4 jaw chuck question

[combustable herbage]

I have been debating on a 4 jaw chuck for my lathe I have been looking at the Sanou K72 series.
My question is should I get the 125mm(5) or the 160mm(6) the 5 would be the same diameter as my 3 jaw, I will have to make a back plate either way and for the money the 6 isn't much more, I can't think of a reason why I can't go with the bigger one but maybe I am missing something?

K72 125 Small 4 Jaw Lathe Chuck 125mm Independent Chuck Lathe with Jaw Hardened Steel for Mini Lathe|Chuck| - AliExpress

Sanou Brand Lathe Chuck 160mm Four Jaw Independent Chuck K72 160 4 Jaw Chuck for Lathe|Chuck| - AliExpress

 

[Tom O]

That would depend a lot on the swing of your lathe because of the jaw extension during turning.

 

[combustable herbage]

Its 10 inches so that should be ok.

 

[YotaBota]

Both the 9" machines I had came with 6" 4jaw chucks. My 1120 came with an 8" 4jaw so you will be safe with the 6 inch chuck but like Tom says, watch the jaw extension.
I read (or was told) somewhere best practice is to give the chuck a spin by hand before hitting the GO button. That way if the jaws are out to far or there is something in the way, like the saddle or cross slide, it's just a light tap.

 

[Tomc938]

Both the 9" machines I had came with 6" 4jaw chucks. My 1120 came with an 8" 4jaw so you will be safe with the 6 inch chuck but like Tom says, watch the jaw extension.
I read (or was told) somewhere best practice is to give the chuck a spin by hand before hitting the GO button. That way if the jaws are out to far or there is something in the way, like the saddle or cross slide, it's just a light tap.

That's a very good idea - take if from someone who didn't!

 

[Proxule]

Funny this is the exact 4 jaw chuck that is in question on my brand new 1236 lathe from KMS.
There is so much unbalance in the body that it dances my lathe - with the jaws removed.
Its the 200mm version.

gluck

 

[PeterT]

I got thinking about your chuck issue, that unbalance may be a factor. I assumed good quality scroll chucks were always balances, but maybe 4J were maybe just 'pretty close' because often times one would have the jaws extended non-symmetrically putting the chuck assembly out of balance anyways. But spot checking a number of decent quality chucks, the 4J's are balanced too, they make a deal out of it.
https://www.precisionmatthews.com/shop/chuck4j-8d1-4/
http://www.bison-bial.com/lathe-chucks/3774-400
https://www.precisionmatthews.com/shop/chuck4j-8d1-5/

I wonder if this is something you could do yourself clamping a dowel pin & supported on the typical twin roller balancing wheels? I think RobinHood did some balancing work, not chucks, but same principle.

 

[DPittman]

I don't know what sort of 10" lathe you have but I have a Asian made 10x22 lathe and bought a 6" 4 jaw chuck to use on it. Together with my excessively heavy back plate I think the chuck is likely a bit too heavy for my little spindle bearings. I use it occasionally when I need the extra capacity but I don't think on my little lathe it would be good to spin it all the time.

 

[Proxule]

I got thinking about your chuck issue, that unbalance may be a factor. I assumed good quality scroll chucks were always balances, but maybe 4J were maybe just 'pretty close' because often times one would have the jaws extended non-symmetrically putting the chuck assembly out of balance anyways. But spot checking a number of decent quality chucks, the 4J's are balanced too, they make a deal out of it.
https://www.precisionmatthews.com/shop/chuck4j-8d1-4/
http://www.bison-bial.com/lathe-chucks/3774-400
https://www.precisionmatthews.com/shop/chuck4j-8d1-5/

I wonder if this is something you could do yourself clamping a dowel pin & supported on the typical twin roller balancing wheels? I think RobinHood did some balancing work, not chucks, but same principle.

Agreed they do make a big deal out of it, Yes they are inherently unbalanced for the most part as you are generally turning odd shaped or sized or weighted parts. I had originally asked in my thread if any one had a static balancer I could use.
My logic was to dial in some drill rod with zero TIR then mount it on a static balancer and add or remove weight until it was happy.

I will post another thread I guess - If king canada decides to not help out
Thanks for the suggestions