wrote: A polarity reversing switch will instantly
change the direction of the motor, (clip)
Then Dan Bollinger wrote: Reversing a single-phase motor will do nothing
since the 'reversing' is
done on the starter circuitry to get it initially spinning in the direction
you want.
^^^^^^^^^^^^^^^^^
First, Nailshooter, regardless of what type of motor it is, it will never
instantly reverse. It may start to decelerate rapidly--but I think Dan is
right: once a single phase motor is up to speed, the direction of rotation
is not controlled by the wiring.

The thing that could be harmful in this situation would be this: You turn
the motor off, and as it slows down, the starting windings kick in. If you
chose to reenergize the windings for reverse rotation before it stops
completely, the motor would draw higher than normal starting current. If
this did not kick the breaker, it could damage the motor, or shorten the
life of the starting switch.

My main concern is not the mechanical lathe parts--it is the effect of
higher than normal current during reversal.

Plan on stopping the motor, for all of the above reasons.