Re: [CAD_CAM_EDM_DRO] Measuring backlash more accurately?

Randy Gordon-Gilmore wrote:

>I have been fighting a problem on the gantry mill. The Y axis seems to
>"drift" so that over the course of macining a contoured part (say 10 depth
>passes of outline and internal pockets) it has moved 20 thou or so. For a
>part that has 30-thou walls, this has created great frustration.
>
>After completely rewiring the controller box, fully separating signal from
>power grounds, minimizing wire runs, twisting pairs, etc. I made another
>trial run this morning, with no help.
>
>I realized that the backlash settings might not be right. (That's about
>the only thing left...) I had measured the backlash on each axis by the
>conventional method (move + increment, zero indicator, move + increment
>again and then - increment and write down the reading) but after a program
>that has maybe 100 Y moves, it seems to me that I need a more accurate way
>of measuring the backlash.
>
>Is there a way to cycle the axis so the backlash accumulates?
>

Backlash does NOT accumulate. It is just the looseness between the
screw and the nut.
(In some stepper systems, the bang when the backlash is taken up could
cause steps to be
lost, but this is not accumulation of the backlash, but the accumulation
of lost steps.)

So, if the error IS accumulating, then you are either losing steps in
some manner, or there
is mechanical slippage in your machine, somewhere. You can mark the
motor shafts and
then run a routine and come back to the same position and see if the
marks have moved.
If they have, you are losing steps. If the marks are in the right
place, but the machine is
NOT, then the slippage is somewhere past the motors (pretty unlikely).

Are you using Gecko drives? They have a different timing requirement of
the direction signal
than most other drives. So, you need to see to it that the software is
providing the right
timing of the direction signal. The problem is that when direction
changes while a step
is still being made by the Gecko, the drive can move one step in the
wrong direction.
A backlash compensation feature might just generate the wrong timing, by
making a burst
of step pulses right at reversal of an axis, to trigger this problem.

Jon