Trouble Machining Ball-screw

I am trying to retrofit my knee mill by replacing the old worn out screws with new ball-screws and ball nuts. I turned the end of the Y axis screw down to the same dimensions as the stock one, but when I tried installing it wit would wobble when turning. At first it wobbled almost 1/2". I then tried adding some material with the welder to create a shoulder to ride on the bearing race. This helped a little but not much. We later discovered that the threads weren't perfectly straight on the screw, because we had to use a die to make them, because the auto-feed on the lathe is broken. So, I cut the screw back some more, cut the old threads off and redid them. This time we used the tailstock to push the die and keep it straight. This seemed to work very well. I then tried it again, but it still wobbled. We then tried adding material again with the welder to create the larger shoulder again. This time we filled the gap in the threads and went around an extra time to make the shoulder taller. This seemed to help even more but still not perfect. I then cut the back of the nut on the lathe to make it perfectly perpendicular to the screw. Also helped a little but still not perfect. So now I am stuck and dont know what to do.

Is there anything I can do to make this screw work? Or should I buy another and bring it to the local machine shop?

Using a dial indicator on the threads of the screw, I measured the High and low spots in the screw. At the low the dial read .0085" and at the high it read .048", diving a difference of .0395" out of wack.

Anybody have any ideas?

-Adam

Picture 1: Screw with Weld before being re-turned
Picture 2: Screw after being re-turned
Picture 3: Dial Indicator showing low position
Picture 4: Dial Indicator Showing High Position
Picture 5: Dial Indicator Setup
Picture 6: Back of bearing mount with shoulder on inner race
Picture 7: Front of bearing mount with nut on inner race

If the screw is straight but running out bend it near the bearing by giving it a gentle whack with a soft hammer NEAR THE BEARING. Make sure you support the the free end with a bag of sand or your hand, otherwise the inertia can create a bend elsewhere because of the twang that will occur. Don't leave the end free, and give it little taps.
MARK THE HIGH SPOT with a marking pen. Don't do it by eye.
I have straightened bent valves while still in a head with just the right hit.
DON'T OVER DO IT. SNEAK UP and keep using the clock to check runout.
If the screw is not dead straight you must straighten it first.
With it not mounted in the bearing mark the low spot of of the bend and where it is along the screw, then holding the end of the screw a gentle whack on a piece of pine can bend it using the inertia of the whole rod.
Get a few bits of steel about the same size and practice first until you are confident you will not over do it. Get some demos from your friendly blacksmith if you can find one. This works for parts of all sizes. Just getting it right is a bit of an art.
PRACTICE FIRST. DON'T START WITH YOUR PRECIOUS SCREW which was bent by the welding.

Ok I will try it when I get home. The weird thing is that the was always out of wack. Only got better after it was welded. Do you think that the screw could have been bent from the factory? It was never dropped or hit by/on anything either. -Adam

Might have started life as slightly bent being from the end of long stock.
Just straighten it. You will then have acquired a new skill. It is easy to do.
Just make sure you do some practice first. Measure twice, cut once.

The second part of this , is what is your lathe set up ? Are you supporting the far end of the ball screw in the lathe tube with a fixture ( IE a doughnut, with the ID of the ball screw and OD of the lathe tube) if not the ball screw will whip and move in the chuck, therefore taking it off center.

Are you using a 4 jaw chuck and indicating in ? If not , and your using a 3 jaw, you need to make a precise split fixture to center in the jaws, or use plain jaws with out serations.no matter how you try,the serations in the jaws will get between the ball screw threads and cause it to mis-center. This is even worse when you center drill with the shaft off center for the tailstock live center, as you are now creating somewhat of a "cam" at the end of the shaft.

The best situation would be a 0 TIR Buck chuck with plain jaws and a fixture for the head stock tube. This is the best set up, because the cutting forces are high and interupted when getting through the hard facing, at least this is the way I did it for 35 years and was able to produce good shafts.

Use lots of coolant, do not let the tool get hot.

Good luck, take your time.

Adobe (old as dirt)

Adobe may be right. If you have more than 8-12" unsupported behind the chuck, it will whip and put a bend just behind the chuck jaws. My Patriot machine has a neat feature called the lathe spider just for doing this work. Whenever I turn steering shafts, panhard bars etc, I support them with the spider. Gives a better finish too, as it takes out the harmonics.

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Originally Posted by smallblock

Adobe may be right. If you have more than 8-12" unsupported behind the chuck, it will whip and put a bend just behind the chuck jaws. My Patriot machine has a neat feature called the lathe spider just for doing this work. Whenever I turn steering shafts, panhard bars etc, I support them with the spider. Gives a better finish too, as it takes out the harmonics.

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My Lathe didn't have a spider....BUT
I made a bushing out some alum stock. The OD of it was turned to the size of the spindle bore with a slight taper so it would go in the spindle bore about half-way and then get tight. I then bored out the bushing's ID to the screw stock diameter.

Jack