John Rhodes wrote about machining urethanes:

> It can be trimmed with a sharp knife, and can probably be ground to
> an accurate cylinder if it is first frozen with liquid nitrogen (or
> possibly with CO2).

Freezing is the right thing to do.  But there is an easier way to get
the stuff frozen: use your deep-freezer.  That works fine, too.  And
do not want to grind too fast, otherwise you will have to re-freeze the
part too much.  Slow is better, here.

By the way, you can use this trick also if you want to machine all
other kinds of synthetic or natural rubbers, as far as known by me.

Jan Kijlstra