Lathe chuck not true.

[JourneymanParticipant @journeyman]

Posted by Martin of Wick on 22/03/2018 21:02:25:

… 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…

Martin

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.

[Brian WoodParticipant @brianwood45127]

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

[Martin of WickParticipant @martinofwick]

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.

[Brian WoodParticipant @brianwood45127]

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

[JourneymanParticipant @journeyman]

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

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