Re: [CAD_CAM_EDM_DRO] Retrofit Backlash

grease122696 wrote:

> I'm converting new Taiwan 9x49 knee mill to CNC using Ball screws and
> steppers.
> There seems to be .001-.002 backlash in the X, .001-.003 in the Y and
> .002 in the z. It seems the backlash is coming from flexing . I put a
> indicator on the yoke up inside the casting to check the Y and it
> seems to .0005 backlash to there but when I indicate at the table it's
> a different story. I have the same problem with the Z. I couldn't get
> a indicator on the yoke to check the X.

Yup, this sounds familiar. I have an ancient Bridgeport that has concave
wear on the bottom of the table, and convex wear on the top of the saddle.
This is the perfect recipe to make the table rock.
The situation may be even worse on the Y axis, like yours.
You should check the gibs on the ways to make sure they are tight.
After some initial wear in of the surfaces, you may be able to take up
a lot of slack in the gibs and hopefully it will be better.

If the yoke is very lightly built, it may be contributing to the backlash.
You might make sure the mounting surface of the yoke is clean, and the
bolts are tight. My yoke actually worked loose on my Bridgeport.
Another cause is bowing of the leadscrew. You want to make sure the
screw is mounted straight in line with the nut, so the screw doesn't
flex. If it is allowed to flex, it will cause rapid wear on the nut, but
also
will cause backlash as the bowing is affected by leadscrew tension.

Jon