Bridgeport MILL - Spindle runout

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Bridgeport MILL - Spindle runout

Post by Maddog55 » Wed Feb 04, 2009 10:12 am

Hi all. I have an older (1972) Bridgeport J-head mill. I noticed the
other day that my spindle has almost .002" of runout.

It seems tight and it runs very quiet. Works for most things just
fine, but lately I want to use it as a seat and guide machine and
.002" is way too much slop for that type of operation. I checked
the runout on the exterior and the interior of the spindle and its
definately out by 2 thousanths.

Can any of you enlighten me as to how I can fix this short of
replacing the head? I really don't want to spend 10 grand on a
seat/guide machine either. (It would take me 20 years to recoup
my investment)

"Those that beat their Guns into plowshares, will
eventually plow for those that did not"

Thomas Jefferson

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Post by lakeside53 » Wed Feb 04, 2009 12:15 pm

If the spindle bearings are in decent shape, all you have to do is drop out the quill (easy) and send it in for regrinding - about $100.

Grinding is required even when just replacing bearings. The result is 0.0002 or better.

I feel your pain.. measured my worst BP again yesterday and it's right on 0.0034 ... but.. too busy to send the quill off until March :(

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Bill Shields
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Grinding when replacing bearings

Post by Bill Shields » Wed Feb 04, 2009 12:27 pm

Just as point of my education, why is regrinding required to replace bearings?

What is there about new bearings that would require a regrind?

Runout can be a function of bad bearings, which can often be tested by putting a bar in a collet and pulling / pushing a bit, worn collet seat area (if the runout was measured there) which can also be checked with the push pull method, or even something as stupid as a bent quill, which pretty much requires a tear down to determine.

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Post by Rich_Carlstedt » Wed Feb 04, 2009 9:38 pm

Sounds like a bent spindle, if the bearings are quiet and the bore is off.
It may also be that someone who owned the mill before you replaced the bearings......and did it incorrectly, or used cheap bearings.

In either case, a tear down is required before a decision can be made.


You didn't say if there was play in the bearings?

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Post by JHenriksen » Wed Feb 04, 2009 11:09 pm

This could be as easy as a loose bottom retainer ring and loose bearings.
Extend the quill at least an inch , loosen the set screw on the back of the quill. ( take it out) try tightening the bottom retainer and see if theres any improvement. When its tight, check to see if there is any gap between the quill and the retainer. .001/.005 is plenty.
I have seen older bprts with the retainers tight on the quill and loose bearings- an indication of loss of preload and worn bearings. I have reground the spacers between the bottom bearings to increase the preload. grind both the same then remove the larger outer spacer and take another .0003/.0005 off the inner spacer and reassemble.
When replacing the setscrew, dont tighten it too tight or you will deform the bottom of the quill and it wont retract smoothly.

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