Lathe cross feed conversion to a ball screw

[PeterT]

Shows a step by step of typical worn leadscrew/nut cross slide conversion. The big revelation to me was FINALLY, someone put a gauge on the assembly & measured (spoiler alert) no backlash by pushing directly on the cross slide. So why has it been 'known' & perpetuated so often, in so many forum posts that ball screws will self feed on themselves & you will have a runaway freight train effect on manual feed machine unless you lock the carriage at every stop? Different ball screw types or pitch maybe? I don't get it., someone please explain.

Spoiler alert #2 (I was going to post my answer but someone beat me to it) he ordered the wrong thread direction so instead of clockwise = in feed, its now the opposite. I'm pretty sure my brain would be slow to adapt like moving to England &driving on the other side. And he kind of glossed over the dial increments no longer makes sense due to different pitch, but he uses DRO anyways. So a few learning here, but it has potential.

 

[DavidR8]

I’m no authority on this but I think the self-feeding ball screw relates to when they are used for the X and/or Y axis on a mill such a a CNC conversion. In those situations the forces tend to act in the direction of travel but are prevented from running away by the stepper or servo motors.

On a lathe the cutting force is largely directed down and not in line with the lead screw.

 

[PeterT]

That's a real good point David. I know I've read articles where people were specifically talking lathe conversions, but maybe the cnc'ers were passing on that info as a possibility?
Let me ask you this, if you push on a cnc router parallel to the leadscrew axis, is it the same result as what he demo'd? (no movement).

 

[Tom Kitta]

Impressive - main problem would be that the dial indicator will no longer be of any use. My screws are totally worn out so I need to replace them. It is not that easy given all the splines and special cuts.

 

[kevin.decelles]

It is not that easy given all the splines and special cuts.

I've got this on my project list too, and the screws from my old lathe have multiple diameters/shoulders/grooves etc. It will be an exercise in part replication.

 

[PeterT]

Kevin, do you mean telescoping leadscrew assembly tubes for disconnect in order to engage taper turning mode? If so, that's a different kettle of fish.

My lathe is quite similar to his & I've had this thought for some time to modify the way the lead screw nut is attached (bolted essentially) to the cross slide table essentially. If that could be temporarily disconnected allowing table to free slide, then the taper push bar will do the rest. But I'm a bit chicken to mill & modify my assembly. The trick is making it relatively easy to re-attach again in a reliable way. If only I just bough the taper attachment feature at the time, oh well :/

 

[Johnwa]

I have a couple of ball screws saved for a Cnc project. It takes a bit of pressure to get them moving but they will definitely self feed.

 

[PeterT]

I wonder if its a function of equivalent pitch. I'm not really familiar with the terminology but looks like around the 10-20mm shaft diameter the most prevalent is 5 & 10 mm 'lead'.
Is that the same as pitch? Like 1/(5/25.4)=5.08 TPI equivalent? 10mm lead would be 2.54 TPI equivalent? If so, that's pretty coarse by typical acme crossfeed screw standards.

 

[Johnwa]

@PeterT my screws are 16mm with a 5mm lead. Lead is just another term for pitch so mine are about 5 turns per inch. Along with the coarse pitch they’re running on steel balls so there isn’t a lot of friction to begin with.