On 21.07.12 14:41, Viesturs Lācis wrote:
> 2012/7/21 Erik Christiansen <[email protected]>:
> >
> > In any event, I'd fix a long slender ballscrew, to avoid whipping, and
> > rotate the nut.
>
> I did this on the last machine I built with this exact intention in my mind.
> The overall result - failure. I seriously doubt I will ever do that again.
> Longest screw was 2800 mm long (other 2 were 1800 mm long), all of
> them - 16 mm diameter, 10 mm pitch.
>
> My main conclusion - the nut housing requires pretty precise machining
> to match the central axis of the nut itself with the axis around which
> that nut rotates in bearings.
> I had some deviations there, so the rotation of the nut caused the
> screw to vibrate, so the max speed ended up to be 5x smaller than
> initially planned just to avoid excessive vibrations.
Viesturs,
Of the other machines that you've built, do any have a horizontal
ballscrew as long and slender as this one, spun at similar speeds? There
is nothing quite like a direct physical comparison, to sort out which is
better. It is a pity that you are do far away. It would be very
interesting to see the effect first hand, just to learn.
Are you game to mention how badly eccentric the nut housing was
machined, in order to create the problem? Is there room to machine a
little off the outside of the bearing mount, and slip on an eccentric
outer, locktited in place with the equal eccentricities opposed, to
cancel them? (Or some other rectification.) Admittedly, an 80% reduction
in maximum rapid speed doesn't mean an 80% reduction in production rate,
but it still seems a big loss.
If the eccentricity is very small, is there possibly a resonance in the
frame exacerbating the vibration? A significant mass tightly clamped in
several different places might help check for that?
Erik
--
Theory is grey, but the golden tree of life is green.
- Goethe
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