Re: Rolled vs. Ground ballscrews

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Jim Colley" <jcolley@g...> wrote:

> 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

Some ADD-on CNC systems use linear encoders from the Mill's DRO.
accuracy is based on the resolution of the scale.

Backlash is the wildcard. if there is any backlash, it can have the
motors banging back and forth or, you have to dial out the sensitivity.

Dave