I came up with a way to square a guard slot with out using a file. It's easy and the corners are nice and sharp.I use a square rotary broach bit.I mill the slot with a 1/8" endmill. Then chuck the rotary bit in the mill,no power needed.Move the bit to the end of the slot and raise the bit. Move the table .002" and lower the bit.This shaves the radius out of the slot.Raise bit move table .002" and repeat. Keep doing this until slot is nice and square.Just the downward motion of the quil.
I hope this helps,