I've noticed several enquiries regarding engraving with a flex shaft
so I wanted to reply with some I hope will be useful.
I have found that engraving or carving with a flex shaft on metal to
be very frustrating, though possible. Because of the usual low RPM
associated with these motors there is a lot of "chatter", skipping
and lines are not continuous. In carving wax though, my Foredom S is
a very useful beast.
For detailing metal I found it necessary to finally take the plunge
and buy a higher priced high speed hand piece. Mine is all metal,
self lubricating and made by a dental supply company. It's highest
RPM is 300,000 and though it does not hog out large amounts of metal
like a flex shaft, it it's peerless when it comes to fine detail
work. Because it has low torque. I am able to move smoothly (albeit,
slowly) through hard material like metal. The only drawback to this
tool is that it only accepts bits with a 1/16 mandrel.
I hope this info is helpful.