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

Hi Adam!

Is it possible that because of the welding of the shoulder the material of the ball-screw has softened? It could also be that your spline has a banana-shape (exaggurated), as a result of the welding. Try to verify it with a dial. If this is the case, you need a new ballscrew... :-( If it has to be machined, try preturning with a low speed. Best to grind it after turning.

Good luck!

Geert Molema (Netherlands)

What does the other end of screw ride in?

Supporting the end of the ball screw with a simple 6003Z SKF ball bearing should be sufficient on most machines. Make sure it is aligned correctly. But if the ball screw keeps wobbeling, it has to be a banana... Check it with your dial.

Try www.kuroda.com for examples.


Good luck!

the proper way to machine a ball screw is to CUT a set of softjaws for your lathe. also cut a piece of material to support the back end of the ballscrew in the tube of your mill.

if you dont support the back you will never get it right unless you use 8"-12" jaws.

if the ball screw is small enough you can use a collet in the front but still support it in the back.

all that will get you close( with in a few thousandths)

if the ball screw came with ground journals than you can indicat off of those right next to the threaded part. they are usually close but not perfect.

The dial indicator you have isnt a good idea to use as cause it will be thrown off when when you turn the ball screw by side play. ( those type of indicators are made for up and down only not rotation) use the other type. however if you only have that indicator get some oil and oil the ground shaft good so the indicator tip runs on something smooth with no friction.

you should also check the threads on the high and low spots (root) to see if the thread is indicated in) shimming may be requiered.

the best thing to do is center drill the end of the ball screw then run a center in it and recut your ods and faces.

oh yeah that one pic were your indicator is on, that numbers you are getting are bogus and you can't check run out that way so ignore it, you have to support each end in order to check run out, supporting one end in a bearing set wont do any good either as the bearing will not support the ball screw perpendicular.