Re: Backlash

So you have .0004" backlash in your radial bearings. Only way
to eliminate that is to either adjust for proper preload or replace
with matched set of angular contact bearings. There are angular
contact bearing sets made specifically for ballscrew applications.
(Read $$$.)
That leaves you with about .0035" backlash somewhere in the rest
of your assembly. I am currently dealing with these problems right
now on my BP. What kind of motors are you driving the Axis with?
Your motor can be contributing to you're error. Servos can have a
a following error. I am not a stepper person but I believe they can
have an error also. Your ballscrews can be stretching a bit. Your
yoke can be flexing a bit. Your bearing mount can be flexing a bit.
Your ballnut can have a bit of backlash under load. Your belt can be
stretching a bit. All these little
things can add up fast. My backlash was .005" in the center of travel
and to my suprise was .011" at the ends of travel. I suspect that
this was because the ends of travel tightened up a bit and caused
a lot of stretching. I am in the process now of having the table
rescraped.
Try loosening up your gibs and measuring your backlash
then. You might be able to see if something is flexing.
How about removing your knee chip guard and trying to get the
DTI on the ballnut itself.

Good luck,
Scott

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Vince Endter" <vince@...> wrote:
>
> I have been trying to chase down a backlash problem I have with my
> Hiwin ballscrews for almost a year. I even sent the ballscrews back
> to Hiwin and they said there is no detectable backlash in them. I
> made a video of the test recommended by Hiwin. In the test I
> attached a ball bearing to the end of the ball screw shaft so I am
> not measuring any out of squareness in the end of the shaft. I am
> using a tenths reading (.0001") indicator on the end of the ballscrew
> and a thousands reading (.001") on the table. I am measuring
> between .0003" and .0004" backlash in the end play of the radial
> contact bearing, and .004" on the ballscrew/table movement. The gibs
> are adjusted to that if I push as hard as I can anywhere on the table
> I can see only .0005" deflection. The mill was recently reground and
> hand scraped by a professional machine tool rebuilder.
>
> There are only two places I can see the backlash coming from. One is
> the yoke, which was supplied by Hiwin (very large cast iron, I can't
> see that flexing), and the other is the ballnut/screw combination.
> The ballscrew has a single nut. Hiwin says they take care of back
> lash in one of two ways, either with a different pitch between the
> nut and screw, or by adjusting the ball size in the nut. They won't
> say which method is used in my installation.
>
> Here is the video of the test:
> http://www.flyingcritters.com/video/Backlash.wmv
>
> Any suggestion would be greatly appreciated.
>
> Vince
>