Re: [CAD_CAM_EDM_DRO] 4th Axis Concept (long)

datac@... wrote:

> I have been searching for some type of Simple "Indexable" Clutch. This
> clutch would be such so that once disengaged, a re-engagement would
> force it back into the "exact same location" as it was prior to
> dis-engagement. (start thinking (2) 360 degree circle/plates with one
> single pin in it for engagement)

Look at the bottom of a 3.5" floppy disk, and the spindle in the
drive, that is by far the most common similar arrangement. I can see
doing this, simply picture a stepper shaft with a half flat D, out to
disengage, just barely touching (may need spring loading, may be doable
without) and spin the shaft to find engagement, then in the rest of the
way in to engage..
I don't see an easy way to engage/disengage without having another
small stepper to screw it in and out. You might could make a solenoid
system that'd work, but I think an extra stepper would be more
reliable. Easy enough to do a small PIC chip system to do this, all
you'd need is an engage/disengage signal, and wait a reasonable time
after the engage signal for it to get locked up..

But, I can also see a better way. Use any clutch system, not
indexed. Much simpler, just has to be locking type clutch so it won't
slip, should be a lot easier to find one of these that will work. Then
have an index wheel on the shaft somewhere, just a plastic wheel with a
hole in it, and an opto-interupter. After the engage signal, send the
command for a little more than one rev, at a low speed, then a wait
period of that long again. Then just have a PIC chip so after the
engage, it looks for the index hole. Soon as it sees the index hole, it
disconnects the step signal for a fixed 1 rev time (that's why you need
the extra wait time, the motor won't see pulses for up to one rev time
afterwards if it sees the hole near the end of the first rev). Then
you'll be set to the index hole, and the pic won't interrupt the signal
again for anything until another disconnect/reconnect.

Another question to ask, is absolute positioning really needed? If
totally lathed, it wouldn't be at all, everything is relative around the
piece. If you want to leave some areas square and have the work in the
round match up to the square portions, it will be, OR use a larger
starting block and machine it square after doing the round, but that'd
be a bit wastefull. But if you're machining it square anyways, you
really wouldn't need indexing, just good motion afterwards..

All in all I'd say it's easy enough to do, and you really wouldn't
need a large stepper to control the engagement. Give it some leverage
and I bet a surplus floppy drive motor would work great to move a much
larger motor in and out, certainly a NEMA 23 type would be enough for
most anything..