RE: [CAD_CAM_EDM_DRO] Homing position

> If you have encoders with index pulses, then you can reestablish the home

> position with the accuracy of a single encoder count.

This is a common method to provide an accurate home position on many types
of machinery. The trick is that you want to make sure that the mechanical
switch (or photo interrupter, etc) is defiantly 'made' before the index
pulse is seen. If the switch sensor is in such a position to be in an
indeterminate state when the index pulse is seen you can get poor
repeatability.



I agree with John that a mechanical switch is not generally very repeatable
w.r.t. its 'on' position. There are some type of 'pogo' switches that are
very accurate however but they are spendy, $50 or so for a cheaper one. With
mechanical actuation you always have a chance of swarf getting in the works
and throwing off the actuation position anyhow.



Many folks I talk to think that just installing some sort of home switch
will give them the instant ability to stop/restart a job. The actuation
position of the home sensor can be very repeatable but what about where your
fixture/parts are? I use different fixtures all the time and mount them all
over my table (to even out leadscrew wear.) An accurate home sensor will not
tell me where my fixtures are. If I have an accurate home sensor and
meticulously record the offset from the home position of each axis to some
index-able position on my fixture my fixture and I know the offset from the
fixture index position to my parts local zero than I can home, enter all the
offset information, and be able to restart very close to where I left off.
Usually though, in my experience it is easier to just rezero on the
fixture/part and go from there.



Some commercial machines have absolute encoders (scales) on each axis so it
is easy to get the machine back to a precise position after a power cycle.
But you still have to worry about maintaining the offset to your work.



Jeff Birt

Soigeneris.com



From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Jon Elson
Sent: Wednesday, March 14, 2012 10:24 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] Homing position





Roland Jollivet wrote:

> Thanks. Thats what I wanted to know.
>
> I will be repeating different parts on different days using jigs, so I

want

> to regain my setup as quickly as possible. I can't imagine trying to run a
> cnc machine without home positions.
>

If you have encoders with index pulses, then you can reestablish the
home position
with the accuracy of a single encoder count. If you only have the
switches, then
you need to determine the actual repeatability of them to know what kind of
accuracy you can get from day to day. Many people report sub-thousandth
of an inch repeatability with micro-switch limit switches, but I'm just
a bit
skeptical.

> I am familiar with cnc mills and lathes, but was uncertain about the
> tailstock on a manual machine. cnc lathes usually run the tailstock on a
> separate slide so it's not in the way of Z travel.
>

One trick I would certainly do is to put a microswitch on the carriage
that would cause
an E-stop before the carriage hits it. In fact, my manual lathe has a
big bolt on the
carriage that will hit the tailstock to protect the DRO scale from being
the first thing
to make contact.






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