Re: Rolled vs. Ground ballscrews

Dan,

So, having used double ballnuts and preloading them, you got the backlash down to about
five tenths. Now, I know that you can never remove all of the backlash, but I'm wondering
if you can get it a little lower at sacrifice of a little freedom of motion. What I mean is,
assuming you preloaded the ballnuts by using two of them with a method of opposing
force separating them (or pushing them together even), that you would run across some
areas of the ballscrew where the lead of the screw deviated more than other areas, causing
the nut pair to bind a bit.

Initially, I thought that the lead error of a rolled ballscrew was a cumulative error from end
to end and that .003in/ft would be a relatively constant deviation across the length of the
screw. However, when I look at your numbers, it seems that the lead error is somewhat
erratic (assuming accuracy of you measurements.)

But, leading back to my orginal question, as long as I can keep my backlash around the
same as what you have, I will be happy. If I were to use a direct measurement system for
axis position, such as linear encoders or rotary encoders driven by a timing belt from the
slide, I would still have to calibrate the axis position indication via some highly accurate
means. Now, assuming that is done correctly, I think it is possible to hold the accuracy of
the machine within .001in. Now, before anyone flame sprays me about thermal variations,
flexure, etc., I'm just looking at what to take in to account for error budget from the drive
system. Any thoughts?

Jim

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Dan Mauch" <dmauch@s...> wrote:
> I installed some Thompson rolleed ball screws (.003 tolerance) on a mill
> drill. I used two ballnuts and preloaded them so there is about .0005
> backlash on the X axis. I then proceeded to check the accuracy by using a
> long drift dial indicator and resetting it. The .0005 backlash was taken out
> before the X axis was moved in the + direction