Re: Digest Number 2937 - Steps per inch

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "P. J. Hicks"
<hickspj467@a...> wrote:

> In following the discussion on calculation vs. trial and error

setting of

> the spi I would like to relate my experience and ask if it makes

sense or is

> way out of line.
>
> I initially set the spi by calculation; sherline supplied standard

lead

> screws 20 tpi X 200 steps per motor revolution X 4x stepping equals

16000

> steps per inch, Correct?
>
> However when I measured the travel (at several places along the

axis) using

> MDI commands my dial indicator repeatedly indicated that 16070

steps were

> necessary to get 1 inch of travel. Similar experience (slightly

different

> numbers) on the other axis. Does this sound reasonable or do I have
> something to track down? And what might it be?
>
> PJH

Make sure you eliminate all of the previous posted causes first.
If the missmatch can be defined as a leadscrew error, would be
about 0.004 something in your example, it would make sense to
compensate for this by adjusting the parameters on your machine
to take this into account.
Higher end controllers can be calibrated to reflect compensation
changes along various points along the travel distance.
Lower end controllers don't usually support this but often can
be calibrated by using one overall stepcount parameter.
In the later you will probably want to measure over a distance
that reflects ~ 75 percent of the total travel distance of that
Axes messured in the middle area.
Calculate the actual stepcount per inch from this messurement.
By messurement of this larger distance you will be able to better
average the varying pitch errors along the way.
Good Luck