Re: [CAD_CAM_EDM_DRO] Re: Bridgeport Ballscrew Backlash

Polaraligned wrote:

>Follow up:
>I just measured the backlash at the ends of travel of
>both the X and Y axis. To my great suprise, the backlash
>of BOTH axis and BOTH ends of travel doubled??
>Opposite of what you would expect. X at the ends of travel
>is .010" and Y is .006". I confirmed that the center of
>travel is half of these values.
>Gibs are freshly set as per specs in BOSS maintenance manual.
>Angular contact bearings holding ball screws appear to be
>properly preloaded.
>
>Any explanations for this?
>
>
>

Something is seriously wrong, here. I can think of a couple of possible
explanations. Obviously, wear is not likely, unless the machine was used
in some very odd way that caused massive wear only at the ends of the
screws.

Did you scrape or grind the ways? If not, the ways may not be fitting
well, as they have wear in the middle. You may be able to detect this
by leaning on the table with a dial indicator showing deflections against
the spindle. Generally, the top of the saddle wears convex, and the bottom
of the table wears concave, allowing the table to rock when the direction
the screw is driving it reverses. This LOOKS like backlash, but is a
totally
different phenomonon. If you have a master precision level, you can lay
that
on the table and reverse the travel direction. If it is rocking, you
will see a
sudden shift in the level right at the reversal.

Another possibility is the screws are not aligned well with the table
travel,
and the bow in the screws is both binding it, and causing a deflection of
the movement. This would be expected at the ends of travel. If you
reground
the ways, the screw and nut will no longer line up, and some adjustment
will have to be made.

Jon