RE: [CAD_CAM_EDM_DRO] Homing position

>You mention having to "meticulously record the offset", but at least EMC

does that

>automatically and brings it back the next time the program starts.

Mach can save fixture offsets too; it does have to be meticulously recorded
by you or the CNC controller. The point is that you have to have a very
accurate home to do this, know what fixture offset you are using at the
time, (what if your program uses more than one fixture offset, what if you
turned off the 'Save fixture offset prompt' tick box and forgot about it),
record the offset, and have a way to re-zero your tool as well. Re-zeroing
the tool may be easy if you have a tool changer and fixed offset, if you're
like a lot of folks and re-zero each tool manually it might be a different
story. What if your general work process is to zero to the top of the stock
but you have milled all that away already?



I'm just playing Devil's advocate Jon. J My point is that yes you can stop
and restart a job but it requires planning in advance and a careful
procedure so as to not screw things up. Like most other things involved with
CNC it just takes a bit of learning and practice to get right.



I cheat at this procedure a lot of time by taking my X and Y axis to the
part's (0, 0) and then shutting things down. When I start back up I just
have to 'zero' the X and Y DROs and re-zero the tool. This is not perfect as
the steppers will likely move a step or two when things shut down and start
up (they will lock to a full step). But when doing things like cutting out
end panels for controllers being 0.001" off from one panel to the next is
unimportant.



Jeff Birt

Soigeneris.com



From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Jon Elson
Sent: Thursday, March 15, 2012 8:19 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] Homing position





Jeffrey T. Birt wrote:

>
>
> 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.
>

The re-homing the next day picks up all offsets and workpiece coordinate
systems
as they were when the CNC program was shut down. This assumes you have
not removed the fixtures from the machine. Of course, you could have some
dowel pins or whatever set in the machine so fixtures could be repeatably
installed on the table.

You mention having to "meticulously record the offset", but at least EMC
does that
automatically and brings it back the next time the program starts.

> 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.
>

The absolute encoder just lets you restart without homing, no big deal
on typical
machines. Takes less than a minute, and re-establishes all offsets to
one encoder
count.

Jon





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