Re: [CAD_CAM_EDM_DRO] move error in MACH3

thanks for the replies guys! (Art, Dan, Jon, Ron...on this and
Mach1,2...forums)

I've replaced the metal nuts on this mini mill with homemade ACME
plastic nuts. My rationale is that the forces are small enough that
the deflection of the plastic (nearly a 1" block) would be minimal and
making tight threads in plastic would still be low enough friction to
allow minimal backlash. We did the X axis some time ago, because the
metal on metal nut was unacceptable. We recently replaced the Y axis
with a thinner piece of plastic and get 0.009, which I am disappointed
in, but that is still better than the metal on metal we used to have.
So this is how we have lower backlash on this little mill.

We measured backlash by preloading a dial caliper (and zeroing it), then
moving another 0.500 into that preload, read the dial (at 0.500) and
then moved in the other direction 0.500 and read the dial again. The
backlash is how far off of 0.000, right? On the X axis we got a
difference of 0.00O. Now that I think about this, I've got to confirm
which screen we used to do the moves, cause the MDI screen seems to be
the one that it isn't causing the move we request. I had a friend
actually measure the backlash and the axis calibrations. I didn't look
at how he did the backlash, although we talked about how to do it, and
I'm pretty sure he understood. He showed me the axis calibration
methodology because he got the errors described, so I have first person
knowledge of that issue.

I think I've found some stuff to use to move a greater distance in X, so
I can get better measurements, but the fact that MACH's axis calibration
gives a 15984.xxx on X and 16000.xxx on Y (we are using Gecko 540's,
and direct driving 8 TPI ACMEs so 16000ish should be correct) leads me
to think that somewhere I have some setting wrong that makes MDI page
behave differently than "axis calibration". And Dan Mauch suggested
using G1 instead of G0. I'm not sure what speed the "axis calibration"
settings uses, but on the MDI page we were using G0. While we don't
want to miss steps at "rapid" maybe that's what is going on??? I'll be
curious about using G1 vs G0.

BTW - as I said I have some thoughts about how to accurately measure
longer moves, but I'd love any suggestions...I have 6" dial calipers, 6"
precision parallels, 123 blocks, a 3" vise, a laser "edge finder", real
edge finder, a machinists precision square, and maybe a precision angle.

more testing this afternoon.

Thanks again! C
12/2/2012 12:02 PM, Jon Elson wrote:

>
> chuck merja wrote:
> > Thanks for the comment. And with the exception of going a long
> > distance, we did do axis calibration. We only have a 1" dial indicator,
> > but using MACH axis calibration in settings at 0.500" move, we get a
> > 0.500" move on dial indicator. BUT THEN when we go to the MDI page and
> > do a G0 X0.5 we get a 0.455" move. That's nearly a 10% error. Motors
> > sound good in both cases (don't hear any missed steps, as we often do
> > when we run up against mechanical limit. And again there is 0.000 to
> > 0.001" backlash on this X axis.
> >
> Make measurements going twice in the same direction. Dial indicators
> can accept
> some overtravel. So, go some tens of thousandths past the zero away
> from the
> indicator, then go to where you have found the zero is, then move 1 "
> more toward
> the indicator. Then, repeat the test by toward the zero, then backing
> away.
> You will undoubtedly get a different reading by reversing direction.
> This is
> the backlash (free play) in your Acme screws/nuts. I'm guessing that it
> may be about 0.05", which is not astonishing on an older machine.
> How have you determined there is only .001" backlash?
>
> If the backlash really is that small, then you need to make a long move
> against
> a high-quality ruler. Do you have a caliper? That can also be used to
> measure
> travel of the machine. But, if no caliper, a good ruler should show a
> large
> error like you are reporting.
>
> Jon
>
>

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