Re: [CAD_CAM_EDM_DRO] Does this make sense? was Re: Bent Nookballscrews ?????

On Tue, 2003-08-12 at 12:01, Jon Elson wrote:

> Kim Lux wrote:
>
> >This discussion might seem trivial and pedantic to you, but to those of
> >us retrofitting/ building machines, these are important details and
> >getting to the bottom of the physics is a very important to high
> >performance machine operation.
> >
> >BTW: our machines are real and operational, used in a production
> >environment, not some dream of an "ivy league engineer fired up on
> >Expresso".
> >
> >
> >
> Hopefully a last word on this subject. I firmly believe that a fixed
> preload is best, when you have a precision ballscrew with very little
> pitch variation.
>
> But, a number of people are using "power transmission grade" ballscrews
> (with bends, bows or even KINKS in them) and 2 separate nuts to make
> a homemade anti-backlash nut system.

We are using 0.004" per thou ballscrews and 0.001" per thou ballscrews.

> I think you HAVE to have compliance
> in these, especially if "tight spots" are discovered in the travel.

>From what I can see, there is no major variation in thread pitch in the

short (2") distance between the ball screws. Even with a bowed
ballscrew, there are no tight spots.

> You don't want a bunch of screen-door springs, you need something with a
> real high spring rate, like thick Bellville washers or something like that.

That are going to induce a whole pile of drag on the ballscrew. Do this
little experiment: mount 2 ballnuts on the screw and preload them to 500
pounds with your spring. Do they turn easily ? I agree the net load
between them is zero, but they still cause a ton of friction, the same
amount as if a 500 pound load as directly applied. You'll need servo
motors twice the normal size to drive this setup.

> If the pitch variation is so slight that you can tolerate the backlash in
> some areas by setting the fixed preload to just not bind on the tightest
> part, that is best.

0.004" per foot refers to the nominal length variation in the lead, not
the lead variation itself. For example, the ballscrew lead on a 5 TPI
might be 12.004" per 60 threads or it might be 11.996". It isn't going
up and down on the same ballscrew, not as far as I can see.

> But, I am totally convinced that putting a heavy preload force between two
> ball nuts by some spring-like mechanism will not cause a torque load
> on the screw of the same magnitude as delivering that same force to
> an external axial load.

It absolutely DOES ! All I have to do is tighten down the bolts on our
2 ballnut setups and the friction goes way up. I've set up 6 axes in
the last few weeks, so I think I know what I am seeing. And this isn't
the difference in the ballscrew lead pitch either.

> If Mr. Lux would care to drop by my shop, I
> am pretty sure I could throw together a demonstration of this in a couple
> of minutes that he would be absolutely unable to refute.

Sorry, I've seen the opposite in our shop.

> Jon
>
>
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--
Kim Lux <lux@...>