Considering rack and pinion

[phantomcow2]

I am really considering going with a rack and pinion for at least my Y axis (gantry movement, not head). Right now, I have a 5/8-.2 ballscrew reserved for the job, but ballscrew whip concerns me.
I believe with my NEMA34 motors, my machine should be capable of some decent rapids. But these leadscrews will need to spin at 1000RPM to achieve only 200IPM. I think that with the torque these motors have, they won't need so much mechanical advantage.
Yes I could buy a larger diameter/pitch ballscrew, but thats pretty costly. I think that for a lower price I could just buy rack and pinion.
One more advantage I can think of is that it would not be vulnerable to MDF dust, which I believe there will be plenty of

What are peoples thoughts on this?

[2muchstuff]

Direct driving a 1" P.D. gear with a 200 step motor will give you .015" per step compared to .001" using your ballscrew. Adding a reduction unit to get the resolution better will bring up the cost, slow your IPM and add weight to you carraige. Hmmmmm decisions, decisions, decisions.

[phantomcow2]

Well .015 per step is a bit more than I was thinking. Regardless, couldn't I always microstep?

[chuckknigh]

Microsteps are not always precisely the same size. It's an *approximation* of a middle step, made possible by energizing 2 coils simultaneously.

Microstepping is used, primarily, to smooth out the motion of the drives. If you value precision, its positional accuracy leaves a bit to be desired.

-- Chuck Knight

[ViperTX]

So, why are you worried about 'whip'....is this a 10 foot long ballscrew.....are you driving it with a huge 10 Hp servo....

[phantomcow2]

This screw is 5/8-.2, just under 48" long.
Wouldn't a screw like that whip around, being a somewhat small diameter?

[ger21]

According to the critical speed charts at www.nookind.com a 48" 5/8 ballscrew is good up to 200ipm if both ends of the screw are mounted in double angular contact bearings.

[phantomcow2]

Well I already have a duplex on one side. Might even have another duplex sitting around, I will try that and see how it goes

[2muchstuff]

If you have a duplex on one side, double angular contact, then a single or even a plain bearing would work on the other end. The duplex is taking up the shaft play in either direction.

[ger21]

Originally Posted by 2muchstuff If you have a duplex on one side, double angular contact, then a single or even a plain bearing would work on the other end.

Read the info at Nook's site. If you want to spin it as fast as he wants to, you DO need both.

[WhiteTiger]

It's not really about shaft play, it's more about the difference between tension loading and column loading. If you have tapered rollers or angular ball at each end and preload the shaft for tension, then any load applied by the nut is a de facto tension load, which is far less likely to induce whip.

If you only have one end fixed against axial shaft movement, then motion toward the opposite end will be column loading the screw, which induces buckle, hence whip.

Best control of whip aside from floating supports is to mount the screw under tension at both ends.


Tiger

[phantomcow2]

WEll i am using a duplex on either end. This was all part of an actuator, so one end is already machined. It will be a 12mm duplex for each end.
I dont think I will bother with my X axis screw, since that one is only 25". I will run that with a duplex on one end and a radial on the other