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

"Steve" wrote in message
om...
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.
snip

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.

In article , Anthony Britt says...

In the procedures I have read previously, it is necessary to apply a
clamping load to the jaws of the chuck.

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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Steve wrote:
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.

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.

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.

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.

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?

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

In article , Jon Elson says...

Problem one. How do you ge the bar accurately center drilled if your
chuck is shot?

3) Center both ends in the dead centers and turn a section ...

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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jim rozen wrote:
In article , Jon Elson says...


Problem one. How do you ge the bar accurately center drilled if your
chuck is shot?


3) Center both ends in the dead centers and turn a section ...


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.


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

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

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.

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

"Jon Elson" wrote in message
...


jim rozen wrote:
In article , Jon Elson says...


Problem one. How do you ge the bar accurately center drilled if your
chuck is shot?


3) Center both ends in the dead centers and turn a section ...


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.



Jon

In article 8hTwb.232368$ao4.831956@attbi_s51, Bob Swinney says...

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?

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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