Cracked chuck -- Ruined or not?

After cleaning the abused four jaw Skinner chuck that came with my 1948 9A (but probably wasn't meant for a SB), I noticed two cracks on the back side, apparently due to a hard drop.



The shorter crack is about 0.4" long and the longer is about 1" long. They appear to be on the surface only. The rim of the casting is bent inward about 0.04" in the middle of a 1.2" flat area.

Is this chuck ruined? If not, should I stop drill the cracks or just leave them alone?

Thanks!

Jon

It's probably still functional although you might consider drilling a "stop hole" at the end of each crack. More important with those old Skinner chucks is the fit of the jaws in the slots. If they're worn so that the jaws are loose in the slots, the chuck will have a hard time holding anything firmly and accurately. I'd say that so long as the slots are not worn, go ahead and use the chuck. We just "surplused" an old Skinner because it was worn to the point that it could not hold anything.

Start saving for a nice Bison 4-jaw though!

Dobermann said:

... More important with those old Skinner chucks is the fit of the jaws in the slots. If they're worn so that the jaws are loose in the slots, the chuck will have a hard time holding anything firmly and accurately. ...

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How loose is too loose?

Put a clean steel bar in the chuck.....maybe 1.0" diameter. Tighten the jaws. If you can see the jaws deflect so that only the rear portion of the jaw is holding the steel bar (rear meaning the part toward the headstock) then the jaws are too loose for accurate work. You can still use the chuck but you'll have a difficult time zeroing a round bar in it and it may slip during heavy cutting.

Looks like the crack is following the section changes in the casting between the screw / jaw carrier metal and the main front plate of the chuck. That rim doesn't do anything much structurally significant but it does help keep fingers and bits out of rotating lumpy parts which is generally considered a good thing. Probably its most important duties were during manufacture of the chuck as the molten iron poured and subsequently cooled down. Having the thick rim makes for a much more stable casting with reduced locked in cooling stresses. Hardly important now as all such stresses should be well aged out but the crack does lie on a stress line which would have made cracking more likely if it was struck when young. The section change also concetrates stress so it could have been a double whammy.

Should be fine to use but I'd be tempted to run some penetrating adhesive down the crack line should any migrate into my toolkit.

Clive

Yan Wo said:

How loose is too loose?

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I've seen larger 4-jaw chucks with so much slop in the jaws, about 1/16", that the jaws clang around turning the chuck. But you still could chuck up and indicate something in within a couple of thousandths. Took a while, but you could do it. On the smaller lathes like a 9" SBL, a few thousandths, .005-.010" slop can give you a time getting stock chucked up and running true. IMHO if you are having troubles truing up a piece of stock or a part, int the 4-jaw, it might be time to look for a new one. Ken

BTW- that cracked place on your chuck, I would not take a chance on it. It could crack the rest of the way through, even if you drill a hole through the web, and become airborne and cause bodily damage, if not put an eye out first. Clamping pressure could put enough strain into the cast iron to cause the crack to pro-grate pass a drilled hole over time. Might not happen today, but twenty years from now, bam!!!
Just my two bits...

Ken

4GSR said:

I've seen larger 4-jaw chucks with so much slop in the jaws, about 1/16", that the jaws clang around turning the chuck.

Ken

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They aren't supposed to do that?.........

I'd throw it away before something bad happens. It looks like a tap with a ball peen hammer would dis-lodge the piece.

Not a high speed lathe here...

Machine a thin ring (say 3/4 in wide), locktite over cracked area..

abarnsley said:

Not a high speed lathe here.....

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It don't have to be high speed if it slings a piece of iron out at you and knock your eye out! Or give you a bloody nose...

But a thin ring would help until a new chuck could be had.