a bit :-) ) driven that machine *hard* into a
> limit/mechanical stop device. Not all machines have been designed
to
> allow mechanical space "behind" the "Trip Zone" of the switch
(though
> all should be).
>
> **Even then, If a machine overtravels when heading back to the
Machine
> Zero location, any deviation in the actual location of the
hardware
> will Force a reset as Limits generally cause all Positional
information
> to be lost. In most programs I have used, Only a homing routine can
be
> allowed to not reset if desired.
>
> I don't suppose this matters much to those who only run a tops of
50ipm
> or so..... But running anywhere between 250ipm, 500ipm and beyond
can
> make a big mess in a really big hurry.
>
> Being an avid beta tester, I can attest that The Limit Step Off
distance
> adjustability has indeed saved more than just a little wear and
tear on
> my stuff. I will not run a software without it, nor recommend one
until
> it has it in it.
>
> Chris L