Grinding jaws of a 3 jaw chuck to run true

How much runout ?

Check the rounout at various diameters, 1" 2" 3" etc. If the runout is the same ammount at any diameter just regrind the jaws. Use a ring so the jaws are under load while truing. If the runout varies at different diameters the chuck is worn out and needs new jaws and scrol. Usualy its cheaper to just buy a new chuck than try to fix one thats worn out. My 40 year old bison 3 jaw has .002 runout. A carbide insert with auto feed will not work very well in an interupted cut like a 3 jaw chuck.

Best Regards Tom.

Scott logan's FAQ on this:

Wrong question. What you should be asking is "where can I find a Buck Adjust-Tru chuck to fit my lathe" (or Benard etc etc)

You might try new jaws..but most lathes will be lucky to give you .003-.005 true with an already turned piece of rework in a 3 jaw.

Or 5c or 16C collets...which would be my suggestion given your ability to score collets

Best to use a tool post grinder rather than trying to turn the jaws. They can be HARD in some cases.

You put a toolpost grinder, die grinder, Dremel or whatever you have on the toolpost. You can use rings and pins or putty or rubber bands to pull the jaws outward against the scroll, just as it would be when gripping a part. Then you use the carriage to run the grinder in and out across the jaws. I usually do this a few passes with the carriage handwheel, then start the lathe running and make a few passes with the power feed. You then disassemble the chuck to remove the grit that has sprayed all over the insides. Protect the lathe bed with wet towels (the water helps trap the flying grit.) We paper towels on cardboard work well, too.

This fixes bell-mouthing, ie. excessive wear at the tips of the jaws. The bell-mouthing allows work to wobble in the jaws while turning, and you get out-of-round work, generally with 3 lobes. If the scroll is damaged or not constrained to center by the chuck body, regrinding the jaws will not help. You may be able to insert a rolled brass shim to constrain the scroll to the chuck center.

I suppose you can do it with a carbide inset, but I've always ground them.

Jon

I use bernerd multi-size collets, they can handle hex and round stock. The multi-size collets have a longer grip than 5c or 16c collets. They are pricey but worth the money.

Best Regards Tom.

This is an interesting thread, 'cuz I learned from someplace (which I cannot now remember!) that you just don't trust 3-jaw chucks to not have runout -- if you want precision without runout you use a collet set, or you use a 4-jaw chuck and spend the time to dial it in each time you mount the piece.

By "run true" do you mean that a round piece has runout, or that it's tilted?

It is tilted, not parallel to axis of rotation. I would not care so much about just being parallel, but off axis

Years ago an apprentice was taught that a three jawed chuck was never expected to hold a work piece exactly "true" and one always started with a slightly oversized piece of stock and when the O.D. was turned to size it was, of course, "true".

The standard rule was that one never removed the work piece from the chuck until it was finished. One could however, usually remove the entire chuck with work piece from the lathe and reinstall it without problems

Are the Jaws in the correct slots ?

Is the chuck screwed on ? or is it a snap fit ? Is it clean and not pushing it aside ?

When I got my Dad's lathe, he had shipped it and just put on the chuck for rough work on something - never leveled it or such. I then was called and told to come and get it - Mom and Dad moved in with Sister and the Lathe was in the Laundry room... I come home with it and found a chip in the threads that shifted the chuck off center.

The swirl inside had a number of chips - I cleaned and greased. Put the chuck back on and it ran true. Cleaned the 4-jaw just for general purposes.

Might be a chip issue.

Mart> I am trying to finish up some unfinished business with my Clausing

Martin Eastburn fired this volley in news:8hgmt.145814$ snipped-for-privacy@en-nntp-15.dc.easynews.com:

(Did he actually call it a "swirl"? )

Lloyd

Old time machinists had a derogatory name for one not so good at the trade. They called him a "Three Jaw Specialist".

CP

20 years ago I trued the jaws in my 3-jaw chuck for my Reed-Prentice. I closed the chuck on a spider behind the jaws and used an air die grinder with a 2" long x 1" diameter stone mounted on the tool post. I took tiny-tiny cuts. The jaws are square and hold .002" easily. And, I haven't had to do it again. Also, I always do the final tightening on one key hole that happens to be the one CCW from the label. One hole will ALWAYS be the "ONE" on any chuck.

Ah, that might be different! What chuck mount is this? If D1, then there may be dirt between the spindle face and the chuck backplate, or between the backplate and chuck body, if built that way. After checking the interface between spindle and backplate, if the chuck can be separated from a backplate, just face off the chuck side of the backplate and then re-mount the chuck. Do the same with any mount where the chuck can be taken off the backplate. Face it until it turns the entire area. This provides a mounting plane that is entirely perpendicular to the spindle axis. Any good chuck should then mount precisely to that plane. Do all this before any grinding operation.

Any time a chuck is moved from one machine to another, you want to check for wobble (technically nutation) and face the mounting surface if required.

Jon

Jaws should not be turned---they should be ground, as they are usually quite hard, and won't turn uniformly as a result.

If you are not familiar with the prescribed procedure for preparing soft jaws, I'd suggest you do nothing until you are. In order for the end result to be useful, there are things you should do to ensure the proper results. One of them is to use a dedicated socket, as each one tightens the scroll differently, resulting in more or less run-out of the jaws. One's choice should be the one which yields better results, so it should be used in all cases, including tightening the jaws on the spider before grinding. This socket should be well marked, so it is the only one used.

If you have two piece jaws, I'd suggest you explore the use of soft jaws instead of screwing around with the hard jaws. They open the door to solving all manner of chucking problems, and can be relied upon to repeat within a half thou or less. If you're not familiar with soft jaws, I can provide a link that will give you a serious amount of information, helping you better understand how they are used. Please ask if you are interested.

Harold