[Shanghyd]

Bridgeport Series 2 Interact 2 milling machine.
I've been having a problem with .009 of backlash that I can't seem to get rid of. I've tried putting .009 in parameter 36 in the TNC control but that doesn't do anything.
I was reading the manual on how to adjust the gibs and measured .001 with a mag base dial indicator. That was measured by placing a dial indicator on the way at the front of the machine and pushing on the table in the y axis direction.
When I put the mag base on the x axis way, and the dial indicator on the table I was able to move the table .009.
I removed a cover and was able to check the bolts where the ball nut hooked to the table. Everything was tight.
Any ideas where the slop can be? Best way to find out for sure where the problem is?

[tc429]

if you have a rotating screw, put a indicator on the end of the screw, push/pull...any reading is bearing slop. if nothing there, the ballnut is likely the culprit.

if you have stationary screw/rotating ballnut, verify the nut holding the screw to the left side of the slide is tight, then put a last word type indicator in the thread pitch, with ma on the table...push/pull, any slop there will be ballnut to screw, or ballnut thrust bearings. if you see slop there, move the indicator tip to the face of the rotating ballnut/recheck... if the nuts moving, the angular contact ball bearings behind the nut are loose- cheaper than a ballscrew, but expensive bearings...

[Shanghyd]

Excellent! I'll try that.
I have a rotating screw. I have put a dial indicator on the end of the shaft and ran jogged the table back and forth before and didn't notice any movement. I try the manual method of pushing on the table and see what the result is.

I have a quote of $650 to $950 to fix the ballscrew. I talked to the tech on the phone and he sounded like he knew what he was talking about. Talked about reballing the nut and if it needed, regrind the screw. Then he said something about preloading the ballnut and there would be zero backlash.

[Shanghyd]

It is offficially the ballnut.
I had the bottom cover off and use a pry bar on the base where the ballnut mounts. I was able to shift the balllnut on the ballscrew about .010.
The .009 backlash is consistent through the entire length of travel so I'm guessing its the balls in the nut.
Anyone ever reballed one of these things? At home I mean. Some guy on the practical machinist site did his on a tree mill and got good results.

[tc429]

Mine(rotating nut) was shot, reballed it, got it down to 1/2 thou...it's still loud compared to other axes, cut the rapid rate to 100 ipm, so far so good, hasn't got any worse.

If it has external tubes, those are easiest to reload, have done many of them at work, internal transfers are a bit more touchy to load, but still not difficult.

With external tubes, after cleaning everything up, procuring new balls, put the nylon seals back in, fish a wire with insulation a bit smaller than the balls (put a magic marker line about a inch or two from the end) into the lead to both hold the pitch aligned and keep a stray ball from passing the pickup hole ( if one gets past the pickup tit on the tube, it will destroy the tube in very short order). Most are 4 or 6 circuit if one piece nut, some two piece nuts only run one tube, but most will have at least 2 per half...push the wire in about 1 rev so it will hold the nut somewhat concentric, start dropping balls one at a time into the other hole...at first they will fall in if the nut is rocked, but soon they will get snug. Use a straight pick to put a little pressure on the ball, twist the nut back and forth slightly, they will rock right in...once it won't take anymore, pull the wire out a inch, see if it will take more, keep pushing with the pick while rocking, once it's got no gaps, it will start pushing the wire out...when your magic marker line shows up, gently pull the wire out, drop balls back in there, pushing with the pick, should fill up easily- stop below the tube shoulder...just BE SURE none pass either hole. Fill the tube, leave room for a ball or three... A few missing wont hurt anything, but one too many will not work at all. Put grease on the ends of the tube to hold the balls in, flip it over, put it in the holes... Temporarily screw the retainer back on if possible, if you can't, just wrap tape to temporarily hold the tube in till all are done. As you go down the line, removing the retainer, installing next tube, just be very careful not to allow any tubes to lift out of the nut at all...if a ball passes the pickup, it will destroy it...hold them in firmly when removing/ reinstalling the retainer, same if taping in, hold tight so they don't lift when un taping
As you go circuit to circuit, the balls get progressively tighter, the last circuit you gotta press each ball in if there's any preload present, just push with the pick, twist the nut back and forth slightly, they'll slide in.
Internal transfers are end loaded, need a long thin pick to push the balls in...put the screw vertical in a vice, you'll need a few inches of thread longer than the nut sticking above the vise, might need a short stepstool and work light to be able to look into the gap between nut and screw for a hour or so comfortably...put the inner nylon seal on, screw it onto the screw until the first pickup is just engaging the thread... Drop in one ball, push it into the pickup- you will find rotating the nut to the exact position where the ball can fall in, yet hit the pickup helps on the first circuit...nut will be real wobbly till that first one is filled... As the thread fills, keep twisting the nut on, eventually you will be at the other pickup, put in as many balls as you can, usually there will be about a 4 ball gap that can't be filled, that's ok, screw it down to the start of next pickup, repeat... The last circuit will be a *****, push/twist/ repeat to skid the balls to fill it up, they will go. After last circuit filled, mark the nylon seal above the setscrew hole, screw it onto the shaft, push it with a sharp pick till the mark is above the set screw hole, snug it just slightly to lock the seal in. The setscrews holding the seals should NOT be tight, you don't want to wear out the nylon...if the screw feels like its loose, maybe a drop of blue Loctite if you're worried about it.
If you have a two piece nut, and it feels loose, you can remove the key, re tighten them snug with feeler gage , shim till key lines up and preload feels ok, make a new shim thickness of old one plus feeler...I've cut shims and tightened that way too, but thin shim might not stay put- better to use thicker shim stock and grind a few thou off the big original shim- be sure it's flat, like zero flat...it's a bearing preload setting thing, must not be crooked. If shimming, be sure to hammer all the edges so there's no stray burrs on the shim stock.

[tc429]

Excellent! I'll try that.
I have a rotating screw. I have put a dial indicator on the end of the shaft and ran jogged the table back and forth before and didn't notice any movement. I try the manual method of pushing on the table and see what the result is.

I have a quote of $650 to $950 to fix the ballscrew. I talked to the tech on the phone and he sounded like he knew what he was talking about. Talked about reballing the nut and if it needed, regrind the screw. Then he said something about preloading the ballnut and there would be zero backlash.

btw, regrinding of the screw is rarely going to do much good...look at the length of the screw vs the nut, usually > 10:1 - the nut threads will begin fretting long, long, before the screw... long as the nut threads arent fretted or spalling away, replacing the balls will usually get it fairly tight. if the inside of the nut is pitted, reballing wont last long...will still help, but usually the thing wont last more than long enough to get a new screw bought.
shoulda mentioned above, often screws have 'spacer balls' that are a couple thou smaller every so many balls- it lets them control drag/preload a bit...if worn enough to need balls replaced, buy all the standard diameter balls from your local bearing house... think mine were 1/8", maybe 5/32, lots at work are varied sizes, but usually a fractional inch or metric size is used... if you have a smooth surface on the nut and screw, some screw rebuilders have offered to sell us both custom/oversized balls and even make replacement pickup tubes- the pickup tubes I dont think could ever be fabbed at home, they are a very special part... just thinking, IIRC, my x had a pickup tit wore off one tube, think I left that one circuit out completely...not right, but none of this really is, guess as long as it works well enough

[Shanghyd]

Thanks for the info. I'm going to try and reball this one. If I can't do it I'll send it off for repair. Replacement balls are about .61 cents each so that won't break the bank.
I'm thinking that I'll order balls the size of my current ones + backlash. Make sense?
It seems to be a shot in the dark without knowing the size of the original balls, then having some sort of gauge to measure the threads in the nut.
I guess thats why it costs so much to have them reworked.

[tc429]

From what I've seen, unless its already been rebuilt, they will be a nominal size, usually recalling will tighten them up quite a bit...bet the old balls are a thou or two undersized of a nominal inch or metric size. If you've got .009 slop, you won't get a .009( or .0045) oversized ball in there...the fit is " funny" in the grooves aren't really a radius, the screw guys call it a Gothic arch or something like that, the balls only contact at the center of the sides of the threads, and as load increases they deform and make more contact...the radius is a few. Thou bigger than the ball radius, preload results in slight compression deformation of the grooves...kinda a 4 point contact unloaded...
Seemed to me 5/32 balls were only about 20 cents apiece from our local bearing house...custom/ ovesize cost quite a bit more.

[Shanghyd]

Ok. I'll measure mine when I get them out and see where to go from there.