Re: [CAD_CAM_EDM_DRO] Re: EMC and lead screw compensation

Bill wrote:

> "Lurkmode=OFF"
>
> Wally replies:
>
> >Smokie, I could not tell if you were kidding or serious in your
> >reply. If you have acme screws and try to circle interpolate a
> >circle you get a axis jump at 4 points of the circle from the
> >backlash. It is very doubtful this big jump could be compensated
> >for as the stepper would have to reverse direction faster than the
> >table would jump.
>
> One way to avoid this problem would be to generate full arcs in 4 quadrants
> so the control will read a new block at each compass point and compensate
> for backlash before executing the next quadrant. This method works very well
> on the Ahha software, I do not know about the other software packages
> discussed in this forum.

This has nothing to do with it. EMC has lookahead, and the backlash compensation
is handled on a step-by-step basis for stepper drives, and a servo cycle
basis for servos. But, it takes FINITE time to get from one side of the
backlash to the other, especially since the motor must slow down
when approaching the other side of the backlash, or it will 'bump' the
table more than intended. Also, since on some machining operations
the table can be moved by the cutting forces, you can't totally assume
you know what side of the backlash the nut is riding on at any moment.

> While this is not a replacement for quality ballscrews and a system with
> ZERO backlash, it is better than having to live with no backlash
> compensation for arc moves in a low cost system.

True. But, if you have more than a few thousandths of an inch
backlash, there are operations you just CAN'T do reliably. One
is anything that is cut in the climb milling direction, the other is
curves that pass the quadrant where an axis has to reverse.
It will cause a discontinuity there, and no amount of backlash
compensation will fix it, because in the time it takes to get
to the other side of the backlash, the table is not being moved.
Unless the curve is cut at agonizingly slow feedrates, there'll
be a bump there. I have about .001" backlash, and I can feel
and see that bump. it isn't big, but its there. It would not make
a good fit for a bearing, for instance.

Jon