Run out on bar

[not done it yetParticipant @notdoneityet]

As per most, accept what you have, if within sensible limits, accept that self-centring chucks are not perfect, don’t buy spot-on material sizes if that dimension is critical and don’t remove the item until all concentric machining is completed.

I actually have enough chucks (160mm down to about 70mm) to be able to swap them out, with the work still in situ if necessary – refitting the chuck is sufficiently precise as the lathe accepts screw-on chucks. But I often use my 4 jaw independent anyway – it can be almost as good as a self-centring chuck by eye (close enough for many operations) and is there, ready to accurately mount most things that come up.

[Salvie CudlipParticipant @salviecudlip79156]

Cheers for the reply’s guys. I managed to get a couple thou out by tightening the Chuck up several times! That seemed to work well!

I managed to turn a bit of bar down! Probably a terrible cut. Wasn’t sure on the angle of the tool relative to the bar. Probably should of looked that up before I started! 🤦‍♂️

thanks again 👍

[Salvie CudlipParticipant @salviecudlip79156]

[Salvie CudlipParticipant @salviecudlip79156]

[Salvie CudlipParticipant @salviecudlip79156]

[Martin ConnellyParticipant @martinconnelly55370]

I think most of the answers on here were not related to the stated problem of "run out at the end of the bar", not at the chuck.

Martin C

[VersabossParticipant @versaboss]

Hmm … when you use a DTI in the same position as the tool which was used before to turn the bar, then it is no wonder that it shows nothing! You have to measure the diameters at both ends with a micrometer.

It wonders me some times that none of our experienced members immediately jumps up on such a blatant error.

Regards,
Hans

[Salvie CudlipParticipant @salviecudlip79156]

This was after I turned the bar down! Wouldn’t 0 indicated that the bar is now straight in the Chuck? Used a micrometer and reading same values either end!

[Graham MeekParticipant @grahammeek88282]

Putting any bar in the chuck without first establishing if that bar is straight, or not, will not prove the chuck to be sound.

Further like all things in engineering the Chuck is made to a set of standards. Manufacturers give "Run-out" figures for their chucks and one of those is for a stated size bar, (usually ground, and not bright drawn), at a stated distance from the chuck jaws.

Thus putting a new chuck on a machine is not going to solve any run-out problems, unless a) the bar is known to be straight, and b) the test is not done to the manufacturers specification. Even then with a 3 jaw chuck the run-out is likely to be about 0.03mm or 0.001" about an 25mm or 1" from the jaws, unless the chuck is a Precision grade chuck.

Lastly the bar above has been machined in-situ, the lathe is doing what it is supposed to do. That is turn a true cylinder. This has no bearing on the accuracy of the chuck I am afraid.

Regards

Gray,

[Anonymous]

Posted by Salvie Cudlip on 12/11/2021 10:39:41:

Wouldn’t 0 indicated that the bar is now straight in the Chuck?

Unfortunately it doesn't tell one anything of the sort. The fact that micrometer measurements are the same at each end implies that the lathe is turning parallel (probably, although the bar could be barrel shaped). The turned part of the work will be parallel to the axis of rotation. But nothing can be inferred about how the chuck is holding the work. If the bar is removed and replaced it is highly unlikely to still have zero runout.

It is very difficult to make measurements that highlight one, and only one, parameter. Assumptions about what is being measured are often incorrect. It helps to visualise the geometry and what is being measured beforehand.

Andrew

[Dave HalfordParticipant @davehalford22513]

Posted by Salvie Cudlip on 12/11/2021 10:39:41:

This was after I turned the bar down! Wouldn’t 0 indicated that the bar is now straight in the Chuck? Used a micrometer and reading same values either end!

Halfway there Salvie. Now you have a straight and parallel turned bar. Take the bar out of the chuck and turn it round to hold it in the jaws by your turned section, then see how much the bar wobbles if at all.

BTW some old worn chucks have been used mainly for small size work and will be bad at those sizes, but they are fine on the unused sections. For example I have a 5" chuck with 15thou runout for sizes under 1", opening the jaws further the chuck key goes noticeably stiffer and the chuck holds like a new one.

[JasonBModerator @jasonb]

Posted by Andrew Johnston on 12/11/2021 11:10:06:

Posted by Salvie Cudlip on 12/11/2021 10:39:41:

Wouldn’t 0 indicated that the bar is now straight in the Chuck?

Unfortunately it doesn't tell one anything of the sort. The fact that micrometer measurements are the same at each end implies that the lathe is turning parallel (probably, although the bar could be barrel shaped). The turned part of the work will be parallel to the axis of rotation. But nothing can be inferred about how the chuck is holding the work. If the bar is removed and replaced it is highly unlikely to still have zero runout.

Andrew

Surely one would get the same reading on a tapered part that had just been turned. Just because the DTI reads zero on the side facing you the back could be tapered, all the dti is doing is confirming that the tool just moved in the same plane as the dti has now done.

As Han's said without measuring the diameter at each end who knows if it is parallel or tapered or even which direction the taper may go.

Edited By JasonB on 12/11/2021 13:01:27

[Dave HalfordParticipant @davehalford22513]

Posted by Salvie Cudlip on 12/11/2021 10:39:41:

This was after I turned the bar down! Wouldn’t 0 indicated that the bar is now straight in the Chuck? Used a micrometer and reading same values either end!

He has

[JasonBModerator @jasonb]

Missed that bit

[Author]

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