RE: [CAD_CAM_EDM_DRO] Reality bites was Re: Step count / inch problem

Hi Ballendo -

The Thomson rolled ballscrew literature shows a graph of the type
of variation you are likely to see with their ballscrews. They
clearly show that the errors DO stack up for longer lengths (they
refer to the "Cumulative Representative Lead"), and they also show
the superimposed error component that is cyclical, like what you
describe ("Lead Fluctuation"). The graph is of a sloped line,
with waviness added. Just imagine taking a ballscrew with
cyclical errors, and changing its temperature. The overall length
will change, and every point along the way will shift
proportionally (with even temperature changes), but the cyclical
errors will still be there. The cyclical errors are in fact small
compared to the cumulative errors. It's just part of the
manufacturing process. I spoke to a Thomson engineer who said if
you could use software to factor out this cumulative error, you'd
compensate for the majority of the error. When they give
accuracies of .004"/ft, they're mostly talking about the
cumulative errors. I tried to get some numbers on the cyclical
errors with no luck. I believe they will map out their more
pricey ballscrews for a fee, however. Anybody have this kind of
data? It would be interesting to compare the magnitude of the
cyclical variations against the cumulative errors, and to see the
error characteristics of an actual ballscrew (slope, waviness,
arcing, etc)

HTH,

Carlos Guillermo
VERVE Engineering & Design

Ballendo wrote:
<snip>
Ballscrew errors, even with the cheapies, do not usually stack
up; but instead are cyclical variations within shorter distances.

<snip>