Re: [CAD_CAM_EDM_DRO] Rolled vs. Ground ballscrews

Hi Jim,

The difference between rolled screws and ground screws is accuracy, not
just backlash. If you use two nuts or other preloading techniques on a
screw that has poor accuracy you will have some tight spots and some
loose spots. This is particularly dangerous when you use oversize balls
and cause the nut to fail very quickly. There are some rolled screws
with higher accuracy specs than some ground screws. Really you need to
look at the specifications of the particular screw you intend to buy. As
long as you can measure the errors in your screws you can compensate for
them in software. The software distorts the toolpath in such a way that
errors in the screw are cancelled out. The tricky bit is in measuring
the errors accurately in the first place.

There is no such thing as *no* backlash. Flex in the machine to be taken
into account. Some high-end machines do use linear encoders but they
usually also have sensors on the motors as well. With a really tight
machine you may be able to get a setup like this to work with Gecko or
Rutex drives if you used very conservative tuning. This would give you
great steady state accuracy but the conservative tuning would give large
following errors when accelerating.

Les

Jim Colley 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
>
>
>
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