When you say it moves doe it get pushed back into the chuck or spin? If it spins where do the jaws leave a mark. If the mark is deeper towards the back of the jaws they are probably worn and bellmouthed

These chucks are very easy to dismantle, clean and regrease, yours may benifit from a little tlc.

If you have some silver steel. of other ground stock like the plain shank of a 1/2" drill in undamaged condition, then you could try chucking it up lightly so you can just turn it by hand, and then wiggle it about. You should be able to tell if the jaws are looser at the front or not. If the are not touching at the front, they are bellmouthed.

Perhaps a cut of 1mm is expecting too much from a little chuck in a Myford.

Edited By old mart on 10/07/2020 20:22:16

Edited By old mart on 10/07/2020 20:25:51

Since no one has mentioned it, you are going around the chuck tightening all the jaws evenly – not tightening from a single position?

A tip given to me by an old machinist to true up the jaws is as follows

Select a grinding stone that has a diameter so it can pass through the rear of the chuck.

Turn up a ring that will fit over the jaws so that when they are opened against the ring ,the grinding stone will pass through.

Now gently stone the jaws till true.

Not done it myself, but he did sort out an old chuck for me.

An alternative might be to source some replacement jaws Pratt Burnerd use to do spares .

I'd have thought you would have ended up with better being the jaws were trued in place.

As for par for the course I'll stick with my less than 1/2thou out of the box 3-jaw and Old Mart can keep his wonky ones

Well done Ro, 3 thou isn't too bad for a 3-jaw. Run-out is mostly a pain when work has to taken in and out of the chuck because each time the job takes up a slightly different position and the turnings aren't concentric. Cutting stays concentric when the job is only set in the chuck once, in which case a bit of 3-jaw run-out probably won't matter.

The main advantage of 3-jaws is speed and convenience gripping round objects, as a breed they're not hot on accuracy. If low run out is essential, switch to an independent 4-jaw because these can be set up very accurately with a DTI. Their disadvantage is the extra time taken to set them up, though some use nothing else and say – with practice – they can be as quick as a 3 jaw. They're also required for holding odd shapes. Collets are good for speed and accuracy, their disadvantage is cost!

Just as a matter of interest, how exactly do you chaps measure run-out? My Chinese certificate calls for a 20mm diameter test bar and the run-out to be measured 50mm out from the jaws. This, I think, is a rather tougher test than finding run-out as close to the jaws as the DTI can get. Rude things said about Chinese certificates, but my chuck was about 0.01mm better than the 0.05mm @ 50mm claimed for it. Not quite so good now unless I fuss with it on the register. Wear and tear on an inexpensive item I expect.

Dave