# Chuck jaw truing



## abunai (Apr 1, 2018)

I have an old chuck that needs the jaws to be "trued".
I was looking at different ways to pre-load the jaws.
Looking through a surplus catalog, I saw something called a "Lovejoy Coupler"


Looks like it would work with a little modifications.
Has anyone tried to use this?????


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## Ulma Doctor (Apr 1, 2018)

that may be a good idea.
not to discourage you in any way, but,
a point to consider is that the scroll may be worn in different areas, thereby giving a variable grip that may translate to differing runout measurements on different OD's of material.

how much runout are you experiencing?


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## abunai (Apr 1, 2018)

Problem I'm having is the jaws don't grip near the top.
Project I'm working on needs to clamp thin round tube, with a mandrel, at the top of the jaw.
I'm thinking the jaws are not parallel anymore. 
It's an old chuck.


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## Ulma Doctor (Apr 1, 2018)

then you have nothing to lose, give your idea a shot!


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## benmychree (Apr 1, 2018)

The jaws should be clamped down on something at the front of the jaws, not the back; if you do it at the back and the jaws are somewhat loose in the chuck body, you will end up with the same situation that you have now, maybe less, but the same.  If your chuck has solid (not two piece) you can drill holes in the end face of the jaw with a carbide drill, insert pins into the holes and clamp down on a steel ring and reach through the hole in the ring to grind the ID of the jaws; if you have two piece jaws, short pieces of allen wrench can be cut and inserted into the allen bolts and likewise clamp down on a ring.


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## Ulma Doctor (Apr 2, 2018)

i was able to put a narrow ring at the rear of the jaws and take very light cuts with a dremel tool and diamond encrusted die grinder tool ,mounted in the toolpost.
it reduced my runout from .020" to .004"  as indicated on a .500" endmill stub (it took forever to grind!).
i didn't check runout on different diameters, but i felt it was a dang good improvement and lived with it until i traded the lathe


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## f350ca (Apr 2, 2018)

A buddy Dave made these to clamp down on while leaving the jaws exposed. He lent them to me to do mine.


There is a small lip at the ends of the radius that catches the jaws.
Ideally you should be clamping at the end of the jaws, but where these are your in front of the scroll so the jaws will tip out.
This chuck was worn at the ends of the jaws as you describe yours. After grinding the jaws gripped evenly and runout was improved.
Beter chucks have one pinion marked, that is the one used when they grind the jaws at the factory and should give the best runout when using it to tighten. Use it when you grind yours.
Wonder if the ends wear or do they get stretched from the force of material expanding as it heats when turning?

Greg


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## abunai (Apr 2, 2018)

This is where I saw the idea.  
By using the "lovejoy" it would save me the trouble of making one.




Drilling holes is a good idea. But with this, I could use it on other chucks and not have to go through the trouble drilling.


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## RJSakowski (Apr 2, 2018)

Here is the spacer that I made for grinding the jaws on my 4 jaw chuck.  I used 1/4" steel plate.  A similar design could be used for a 3 jaw.


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## Boswell (Apr 2, 2018)

just curious, how strong does the "pre-load" spacer thing have to be? It would be quick work to make one on a 3D printer if it could be made from PLA (plastic).


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## Kernbigo (Apr 2, 2018)

i use 3 gage blocks


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## abunai (Apr 2, 2018)

All great ideas.
Boswell brought up a good question. 
I wonder how hard it need to be???
I was also wondering how even the fit on the three jaws needs to be????
I would think it would need to be as even as possible, so one jaw is not flopping around.


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## tertiaryjim (Apr 2, 2018)

This uses 1/4" flat stock and the circle just keeps the spacers square to the chuck face as well as acting as a safety so things don't get exciting.
Nothing fancy is required, but there needs to be even tension between the chuck jaws so the widths of the spacers might vary  just a bit.
That depends on the way the chuck is stressed.
Notice how the pictures and explanations of the members all have the spacers at the outboard end of the jaws.
The chuck should be "fully" disassembled and cleaned before
	

		
			
		

		
	



	

		
			
		

		
	
 and after the operation so chips and grit don't cause a problem.


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## abunai (Apr 2, 2018)

That is nice.
I ordered a Lovejoy off of evilbay.
We'll see if it works


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## abunai (Apr 10, 2018)

looks like it will not work. 
Thought I could get away easy. 
The fingers need to be wider. These are way to narrow.
Back to the drawing board.


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## Kernbigo (Apr 11, 2018)

i use 3 square gage blocks held in place at the relief angle on the jaws


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## abunai (Apr 13, 2018)

Maybe I'm looking at this wrong.
The jaws are true. Not enough run out for the work I do.
Problem I'm having is the outer part of the jaw will not hold thin material.
I am trying to hold a thin tubing, with a mandrel for support. 
Because the jaws don't hold all the way out, the tubing sometimes spins in the chuck.
This is why I think the jaws are not parallel when clamping towards the outside.
Does this make sense????


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## tertiaryjim (Apr 14, 2018)

If the jaws aren't parallel, they aren't true.
If the thin tube is supported with a mandrel inside of it, the jaws should be able to clamp on it even if they don't run true.
Unless; the back of the jaws are closing tight on the mandrel and the front of your jaws are still loose over the tube. Lotta warp there.


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## f350ca (Apr 14, 2018)

abunai said:


> Maybe I'm looking at this wrong.
> The jaws are true. Not enough run out for the work I do.
> Problem I'm having is the outer part of the jaw will not hold thin material.
> I am trying to hold a thin tubing, with a mandrel for support.
> ...



Thats the exact problem I had. Grinding the jaws fixed the chuck.

Greg


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## Kernbigo (Apr 14, 2018)

Your jaws are tapered, worn at the outer edge, they need grinding. Use 3 gage blocks at the relived angle of the jaws to lock them in place.


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## MrWhoopee (Apr 14, 2018)

abunai said:


> Maybe I'm looking at this wrong.
> The jaws are true. Not enough run out for the work I do.
> Problem I'm having is the outer part of the jaw will not hold thin material.
> I am trying to hold a thin tubing, with a mandrel for support.
> ...



It's called bellmouth, and it's common enough that it has a name. The jaws/scroll have been tweaked/worn from clamping parts at the outer end. It may not be apparent unless the jaws are loaded (under pressure).


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## abunai (Apr 14, 2018)

Bellmouth.  Makes sense.
Question.....
Does it make a difference if I hold the jaws open at the bottom or top?????
I would think that if it's bellmouth, you would want to hold them open at the top.


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## NortonDommi (Apr 14, 2018)

I've posted these before, an oldie but goodie.


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## markba633csi (Apr 14, 2018)

I watched a YouTube recently where a guy was truing a Rockwell's 3 jaw by using some rubber pieces jammed in there- I wish I knew where the link
was


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## ddickey (Apr 14, 2018)

Here you go.


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## abunai (Apr 20, 2018)

That is interesting.  I'll give it a try.


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## abunai (May 13, 2018)

Haven't had time to work on it.
Need to make a jig to hold the grinder first.
Thinking of using a Dermel. But i don't know if the stone shank is long enough.
Any ideas????


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## C-Bag (May 13, 2018)

There is a very knowledgeable gentleman named olderdan that posted this on another site for putting tension on the jaws to grind them true for curing bell mouth.

He made them for 3 jaw, and 4 jaw.




His cure for runout was .008" relief in the backplate register. I have yet to do the process as I'm still making the mount for Foredom grinder. But I believe this is the way I'll go when I get time to finish the mount.


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## KBeitz (Sep 2, 2018)

I had to true up some new chucks for a business where I worked at.
I took 3/4" bearing stock and slathered it up with valve grinding compound.
I fixed the bearing stock so it could not turn and lightly tightened the 
chucks jaws down on to it. I would turn the lathe one and let it run for
about one hour. It dd a great job. Make sure to disassemble the chuck
and wash out all the grinding compound when done.


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