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

I think you are both right....in a sense.

I think it's easy to imagine that if we load down those preload setup screws
tightly enough, the thing won't move. In that case, it would take more than
what the motor can deliver to move it, hence the motor "sees" the preload
force at the screw end. So much load is applied that the balls and thread
are deformed aas previously mentioned.
On the other hand, one can imagine that more force "between" the preloaded
nuts can be realized with a small preloaad added, that will not necessarily
be "seen" in its entirety by the motor due to the canceling out of the
push-pull arrangement as was previously discussed.

I suspect that if we just stay in the middle, with "just enough" tension so
as to eliminate the backlash, thru field trial, that is where we need to be.
Kim, I understand that's what you did initially, it sounds like it's still
working perfectly well, minus the problems with chaff entering the nuts.
I am highly impressed by this whole design arrangement. I was totally
convinced a week ago that I needed to plop down almost a grand to get 2 axes
setup with preloaded ball screws, and now for about $300 I have all the
materials to not only add the screws to my full sized mill, but make it
absolutely zero backlash to boot. And even though one of the screws is
slightly bowed, it can be corrected, and the installation is probably going
to be quite precise and long lived. Isn't that where we ideally need to be?
R. William Kichman, P.E.
Kichman Engineering Associates
103 Old Furnace Road
Cornwall, PA 17016-0643
tel/fax 717/270-0714

----- Original Message -----
From: "Jon Elson" <elson@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Tuesday, August 12, 2003 2:01 PM
Subject: Re: [CAD_CAM_EDM_DRO] Does this make sense? was Re: Bent
Nookballscrews ?????


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