Here's the neatest solution I've ever heard of:
Get some valve grinding compound. I suppose fine sand would work.
Mix a little oil with it. Dab it into recess in the set screw. Get a
new, proper size wrench. Now wiggle the wrench into the hole so the
gooey grinding compound can squeeze up to fill in the possible rounded
corners of the set screw head. Then loosen. This works pretty well.

One other poster mentioned tapping on the wrench, (but not too hard) to
"jar" the screw threads. I like this one, too, but I go for hitting it
pretty hard. In order to do this though, you can't realy hit an L-
shaped "allen wrench" straight enough to do the job.
I cut one leg off the wrench so I can hit straight down. This, of
course means you'd have get an extra wrench. I'd cut off the old,
rounded wtench to use as the "driving" tool. Support the under side of
the pulley so you don't bend the shaft. If you are not an accurate
person, hold the one-legged wrench with a pair of vise grips. And, of
course, wear eye protection.

Pete Stanaitis
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cc wrote:
I am sure this has a simple answer

Was tuning up my band saw and adjusting motor pulley, which uses a set
screw. Guess I turned it too tight, rounded off the allen wrench and
now it just turns. It is a metric 3.0 wrench. I also assum the set
screw itself is probably also damaged.

My only solution I see is to eposy the wrnech to the set screw and
then replace both when I get it loose, but don't know if you can buy
individual allen wrenches..

any helpful suggestions?