I ground a narrow bit to approximately the correct angle on both sides
(like an Acme threading tool) and rough out the groove by wiggling it
from side to side while slowly advancing. The angled sides are short
and not ground to cut well, so they serve as stops. The chip is small
enough that a large spoked pulley pressed onto a mandrel doesn't slip.
Then I finish the sides to the proper angles with the compound.
Jim Wilkins