Extending the outer jaws past the edge of the chuck - how far before unsafe?

[WobblyHand]

I have a 6" four jaw chuck for my G0752/G0602. Just bought an 8" diameter disc 0.8" thick of 6061 to make a fixture plate for a recently purchased rotary table. How many threads of engagement should I have in each jaw? The jaws look far enough out that I'm wondering how safe it is. Obviously I will check for clearance, but my question is really how many threads are needed. The 4 jaw screws have two sections of thread, an inner thread and outer. The jaw does not engage the inner thread at all.

My plans on the lathe are to face the plate (it is saw cut, but not bad) both sides and to bore a 30mm hole in the center. Once I have done that (and some chamfering) then I will do the rest of the machining on the mill.

Don't want to do anything ridiculously unsafe, yet really have no need for an 8" chuck (I think). What is reasonable practice, and what would you do?

 

[RJSakowski]

I don't extend the jaws past the first step on my 602 3 jaw chuck. I checked the four jaw chuck and you may be able to get away with mounting an 8" disk but proceed with caution. I would keep the spindle speed down so if something does come loose, the result would be less of a catastrophe. Since you are boring a center hole, you most likely wouldn't lose the disk in that operation. I would use light cuts for the facing operations and check the jaw tightness several times. If you bore a smaller hole, you could run a rod though to the tailstock chuck as a precaution against flying disks for the facing operation.

 

[WobblyHand]

I don't extend the jaws past the first step on my 602 3 jaw chuck. I checked the four jaw chuck and you may be able to get away with mounting an 8" disk but proceed with caution. I would keep the spindle speed down so if something does come loose, the result would be less of a catastrophe. Since you are boring a center hole, you most likely wouldn't lose the disk in that operation. I would use light cuts for the facing operations and check the jaw tightness several times. If you bore a smaller hole, you could run a rod though to the tailstock chuck as a precaution against flying disks for the facing operation.

Good idea on the rod. As for spindle speed, eek, going to run this rather slow. I haven't turned anything this big before, so I'm going to quadruple check everything. Going to be an interesting day tomorrow. Thanks for the advise.

 

[WobblyHand]

Managed to face off the plate this morning. First face was uneventful. I ran slow, started out at 100 RPM, as I really wasn't sure what would happen when I turned the switch to forward. There's only about 1/8" clearance between the jaws and the ways. Second face was less of a true cut of the saw. Could really feel the uneven cut as a slow thumping, which was a function of facing feed rate (manual). Once the face became relatively true, the thumping went away. Had to speed it up when I was turning in the center section, as I was getting noticeable grooving.

This is not something that I will do often with this lathe. Jaws are really extended. But, knock on wood, so far, it's working.

 

[WobblyHand]

How extended are the jaws? This far:

I think there are 3 - 3.5 threads of engagement, 2/3 of the jaw is outboard of the chuck. This chuck really wasn't meant to hold such a large piece. Knock on wood, it is still holding. Just need to chamfer the edges and bore a hole.

 

[MrWhoopee]

How extended are the jaws? This far:
View attachment 397251
I think there are 3 - 3.5 threads of engagement, 2/3 of the jaw is outboard of the chuck. This chuck really wasn't meant to hold such a large piece. Knock on wood, it is still holding. Just need to chamfer the edges and bore a hole.

Looks like that chuck is perfectly sized for your lathe. I wouldn't feel safe with the jaws any farther out and you can't go there anyway.

 

[WobblyHand]

Looks like that chuck is perfectly sized for your lathe. I wouldn't feel safe with the jaws any farther out and you can't go there anyway.

I'd agree, it's a good size match for the lathe. The jaws are slightly camming towards the headstock, since there's only 1/2" in the chuck. Not something I'd want to do regularly, but ok every once in a while.

 

[Bi11Hudson]

You seem to have the matter well in hand. The only comment I can make is about my personal "standards". My machine is an Atlas built Craftsman 12 inch machine. So, in theory at least, I have 6 inches from the center to the shears. Up front of course is to turn over the chuck by hand to make sure nothing catches on the shears. That one's a given. . .

I have a number of chucks, from 2-1/2" (65mm?) to an 8 inch 4 jaw. All with inside and outside jaws. My personal preference is to have at least 2 teeth on the scroll or adjuster. I prefer 3 but sometimes you get into a situation where 3 isn't enough and 2 will involve a lot of work changing the chuck for one cut. I have made a cut where only one tooth was made up. I didn't like it, kept the speed down with light DOC, and had a rod in the tailstock just in case. And haven't done it often, that's one reason behind so many chucks now. But I got away with it, this time. . .

.

 

[WobblyHand]

But I got away with it, this time. . .

I hear you, loud and clear. Honestly, it makes me very nervous to operate the lathe this way. Once I bore out the hole, the lathe operations should be complete.