Ball Nut Holder

I am sure somebody can tell me why this is a stupid idea, but I needed
to get this ball nut off the ball screw without dumping the balls all
over the floor. I don't know if the screw is salvageable or not. Its got
a bend in it somewhere. If its just in the machining in the end I might
be able to turn it down and machine some bushings. If the screw is bent
in the main span its done. I might be able to straighten that, but it
wouldn't be very accurate anymore. Normally guys screw the nut off onto
a piece of cardboard tube, but I have bad luck with that. This hand made
stub of aluminum "ballscrew" mates up with the end of the ballscrew I
need to check, and I was able to just screw the nut off onto the stub. I
had to hand grind a radius tool out of HSS in order to thread the
holder. It took me a couple hours, but it worked out perfectly. Of
course that time is wasted except for the learning experience if I can't
save the screw.
Anyway, now I can roll the screw on my granite surface plate and
determine where its bent.
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Reply to
Bob La Londe
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Greetings Bob, If the screw is indeed bent but the bend is a large radius I think the screw might still be OK after being straightened. I used to straighten shafts used in piston to turbine engine conversions for small planes. The shafts would need to be straight within a couple thou over about 40 inches. After straightening I had to check the shafts pretty much completely along the length of the shaft. And I never saw localized small radius distortions, they were always over long distances. So maybe the same thing would hold true for your screw. As I see it a sharp bend would cause the ball groove(s) in the screw to be compressed in one spot and expanded in another, opposite spot. A large radius bend would cause the same effect but it would be much less. If the shaft is then straightened I think the distortions in the ball groove(s) would be lessened to the point that it wouldn't matter. Eric
Reply to
etpm
The screw doesn't seem to be significantly bent. I got maybe 0.00075 variance from one end to the other by resting the screw on a granite surface plate and passing an indicator over the top of the thread. I checked on multiple lines along the screw. Then to double check myself I grabbed a piece of .002 thin shim stock and tried jamming it under the screw all along the screw at 4 different rotational positions of the screw. I know the indicator is better, but I can get a feel with the shim stock. A sort of confirmation.
Then I ran the indicator over to find the high point of each turned step/shoulder. The results were about the same. I double checked myself by using .001 difference gage block stacks as improvised go-nogo gages to slide under the turned surfaces. I confirmed it's within .001 all the way around.
I'm at a loss.
The thing is I could see the screw bind and release before it was removed, and I could feel it bind and release while I was moving the machine back and forth by hand. I'm totally at a loss.
The only thing left I think is to go ahead and dump the balls out of the ball nut and check it out. The thing is it felt perfectly fine once the screw was out of the machine. Maybe it has alternating ball sizes and at some point in the past somebody packed the balls without alternating them creating the same affect as a bent screw. I think that is a shot in the dark, but I don't think it can hurt anything to check.
I'm just guessing now.
Reply to
Bob La Londe
On Jan 26, 2018, Bob La Londe wrote (in article ):
It sounds like a piece of dirt or swarf is stuck somewhere, and probably flattened by the passage of balls. Clean everything very well, and see if that helps. It may require chasing the thread with a scriber point to get stuck swarf loose.
Joe Gwinn
Reply to
Joseph Gwinn
I asked because I think a problem with a ball or bearing might not repeat at the same frequency as a bent screw. -jsw
Reply to
Jim Wilkins
Oh the bearings were definitely bad. I think they were what allowed it to work at all before. I'm putting it back together, but I drilled out the bolt holes and pulled the alignment pins for the front cover that holds the thrust bearings for a little slop. I ran the saddle forward and snugged down the cover mounting bolts. (Kind of like the machine to fit approach you have to do when fixing a Chinese mill.) So far its smooth, but the ball nut mounting bolts are only t-handle tight. I have not gotten in there with a long handle L key to get them good and tight. Maybe tomorrow. I'm done for today. I can't turn the screw by hand, but almost. If I could get inside and get a good grip with both hands maybe I could. The table and saddle are heavy and the the balls are still nice and tight in the ball nut. It spun all the way up and down the screw, but I only get about 1 to 1-1/2 turns on the spin test. Like I said. Nice and tight. I have the end of the shaft wrapped with leather and gripped with locking pliers. I can turn the screw with two fingers, and its not pulsing or binding so far. The cover did snug down shifted over very slightly. There had to have been a little continuous bind on the screw before.
It wasn't a super productive day, but I did get a mold finished and shipped, and I got the CAM done for three more steel embossing dies. I'm not counting anything on the Hurco as an accomplishment until I'm sure I don't have to tear it down again. If I can get it going tomorrow I might reward myself with a day out on the river trying to drown some rubber worms.
Well, I think its about beer:30. Maybe even beer:45.
Reply to
Bob La Londe

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