Re: Backlash

It's not in the motor, belt or gigs. If you look at the video you
can see the pulley rotate about 4 degrees before the table starts to
move. I have another cnc mill with .0006" backlash, almost all of it
in the bearing and it is set up almost the same, but with a different
brand of ballscrew.

Vince

>
> 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
> >
>