Re: [CAD_CAM_EDM_DRO] B/S vs. acme was Re: What makes a machine CNC ready?

At 03:14 29/06/2006, you wrote:

>How is any thread or mechanism immune from contaminants without some sort
>of shielding/wiper system? Manual machines with acme's and handcranks
>stiffen up occasionaly when debris is pulled into the Acme nut. It remains
>that way until running it back and forth clears it. Same as debris in a
>standard hex nut, it locks up. I've even had mic's tighten up in dusty
>environments. All the ballscrews on my machines have wipers on them. Just
>a UHMW plate that locks into slots in the ballnut. Also, bored/threaded to
>fit the ballscrew.
>
> Just curious what the "Advantages" are to an Acme screw.

I think you just described them pretty well. No thread is immune; some cope
better than others. You need far less contamination to mess up the
operation of a ballscrew than a Vee or Acme screw. As you say, you *can*
clear an Acme nut that is clogged with debris simply by running it back and
forth (maybe with the assistance of a bit of light oil/kerosene etc. on the
screw); if you contaminate a ballscrew the fix is generally more involved,
requiring stripping down the nut and re-assembling it afterwards. Yes,
wipers on a ballnut can help, but they are not a complete fix - fine swarf
gets past the wipers. A friend of mine converted a mill using ballscrews
fitted with the manufacturer's standard wipers on the nuts; he uses the
mill mostly for clockmaking stuff - machining leaded engraving brass. The
swarf that this generates gets everywhere and finds its way past the wipers
very easily; he periodically has to strip the machine down and clean the
leadscrews/nuts to keep them operating smoothly. I suspect that with Acme
screws he wouldn't have to bother.

On my own Taig mill, (Vee thread, not Acme, but same general principle),
the Z leadscrew is exposed at the bottom and regularly gets covered in
swarf from whatever I am machining at the time; however, the wiping action
of the nut means that I have never had to disassemble/clean it. I very much
doubt I could say the same if it was a ballscrew.

Regards,
Tony