Re: How does one adjust Ballscrews on a Bridgeport

I think Jon has it right on that detail. The floating bearing on the
right end (except for yours, which was switched :) is held only on its
inner race, between a shoulder on the ballscrew and the sleeve of the
handwheel. The outer race has some float to handle the CTE mismatch,
as was pointed out. It does look from the outside like its fixed though.

As far as refilling a ballnut, I feel your pain. I bought a set of
replacement precision balls and did it several times until I was sure
I had it right. My wife could ascertain my progress (or lack) by the
yells and curses coming from our basement. She even had the rare
luxury of seeing my miserable aS* sticking out from under the lathe
(which I used to hold the ballscrew during) after dumping a pan of
100+ balls. Yea, fun..., but those ballnuts run sweet now!

>
>
> Jon Elson <elson@p...> wrote:
>
> all the backlash adjustment is attained between the two endplates.
> >
> No, Bridgeport, and most other machines with rotating screws and fixed
> nuts use a double bearing
> at one end, and a "simple" bearing allowing some axial travel on the
> other end.
>
> // I must have an unusual one, just looked at it again and thats how

it is. Possibly thats how it looks and I think it is and its actually
not. The toothed belt hub threads on one end tightly against bearing
and is secured by setscrew or rollpin. Either method would have the
same result. I do recall one endplate containing two bearings (servo
end) and the other only having one. In retrospect, it must have a
shoulder on the far end that the bearing heels against. Because the
handwheel hub on the undriven side just slides on the shaft up to the
bearing and overtightening the handcrank nut would push/pull the
endplates out of square with the table. My retrofit was originally
done by no telling who and I'm sure different builders had different
ways of preloading. I turned my ballscrew around to get the
ridiculously long servo out of the aisleway and potential path of the
hi-lo towmotor. What a nightmare that was to re-assemble. Little balls
everywhere. .///

>
> This is to allow
> different thermal expansion between cast iron and steel, without any
> possibility of bowing the
> leadscrew. One machine that does use the double fixed ends is the
> Bridgeport BOSS, where
> the leadscrew is bolted firmly to the table, and the nut rotates on a
> set of bearings in the
> saddle.
>
> So, normally, all preload is between two bearings that are within an
> inch of each other.
> That way, temperature has no effect on preload.
>
> Jon
>
>
>
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