How to center ACME lead screw in the lathe

This topic has 12 replies, 8 voices, and was last updated 6 October 2017 at 19:16 by Neil Wyatt.

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4 October 2017 at 20:10 #320045
Martin Dowing
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@martindowing58466
I got an 8 TPI leadscrew rod and I need to adapt it to machine so some work need to be done on both ends.

I need to center it in the lathe to do necessary jobs.

How to make sure that it run concentric? I do not have a Grip True chuck and my 3 jaws chuck runout is 2-3 thou, depending of socket used and bar diameter.

So maybe there is a way to set it true in 4 jaw?

Run clock for TIR check with lathe set to screwcutting, to follow crest? Any other ideas?
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4 October 2017 at 20:10 #8896
Martin Dowing
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@martindowing58466
4 October 2017 at 20:17 #320047
Rainbows
Participant
@rainbows
Use an indicator with a large foot so that it cant fall into the thread root.

Make a ring with a bore to closely match the thread OD, turn OD and ID in same set up to guarantee concentricity.
4 October 2017 at 20:23 #320050
JasonB
Moderator
@jasonb
Bore out a bush in the 3-jaw to accept the screw OD, mark its position so it goes back into the chuck the same way, Take it out and cut a slot in it, then use to hold the screw. Make it from aluminium and you won't damage the thread which heavy handed use of the 4 jaw may do.
4 October 2017 at 22:15 #320062
John Hinkley
Participant
@johnhinkley26699
If it has accurate centres in each end, hold and turn it between centres. Of course, you'll need a drive plate, dog and live and dead centres, too.

John
5 October 2017 at 04:58 #320077
Hopper
Participant
@hopper
Don't you have a four jaw chuck? If not, you may be able to shim your three jaw, using paper between one jaw and the job, to get it running true.
5 October 2017 at 08:09 #320084
John MC
Participant
@johnmc39344
For absolute accuracy the screw should be centered from the threads PD (pitch diameter). A tricky thing to do. The way I would do this is, as suggested with a DTI with a "big foot" on the outside diameter, then check with wires what the PD is doing. A bit of a faff but well worth doing for absolute accuracy, especially if the method of manufacture of the screw thread is unknown.

Incidentally, the same applies to gears, true up on the OD initially, then check the PCD.

John
5 October 2017 at 09:09 #320089
Martin Kyte
Participant
@martinkyte99762
So a suitable spring wound into the threads to stand just clear of the crests and a snug fitting concentric tube pushed over the lot.?

Could use this in a fixed stady to centre the ends if required, maybe.

regards Martin
5 October 2017 at 11:29 #320114
John MC
Participant
@johnmc39344
Could work if you can find a spring with suitable wire diameter. The wire needs to contact the flanks of the thread about halfway up the depth of the thread.

John
5 October 2017 at 14:00 #320123
Martin Kyte
Participant
@martinkyte99762
Actually its only going to work if the wire diameter is a little over half the pitch. Acme threads are pitch/2 deep and at half way up the flank they must be pitch/2 wide. So an exact size wire will touch the bottom on an unworn perfect thread. So maybe somewhere around 70 thou for an 8 TPI thread?

All said it's probably easier to chuck it in the 4 jaw and clock against a wire. (As usuall I get to musing about strange ideas).

regards Martin
5 October 2017 at 19:07 #320163
Martin Dowing
Participant
@martindowing58466
Many thanks for comments.

@Rainbows – I like this method – If consistent readings are secured I will stick to it.

@JasonB – I think, too much is left to chance here

@ John Hinkley – no centers. Leadscrew is rolled. Anyway, I would need to extend bedways in any case.

@Hopper – I do have 4 jaw. I do not have Grip True. It is still not easy to center an awkward object in 4 jaw – hence my querry.

@ John MC & Martin Kyte – Method of production *is* known. It is rolled precision leadscrew ex McMaster Carr, 45-50 HRc hard, It has nice deep brown to black shiny finish on its surface. Do not know about any possible surface hardening. It is made of 4140 Alloy Steel and Mc Master Carr catalogue No is 98940A733. Do you have any ideas, how to approach the issue with this information? For example how *rolling* of the screw affect any possible irregularities? I would really appreciate any further ideas here.

I would really like to center it well – It will be a leadscrew in my lathe after all and needs to be kind for end bearings and clasp nuts.

Martin
6 October 2017 at 00:51 #320206
Hopper
Participant
@hopper
You have a four-jaw, so use that. Wrap a piece of sheet metal around the thread to protect it where it is gripped, or a piece of pipe bored to fit with slot hacksawed in it etc. I'd then use the suggested method of the large diameter "elephant's foot" on the dial indicator spindle end to read off the tops of the threads to get it running true. Keep it simple and git 'er done.

Edited By Hopper on 06/10/2017 00:52:16
6 October 2017 at 19:16 #320310
Neil Wyatt
Moderator
@neilwyatt
Posted by Martin Dowing on 05/10/2017 19:07:52:

For example how *rolling* of the screw affect any possible irregularities?

As I understand it rolled leadscrews have better surface finish and are harder wearing than ground leadscrews.

Ground leadscrews are more accurate.

Precision rolled leadscrews should be fine for a lathe, but note that any screw you cut can't be more accurate than your leadscrew*.

I think the best quality are rolled leadscrews finished by grinding.

Neil

*Except for some very special lathes with compensation for leadscrew errors or servo control of tool position but I'm guessing you don't work for the BSI.
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