[Vern Smith]

This comes under the heading of probably not doable but I figured if there was anyone who did it they would be here. I have a 3 jaw on my lathe that runs out about 4 thou. (It is not a set true chuck). I would like to true up the hard jaws if possible. Is there some type of boring bar/stone/file/etc. that I can put in the turret and take a couple of fine passes to true up these hard jaws?

I know I will have to preload the jaws but I can easily make up a three pointed star affair to put behind the jaw faces and catch the bottom of the scroll feet. This is all my off hand terminology, probably not proper machinists lingo.

Vern

[Donkey Hotey]

You might want to check a few things before proceeding:

• Take the chuck apart and make sure everything--especially the scroll--are clean and oiled. A chip caught in there could be causing the problem.
• Make sure that it's out across the range. Clamp a variety of diameters to verify this. I've seen chucks where the scroll was the problem and it was out more in some diameters than others.

After that, yes, a friend did the very thing you're talking about. He clamped a hand die grinder on his toolpost, spun it up, then went inside and kissed the faces of his jaws as the chuck was spinning. Of course, this made a grinding mess that he had to be careful to clean up but, it did the job.

[HelicopterJohn]

Hi Vern,

My question would be if you grind the jaws with a piece of material in the rear section of the chuck jaws to hold proper tension during the re-machining operation and say that piece is 1.0 inch in diameter, would that accuracy still apply to working with different diameter stock. I realize that after the re-machining the rear section of the jaws would have to be slightly undercut.

Perhaps someone else here could shed some light on the principals of the scroll chuck and if indeed that scenario might??? be applicable.

John

[Geof]

As suggested, take the chuck apart and clean it well. Check the mounting plate for embedded chips, burrs, etc. Check the run out on the mounting plate. Make sure the jaws are indeed in the correct numbered slot. If there are three bolts attaching the chuck and no dowel pin it could go on in three positions; check all three.

If, after doing all this playing around you cannot get better than 0.004" then consider trying to true the jaws...incidentally is that 0.004" TIR, in other words the chuck is 0.002" off center or is it 0.004" off center.

If it is 0.004" TIR you may be better off living with it. It is difficult to load the jaws correctly to grind them because a ring at the back does not load the front. Also it is almost certain to vary with diameter as the scroll will have some wear.

[Geof]

There are actually a couple of possible work arounds for a worn chuck that is not gripping true; both of them requiring judicious use of a hammer, preferably dead blow.

First one is to simply true the jaws while they are gripping the work piece. Clamp things up tight, locate the jaw nearest the high point and give it a good tap. There is some clearance between the scroll and the chuck body and you will sometimes move things a thou or so. Don't be too worried about giving a fairly healthy tap but don't do it directly with a regular hammer, use a brass or aluminum drift.

Second approach if the chuck is bolted to a backing plate with a locating spigot is try tapping the entire chuck body with the clamping bolts only gently tight. Again there is some clearance and it is possible sometimes to get a thou or so of movement and then pull the bolts down tight.

In both cases if you then take a hulking great intermittent cut you will probably find things relax back into the out of true condition but these 'fixes' can work for light work.

[Vern Smith]

I appreciate the help. I kind of boxed myself into a corner with this deal. I added an aftermarket 3 jaw to my GT-10 (A2-5 spindle) and had to use an interface plate supplied by the chuck manufacturer (Bison). In a fruitless effort to get the thing to run true and stay there I put the interface plate on the mill, probed it in, and located 2 dowel pins in it. I then did the same with the chuck and reamed holes .001 larger to locate the chuck on the plate. So ---- hammering on it should not move things much Of course, all this assumes the plate OD was true to aligning taper in it, that the OD of the chuck was true to the scroll/jaw location, and that I did not screw it up somewhere.

I thought about gluing my Dremel tool to the turret and grinding a little and that's starting to look like the best solution.

Everything was brand new but the scroll might be off as well but I would hope their tolerances were better than that.

Vern

[luv2pla]

Mine was running 5 thou. out. Then some one had a bright idea to grind each jaw individualy. Then it ran 20 thou. out.

I ended up boring the hard jaws out with a PCD insert. Just pre-load the jaws at the tips and take light passes. It work like charm and running under 1 thou.

[roundman]

You could use machinable Jaws also for a specific job and number them so that the next time you use those Jaws it will run true for that size. I mark my jaws with customer part number. Our sharpe lathe chuck has an issue it runs out about .005 thousanths. There were times when I needed to hold a part tir much beter than that and so I shimmed one jaw to bring it in. It is one jaw that is off. If I cant get it like that I also have used the 4 jaw chuck on it. I think the time may come when we order a better chuck. (yeah buddy) When I bore my jaws for a specific part I also write on the jaws (besides #1 2 or 3) I write what diameter it is cut for.