Model A Chuck Size?

[Darkside]

the 9a that I've been restoring came with 6-1/2" chuck. guess I should start looking for a 5"

 

[Aaron_W]

So what's the objection? Legions of people use 6" chucks on South bend 9" lathe, in fact I don't think I've seen one with a 5" chuck before. The jaws clear the ways when they're at their maximum diameter, the weight is trivial relative to the size of work piece the lathe can work and then only downside I can think of is that you'll lose a wee bit of bend length relative to a smaller chuck.

Saying an 8" chuck would fit is asinine, it clearly wouldn't as the jaws won't clear. Are you simply being dogmatic due to a short passage in HTRAL?

In general roughly 50% of swing is recommended for a lathe chuck, which would be a 4" or 5" on a 9" lathe. Four jaw can generally go a size larger.
I think this is mostly to ensure the jaws clear at full extension, so if your 6" 3 jaw clears, great, but maybe that is not universal to all 6" so they recommend a known safe size.

My Sherline is a 3.5" swing, but Sherline sells 2.5" and 3.1" chucks for it when the conventional wisdom would suggest 2" would be the right size. Their 3.1" is of their own design and clears the bed, but who knows maybe some generic 3" chucks would be too large.

Weight can be an issue, but I've also seen some suggest the greater weight of a "too large" chuck is also a benefit acting as a flywheel to maintain speed on a large piece.

I know quite a few people also choose a 6" 3 jaw instead of the recommended 5" for the Chinese 9x20 lathes which have a 1/2" smaller swing than a SB 9 (8.75" vs 9.25").

Rules of thumb are just handy guidelines, not laws of physics.

 

[FOMOGO]

Many of the earlier US made lathes will swing considerably more than than their stated size. My 1940 14" Sidney's actual capacity is 16.5".

Rules of thumb are just handy guidelines, not laws of physics.