Brass stock

[Roger HulettParticipant @rogerhulett83124]

I have bought a piece of round brass stock from a well known metal supplier. It is 1 1/2" diam 9" long. When I put it in the lathe to turn it down I found that if I trued it up at the end then at approx 4" away from the end close to the chuck jaws it was several thou out. If I trued it up close to the chuck jaws it was then several thou out at the end. How should I ascertain the correct point for turning ? I tried turning the brass to different positions both in location in the chuck and in the amount sticking out from the chuck but it made no difference. Your advice will be most welcome.Thankyou.

[Roger HulettParticipant @rogerhulett83124]

[David Standing 1Participant @davidstanding1]

If your chuck is true, I would ask for a refund/replacement!

[Speedy Builder5Participant @speedybuilder5]

I am not quite clear of what you are saying. Is it that the bar stock is a few thou out of round (may even be a fast spiral shape) or slightly bent, or is it that your lathe is not cutting true. I think it is the former in which case that is quite normal. If you find that is not acceptable, you may get away with truing up the tailstock end and putting a centre in it, then turning the bar around and centre the other end. Then you could turn between centres (if you have a headstock centre and dog to drive the bar, or you could make your own centre in the chuck. I guess most of us allow as least 0.010" to 0.20" to true up a bit of bar stock at that sort of size.
BobH

[Roger HulettParticipant @rogerhulett83124]

Yes,Speedy Builders, that's the way to go. I should have thought of that myself but have only limited experience. Thank you very much.

[Martin ConnellyParticipant @martinconnelly55370]

The chances of putting a large piece of metal in the chuck jaws without it being off axis is very high if you don't use a centre drilled end and a tailstock centre. Set it to minimum run out near the chuck, put the indicator on the far end and tap the stuck out bar with a mallet to get it running with minimum run out at the end as well. Don't have the indicator in contact with the bar when you tap it.

Some people have a bearing mounted as a tool that can be used in the toolpost to push round bars to get them to run true easily. There are videos of them in use on YouTube.

Martin C

Ps search for lathe centering tool on YouTube

Edited By Martin Connelly on 16/11/2017 19:04:03

[Stephen FollowsParticipant @stephenfollows82099]

Experience has taught me to buy a larger diameter than I need then turn down to the desired size. Having seen the machines that straighten metal bars I would be surprised if there was a truly straight one ever produced.

[vintagengineerParticipant @vintagengineer]

I was always taught to use 4 jaw chuck. 3 jaw chucks where for production work! Soon learnt to set a job up to run true in a 4 jaw than a 3 jaw.

[John ReeseParticipant @johnreese12848]

If you received continuous cast bar it is quite likely it will not be either truly straight or round. Extruded bar may not be straight unless it was "stretched". You need to take that into account when setting up the job.

[Enough!Participant @enough]

Posted by vintagengineer on 23/11/2017 21:50:32:

I was always taught to use 4 jaw chuck. 3 jaw chucks where for production work! Soon learnt to set a job up to run true in a 4 jaw than a 3 jaw.

I don't doubt that that would work just fine as you say and set up the bar very accurately when measured close to the chuck …. now go and check the runout at the other end of this 9" long bar. Which I think was the point.

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