Here is a homemade version of what Fred has described. I haven't made
this one yet because i can't for the life of me figure out how you
center the bowl into the clamps before turning the dang thing on (I
prefer to understand how something works prior to spending time
building it). Good luck.
http://www.crwoodturner.com/longworth/
JD

It's self centering, because the spirals on the back plate are a
mirror image of the spirals on the front plate. It HAS to be centered
when you tighten it. Just print two copies of this
http://www.fholder.com/Woodturning/dcp01158.jpg , cut out the spiral
slots with razor, flip one of them face down and put a pin through the
center of both together. Rotate one page relative to the other and you
will see how it works.