Lathe chuck not true.

Brian,

Thanks, I will check that later tonight and report!

Regards

Martin

Three thou run out on a 3 jaw isn't bad! I cannot emphasize enough that the chuck and spindle nose should be ABSOLUTELY clean. In my early days at using a lathe, I was plagued by variable run out of anything up to 10 thou! Cleaning both the chuck thread, scroll and the spindle nose made the run out about a consistent thou.

If you genuinely have a 3 thou run out, then you can always convert the back plate to a "sort" of Burnerd Grip Tru chuck. I think Commander Barker was the first to describe the mod and then GHT wrote it up. You can buy a kit from Hemingway to convert your chuck, but best read GHT's manual of workshop practice, it is described therein.

Grinding jaws is a last desperate resort for a worn out chuck as far as I am concerned and then it only works for one diameter.

Andrew.

Brian,

You Da MAN! !!!

The back plate of the integral Myford chuck comprises the register only as the threaded portion is in the chuck body. However, that register portion was a good fit on the mandrel nose and not going to wobble from the pressure of a DTI.

Even so I checked several times in differing positions and the result was always the same - 3 to 4 thou run-out in the plane normal to the spindle axis. Moreover, the low and high positions corresponded to the low and high chuck jaw run-outs, so well spotted.

I can see how this part not being truly normal causes the problem, but cant understand how it ended up in that condition - or was it like that from day 1?

Next thing to do is make up a slim tube/top hat 11/4"W12 nut arrangement to secure the register plate tightly to the mandrel for skimming - that will be an interesting internal threading project! If my dodgy geometry is correct skimming the chuck back register should eliminate 80 to 90% of the chuck run-out at the jaw location.

It is great to know what the real cause of the problem was and you have saved me committing the unspeakable crime of jaw grinding!

So thanks very much Brian!

Regards

Martin

Talking about round stock being "out of round",we get that quite a lot over here. We just deal with it and get on with the job.

Martin, unless you particularly want to cut the internal thread you could probably do a similar job with a plain tube, a couple of large washers and a length of studding through the spindle with a couple of nuts either end. Should be plenty strong enough to hold the backplate securely against the shoulder whilst lightly skimming the face.

John.

Hello Martin,

Nice to hear that my diagnosis was correct. I don't fully follow the build of your chuck though, I have a Myford 4 jaw which has integral mounting features and thread but it is all part of the chuck body.

There is another way of rectifying your chuck as I can see holding the flange will be difficult.

If you have another chuck, turn up a decent diameter spigot on a piece of bar held in that, at least 1 inch diameter or so with a bit of length on it and then grip that fully in the jaws of your 3 jaw chuck as a full assembly and take a very lightest skim off the vertical register of the 3J. It will have the same effect as re-machining the other register.

As others have said, getting a 3 jaw chuck to hold repeatedly to 3 thou or less is about as good as you might expect, you begin to chase diminishing returns from that point and even run the risk of making matters worse.

How things moved is a question I can't answer other than to suggest the relaxing of some built in stress after machining, it can and does happen.

Regards

Brian

Brian,

Thanks for that, It also occurred to me that although the chuck back looks relatively substantial, it might be possible to distort it if there was a nasty dig in or lock up (but not on my watch - to timid!)

Two halves of myford integral chuck shown in photos below, with the offending back part placed on lathe spindle in top photo - the threaded part is in the business end of the chuck in bottom photo.

There is about 20 thou clearance between the spigot on the back part of chuck and the rotating scroll, so should be ok to skim 3 thou of the bolting flange (might also have to check the spigot clearance from the front of the chuck also)!

If I can get to 3 thou run-out I will be delighted! (better than current 9 thou TIR).

Journeyman, thanks for brilliant suggestion to use some stout tube and threaded rod through the spindle - that would make for a very quick job.

Bets regards and many thanks for the excellent advice,

Martin.

Hello Martin,

Now I understand. I think Journeyman has the right approach, a good thick walled tube should pull that down tight enough for a light facing cut.

Castings do move, generally they are left to age after rough machining for that sort of movement to take place and then finish machined, but perhaps in your case that wasn't enough

Good luck, I'm sure it will be greatly improved after this work

Regards

Brian

Odd way to make a chuck I can only think they did it like that to make the chuck as short as possible and get it nearer to the spindle. Looks easy enough to take a really fine cut off the face just make sure everything is really clean before bolting up.

John