Re: [CAD_CAM_EDM_DRO] Re: what the world needs now ... backlash aware stepper motor drive r

Right On Peter.....

Backlash seems to be a concern for many. Yet, unless it is a very large
machine with a lot of mass, Backlash Compensation has some downfalls (with an
economy control). On a smaller machine without the usual weight and mass,
backlash can end up as you describe - Messy.

I think it only works on a non-contact machine if there is enough friction in
the machine to allow it to ride "one way or another", letting the software
straighten things out, Or a much heavier machine where you can be assured that
an Axis simply does NOT MOVE unless the motors moved it !

One thing I do not think inexpensive control "users" really think about, is
that backlash is NOT just "add a few steps - subtract a few steps" according
to direction. Especially if and when Continuous Contouring is applied. I
had a very good discussion with Art at Ability Sytems once, as he explained
what is involved with doing compensation **correctly**. A straight line
motion, one way, then the other is real easy - a no brainer. But, when you add
in all of the moves a CNC machine >really< makes, the "add some-subtract some"
falls flat on its face. Thats where the marks come in.

Even acceleration and deceleration factors as applied to cutting force and
chip removal will affect just "What" amount of backlash is necessary. How do
you make such a calculation to hit that correctly ??

Thus, its why Indexer does not yet have backlash comp. Art said it can be done
right, but will take some time. He plans on adding it in the future, but even
then as the problem really is described, I don't see how it could ever be a
perfect situation.

I know a lot of guys are caught stumbling all over the issue of backlash comp,
but, In many circumstances, that very simple "add-Subtract" technique is the
one applied. Don't be fooled by some "Label" on a product indicating it
*includes backlash compensation* and expecting miracles no matter what the
cost of the control. Things will vary from installation to installation and
machine to machine.

That is why I am such a proponent to just stick in something better for a
leadscrew right off the bat.

To each his own................... Ballscrews are nice..... But maybe the
Rolled Leadscrews and A/B Nut will fit your hobby budget better.

If your really budget minded (or your wife will throw you out if you spend too
much on this "thing" your building), well, I guess the threaded rod at the
hardware store looks pretty good too.

Peter wrote:

> Having backlash compensation may be ok for light duty (i.e. low side
> force) CNC applications like plasma cutting. My experience with
> backlash compensation in my Flashcut CNC system is that while you can
> completely compensate (in theory) for the backlash in the acme screws,
> it still results in poor machining surface finish.

(snip)