I have a new set of ballscrews for my Bridgeport Series I machine that
purportedly are zero-backlash. I bought these for my CNC upgrade
project. I'm trying to verify that these are indeed zero-backlash.
This set is a direct replacement for the stock acme lead screws.
One might think the easiest thing would be to look up the part numbers,
but these are apparently new old stock from 20 years ago, made by Han
Jiang Machine Tool Works in good old China (PRC). They do look nice and
have an inspection certificate that, if true, is pretty good ground
quality. Also came with a replacement yoke to fit the ballnuts. But no
specs for backlash. Google (and Google groups on rcm etc) turns up
nothing on the part numbers (BS-001A, etc) for that manufacturer,
although this seems to be the business renamed Hanjiang Tool Co., Ltd.,
that makes various precision machines today
. After 20 years,
the cosmoline was as hard as shellac in places, so it was quite a clean-
up job, but it did preserve the steel perfectly.
Back to the backlash issue, I thought first to install the Y screw to
just give it a try and hope for the best. My dial indicator showed
0.002" or 0.003" of backlash. Blah. It occured to me that this might
be just the wrong kind, or worn out, bearings. Do you need different
bearings to eliminate backlash on the table screws? Or perhaps a shim
washer or something? I suppose that much backlash in the bearings
wouldn't matter with acme screws, and it might just be a "feature" of
the stock bearings.
I suppose springiness in the yoke could be another source, but that much
I should have tried pushing the table to isolate the backlash possibly
to the nuts themselves, but I didn't think to do that before tearing the
machine down for some way scraping (another story).
After some more musing, I thought clamp the screw in a soft-jaw vise and
measure the backlash directly with the dial indicator. I was encouraged
to see that there was no perceptible play indicated by pushing the nut
axially. There is a tiny sort-of backlash in that when you reverse the
direction of the nut (the screw being fixed in the vise) it takes a tiny
bit of rotation, perhaps 1 degree or so which translates to about
0.0005" travel, before the axial motion actually reverses. Is this
perhaps normal zero-backlash behavior, just part of the springiness of
the mechanism (again, no axial play is present as hard as I can push by
It seems to me that consistent backlash in the drive train (as I found
in the nut reversal, or would be found in timing pulleys and belts on
the screws) doesn't really matter if you have CNC compensation, what
matters is the lack of axial play from preload force, and that seems to
be below my indicator's resolution.
Another confirmation is that the nut (a single nut, with two
recirculation tubes) requires a few in-lb of torque to turn; this would
seem to be a characteristic of preloaded ballnuts? My understanding is
that non-preloaded nuts will spiral down freely just on their own weight
if held vertically.
So any expertise in proving zero-backlash, and just how close to zero
that should be in practice on the retrofitted machine, would be a big
help to me.
Richard J Kinch
Palm Beach County, Florida USA
18 years ago