On Fri, 14 Jan 2011 15:35:21 -0600, Terry
wrote:

My 13" SB metal lathe has been retrofitted with a PM DC motor. It's
not a treadmill motor, it's continuous duty according to the tag and
can handle up to 180 V. I assembled a speed control from a
treadmill-type board. Unfortunately it doesn't appear that the
controller provides enough oomph.

What I'd like to try instead is a 120v 20amp variable transformer,
output to a bridge rectifier, then filter the DC and send it to the
motor. By happy coincidence I have all the parts, total cost will be
simply a bit of time.

The rectifier I have is a GBPC1210W; data sheet says 12 amp, 1000 v.
It is rectangular with a hole through the center. It looks like it
ought to be mounted to a heat sink, but the data sheet I saw doesn't
say anything about doing so.

The question is: should a bridge rectifier be heat-sunk (sinked?) in
such an application? If so, how big (roughly) should the heat sink
be? Would it be sufficient to bolt the rectifier to the 4" square
metal enclosure? Thanks for your input.

Have a look at this datasheet:

http://www.vishay.com/docs/88612/gbpc12.pdf

Figures 1 and 2, especially (they show some example heat sink
dimensions). You can derive this by looking at figure 3, and at the
degrees C/watt for various commercial heat sinks.

You'd probably be safe up to 5A continuous, for much more I would go
to a much bigger heat sink and a 35A bridge rectifier (only a few
dollars).