Hi Bill

I think I would call your jam chuck a friction drive, and maybe that's
not the right name either, but it is cheap and readily made, and
probably used by almost every turner in one way or another, be it just
a chunk of wood or in some more elaborate dimension.
The question why should I have both, well to take just my case and this
also would be so for everyone who moves the headstock to the end of
their lathe and also if a swung headstock is used, we are not able to
use the tail stock to steady the turning.
I do use the Mega jumbo jaws a lot, they are very quickly put on and
very flexible, from 5" to 15" by just moving the rubber bumpers, and if
larger is needed I have 4 1/4 pies that I will bolt on and the bumpers
are then screwed into those pies, I have used them for larger than 20",
and yes I am very careful when working with those large sizes.
Yes I do want both, friction drive and jumbo jaws.

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Have fun and take care
Leo Van Der Loo