Ball Nut Holder

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Robert LaLonde

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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.

Ball Nut Holder.jpg
 
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Seems to have worked so it's gotta be a great idea.
 
Have a hydraulic press? Set up and straighten the ball screw. That's a learning experience, too.
 
4gsr said:
Have a hydraulic press? Set up and straighten the ball screw. That's a learning experience, too.
Click to expand...

I have a hydraulic press. Two of them actually. However I am concerned if the main travel of the balls screw is bent that straightening it would affect accuracy. The machine this screw goes on holds decent tolerances. Of course to matched that with a new screw would be pretty expensive, and a cheap screw might be no better than a straightened screw. One thing at a time.
 
go watch keith Fenner's you tube on how to straighten shafts. He has to do it a lot for prop shafts on boats. They need to be very straight or they will vibrate the crap out of the boat and rip the mount out.
He does 2 methods, presssing , and heat. The heat is very cool, it's quick and does not mar the shaft like pressing could. The heat is followed with cold water and air to rapidly cool and pull the side into straight.
 
I'll probably roll the screw on the granite plate today or tomorrow to see how bad it is and where. It was bad enough that is was binding in the new thrust bearings and causing pulsing in movement. First I have to clear the area around my surface plate. Stuff tends to get parked in front of it since it doesn't get used every day. I can make aluminum shoes to fit the press that are a good fit, so marring the screw is not really an issue. Still I need to see where its bent.
 
woodchucker said:
go watch keith Fenner's you tube on how to straighten shafts. He has to do it a lot for prop shafts on boats. They need to be very straight or they will vibrate the crap out of the boat and rip the mount out.
He does 2 methods, presssing , and heat. The heat is very cool, it's quick and does not mar the shaft like pressing could. The heat is followed with cold water and air to rapidly cool and pull the side into straight.
Click to expand...
Not a good idea with a ball screw; heat would partially anneal the screw and it would be subject to wear and roughening (Brinelling) in that spot.
Heat/quench straightening works well in some situations; I straightened a bunch of 5" diameter TG&P (turned, ground, & polished) shafting many years ago, in a lathe. I guess they were about 16 ft long, some had simple bends in them, some had spiral bends, I just indicated the runout, marked the high spots, and heated and quenched the spots until they were within tollerence
 
Bob La Londe said:
It took me a couple hours, but it worked out perfectly.
Click to expand...

Not a bad idea! When I bought some ballscrews from TBI, I received the ballnuts in specially sized mandrels. They are basically a piece of plastic tube with the "right" OD. I suspect the OD is sized to closely match the root diameter of the ballscrew but I didn't bother to measure.
 
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.
 
so what if you put it in the lathe jaws (carefully) and turn it. See if it wobbles. It may seem flat on the plate, but what does it do from end to end.
Or put 2 pipes down on a bench parallel, and roll the screw over them. See if it rollls straight or you see a lift at the end or in th middle.
I think you would see it better rolling than on a plate.
If you rolled it on the plate then ignore my advice.o_O
 
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