Re: [CAD_CAM_EDM_DRO] Rolled vs. Ground ballscrews

I installed some Thompson rolleed ball screws (.003 tolerance) on a mill
drill. I used two ballnuts and preloaded them so there is about .0005
backlash on the X axis. I then proceeded to check the accuracy by using a
long drift dial indicator and resetting it. The .0005 backlash was taken out
before the X axis was moved in the + direction
The results were surpising
x =0 on manual dial dial indicator 0
x1.000 x.998
x2.000 x2.000
x3 x2.997
x4 x.3.999
x5.000 x.5.0
You decide it it meets your needs
Dan

----- Original Message -----
From: "Jim Colley" <jcolley@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Sunday, September 18, 2005 3:25 PM
Subject: [CAD_CAM_EDM_DRO] Rolled vs. Ground ballscrews


> Alright guys, maybe some of you are open to a little debate.
>
> I am in the design phase (still) of a 5-axis mill and have purchased
> nearly all of the pricy stuff except ballscrews. Now, obviously
> ground ballscrews are more expensive due to their higher accuracy,
> etc.
>
> My question is, exactly what are you getting for your money and is it
> posssible to achieve the same end goal using rolled screws and a
> little clever design?
>
> If the major difference is the accuracy of the lead and backlash,
> then wouldn't it be possible to eliminate the backlash in a rolled
> screw either by using a double nut design (preferred) or repacking
> the ball nut with larger diameter balls? Then, once the backlash is
> minimized, the only real concern is the accuracy of the lead.
>
> Now, my thoughts here are that this is only a concern if you are
> using the servo motor's encoder to close the feedback loop by
> interpolating the axis position from the number of rotations and the
> diameter of the screw. However, if you used linear encoders on the
> axis (preferred) or used an arrangement with a timing belt fixed to
> the slide, turning a pulley which drives a rotary encoder, wouldn't
> that give a very precise readout of *actual* axis location? Provided
> there is *no* backlash in the ballscrew arrangement, there should be
> no problem with hunting on the servo mechanism, etc.
>
> As I have said before, I base my statements on absolutely zero
> experience and no intelligent thought whatsoever. Just an idea
> I had.
>
> What do you guys think? Any problems? Better ways?
>
> Jim
>
>
>
>
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