4 Jaw Chuck Issues - Help!

My headstock has been measured with an MT3 Bar and is perfect. My 6" Bison 3 jaw has very little angular runout on this lathe (I forget the exact number.)
You are obviously correct that longer items will probably have tailstock support except in the unusual situation of turning an eccentric. I like your idea though and I think I will give it one last try. I appreciate the encouragement!
 
If all four tracks are angled the same, then doing a dremel/TPG grind from the inside should do the trick. If they are different angles you may have to mark the jaws and/or have a "least inaccurate: range near the diameter you ground at.

BTW, a 3d printed spacer that catches the angled sides of the jaws is great for preloading.

I would put a big pin in the jaws, center it, then put an indicator on one jaw, open it to pull the pin, and return it to where it was before using the indicator.

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Weldingrod- Preload would ideally be applied at the front of the jaws. Not possible when grinding. Applying preload at the back of the jaws would cause increased error due to jaw tilt in the wrong direction.
It's not a scroll chuck the position of the jaws when grinding is not important. I am not going to rotate the lathe spindle while grinding, just work on each jaw individually. I could do it as you suggest, I am just not sure it is necessary. I was trying to avoid a convex grind since it would have a pretty small radius no larger than the center hole of the chuck.
I do appreciate everyone's help and suggestions.
I am pretty discouraged about this chuck at this point. I think I am going to put reworking this chuck on the shelf for a rainy day and work on a new chuck.
 
I had a problem similar to yours with my Grizzly four jaw. Runout at a distance from the chuck was unacceptable and it could be changed by tapping the workpiece with a hammer. I expect that the jaws are ground in an independent fixture as jaws from Grizzly are replaceable as a separate item. In your case, since your setup is identical for all four jaws, the isuue may be with the ways on the chuck. If they aren't found perpendicular to the spindle axis, you can't correct the problem by grinding the jaws externally. If ground in situ and the jaws are always installed in the same location, it should be possible to correct the problem.

Here is how I corrected my chuck. I made a ring to capture the 45º sides of the jaws and ground all four jaws at once. My wobble went from .014" to .0001".
www.hobby-machinist.com

Grinding My Jaws

I recently took a critical look at my Grizzly 6 -1/2" 4 jaw chuck and found the jaw grind to be seriously deficient and I decided to try to regrind my jaws. I have a Craftsman 1/4" die grinder that I would press into service but I needed to come up with a stable mount for the grinder. My...
www.hobby-machinist.com www.hobby-machinist.com

If you desire to have flat faces on the chuck, you can grind the jaws in situ which will ensure that they are parallel to the spindle axis and them remove the jaws and grind them flat using the bottom of the curve as a reference. Blue the surface and grind so that all traces of the bluing are removed. You should then have a flat surface which is parallel to the spindle axis.
 
Is the back plate squared to your lathe? I'm surprised that the ways for the jaws are so far off on a select-grade import like that.
 
RJ- That is super clever with the cutout plate! Thanks for sharing. Yeah, I think the slots in the chuck are too far off to go crazy grinding the jaws. I could probably get closer but I would never like using this chuck.
I'm wondering if your chuck is the same as mine rebranded?!

I'm looking at this:
www.ebay.com

EXCELLENT! SOUTH BEND NO.PT2781 1 4 JAW 6" LATHE CHUCK 4"D1 MADE IN GERMANY | eBay

4 JAW 6" LATHE CHUCK. IT IS MARKED ON THE BACK 4"D1. ~ SOUTH BEND ~. MADE IN GERMANY.
www.ebay.com

I'm a little afraid to stay with South Bend but I am thinking that chuck is made by Rohm. Looks too similar. It looks to be a D1-4 mount so I would have to swap back plates.
 
My chuck came with the lathe and it, along with my three jaw, have a stylized "H" logo.
 
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I am also considering a Bison 6". Like this:
1690760352338.png

1690760114874.png
Regarding mounting this. Is the reference surface that sits on the back plate the inner recessed area or the outer area? Given that the holes are in the smaller area I would assume this should touch the back plate? Otherwise you might "dish" the chuck or back plate.
 
rwm said:
I am also considering a Bison 6". Like this:



Regarding mounting this. Is the reference surface that sits on the back plate the inner recessed area or the outer area? Given that the holes are in the smaller area I would assume this should touch the back plate? Otherwise you might "dish" the chuck or back plate.
Click to expand...

The critical surface is the face of the deep bore, it is all that matters other than concentricity. The rest of the back face does not need to be supported, unless your mount pattern requires more room- With those, I leave a thou or five of air so nothing influences the chuck's axis.
 
Thanks Pontiac! I have a 6" back plate. So I should turn a boss down so that if fits inside and touches the reference face where the bolt holes are? I will leave a small air gap for the rest or maybe turn it down to 4-1/2". I hate cast iron dust...
I'm gonna have to buy a bottoming tap for the mounting holes that do not go all the way thru.
 
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