Jim Stewart wrote in message ...
I've been trying to cut 28 tpi threads in a length of
W1 5/8" drill rod stock. So far, they come out
ragged to the point of uselessness. I'm a bit
frustrated because the steel turns beautifully.

I'm using hand-ground and diamond-honed HSS+10%
cobalt tools. I've had good luck cutting threads
in brass, bronze and AL. I've also made passable
large form threads in steel.

Anyone have any pointers?

Try using a bit more rake, slightly less clearance than you normally
would. Possibly changing speeds, going just a bit higher, and using
regular old thread cutting oil. W-1 seems to like high sulfur oils.
LIghter cuts for the last little bit seems to help too. MIght want to
check that the back side of the tool isn't dragging, 5/8"-28 isn't a
whole lot of helix angle.