Run out on bar

VERY few 3 jaw chucks will hold anything absolutely concentric, even if very accurately matched to a backplate that has been finish turned and face on the lathe in question.

Expect something of the order of 0.075mm eccentricity for a good one, when new.. The concentricity will vary according to the diameter being held.

You are depending upon:

the accuracy of the scroll location within the chuck body,

the accuracy of pitch the scroll,

and the accuracy of the jaw face relative to the "teeth" that engage the scroll,

not to mention all the clearances.involved

Start adding up clearances, of say 0.012 mm, at each interface, and the tolerances on the dimensions of each part. You should be agreeably surprised at what you have found for concentricity!

(I have only ever seen one 3 jaw that held work truly concentric ).

A chuck that has seen a lot of use / abuse, may well have jaws that are bell mouthed and gripping less well. or accurately.

Bar may well not be truly round. It is not unknown for centreless ground bar to be trilobular, rather than absolutely round. Check some with a DTI when resting in a Vee block

If it has been ground between centres you stand a much better chance.

If you hold the work in a 4 jaw, independent, chuck the eccentricity will depend on the operator; on how much trouble and time they are prepared to devote to bringing the work to run concentric.

Generally, if it is within 0.012 mm near the chuck jaws, I am satisfied.

Also, remember that most of us have hobby, abused ex college, or worn ex Industry lathes, rather than a well cared for toolroom machine.

Howard

I think a perfect chuck would open and close satisfactorily on one key, but real chucks are always slightly slack. Going round the clock takes out the slack and allows the chuck to be fully tightened.

When tightened on one adjuster, the tiny misalignments in the mechanism tend to wedge and stick the chuck. Shifting to another turning point breaks the wedge and the chuck gets slightly tighter before wedging again. Moving to the third adjuster shifts the wedge again, allowing another squeeze to be applied, and it should be all good.

Dave

I was in a similar position with 3 Jaw chuck. After reading up on it, and thought about the options done by others, I decided not to:
1-Swop the jaws around.
2-Not to grind the jaws.

I decided to modify the 3 jaw chuck with a setscrew across each jaw, so it has 3 adjusting points, ok this is a bit differrent feeling than a 4-Jaw with 4x adjustments possible. You can reach a point where you have to repeat to get the movement you want. So far I ajm glad I did it. Some 3 jaw chucks comes with such an option.
The holes were drilled at around 1-2 degrees angle in chuck and treaded, setscrews used.

After that I carried out an experiment at different diameters, and I found that say I adjust the chuck true at 8mm, it was out at other different diameters, some like at 25mm more and others minimal difference, due to how the chuck works inside, this is the reason I did not grind the jaws at a cerain diameter. I mainly use this if I have keep trueness after chucking up, or if I turned something and have to reverse it in the chuck and keep my previous cut still true to the one I amk going to turn.

So I just leave the chuck at the settibg it was, untill needed to true up.

Note: I fitted 3x split washers to chuck fastning bolts, this give me a squeeze fit to adjust chuck, if its loose you may battle tightning the chuck and keep it true..

Now putting in a long shaft, you must 1st drill a true hole at the end for tailstock to mount, and that in turn must be at the centre of the spindle, then extend the shaft to tailstock to hold it.
Now after this you also faced with a a twisted bed issue if lathe was bolted to a table or bed, now its cutting and shimming. This is what I experienced. So theres more than one factor getting a longer shaft cut not tapered also.

Edited By Chris Mate on 11/11/2021 16:31:34

If you MUST have concentricity with a 3 jaw chuck, then make a collet to fit in it, by boring to the size of the work that you want to hold. It may be worth leaving a short flange on the outer end tom prevent it slipping through the chuck jaws when in use.

Before removing it from the chuck, clearly mark it adjacent to one jaw, so that it can be replaced in the same position.

Remove the embryo collet, and slit it lengthways, so that when in the chuck and the jaws are tightened it collapses to grip the work, and deburr..

Replace the collet in the chuck with the mark against the original jaw, and insert This should give minimum eccentricity of the workpiece.

Howard

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.

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 👍

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

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

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!

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,

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

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.

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

He has

Missed that bit