Hello Nige, you have done very well to achieve that. 4 jaw work is often a bit long winded but for many jobs such accuracy is not needed. For example, if you used the 4 jaw to hold a piece of square bar for turning then it might be OK within 5 thou, it depends on how well the parts need to line up.

If setting up a casting, the only requirement is to ensure that everything cleans up.

Brian

Nige,

That is likely far better than most 3 jaw self centering chucks will achieve.

If you plan your work such that the item does not require rechucking, the initial chucking may not need to be centred perfectly – as long as the stock is sufficiently oversized – just the first cuts may be intermittent.

Turning between centres is the way to go if parts need to be removed from the lathe and replaced several times without fear of concentricity problems.

Trial and error is always the way it is done. Think here, if you trial it (measure) and reduce the error (run out) by one half each time, you should only need a maximum of eight repeats to reduce the error to close (enough) to zero (a bit like folding a sheet of paper in half and half again, etc). It is only that some can get there more quickly than others!

Check out the recent competition between Keith Fenner v Abom79 at the Bar Z bash 2017.**LINK**

Even they can be set a rediculous task to test their doubtless high skills.

I'm amazed you managed to find a bit of steel as straight and round as that.

;0)

Martin

A very good point. I took my 3-jaw apart to clean it after the run out went to pot. It turned out that the length of silver steel I was using as a reference is slightly bent! (I may have dropped it earlier.)

Dave