4 Jaw Chuck Issues - Help!

[rwm]

If you have a tool post grinder you could preload the jaws in the chuck and regrind them on the lathe. If you don't have the TPG you could possibly rig holder for a die grinder to do the same.

Getting them preloaded is the tricky part. You could make 3 spacers and put between the jaws so that they press against each other.

Preload would be more important for a scroll chuck since all the jaw faces have to be ground together. It is important here to extent that the jaw could tilt a little under load. I am not sure how to achieve preload. I could potentially put a piece of wire in the small notch near the front of the jaw and put tension on it with a weight? As I said earlier, these jaws don't have much slop in the rabbet.

Edit: the way I'm gonna grind these, the weight of the jaw and the down force of the grinding wheel will add some preload. I think it will be sufficient to get within a couple of tenths.

I made a mandrel to hold my diamond wheel in the mill:


Here's how I did the rough grind:


Scary I know! But after this rough work the jaws look a lot better and will tightly hold a dowel pin

 

[rwm]

Who's taking bets?

 

[rwm]

Grinding with the diamond wheel was actually very easy. I got a good finish on the material. I did a rough test by clamping a dowel pin between opposing jaws. Definite improvement.
Now to mount this and do an actual test.

 

[rwm]

Actual test:

That test bar I know to be straight. I also have another piece of precision ground shaft that I confirmed with. Unfortunately, I get identical readings. I am seeing .005" angular runout at 6" from the jaws. I should note that runout measured at the chuck body is 0.000.
This is a DEFINITE improvement from the original condition when the angular error was visible to the naked eye!
However, in my book, I have taken a boat anchor and made it into a more precise boat anchor. The only problem now, is the shackle on my anchor chain will not fit through the center hole of this chuck. Alternatively, this would make a great wood working chuck...

Sigh.

So on to more expensive options.
A PBA chuck would fit my D1-3 due to the larger bolt circle. I cannot seem to find one easily. I think I may have found a way to mount a Bison chuck. They make a thicker back plate intended for use with Settrue chucks but since it is 1" think I could drill short mounting holes and use a shorter screw that would not go through the back plate and interfere with the spindle setup. Bison has good availability for 6" and 5" 4 jaws new and on eBay.
Again I am open to suggestions on a chuck if anyone cares to comment. Anyone have a Monarch EE?

 

[mksj]

Before you give up, I would try a die grinder in the cross slide, and lightly run it the length of each jaw. You are talking a very small amount of material to remove from each jaw. Worth a try. I also would check the run-out of the back plate and chuck face. There may also be an issue with how the jaw slots were milled. I would attach a test indicator to the cross slide indicate the jaws tips from the back to the front for each jaw. There are a number of vendors that sell semi-finished extra thick back plates, a lot less expensive then the PBA.

6-1/4" D1-3 Back Plate - Thick at Grizzly.com

<h1>SB1392 6-1/4" D1-3 Back Plate - Thick</h1> <p>Made thicker for use with adjustable chucks, this extra thickness allows a large step to be machined on the front of the plate for use with the adjusting screws on the chuck.</p> <p>The remaining material safely supports and secures the...

www.grizzly.com

 

[stupoty]

Like what rwm said. I have only seen the jaws ground flat , never concave. Unless you are chucking something up that is that particular diameter it won’t hold the part properly.

All the chucks I have , the jaws have a concave or convex depending if interior or exterior hold.

Might just be different manufacturers preference.

Stu

 

[rwm]

That's the exact back plate I am looking at. That should allow me enough space to mount a Bison 6". For those who aren't familiar, the D1-3 has a stud circle of 2.779" and the Bison chuck has a bolt hole circle of 2.75". With 3 and 4 holes respectively you cannot clock the chuck to avoid interference. That plate is over 1" thick so I can drill it partially.
I will do some measuring of the jaw slots. I fear they may be off also. It's a shame this didn't work out better. The jaws look really good as ground and the jaw faces seem really parallel to each other. Just somehow the angle is off. It doesn't make sense that the faces are parallel to a few tenths but the angle is off by about 1 thou? (5 thou over 6" is about 1 thou for each 1-1/5" long jaw.) Given the method I used there are a lot of possible errors!
I have an 8" Bison and the teeth are convex. This is a problem if you are deliberately trying to offset a work piece to create and eccentric part. It's great if you are trying to dial in a shaft precisely.

 

[rwm]

I made some measurements of the jaw slots.

I set the DTI to 0 at the inside edge of the slot.
Here is the middle:


Here is the outside:

I would say this is a valid reference surface and .004 of variation is not acceptable.
That is jaw 3 which is the worst. Jaw 1 has about .002 error. Jaws 2 and 4 are .001 or less. Still I think this is part of the problem. I also think jaw 3 will change its angle depending on where is is located in the slot!.
FYI, runout of the back plate and chuck body are 0.000

 

[rwm]

When I measure the jaw faces installed they are definitely off front to back by up to .002. I am not sure why but something was wrong with my grinding geometry. Because I ground from the outside, any error in the slots would be doubled when I reverse the jaw? My 90 deg angle plate could also be off. I do not have a great way to check it.
I may give this one more shot using the diamond wheel, in situ on the lathe. If I grind one jaw at a time I think I can get into all the teeth. Plus the fine coating of diamond dust will be protective of the lathe surfaces. (actually the diamond loses a lot less grit than standard grinders)

 

[mksj]

Agree that it appears the issue is the jaw slots are probably the issue, but in theory if you mount the jaws and lightly grind them in there respective slot it should improve the TIR. Most likely larger stock will not have a 6" stick out so maybe try to touch up the jaws at something like 2" diameter opening may be sufficient. Anything sticking out 6" or more one would use a center in the tailstock for support or a steady-rest. I have had stock Chinese chucks that had a TIR of 0.007" at the chuck, and worse further away from the chuck, so a TIR at 0.005" at 6" isn't terrible. There could also be some element of headstock alignment.