Subject: Re: Fixing 3-Jaw Chuck?

From: [email-masked] (Gordon W J Sorensen)
Date: Mon, 19 Jul 1993 19:50:43 GMT

Bob Neidorff ([email-masked]) wrote:
: 
: I have a Logan lathe with a 3 jaw chuck.  The chuck is in bad
: shape, and the lathe is in slightly better shape.  I bought it
: a while ago, used and abused, and have been bringing it up slowly
: over the past two years.
: 
: When I chuck a 1" rod, it wobbles a bit (20 mils on 8" end).  I assume 
: that the  problem really is the chuck.  I have replaced the arbor ball 
: bearing cartridges, and it spins fairly smoothly now, with no play.

3-jaw chucks or any none independent jaw chuck will always have some run out.
Very expensive chucks are better then cheap ones, although .020 inches is a fair amount.  Having replaced the headstock bearings you would have introduced some
run-out to the spindle but not .020.

Assuming the run out on the spindle is not excessive less than .002 your problem
is in the chuck itself or how it is mounted to the backing plate.

: 
: It is also possible that the lathe bed is twisted, but I don't
: think that this would cause the chucked part to wobble.  I have
: confirmed this wobble with various rods.  I don't own a precision
: level, so I haven't measured the bed yet.  I was going to try to turn
: a 12" shaft at two points and compare the diameter, but the wobble is
: so bad that it would be a waste of time.
: 
: Can chuck jaws be ECONOMICALLY remachined or replaced?  I don't
: know the brand of the chuck, but the lathe is from 1953, so the
: chuck is also probably 40 years old.  The chuck is a 6" three-jaw,
: very conventional looking.  

Assuming that the chuck has not been damaged internally (ie,. the scroll is not 
messed up) there are things that can be done to true it up.

: 
: I haven't tried to remove the jaws.  I assume that they just fall 
: out if I twist the tightening screw all the way.  They are numbered, 
: so I assume that I can get them back in the right spots.  

I would dissassemble the chuck so at least you can see if the scroll is okay and
the jaws are not damaged. Make sure that jaw #1 is in slot #1 and start with the
#1 marked jaw first.

The second thing to look at, is to check the spindle run out on the surfaces 
where the backing plate of the chuck mounts to the lathe. If you have .020
run  out there than you have a problem with the headstock.

My guess is that the backing plate was not mounted properly or it was originally
for a different lathe.

If there is sufficient material in the backing plate it can be trued up for your
lathe and the chuck remounted on it. You can do this by removing the backing 
plate from the chuck and intalling it minus the chuck on the lathe.  Make 
sure that all mating surfaces are clean and free of dings, nicks etc. Then
face off the backing plate and re cut the stepped mating surface for the 
chuck.  By doing this you will machine out most of the misalignment of the 
mating surfaces that are part of your lathe and backing plate.

Remount the chuck and see if the run out is improved.  

Chuck repair gets a bit more sticky. If the internals of the chuck are okay 
but somebody botched the back side, you can true this up using another lathe.
Chuck a hefty piece of steel shafting in the second lathe (~2" diameter) and
do a clean up pass to make it true.  Then take your chuck and clamp it on the
stub piece of shaft that you just cleaned up. Make sure everything is nice 
and clean and TIGHT. Now true up the back side of the chuck (I am assuming that
the chuck is not hardened steel) and then refit it to the backing plate.

The only other thing that can be done is to regrind the jaws, which is a bit 
tricky as they have to be preloaded and still give access to the grinding wheel.

Hope this helps.

: 
: I install #1 first, right?
: 
: I've been thinking of making a 5C collet adapter for this lathe.
: Perhaps this is the motivation I need.  :-)
: 
: Perhaps I should take a dial indicator to the bare spindle first.
: I guess that the arbor itself could be bent a slight bit.
: 
: Anyone have thoughts?  Advice?
: 
: Thanks for your patience with me.  This has been a real learning 
: experience, and I'm still learning.  Lots of learning to go!
: 
: -- 
: Bob Neidorff; Unitrode I. C. Corp.  |  Internet: [email-masked]
: 7 Continental Blvd.                 |  Voice   : (US) 603-424-2410
: Merrimack, NH  03054-0399 USA       |  FAX     : (US) 603-424-3460

---

From: [email-masked] (ron ginger)
Date: Tue, 20 Jul 1993 12:24:07 GMT

I wont include all the previous reply, but it is an excellent description of 
fixing up the back side of a chuck.

However, my old Logan chuck was OK on the back side, but buggered on the jaws.
I improved it by the following.

First I needed a tool post grinder. I made a wood block to bolt to the compound
and bored a hole thru it to fit my Dremel tool. I then clamped the Dremel in
the block with a 1/4" dia grinding wheel.

The chuck jaws must be loaded while grinding. I took three ground all over
1/4" square tool bits and placed them between the faces of the jaws. The jaws
are beveled on the ends, so that they can come together when closed way
down. The three tool bits lay on the face of the chuck, I used some duct
tape to hold them while clamping up. With them in place I could tighten the
jaws, but leave the center hole open. 

Using the Dremel, while also rotating the chuck at a moderate speed, I was able
to clean up the clamping face of the jaws. Obviously the Dremel is a VERY
light duty tool, so cuts were tiny and slow, but it did improve the chuck
a lot.

Id guess a 3 jaw with .002-.003 runout is about as good as they get.

ron ginger

---

From: [email-masked] (Bob Neidorff)
Date: Tue, 20 Jul 1993 13:29:07 GMT

Well, I have received very good suggestions so far.  Ideas like
chucking a solid rod in the BACK of the jaws and using a tool post
grinder or Dremel tool to grind the jaws.  

I have also had an offer of a used chuck if I want it.  However,
I'm too pig-headed to give in that easily.

If you have patience, could you read on and give me more advice?

I spent some time with a dial indicator and the chuck last night.
First I put the dial indicator in the tool post and looked at  the
threaded nose.

------
       \
        |A
HEAD    |----/\/\/\/\
                     |
                     
                     |C
        |----\/\/\/\/
        |  B
       /
------

I measured runout on spots A, B, and C, in the above picture.  Both
A and B ran better than 0.0002" on my dial indicator, which I considered
within my measurement ability.  Point C was a bit worse, but not much.
Perhaps 0.0005" runout.  Again, this was a good plunger dial gage with a 
solid steel round point, so there was some friction interfering with 
the measurement.  It would have been better with a pivot-point dial,
but I don't have one.

I suspect that the above measurements tell me that my "nose" is good.
Comments?

I then looked at the chuck.  I was marked Logan 430, Made in USA.  There
were three jaws, and they were very clean.  Not badly scored or gouged.
Either virgin surfaces or work-polished.  The jaws were numbered 1, 2, and
you guessed it, 3.  They were shoved in the wrong slots, too.  Here's a
problem I know how to solve!

I moved them to their correct slots and chucked a 1" mandrel.  Now my
dial gage indicated 0.014" out of true along the entire length of the
mandrel.  Not good, but perhaps better than before.  I wish I had
taken the runout measurement before moving the jaws.

So I still have a bad chuck.  However, I wonder.  The chuck is bolted to
a threaded back plate with six allen screws.  

Could I perhaps align the chuck better by loosening the screws and 
repositioning it on the plate?  Or just rotating the chuck on the back plate?

Is there a right position for this sort of thing?  Or is it perfect in every
position?  Is there a good procedure for positioning a chuck correctly?

If the previous owner (a high school) couldn't put the jaws in the right
slots, there's no telling what else I will learn.  I have found other parts
of the lathe assembled wrong, such as a shaft inserted into the gear box
backwards that prevented access to some of the gears.

Again, thanks to all who have advised me so far and thanks for any more advice!

-- 
Bob Neidorff; Unitrode I. C. Corp.  |  Internet: [email-masked]
7 Continental Blvd.                 |  Voice   : (US) 603-424-2410
Merrimack, NH  03054-0399 USA       |  FAX     : (US) 603-424-3460

---

From: [email-masked] (gary preckshot)
Date: 20 Jul 1993 15:36:09 GMT

In article <[email-masked]>, [email-masked] (Bob
Neidorff) wrote:
> 
> 
> I have a Logan lathe with a 3 jaw chuck.  The chuck is in bad
> shape,
[stuff]
> When I chuck a 1" rod, it wobbles a bit (20 mils on 8" end).
> I assume 
> that the  problem really is the chuck.

I wouldn't.

> Perhaps I should take a dial indicator to the bare spindle first.
> I guess that the arbor itself could be bent a slight bit.
> 

That's the first thing you should check.  If the spindle is bent, your
options are limited.  Your new bearings will probably not last long in this
case.

> It is also possible that the lathe bed is twisted,

But it wouldn't cause lack of concentricity, which is what you report.  By
the way, the bed probably is twisted and may be worn just in front of the
headstock.  Lathes of the approximate vintage of yours had to be set up
with leveling jacks to remove stress on the bed.  You should assume bed
twist until you prove otherwise by set up with a precision level.

> 
> Can chuck jaws be ECONOMICALLY remachined or replaced? 

It's probably cheaper to buy a bare-back two-piece jaw import from Enco and
cut your own backplate.

> Anyone have thoughts?  Advice?
> 

Check the spindle first.  If it is true, then check the jaws for true
closure and hold on an accurately round test piece.  The jaws should be
tight and you should not be able to detect any looseness.  If you do, then
the jaw ways or the mating grooves in the jaws are worn and the only
economic way to fix it is to buy an new import chuck.  Keep the old one for
rough work.

If the jaws are tight, the scroll may be worn and the jaws are closing
off-center.  The economic solution to this too is a new chuck.

If the spindle is ok, the jaws and the scroll are ok, then the backplate
may be bent or may have dirt or junk wedged in the threads so as to be
driven off-center.  The solution is to clean out the junk, or to buy a new
backplate blank and turn a new one.  This is relatively cheap and you may
even be able to get a large junk steel round from your local surplus metals
outfit for 10 cents a pound or so.

G.

---

From: [email-masked] (gary preckshot)
Date: 20 Jul 1993 15:50:17 GMT

In article <[email-masked]>, [email-masked] (Bob
Neidorff) wrote:
[stuff]
> I'm too pig-headed to give in that easily.

Man, unlike animals, sometimes is able to learn from other's experience. 
Sometimes.

A new chuck would cost about $80 from Enco, and you could get hardened
two-piece jaws.  Believe me, it isn't worth the hassle trying to fix a
chuck.  The results won't be that great and the effort will be.

> I suspect that the above measurements tell me that my "nose" is good.

Yes, your spindle is true.

> problem I know how to solve!

Not unless you can regrind the scroll and the jaws.  The scroll has been
abused by teeny boppers and there's no way you'll get it right again.

> So I still have a bad chuck.

Yes, you do.

> However, I wonder.  The chuck is bolted to
> repositioning it on the plate?

Not if the mounting was done right in the first place.  The backplate
should have been turned to fit exactly to the back of the chuck body.  Buy
a backless chuck and adapt your current backplate to that.  Or get a
backless chuck and a backplate blank and turn to fit your Logan spindle. 
You can use the old chuck to hold the blank for this because you will spin
the new backplate on the spindle and turn to fit the back of the new chuck
in place, thus getting perfect accuracy.  A new 3-jaw should center within
.001.  With hardened two-piece jaws you'll have a better deal anyway.

G.