lead/ball screw end machining

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caspaincmonster

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Hi I am building a CNC lathe and have a question about the best way to deal with the ends of the screws. I have a swiss made 12mm ball screw (at this point annealed but not machined) and a 10mm bearing, this means that when machined down there will still be some of the thread roots on the bearing surface which I think is going to result in a poor fit in the bearings. (I also have a Chinese screw that came machined and it has this issue and it didn't fit very well) Should I either sleeve the end of the ball screw or perhaps make some sort of very accurate connection between the end of the ball screw outboard of the bearings. The end could be, glued, keyed, pinned, heat shrunk, silver soldered etc or I could use an ER collet holder through the bearings and thread the end of the shaft to pre-load the bearings and connect it to the motor with a flexible coupling of some sort.

I attached a couple shots of the motor mount

thanks

Luke
 
I think you will be fine with some thread root showing on the leadscrew. The important direction is axial to set the pre-load. The radial fit in the bearing is less important, there should be more than enough surface area to support the leadscrew in the radial direction. Just size the bearing surface for a tight slip fit.
 
I agree, you should be fine. If you do choose to sleeve, I would stay away from solder, as you risk loosing the temper in the rest of the screw if you are not careful.
 
The small amount of material missing under the bearing woln't make any difference. Even under a decent radial load, as long as there's enough material to support the load without deforming it's not a problem, and for a ball screw application there shouldn't be any real radial load to begin with. Non-issue.
 
Thanks for the input folks. The issue I see isn't with load, it is with the ability to get a good sliding fit into the bearings with all those little edges in the middle of the surface. I have some extra annealed length on the end of my screw so I will try it and if that doesn't work I can cut it off and now that I have a torch I can shrink fit and pin on a sleeve or exestention (heating sleeve and not the screw) and then machine it.
L
 
The loctite bearing retainer (638) works quite well for such things. You may also consider that as an option.
 
Sounds like it's just a bit of flash from the machining, right? Seems like a bit of touch up with a file or something like a dremel should fix that real easy. Unless I'm much mistaken, there's no real reason you need an exacting radial fit anyway. The critical loading is thrust, all the radial support is doing is taking loads from weight of the screw and and side thrust from a drive belt or whatever.
 
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