Well ... lots of possible ways to do it. One would be a spindle
which passes though the main spindle and is held in the large chuck.
That one would be driven by an outboard motor which couples in through
the back of the spindle.
Another would be a spindle (and bearings) held in the big chuck
with a pulley on the workpiece side of the big chuck, and with a belt
going to a motor behind the bed -- and likely behind the headstock as
In any case, you want to lock the spindle by engaging both back
gears and direct drive (unless this is purely gears inside the
headstock, in which case I would look for some form of spindle lock on
the headstock. And interlock it so nobody can power up the main
spindle, because that would beat the motor and the bed to death.
Without seeing the lathe, I don't know what would work best --
and it would also be dependent on what parts you could find. For a
really small setup, something like the spindle cartridge from a Unimat
SL-1000 (goes for insane prices these days, because they are collector's
items). For that, you would have to make a drive tube which threads
onto the outside end of the Unmat spindle cartridge in place of the
pulley and nut, and extends outside the far end of the main spindle.
Install a bearing on the outboard end of the drive tube which is a nice
fit inside the outboard end of the big spindle. This would then let you
use some small three and four jaw chucks, and with creative work and the
WW (watchmaker's) spindle for the Unimat, even really tiny collets.
I wonder whether you could take the spindle cartridge out of a
Taig lathe -- a little bigger than a Unimat SL-1000, and a lot more
affordable, since it hasn't become a "Collector's Item".
What you would lose would be any existing power feeds on the
lathe, and have to come up with some other way to do those, too.
Maybe mount a small cross-slide and compound on the big one, to
give you finer feeds.
Just some speculation. :-)
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