On 20.07.12 16:18, Jeshua Lacock wrote:
> I am sure with a 10-foot span that the horizontal ballscrews will sag
> a little from gravity. Is this much of a concern? Does anyone know of
> a trick to put some kind of support in the middle of the span? I can't
> think of any practical way.
One method which has been used on very long lathes is one or more
pivoting supports holding up the leadscrew. Imagine a cross-drilled rod
with a weight on one end. It will hang vertically on a bolt through the
hole. Now a segment of an arc (imagine a slim pie-slice of a V-belt pulley)
welded on the other end will support the leadscrew in the V, but be
pushed aside by the passing leadscrew nut housing, and slide under it,
to bob up again when it has passed.
While that has been used in industry on older long lathes, it doesn't
seem ideal, since a cnc machine is likely to whack into it at speed, or
reverse at that one tiny point where the shape of the ballscrew nut
housing hasn't quite let it bob up, but sliding back under could be at
least high friction, if not an outright jam.
Those left/right alternating support legs, where striking the left one
from the right causes it to fold down, and pull up the right one behind
the leadscrew nut housing, by means of a link, are perhaps also not at
their best when struck at speed.
I'd be tempted to go for a horizontally retracting flat support,
reaching out under the ballscrew from the adjacent frame. Imagine a bit
of flat in a long slot, with the corners of the exposed edge cut
well back, forming a ramp nearly to the middle of the edge. When the
(also heavily chamfered) ballscrew nut housing whizzes by, it
progressively pushes the support strip back into the slot in the frame,
and progressively re-emerges as it passes. If the slot is deep, smooth,
and well lubricated, then the support ought to retract much like that
angled spring-loaded brass catch on domestic external doors. (The one
which locks you out without your keys, even if only once in life.)
If even that is too noisy when hit at speed, then the support could be
automatically retracted when the carriage is near (reed switch or
optical sensor), either pneumatically or by means of a small motor.
In any event, I'd fix a long slender ballscrew, to avoid whipping, and
rotate the nut. It is hard to image how vibration of a very long slender
ballscrew can be avoided if it is spun at speed.
Erik
--
Re: Graphics:
A picture is worth 10K words -- but only those to describe the picture.
Hardly any sets of 10K words can be adequately described with pictures.
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