Bridgeport M head Dis-assembly

A while ago I purchased a combination Hardinge TM mill with a Bridgeport M head. I was surprised at the performance of the mill and measured certain performance parameters. Best I could determine the TIR inside the M head spindle was ½ tenth (?) - truly amazing. Unfortunately, later had a collect stuck in the taper and couldn’t remove it using the two wrench points. In a fit of anger, I hit the top of the drawbar with a hammer which did release the collet. But the performance hasn’t been the same.

The TIR of the M head has risen to 3 tenths (which isn’t bad for a mill) along with increase noise when operating at medium or high speed. The operating results are more telling when compared to my larger Bridgeport J1 mill. Boring into durabar type cast iron was really rough, a crude surface finish resulted with milling 12L14 and drilling in 12L14 produced a finish like a dull drill in 1018.

Consequently I started to dis-assemble the head following descriptions found in this forum. Three problems have cropped up:

1) I have removed the two bearing set on the top of the spindle. Both bearings were noted as R MM 205K. But these don’t seem to be your “off the shelf” Conrad bearing. The “R” was scribed rather than a stamped letter. From the literature on the web, “R” represents a non-standard internal fit, MM designates ABEC 7. Going to Alpine Bearing (outside of Boston) could probably find the ABEC 7 205K – what about the internal fit? I was considering using a set of 205 (7205) 15’ contact angle DUL P4 bearings with the same spacers (both measured as best I could as 0.3747 -0.3748 inches). Thus there would be two sets of 205DULP4 in the spindle - would that result in thermal expansion problems?

2) The larger diameter spacer looks in excellent condition except for some brown discoloration. As best as I could measure with a tenth reading micrometer, the ring was a uniform 0.3747 around the circumference. Not so with the smaller diameter spacer. There were some small burrs within the chamfered edge (don’t know if this extends to the actual surface) but the thickness varies slightly 0.3747 to 0.3748 inch. Next ran both rings under a tenth reading dial test indicator on a surface plate- essentially verifying the micrometer results. I am hoping that the burrs are not interfering. Assuming the burrs are not interfering, is this variation satisfactory?

3) The online directions for removing the lower bearing set is “press set No. 1 with and arbor press.” Do you press on the Morse taper end or from the top? I ask this because there is some cup shaped piece between the end of the spindle and the lower bearing set. Is this a separate piece (not shown on any of the M head parts diagram) or is this just a integral part of the spindle?

The lower bearing pair and lower spacer *must* come off the spindle the same way the upper pair came off.

The taper socket is part of the spindle, not removeable.

The upper pair really can be electric motor grade. The lower pair need to be an angular contact pair, and the OD spacer
and ID spacer for the bearings have to have the same hight with a few tenths. Also good parallelism for the preload
to be developed correctly.

The original M head bearings were actually specially altered radial bearings, where bridgeport actually ground the race
surfaces of the standard bearings to create what amounted to preloaded angular contact pairs, when the outer races
and inner races were separated by spacers of exactly the same length.

Removing the lower pair, once they are installed, requires pressing the bearings out, through the balls. Doing so
ruins the bearing, basically. When you go to install the new bearings, be sure they're going on the correct way (manufacturer
will give marks showing which way the races point) as doing it wrong means ruining a brand new pair of abec 7 bearings.