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
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 seems excessive.
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 hand).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