Re: [CAD_CAM_EDM_DRO] Encoder index and homing

cnc_4_me wrote:

>[sinp]
>Home on the other hand is merely a position to find travel limits and
>possibly a position to move to, to get clearance to change parts or
>tools. Because of this it seems to me that home does not need to be
>very repeatable.
>
>

It depends on what accessories you have on your machine. An automatic
toolchanger (mounted to the table) would require a precise home
location. Also, software can then have "soft limits" and do things like
stop and wait for oprtator input if asked to exceed these limits (better
than a limit switch, which is often an e-stop event).

>The only instance I can think of were precise home repeatability
>would be of any concern. Is if you command the machine to home after
>you have set your zero and your CNC control software automatically
>zeros its position. And since I have never used any CNC software I
>do not know if this happens…
>
>

That's what homing is - the machine moves an axis until the home switch
input turns on, and zeroes the position for that axis.

>As for using the encoder index pulse for zero position, this would
>not be feasible. Let's assume the encoder is mounted on a 5TPI
>ballscrew. That means the index pulse comes around 1 time per rev,
>or 1 time every .200". There is no practical way to sync this
>position to your vice or stops.
>
>

Well - you can do it, you just wouldn't want to have to every time you
set up a workpiece. There are corner blocks available that you clamp on
to the table, which act like an end stop. If you go through the setup
carefully once, you can drop a bunch of pieces at home pretty
repeatably. (not that this is common practice - I don't know about that
- but it is possible)

>To sum things up, home is a rough position for table and spindle
>limits found with limit switches. And zero is a precise position
>defined by the machine operator using the keyboard.
>
>Wally
>
>

This operator zero is a "relative" position. I have used a Bridgeport
CNC (I'm not sure what model), and on that unit you could hit a zero
button to set the DRO displays to 0.0. This gave you relative
measurements (and for all intents and purposes was the "real zero" - NC
code would use that as zero, even for absolute commands) until you hit
another button to turn that mode off. At that point, the displays showed
the "real" position. (sorry I can't be more specific about the machine /
controls - I was making parts for my company at a machine shop that was
shorthanded, so I didn't do the setup for the part run)

Think of it like absolute zero and the temperature that water freezes.
There is a "real" zero, which we often don't care about but sometimes
comes in very handy. For all other circumstances, we use some arbitrary
zero, and reference everything to that.

- Steve