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6 jaw very weird issue


Dec 30, 2023
ive been at the lathe for many years making trumpet mouthpieces and trumpet parts. I just got a cheap Chinese 6 jaw chuck and it works pretty nice except for one weird thing.

its a set-tru and I easily dialed it in to .0003. The jaws feel like they positively Grab the material and give very repeatable runout of .0003.
when the work is spinning true, so is the chuck Body and the backing plate. What is NOT true is the outer edges of the jaws. When you look at the spinning chuck it’s VERY obvious that the side of the jaws facing out, opposite the work, are not at all swinging true. The wobble is maybe .010 or so. That seems like it must be unbalanced or somehow dangerous. The jaws are reversible two piece jaws.

the first thing I did was to check if the jaws were numbered and the slots were numbered. You can’t see the numbers until you take the jaw out, but they were in the right slots. is it possible the removable parts of the jaws are mixed up? I have not taken them off yet but don’t see any numbers. Should there be numbers? They must must be hidden if there are. Is it just possible that this whole scenario is more normal than I think? I’ve never had a 3 jaw look this weird spinning around. I have a video by the way
thanks for any help:)
Sure, Did you verify by measuring, or hand rotating chuck as safe distance to a fixed object?

(not at all to be mr obvious)

It's great it does what's intended and you can get great trueness
Sure, Did you verify by measuring, or hand rotating chuck as safe distance to a fixed object?

(not at all to be mr obvious)

It's great it does what's intended and you can get great trueness
Well all the jaws and jaw pieces are in the correct places. Work holding and runout etc seem to be very nice. Using an indicator on the outward facing parts of the jaws two are the same and the others vary by either .015 or .030. Do all six jaw reversable setups do that? It must be normal I guess?
Well all the jaws and jaw pieces are in the correct places. Work holding and runout etc seem to be very nice. Using an indicator on the outward facing parts of the jaws two are the same and the others vary by either .015 or .030. Do all six jaw reversable setups do that? It must be normal I guess?
Depends on the jaws. There's i.d. jaws, o.d. jaws, some that can do both, more or less, and soft jaws. Are these two-piece ? I'm guessing not.

Anyway, doesn't matter, you are being silly. The only parts that count are where you grab the material. If you really care for some reason, toss a toolpost grinder up and skim them.
They need to be concentric, hardly i need say.

Lathe chuck jaws may hold an object externally, or internally.
It's conceivable they would never be used, in some application, to hold an object, like a bar, internally.

An example would be the brake rotor, gripping the rotor's center hole for locating on the vehicle's hub.

Rando internet picture, non reversible jaws-
designed, obviously, for that very purpose, to hold object externally.
My jaws are reversible. When the jaws swing around with the outer edges as a much as .060 different from each other, it is disconcerting to look at the jaws swing so lopsided. It can’t be balanced very well either.
if anyone has a chuck with reversible jaws, do your outer edges swing evenly or is there definite wobble visible easily by glance? the only thing that makes sense to me is that when the jaws are reversed then the outer edges will be true and the edge that was true before reversing will not ne true. That makes no sense to me.
I suppose it’s also possible that when the outer jaws were ground in at the factory, the jaws were in the wrong slots, but when the inner jaws were ground in they were in the right slots.
in any even, if someone has a reversible jaw chuck take a possibly they look and see if their’s does this?
Here are three videos. Once again I basically asking if other people have noticed this on their chuck. The main jaws (the part that stays in the chuck when you reverse the top piece of a two piece jaw) all indicate that the outer edges are swinging in the same circle as each other. The outer edges of the removable reversible top jaws are swing quite different. You can see in the 1st video. The strobe effect of lights and video frame rate makes the effect look slow motion.




Too jaws are crap.
Cheap parts give poor performance.
Perhaps a mixture of parts from the factory, maybe just no QA.
Top jaws should all be the same length overall ( yours are obviously not) they also need to have the same length from the locating key to one end for each jaw.( measured in the same direction)
What you see is not how a quality chuck would run. Should not have any difference in the circle the jaws describe on either end of the jaw set, ID, or OD.

Cheers Ross
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To directly answer your question, No, the two-piece jaws I've used DO NOT do this.

Even a 'Cheap' chuck would not do this.

Again, it machts nichts whether jaws are single piece, or two piece. They MUST be able to hold stock externally

There's absolutely nothing that says the jaws would EVER grip internally, I'm sure there's some application, somewhere, where a lathe chuck has never held a bar such as in your example, but always held stock by external surfaces.

Why does your chuck have such runnout? Who knows
Does it matter if you'll only ever use the machine as you are now??


If the machine is doing as you will, and there's a gross defect that affects nothing, worry about something that does affect something.

priority #9,999 of 10,000
Have you actually tried the jaws reversed to hold a disc or whatever, the way they're supposed to? As it is you are complaining about an irrelevance, unless you are running at 5 or 6000 rpm. What's the speed limit marked on the chuck? Those 'errant' jaw faces are just there for the ride, they are NOT configured to grip internally, note the gripping surfaces are concave. Try them as they are meant to be applied, in the situation you present it's like complaining about the balance of the spare in your car's boot/trunk. I admit it doesn't look nice but you may find, reversed for actual use, they're OK but the unused face still 'runs out' .. big deal .. unless your lathe is dancing across the floor. EDIT: you beat me to it Sport.
Note: The chuck jaws shown in UptownSports' picture are designed for both internal and external gripping of a workpiece.
OP said "reversible" and I am trying to imagine it. Have had several 6-jaw chucks, still can't imagine how a reversible jaw would work.

Lipshurt said:
Here are three videos ... dropbox ...
A lot of - or at least some - browsers don't work with that stuff. Easiest and most reliable is to put a photo up here. Then it stays, for however long the site lasts, and people can see what you're talking about. I still don't have a clue, because dropbox doesn't work with this version of safari or a newer firefox or in fact anything that runs on el capitan. And I am sorry but not going to buy a new computer to see your videos.

In future, a simple photo does a better, more universal job of explaining your question.
I would like to thank everyone who offered to help me understand this (even though I believe i will think twice before asking again…not the frendliest forum). I believe i have it figured out.
Firstly, i have bought plenty of cheap Chinese tools, and never had any trouble with any of them, so it was natural for me to think my situation was somehow normal.

As it turns out, it is correct and normal. For removable/reversible two piece jaws to be able to run true on both the inner surface and and also when reversed, the top jaws have to be different lengths from the inner grabbing surface to the outer grabbing surface. And by kind of a lot. That dimension varies by .025.
The outer edges will track circles that are up to .050 different from each other. That is VERY noticeable the first time you see the chuck spin. It’s pretty weird looking, as you can see in the videos i posted. Still, it is correct. the chuck is true when using either grabbing surface (true on both orientations of the top jaws.

when you see a chuck with one piece jaws that are meant to gab externally and internally, everything swings true at the same time. It looks very true and still when spinning. Apparently two piece reversible jaws don’t look like that. And if you have a light shining on the chuck that is strobing 60 times per second (alternating current… before someone jumps in to say that is silly, note that record players use the 60 cycle strobe of light bulbs to tell you if the speed of the turntable is correct) that strobe slows down the lopsided swing of the jaws and its pretty weird if you are not expecting it.
Funny, didn’t read any real negative or hateful responses.
Ask questions from a site populated by guys that cut metal and work with their hands and you should expect to get direct no nonsense and likely impatient responses.
Personally I wondered why this was a question at all.
Anyone with a pair of calipers and the ability to measure could have diagnosed this as a problem, and its root cause in about 5 minutes.
Apologies for the “ unfriendly “ response.

Cheers Ross
I was actually looking for a person that has a similar chuck to tell me if theirs looks like mine. Which it would. Nobody gave that direct answer. Regarding calipers, the responses generally said that the jaws should all be the same which is not correct. I also thought that the jaws should probably be the same depth inside to outside. The first thing I did was measure the distance and found that They were all different which seemed weird to me, so I posted on a forum where someone may know the answer to my question. its OK that that did not happen. In my field I’m known as a world class craftsman, but I had never used a two piece jaw before or seen it spin up close. Everyone has something they don’t know.