Re: [CAD_CAM_EDM_DRO] Rolled vs. Ground ballscrews

Les,

If the servo control loop has feedforward terms (eg velocity and acceleration feedforward) then you can have the best of both worlds ie tight steady state errors and whilst in motion.

CS

> Les Newell <lesnewell@...> wrote:
>
> 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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> OFF Topic POSTS: General Machining
> If you wish to post on unlimited OT subjects goto:
> aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach it
> if you have trouble.
> http://www.metalworking.com/news_servers.html
>
> http://groups.yahoo.com/group/jobshophomeshop I consider this to be a
> sister site to the CCED group, as many of the same members are there,
> for OT subjects, that are not allowed on the CCED list.
>
> NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING THEM.
> DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
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