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Using Brass Round to Lap Chuck Jaws

I was reading up on lapping chuck jaws and came across the thread about using a brass bar and valve lapping compound to do the job. I want to make sure I understand the process before I try it.

1) Get a brass rod 18" long and 1.25" in diameter. Face and center drill both ends.

2) Put a dead center in the spindle and another in the tailstock.

3) Center both ends in the dead centers and turn a section long enough to make sure that it is perfectly parallel to where the chuck jaws would clamp onto it.

4) Tighten the jaws onto the brass bar to where you can barely turn the rod by hand with the rod still being held between centers.

5) Using a screwdriver paste on some 320 grit valve grinding compound. Turn on the lathe at one of its slowest speeds and grind the jaws until the bar loosens up on the jaws.

6) Tighten the chuck jaws a little and grind again.

7) Grind until you can see grind marks all along each jaw.

Chuck up a drill blank of the same diameter you used to turn the jaws and check for runout.

Is this correct?

Where do I get the adaptor to hold the dead center in the spindle? Would it be possible to use a smaller lapping bar and use a collet to hold the dead center with the draw bar and then just knock the collet out after use using a knock bar though the spindle?

Thanks, Steve

Reply to
Steve
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In the procedures I have read previously, it is necessary to apply a clamping load to the jaws of the chuck. This normally consists of a ring of metal clamped by its internal diameter on the steps of the chuck jaws. This leaves the insides of the jaws free to be lapped. If the jaws were lapped without a calmping load applied, then any wear between chuck jaws and scroll would allow the jaws to move apart, and cause uneven lapping.

HTH

Anthony Remove eight from email to reply.

Reply to
Anthony Britt

In the procedure he mentiones (which I had never heard of before, btw) there *is* a clamping load applied. By the jaws bearing down on the brass stock.

It actually sounds like it might work, given that the rod is constrainted by the two centers.

For the original poster, the reason you don't want to use collets is that the closer will probably interfere with the chuck's mounting on the threads of the spindle. Depending on what taper is inside your spindle bore, you can obtain MT adapters that go down to the largest size dead center you have.

Use the largest diameter brass rod because the rod will wear some as well.

The ultimate innaccuracy in your chuck is probably due at least in part to the wear in the scroll plate inside the chuck. Lapping the jaws will make it center true at that one diameter that the procedure is done at, but the wear will still be present in the scroll so chucking other diameter stock will still show an error - of some magnitute. I doubt you will get a worn chuck to do better than five thou TIR over a range of clamp diameters. If your chuck is running out ten or twenty, then this will make things better.

Be sure the backplate and mounting of the chuck to same are in good order before doing any of this, of course.

Jim

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Reply to
jim rozen

Problem one. How do you ge the bar accurately center drilled if your chuck is shot? I guess you can still do it with a steady rest.

Yeah, this is going to be a SLOW process. You may need to cut down .010 to .020" (.25 to .5 mm) on badly bellmouthed jaws. Mine were like that on one old chuck. This could take DAYS with lapping compound. If the problem is only one jaw being a little high or low, it might not be such a big job. But, that is most likely due to chips in the scroll, not wear.

Most lathes accept a dead center of the right taper directly into the spindle. You just tap it in with a lead ball.

A far better process is to adapt a grinding tool (toolpost grinder or Dremel, etc.) to the toolpost and grind the teeth. You can even cut the teeth with standard boring bars.

Jon

Reply to
Jon Elson

He was turning a diameter in situ. This would give an accurate diameter, concentric with the spindle axis, to lap against. I thought this was one of the nicer features of this approach.

Too bad I tossed out all my clapped out three-jaws, I would have tried this.

:^)

Jim

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Reply to
jim rozen

It sounds like it would take many hours, With a decent grinder already adapted to the toolpost, it only takes 15 minutes or so to true badly worn jaws, and maybe 5 minutes to do some with light wear.

Jon

Reply to
Jon Elson

I used this technique to lap in a 3 jaw that had worn jaw tips but was a good chuck otherwise. It worked very very well. I had previously tried to bore the jaws as was suggested here and the boring bar would not touch the chuck jaws.

I recommend using the largest brass round stock that you can get into the chuck because your jaws will be curved not flat. When you grip something larger than your brass rod, it will be grabbing on the edges, not in the middle of the jaw.

Also run the spindle as slow as possible to keep from throwing grinding compound all over. I think I used a fairly course grit, probably 180. Can't remember if I also used a fine grit afterwards.

chuck

Reply to
Charles A. Sherwood

Yeah, but how long would it take to clean the grinding dust out of the chuck and any that may have fallen on the lathe's ways?

Bob Swinney

Reply to
Bob Swinney

Here I'm figuring that you are commenting specifically about using a toolpost grinder to to the jaws - but of course, using grinding compound to lap them in has a similar downside. Crunchy goop all over the sliding parts. :(

I guess I've never seen that holy grail of worn chucks, the kind with all the wear concentrated at the tips of the jaws. All the ones I've ever come across look like they've been run over by a train, the scrolls are half gone, and the jaws flop around in the body like loose teeth.

Jim

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Reply to
jim rozen

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