Re: [CAD_CAM_EDM_DRO] Step count / inch problem

jeffdavis516 wrote:

> I have a question about why the math did not work out for my step
> counts per inch. On my servo system I just finished and using the
> calculations I got from help here, see below
>
> servo with 250 cpr, .2 pitch ball screw, 2:1 pulley system.
>
> 250 x 4 x 5 x 2 = pulses per inch or 10,000
>
> When I place this figure into my controller software and test it with
> a dial indicator. (backlash taken into account) I come up a few thou
> short. and a little more when I move 2". I had to set the counts in
> the controller software to 10,010 to make it come out dead nuts on at
> 1" and very close at 2" (This was the X axis on the mill)

How accurate is your screw? Is it guaranteed accurate to .001"
per inch or better? You may need to map out the whole length
of the screw with gauge blocks and then figure out the best average.
Or, if your software supports machine error compensation, you can
enter those numbers into the compensation table.

> When I checked the Y axis I only needed to alter the value to
> 10,005. The Z was fine at 10,000. What would cause this? Could it
> be that I did not purchase expensive ground ball screws?

Of course. Rolled screws are good to .004"/foot, on average. Who knows
what the short term variation is, though. Did your screw come with an
inspection report, or is this a McMaster-Carr power transmission screw?
If the latter, as long as it doesn't bind in the nut, it is probably
considered
good enough. You should measure a longer length to have any real
idea of the true lead of the screw.

Jon