Hi Bill

Like You say most lathes don't need the set screw, except when reversing
or for those lathes that have braking on them to slow down the spindle,
as for the adapters they are specifically designed to get around all the
different thread ends of all the different lathes, can you imagine the
inventory needed to keep some stock on hand for the whole range of lathes ??
Now they only have to keep a number of bodies and popular adapters on
hand, any special threaded adapter is quickly machined.
The unthreaded portion of spindle comes from the metal working lathes
were reverse running is almost universal, tapping threading etc. and you
better not forget to lock down your chuck.

http://www.oneway.on.ca/chucks/adaptors.htm

Have fun and take care
Leo Van Der Loo

billh wrote:

Ron,
The adapters for Oneway chucks are actually designed for the spindle on
Oneway lathes which have a mating unthreaded portion that is a bit recessed
from what I understand. Some, usually more expensive lathes, also have
hardened spindles which means the screws won't damage the thread or are less
likely to damage the thread.
My reversing General has an unthreaded portion on its spindle but it isn't
wide enough to mate with the chuck screws so they do impact on the threads.
I bought some brass screws and cut pieces of them to fit between the chuck
locking screw and the lathe thread. The brass won't damage the thread. I
don't really use the lathe much in reverse to it is far from a big deal to
me.
If you aren't runnin the lathe in reverse there is no need for the locking
screws.

Billh