Lathe Chuck Mystery

[G-ManBart]

Edited to correct the jaw numbers and add the clarifying question at the end.

I know this will be a lot of reading, so I'll apologize in advance! I've only got $85 total tied up in this, so I'm not at all worried about the outcome, just curious about what's going on. Here's the scenario:

I recently bought a used Buck model 4831 3-Jaw Adjust-True chuck (8"). It was at an auction I was able to preview and saw it was a Buck AT...the auction pictures didn't show the brand, and just said 8" 3-jaw chuck so I got it cheap. It came with inner and outer hard jaws. The inner hard jaws obviously came from another chuck as they were marked 1, 3 and 5, with the #3 being in the 2nd jaw slot and the #5 in the 3rd jaw slot, so I'm assuming they came from a 6-jaw chuck. I couldn't see this at the preview as it had dried cutting oil and dried dirt.

I had a backplate that would work, so I fit the chuck to the backplate, adjusted it to zero the runout and was okay with the results. The inner jaws have a trace of bell mouthing so I could only get the runout down to like .0015" which is three times what I get on my other 4831 that's on a different lathe. The outer jaws show runout well under a thousandth, so those are good and appear to be the original, correct jaws for this chuck.

While adjusting the runout I noticed I had to use pretty much all of the range available in one direction but didn't think much of it as I only left .010 clearance (diameter). When I ran the lathe with the inner jaws mounted (holding a piece of stock) I got a significant vibration at around 800rpm. If I slowed it to 700 or below, or went 900 or higher it was much better and at some speeds not noticeable at all. I stopped running it almost immediately...just enough to play with the speeds and check that nothing obvious was loose like one of the leveling pads...so maybe a minute of run time.

I put an indicator on the side of the backplate and saw well under a thousandth in runout. I put an indicator on the chuck body and it's like .010" runout. Putting a straight edge on the chuck body showed it was flush with the back plate on one side and a very visible gap on the opposite side. That tells me the vibration is due to the chuck being off center, so it's not balanced.

My initial thought is that the incorrect inner jaws are so far off that I had to move the chuck body a long way on the backing plate, but when I swap for the outside jaws they are spot on. I would have expected the outside jaws to be way off when the inside jaws are zeroed if the inner jaws are the problem. Clearly I'm missing something, but what?

I've got so little into it I'm not worried, and I'm considering talking to the folks at Kalamazoo Chuck Manufacturing as they mention being the service center for older Buck products. I expect it won't be cheap to have them go through the chuck and make new inner jaws, so before I head down that path I want to kick around what could be going on.

I'm not focusing much on the fix, as much as how it could be possible that both sets of jaws could be dialed in while the body is offset from the back plate. It would seem like if it was zeroed with one set of jaws the others should be off by a lot. Thoughts?

 

[durableoreo]

Could you bring the chuck back to center and grind the jaws true? It's quite a project, if you factor in the fixturing for holding the jaws under pressure. Time & trouble vs money, I suppose.

 

[G-ManBart]

Could you bring the chuck back to center and grind the jaws true? It's quite a project, if you factor in the fixturing for holding the jaws under pressure. Time & trouble vs money, I suppose.

That's certainly something I've thought about....physically center the chuck on the back plate then grind the jaws. I have at least one other chuck I've considered doing that to, so I could set up and do both to help offset all the time it takes to get the lathe covered and a vacuum system for the grinding swarf, etc.

Still, it makes me wonder if the outside jaws would then be completely off...not sure.

 

[jwmay]

If it’s me, I’m tossing the jaws I know don’t belong in this chuck, and I’m using it with only the correct jaws.
Then I’m looking for another chuck with inside jaws.
In most cases, it’s faster to change chucks than changing jaws.

 

[MrWhoopee]

I guess you're lucky that they were able to come up with jaws that would fit the chuck, even if they're not the right jaws.

I would, as you suggested, center the chuck body and grind the jaws. You have nothing to lose.

 

[CJ5Dave]

Drill holes and balance the chuck?

 

[Cadillac]

Good thing is you have jaws that fit the scroll on your chuck. Bad thing is your missing the #5 jaw instead you have a #6 jaw.
In order for your jaws to be properly centered they are timed to the scroll. This is why you have to insert your jaws in order when reassembly. If you take a good set of jaws out of a chuck and flip them over so you can see the teeth. Line up the outer surface. Then look at the distance from the first tooth to the inner working surface of the jaw. That distance gets larger with each jaw starting from jaw 1.
With having that jaw #6 that’s the oddball throwing the center off. If you have 1,3,5 or 2,4,6 you’re good. Anything else you’ll have problems.

You can grind the jaws face back but you’d have to grind all the angles also. Not just a quick bore job. If you close the jaws all the way do they nest well?? I would think you should see the difference there.

 

[Cadillac]

I’ve contacted buck about a 3” I have wanted new inner jaws and it was just shy of 300. For master jaws I’d say north of 500.

 

[G-ManBart]

Good thing is you have jaws that fit the scroll on your chuck. Bad thing is your missing the #5 jaw instead you have a #6 jaw.
In order for your jaws to be properly centered they are timed to the scroll. This is why you have to insert your jaws in order when reassembly. If you take a good set of jaws out of a chuck and flip them over so you can see the teeth. Line up the outer surface. Then look at the distance from the first tooth to the inner working surface of the jaw. That distance gets larger with each jaw starting from jaw 1.
With having that jaw #6 that’s the oddball throwing the center off. If you have 1,3,5 or 2,4,6 you’re good. Anything else you’ll have problems.

You can grind the jaws face back but you’d have to grind all the angles also. Not just a quick bore job. If you close the jaws all the way do they nest well?? I would think you should see the difference there.

I was wrong...I have a 1, 3 and 5. I'm not sure why I was thinking it was a 6 (I'll edit my post above). They do line up pretty well.

I'm not worried about the fix honestly. With the relatively small amount the chuck seems to be out (maybe .010") I think if I mechanically centered the chuck on the back plate and used the TPG it would be within range of just boring the center...maybe not, but it would seem that way.

What I don't get is how both sets of jaws could zero out with the chuck body offset .010" or so. In theory one set of jaws should be zeroed and the other should be off by a lot. That's really the bigger question for me.

 

[G-ManBart]

I’ve contacted buck about a 3” I have wanted new inner jaws and it was just shy of 300. For master jaws I’d say north of 500.

I'm under the impression that Buck won't work on the older chucks like mine, but I haven't confirmed that. I know KCM says they're the only source for parts/repair on the older, legacy Buck products. It's a bit of a drive, but I could actually drop mine off at KCM (assuming they'd allow that) so maybe save a bit on shipping.

Either way, $300-500 plus the original $85 wouldn't be terrible to have an 8" Buck Adjust True in good condition considering what comparable chucks are going for these days.