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

bryan b wrote:

>
>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.
>
>

Well, then there isn't a whole lot of speculation left. Either you guys
reassembled
something wrong, or the bearings are shot. If there is only a couple
thousandths of an inch of preload put in the thing (and that's what I see
in the bearing sets on my Bridgeport) then wear, especially on a CNC
machine,
could wipe out the preload. It is pretty hard to believe you could have
wear
of .025", though!

>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.
>
>

Are you positive that the outer bearing's inner race is free to slide toward
the other bearing? Are you sure the keeper that presses on the inner
race is not ALSO pressing on the end of the leadscrew? IF it hits the
end of the leadscrew, it can't maintain force on the inner race. This would
mean the length from the wider step for the inner bearing to the
place where it steps down near the outer bearing is too long. A spacer
with an ID equal to the bearing ID might fix it. Without seeing the
whole setup, all I have to go on is how Bridgeport did it.

Jon