Mr. Goodman is absolutely correct in regard to the knife switches and
their originality. However, I personally don't go so far as to say,
"don't replace them." When that knife switch was new, it didn't get hot
because the blade that drops between the contacts still had all its
metal. Today, if it gets hot, the problem is that you are trying to run
the motor through a resistive knife switch, usually because it has been
worn away in time. The "originality value" of a dangerous switch is low
in my book. I suggest building another switch. It's almost too simple
to even describe. Once you see it, then how to do it is easy.

   Getting the screws out of the porcelain or fiber may be difficult,
and if it has rivets, then it's aggravating, but you'll think of
something (chuckle).

   There is another way to use the original worn-out knife switch. Add
a relay and let the switch operate the ac relay, which in turn operates
the ac motor.  I say "ac" because 110v dc is difficult to control with
today's relay contacts. You need to modify them if it's a dc machine,
or they will turn into funny looking little lumps of plastic (that
conduct equally in both directions).

   Adding a relay doesn't detract from the originality unless you have
modified something else first to install it. That isn't necessary, so
the instrument remains completely original and it adds great peace of
mind.

Craig B.