Re: [CAD_CAM_EDM_DRO] Re: Backlash

I just checked one of my machines with two year old rolled Rockfords, zero backlash on X and Y. Checked with an Indicol and Interapid. Step jog + .001 .001 .001 - .0005", .001", .001" on every test. Using the flimsy Yoke that comes with them.

If the original poster thinks its in the screw, .004" would be easily seen just handling the screw. Pull it out of the machine clamp the ball nut to a table and set an indicator on the end of the screw and push pull test it. If there's backlash, that will display it.

With that much play the ballnut would be loose and could be heard audibly.

Ron

Jon Elson <elson@...> wrote:
Vince Endter wrote:

> The backlash is the same regardless of where in the travel the
> ballscrew/nut is. For the backlash to be caused by flexing, the
> ballscrew would have to deflect .215". I can garentee that the
> ballscrew is not flexing that much. As for twisting, the pully in the
> video rotates almost 4 degrees before the table starts to move. The
> table takes about 40 pounds of pressure to get it moving

What? Something is seriously wrong, here! No way should it
take that much force! I have a badly worn Bridgeport that has a
very definite arc-like motion due to concave wear on the bottom
of the table and convex wear on the top of the saddle. I can
easily see this if I mill a full circular track on a piece of
scrap. (I mean I can "see" it with a dial indicator.) It was
last scraped at the Bridgeport factory in 1938!

But, even with all that wear, I can get the table moving with
5-10 Lbs of linear force, and once static friction is broken, it
will glide with less than 8 Oz of force! I have only handled a
Bridgeport with really good ways once with no screws mounted,
but the table was amazingly free to move, and you could slide
the table almost clear off the saddle with one finger.
and about 5

> pounds to keep it moving. Hiwin said their ballscrew have a working
> range well over 400 pounds.

Just because the screw can deliver 400 Lbs linear force safely,
doesn't mean that substantial forces delivered through the screw
won't deflect the system. For instance, a 50 Lb force on a
manual Bridgeport's quill deflects the cutter at least .001"
This is flex in the structure that mounts the head to the base.
You think of these things as being absolutely stiff, but there
is no such material.

Jon





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