Ballscrew and nut mismatch

[rkcsx4]

Probably a silly question, but can you use a 0.2 inch pitch ballnut with a 5mm pitch leadscrew?

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[Jim Dawson]

No, it won't work. There was a thread on this a while back, somebody bought the wrong ballnut and it jammed up.

[rkcsx4]

That's what I thought, but the supplier of the conversion kit I bought claims it's fine... 5mm is 0.0196 inches so they are pretty close...

They do seem to be fitting together ok, albeit with a bit of play (I don't know if that's normal) but I don't want to fit it all to my mill if it's going to cause me problems later.

[Jim Dawson]

The whole idea of a ballscrew/nut is that they don't have any play, that would not be acceptable to me.

I don't remember if the poster of the thread I mentioned had a 5mm screw or 5mm nut. But I know it worked once he got the correct nut, but did not work with the wrong nut.

[rkcsx4]

Thanks. I'll see whether I can persuade the kit supplier to provide the correct nuts. It seems reasonable (to me at least) that if a kit says it includes leadscrews and ballnuts that the ones supplied should be compatible with each other!

[skrubol]

I'm surprised there would be play. Normally they would jam.
It's not 5mm balls in place of .2" balls is it?

[rkcsx4]

The nuts are https://www.thomsonlinear.com/en/product/8106-448-006 (according to what is printed on them)

The screws are Thomson #RM1605 (according to the kit vendor)

I don't know whether anything was done to modify the nuts but I doubt it.

Would any 1605 ball nut fit those screws?

[Jim Dawson]

Would any 1605 ball nut fit those screws?

Maybe, but I would rather have a matched set. I would be having a chat with the vendor.

[rkcsx4]

I have had a chat with the vendor, but he's not offering any solution other than send the kit back for a refund (which would leave me significantly out of pocket for shipping and customs fees). So I'm trying to see if there's a way I can make it work.

[Jim Dawson]

If you paid with PayPal or a credit card you may have some leverage there.

Maybe you can find the correct Thomson nut for the screw.

[rkcsx4]

I fitted just the X axis so I could measure the actual backlash in use using a 0.01mm dial indicator and taking 0.01mm steps.

There is 0.29mm of backlash where the carriage remains stationary when changing direction.

I don't know what would be considered acceptable backlash for a ballscrew-driven mini-mill (it's a SX2.7) but that seems way too high.

[Jim Dawson]

0.02mm backlash MIGHT be acceptable, but 0.29mm is way too much. My mill has 0.01mm backlash in X and 0.04mm in Y. I need to adjust the Y ballnut preload, one of these days.

[Eldon_Joh]

there is a chance you can fit oversize bearings into it but the oversize bearings are going to contact the nut in only the outer portions of the helix, so its going to wear out quickly, literally only a handful of balls will be in contact, rather than something like 60 of them.

also oversize balls may cost you as much as just buying a replacement screw, or a replacement nut to fit correctly.

[rkcsx4]

How do I adjust the ballnut preload?

I suspect my best option is going to be to cut my losses and return the kit, and start again somewhere else.

[underthetire]

Most recirculating ball screw nuts use a staggered ball, one large, one a little smaller, then the large etc.. either way, thats not a matched set, so even if you get the backlash out, it's probably going to end up binding. Split nut screws are much easier to set preload, though it takes grinding the spacer.

[Jim Dawson]

How do I adjust the ballnut preload?

I suspect my best option is going to be to cut my losses and return the kit, and start again somewhere else.

To adjust the preload on a single nut system you have to reload the balls with a larger size. But as underthetire said, if you do get it to preload it will probably jam up. My mill has double nuts, so is adjustable.

I suspect returning it might be the best option.