Re: How does one adjust Ballscrews on a Bridgeport

Hi Jon, (et tu SWP),

I'm the other guy Steve mentioned earlier, with the other B'port,
actually #2 of 2 that I own. My older, non CNC machine has been apart
and together several times with no such problem. It has slop in the
acme nut, but once pushing against a locked table, not spring whatsoever.

This CNC machine was a complete Southwest Industries Trak, mid '80's
vintage, with ground ballscrews, and some very classy taiwanese
ballnuts, and a solid cast iron yoke, with the balls trammed very
closely to table level, direction of travel and centers.

The drive end carrier of the long ballscrew had not been apart, so I'm
sure of the orientation of the Fafnir M204KT4 bearings facing each
other. Before Friday, they had probably not been apart since the
original factory build. The grease is clean and coffee-with-milk
color. I'm waiting on a company in Dallas to provide specs on whether
these are angular contact, and if so, what angle spec. The floating
(right) end, as Steve mentioned, has a Fafnir 204KTT, and has a manual
crank handle, not keyed to the ballscrew.

The drive end is a stack of aluminum spacers on either side of of
toothed belt drive gear. Looking closely, its clear that when torqued
to about 50 ft / lbs, the outer races are in definite contact, while
there is still a gap between the inner races. (This is clear by the
grease pattern on the the bearing races when removed from the drive
bracket pocket.)

The other thing thats clear is that when the table is locked, the
spring in the system is in the inner races. The inner race on the
inner bearing is against the ballscrew shoulder, and the inner race of
the outer bearing is visible with the keeper removed. The spring is
about 1/10 of a turn of the ballscrew, 1/8 X 0.200" / turn = ~0.025"
of spring. Having the keeper off allows the spring to be seen in one
direction; the other direction slides the bearings out, of course.

As a side note, its god awful hard to put 50 ft /lbs torque on the
nuts; we ended up with a pair of vise grips with a copper chafe on the
ballscrew, and held both ends with wrenches, chicken blood, curses,
incantations, etc.

My B'port is as it was used for years for CNC, and I'm just not sure
about torquing those nuts any further. Stripping the ends of the
ballscrews would sUcK. What are we missing.......