Re: [CAD_CAM_EDM_DRO] How does one adjust Ballscrews on a Bridgeport

Jon Elson wrote:

>Stephen Wille Padnos wrote:
>
>>> <> Yes - it does appear to move in the bracket. We put one heck of a
>>> preload on the end bearings - we used some Aluminum tube as a shim, and
>>> tightened the end nut *VERY* hard. We didn't want to go too far, since
>>> there are only 5 or 6 threads on the nut. Do you have some idea of the
>>> amount of torque these should have on them?
>>
>You actually should have 1000 Lbs or so of preload on the bearings, so they
>don't "give" when the screw is pushing on them. That doesn't take a whole
>lot of tightening on the nut, though. What "nut" is this? Are you sure
>it is
>really clamping the inner races of the bearings?
>
>

The nut at the end of the ballscrew, which holds the handle on.

>>>Are you sure you installed the bearing sets exactly as they came out of
>>>the machine?
>>>There's a plate that screws on over the outermost bearing that applies
>>>the preload
>>>to the outer races. But, something has to hold the inner races together
>>>and tightly
>>>to the screw. It looks like in the stock setup that is the graduated
>>>dial hub.
>>>Do you have a similar part on your ballscrew setup? If the screw can pull
>>>inward until something else jams against the inner races, that might be
>>>your problem.
>>>
>>>Jon
>>>
>>>

I'm pretty sure the bearings went in the same way they came out. I'm
not sure if they were correct when I got the machine though - it was used.

>I have the Bridgeport manual here, and looked to refresh my memory of the
>mechanism. (I don't have a real maintenance manual, but it does have an
>illustrated parts breakdown of the leadscrew brackets. I don't see any
>shims
>in there.
>

Does the diagram look like this one:
http://www.frugalmachinist.com/bp1jlongitudinalscrewinfo.gif ?
It does look like the contact angle is "outward", though that seems
backwars to me. If you push the screw left, then the right (inside)
bearing "loosens", and the outer one should "tighten". But, it seems
that you could just push the inner race out of the bearing, since you're
pushing toward the wider side of the bearing. (I realize it would take
a lot of force, but still...)

> But, it is pretty clear that if you have the inner races
>clamped hard
>together, then they have simply hit each other, and not given any preload.
>IE, the inner races should have a gap between them, transferring force from
>the inner races to the outer races, and pushing them together. This is
>obviously
>not happening.
>
>

The outer races are already touching. I'll have to look carefully at
how the inner races are compressed - maybe the dial hub needs to be
shorter??

>Is there any possibility it was like this before?
>

It could have been that way beore - I only had it together for a week or
so before I pulled the power feed and started disassembling in earnest.

>The Bridgeport M-105
>manual
>lists the bearings as Norma Hoffman XF-12 felt sealed ball bearings.
>
>

(They're Norman Hoffman on the left side :)

>The cross
>section drawing makes them look like they probably are angular contact,
>with the
>wider gap in the races facing outward from the bearing pair.
>

Yup - looks that way to me, too. I still don't quite understand why
they would be in this configuration - it seems to me that the wider gaps
should be facing toward each other. It's hard to tell from the online
image which race the dial hub is supposed to push on.

>If these
>bearings
>are just standard radial ball bearings, then the inner and outer races
>will have
>the same width, and no amount of pushing on the inner races will
>actually preload
>them. I'm just wondering if the bearings have been replaced incorrectly, or
>with the wrong type.
>
>Jon
>

The races have a noticably different thickness from side to side, which
would lead me to believe that they are angular contact bearings.
According to an old usenet thread, the bearings are normal radial
bearings, but in a custom thickness (true - they are 15.5 mm vs. the
standard 14 for this size).

I'll look at it again over the weekend.

- Steve