3 jaw and 4 jaw Chuck sizes

[Just for fun]

It seems that the standard 3 jaw chuck for 12x36 lathe is the 6" but yet the standard 4 jaw is an 8". Why is that? Why are both chucks not 8".

 

[benmychree]

Tradition/convenience; 4 jaw chucks accommodate larger more irregular work, 3 jaw chucks are more used for smaller more finished workpieces.

 

[pontiac428]

The 3-jaw scroll chuck can hang the jaws way outboard and still fix parts. The independents need over half of the jaw engaged with the lead screw. The scroll can go out as far as it can as long as it still has a few teeth engaged in the scroll. So the 6" scroll has a similar swing diameter as a 8" independent.

 

[benmychree]

The 3-jaw scroll chuck can hang the jaws way outboard and still fix parts. The independents need over half of the jaw engaged with the lead screw. The scroll can go out as far as it can as long as it still has a few teeth engaged in the scroll. So the 6" scroll has a similar swing diameter as a 8" independent.

And that abuse is why we see chucks with broken teeth on the jaws and scrolls, having said that we all have done that more than once; personally, I have never broken any chuck parts.

 

[pontiac428]

Perhaps, but where is the line between use and abuse? I'm sure a little stickout is use, and too much stickout is abuse, but why the hard take?

Personally, I never have to run the jaws past the shoulders, because I have enough chucks to cover my lathe's capacity. But I've seen 8" scroll chucks with 3" wide jaws, so does that mean it's only good for 2" internal capacity before it's abused?

 

[benmychree]

Agreed, the line between use and abuse is a fine and blurry one.

 

[verbotenwhisky]

Agreed, the line between use and abuse is a fine and blurry one.

Perhaps, but where is the line between use and abuse? I'm sure a little stickout is use, and too much stickout is abuse, but why the hard take?

Personally, I never have to run the jaws past the shoulders, because I have enough chucks to cover my lathe's capacity. But I've seen 8" scroll chucks with 3" wide jaws, so does that mean it's only good for 2" internal capacity before it's abused?

Back years ago when we were still drawing on the board with a pencil we use to say the trick is to pick up on the pencil just before the lead broke... I suspect that is similar the the line between use and abuse.....

 

[pontiac428]

Maybe rules of thumb are no substitute for the manufacturer's spec sheet. When something's funny, stop and reassess, of course. A chuck jaw would make a terrifying projectile. There are so many chuck types out there, I can't imagine they all have the same capacity for a given diameter. At least with the chucks I've owned, the scrolls have the greater capacity over the independents for the same body size. They engage differently, scrolls and lead screws.

This probably disproves my point, but a Kitagawa 8" independent chuck's max capacity is listed at 75mm on the inner jaws, and the 8" scroll is spec'd at 68mm. Bison does not publish capacities. It probably comes down to using good judgement with the chuck and the work you've got.

 

[verbotenwhisky]

Maybe rules of thumb are no substitute for the manufacturer's spec sheet. When something's funny, stop and reassess, of course. A chuck jaw would make a terrifying projectile. There are so many chuck types out there, I can't imagine they all have the same capacity for a given diameter. At least with the chucks I've owned, the scrolls have the greater capacity over the independents for the same body size. They engage differently, scrolls and lead screws.

This probably disproves my point, but a Kitagawa 8" independent chuck's max capacity is listed at 75mm on the inner jaws, and the 8" scroll is spec'd at 68mm. Bison does not publish capacities. It probably comes down to using good judgement with the chuck and the work you've got.

I keep looking a the 6 jaw chucks, I hear they center better than the 3 jaws do and have some adjustment for dialing them in....

 

[Jake M]

It seems that the standard 3 jaw chuck for 12x36 lathe is the 6" but yet the standard 4 jaw is an 8". Why is that? Why are both chucks not 8".

I think that for most work, in the most general sense, that is just the most practicable way to equip a lathe. The 4 jaw somewhat larger than the three jaw. So most pre made packages come that way, and most lathes that weren't a package deal just end up that way by default. There's no law that says you can't do different. It's just the most common sweet spot for the best functionality overall.