Just a little addendum.......
I've been ploughing........ Roughing a
lot of metal from a couple of flywheel castings.
Started at 5" diameter, with a 1" wide rim, alloy castings.......

Around 200rpm.
The speed control, set around 1/3 of it's travel. Which is not the best position for motor cooling......

Not a ha'p'orth of bother! Just ploughed on. Half a mill per pass, o/d. 30thou per pass, facing.
Motor felt slightly aired. Same with the extractor fan outlet. No alarming smells from the windings.

The result.....

I reckon we've cracked it!

Thanks once again Lads, for all your help and encouragement. I couldn't have done it, without you all out there.

David D