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Out of round turning

Help needed

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Mark15/08/2010 13:29:50
1 forum posts
Hi, I'm just starting out learning turning, I've managed to get my work parallel but could anybody tell me why the mild steel bar I have turned has come out 0.03mm out of round, i.e oval shaped.
 
The piece is 150mm long, 40mm o/d turned down to 20mm over a 50mm length. The piece was supported in a 3 jaw chuck and tail stock and I used small depth of cuts with a feed of 0.15 per rev.
 
Ant advice would be gratefully received.
 
Mark
Richard Parsons15/08/2010 15:12:04
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645 forum posts
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Mark,
Where did you get the steel?
This looks like 'internal stresses’ coming out. 
If you are having problems try ‘Rough turning’ to about 0.2mm oversize and then ‘stress relieve’. This can be done by leaving for a day or two ,of if SWMBO is ‘out’ or does not object leave it in the kitchen oven over night at about 180-220°C. 
Reset in the lathe anf finish turning. That should fix it!

 

Edited By Richard Parsons on 15/08/2010 15:13:54

KWIL15/08/2010 16:31:22
3681 forum posts
70 photos
Normally you do not get problems round turning from round stock, it is when you start machining BMS bar that stress problems arise.
 
Other than round turning from round I always stress relieve by fire (torch), cook  to red, hold for 20-30 minutes per inch thickness and allow to cool as slowly as possible.  This does not apply if you start from "black iron" rather than drawn bright mild steel.
Gone Away15/08/2010 16:50:46
829 forum posts
1 photos
Any chance that this is a simple case of leadscrew misalignment ?
Ramon Wilson15/08/2010 20:54:43
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1655 forum posts
617 photos
Hi Mark,
Given the size of the material you are turning it's unlikely that the material is exhibiting any form of stress release. That said if the skin of any material exhibits a relative hardness then the initial cuts can sometimes produce the effect you are getting but should be long gone by the time you get to the finished size you quote. 'Black' hot rolled bar is not very round at its best so is usually skimmed before final turning.
 
Normally though, if something turns out of round it's usually a, the tool or  b, the work  'moving' in or out relative to each other. If the headstock and tailstock are 'sound' then it may be caused by the cross slide and/or the top slide gibs needing a nip up. The run out of 03 is quite small so movement wouldn't need to be much.
 
There are other factors that can come into play also  - a faulty tailstock live centre - though this normally would produce a tapering effect down the length of the work - and/or worn jaws or scroll in the chuck. Is this 03 run out consistant down the length of the reduced diameter?
 
If the .03 run out is still there after a final .05 cut - take two passes at the same setting - then you probably have to look at the set up. Start with the chuck as movement of the tool is unlikely with such a small depth of cut.
 
Hope this gives you some idea from where to start to look
 
Regards - Ramon
 
Stub Mandrel15/08/2010 21:59:37
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4318 forum posts
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Mark
 
What measuring tools are you using and do you have?
 
How are you measuring the work? It's possible to get inconsistent results from a micrometer and assume they are variation in the work.
 
If you have a DTI, for example, you can use that to check for play - fit it to the toolholder, with the probe touching the work. Push and pull various bits and see if you can spot anything loose.
 
Another idea is to turn a short test piece without tailstock support and see if this comes out round.
 
If an old lathe look for worn headstock bearings, rare, but I understand it can cause such problems.
 
Neil

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