Problem with stepper system cutting oversize.

I am having a problem where the system is cutting over size.

I am running:
Debian Wheezy
2.8-pre
Gecko 201X drives
Mesa 7i76
Intel mini ITX MB
5mm per rev ball screws
stepper motors are 200 steps per rev

At first it was cutting under-size by quite a bit so I played with the acceleration settings and it no longer cuts under-size.
Now it cuts oversize by what looks to be about 3 steps per inch. In doing the math it should be 1016 steps per inch X 10 for the microstep on the Gecko drive. That makes it 10160.00. steps per inch. where am I making my mistake in my calculations? How does the size grow?

Jim

Your calculations look correct

Are the errors consistent?
That is, if you move back and forth does it return to the same place?

If the errors are consistent and proportional to distance that suggests a ball screw tolerance issue

If you get inconsistent readings, it could be a lot of things:

Lost steps from wrong timing or too high acceleration

excessive backlash etc etc

5 TPI is a common screw lead and is very close to 5mm (it is 5.08mm)

If it is not a metric/imperial thing, I would test the axis using a metal ruler and a programmed known distance. You can calculate the percentage error and convert that to steps and adjust. If you use move relative commands you can do multiple passes. When I had a similar problen due to non standard gear rack, I got it close against a 1.0m steel rule then set a dial indicator up so I could measure the error after 20 metres of travel.