RE: Re: backlash reduction

Charles,

Not all ballscrews used in milling machines are preloaded.
Some mfg rely on backlash comp to do the job.

Bob

At 08:53 AM 1/26/00 -0500, you wrote:

>From: Charles Hopkins <chopkins@...>
>
>The used screws may be worn, but still should not have backlash. The ball

nuts should be pre-loaded against one another. If the pre-load is loose you
can readjust the spring. This will not help the wear, but should eliminate
any play in the nuts.

>
>Charles Hopkins
>
>----------
>From: Jon Elson
>Sent: Wednesday, January 26, 2000 12:48 AM
>To: CAD_CAM_EDM_DRO@onelist.com
>Subject: Re: [CAD_CAM_EDM_DRO] Re: backlash reduction
>
>From: Jon Elson <jmelson@...>
>
>
>
>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
>
>
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