They make a tool just for straightening lens threads and filters.
Porter's Camera used to catalog it, Shutterbug used to have a small ad
in the back for the manufacturer. At the price of some lens
accessories, one use would pay for it. It's kind of a flat jackscrew
with curved surfaces to bear on the dent(s). You could also cut a
male thread on your lathe and use that to gently restore the threads
back into usable condition.

Stan