Re: Re: backlash reduction

Dan Falck wrote:

> From: Dan Falck <dfalck@...>
>
> I probably would have left the Z axis alone, like you suggest, but my
> machine came with a rack and pinion, with a lot of backlash already built
> in. It seemed to have about 3/4 of a turn of backlash in the handwheel
> alone. I didn't even bother to measure how much that is and proceeded to
> just tear everything out, cut a 3" diameter hole in the top of the column,
> placed a 1/2" blanchard ground steel plate between the column and base and
> then mounted the ballscrew between top and bottom. If it had the acme
> screw in there to begin with, it would have made things a lot easier. As
> it is now, I have measured the Z and have zero backlash. I do plan on
> doing contouring with it, so this is good.

Well, that sounds awfully good. I installed a precision ground anti-backlash
ballscrew about 2" in front of the quill of my Bridgeport J head, just a little
further out from where the stop rod used to go. I made a one-piece bracket
that goes where the stop ring used to go, and connects the nut to the quill.
And, I built a pretty massive housing that attaches to the front of the head
at 3 points. I have angular contact bearings to restrain the screw from axial
movement. But, I still have about .0015" of backlash, as measured by
comparing a dial test indicator against the CNC readout from a shaft encoder
driven through an instrument-grade helical-cut shaft coupling.

I can't find where the flex is coming from. About the only thing I can't easily
check is if the bracket from the nut to the quill is rocking or flexing. It
can't be flexing, it has a 1" x 2" cross section most of the way.

I have about .001 - .002" backlash on all axes, and I really can't tell where
it is coming from on any of the axes. The X and Y are used precision
ballscrews, though, so it could easily be wear. But, the Z screw is new.

Any thoughts, anyone?

Thanks,

Jon