For cutting screw head slots, I have a length of fine cut hack saw blade from which I ground almost all of the set from both sides. I mount it in a small frame with the teeth reversed so they cut on the pull...much better accuracy with a pull cut. I sometimes use a knife file to give the slot a more historically correct 'v' but I like a fine slot.
This is my method as well with a small frame and 6 inch blades and I use a tiny "V" shaped file
with a very acute angle to give the effect I want.
Bob Roller
