4 Jaw Chuck - Safe Diameters

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BillWood

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Hey,

Am looking at holding a job on my 4 jaw chuck

The workpiece is a 1" thick by 6" diameter piece of cast iron. The chuck is 6" Pratt-Burnerd

Any guidelines or thumb rules such as "Always mount a workpiece of smaller diameter than the chuck ?"

I guess the obvious one is "Check the Jaws dont exceed the swing - test by hand before turning power on"

It seems sturdy enough but am wary of any newbie traps.

Bill
 
It will be safe if the workpiece is held firmly and the rpm is moderate.
 
Hi Bill,

Caution is healthy with unfamiliar operations.:encourage:
Ensure that the when you have it in the chuck that the jaws are still firmly engaged in the chuck. ie. Not hanging on by a few threads.
Watch the rpm as that large dia. means high sfpm at the circumference.
Although it's a heavy piece, it's right at the chuck without much "stick-out" and should be held very firmly.
Facing should be no issue(light cuts), a large drill with too much feed or without step drilling could try to pull it out of the jaws.
-brino

Edit: be sure to clean up the cast iron chips, they tend to stain the ways if left behind.
 
It is actually a very good question that I have never seen the definitive answer to.

On a scroll chuck, one jaw will be the first one to disengage. How much of the scroll tooth should be engaged? Should multiple teeth be engaged? There is always a temptation to open the jaws up "just a little bit more" to squeeze in that slightly oversize piece.

The manual for my Grizzly G0602 shows some pictures of "proper and improper use of the chuck" but doesn't really show when you reach them. When I am working close to the point of jaw disengagement, I back #3 jaw out until it is free and make sure I have at least a quarter turn on the chuck key to tighten. This gives me full engagement of one tooth on the scroll.

Another question is how is chuck concentricity affected when operating in the outer limits?

Similar questions are posed for four jaw chucks.
I expect that the answer will vary with different chuck manufacturers but it would be good for all of us to have some guidelines.
 
When I was younger and much less experienced I mounted a workpiece that was large in chuck, it fit with no problems, I assumed wrongly that if the stock chuck for lathe would take a piece that there was no chance of a crash, I was very wrong. My lathe got a nice nasty scar where the back side of extended chuck jaw contacted the cross slide. Now any large workpiece gets a one revolution hand turn to make sure it clears everything.

Just a gentle heads up
michael
 
A good rule of thumb I use when the jaws have to leave the chuck body.
"Never let the jaw extend any more than 1/3 of the length of the chuck jaw."
You want 2/3's of the jaw length still engage with the chuck body.
So pretty much if the chuck body is 6" OD, you don't want to chuck on anything greater than 6" to 6-1/2" OD.
 
Thanks for the responses.

That job is now complete - made a pseudo faceplate out of a gym weight.

I will now use the gym weight with some screw holes in it as a jig to hold a 7" piece and then a 9" piece instead of attempting to use 4 jaw for either of them.

Bill
 
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