Having that leg of the angle iron that you are milling unsupported really isn't the best setup. I'm glad that it worked for you but you should really hold pieces completely.
That formula for calculating spindle speed is wrong too. This is the real formula
(PI * DIA X RPM) / 12 = SFPM
Using your formula it says to run a 5/16 inch cutter at 550 RPM which is only 45 SFPM, less than half the speed you should run. You should mill steel at 100 SFPM or more.
How To Mill Slots On Milling Machine
Having that leg of the angle iron that you are milling unsupported really isn't the best setup. I'm glad that it worked for you but you should really hold pieces completely.
That formula for calculating spindle speed is wrong too. This is the real formula
(PI * DIA X RPM) / 12 = SFPM
Using your formula it says to run a 5/16 inch cutter at 550 RPM which is only 45 SFPM, less than half the speed you should run. You should mill steel at 100 SFPM or more.
How To Mill Slots On Milling Machine
How To Mill Slots In Steel
I'll mock up a better setup. Ignore the burn holes in the steel. It is the first piece of scrap angle I found laying around. Anyhow, the hold down on the leg keeps it from fluttering around. Chatter is a bad thing when machining.
If I cut a slot in a piece of material.123' and NEED a slot.260', then I need to remove.137' MORE. But I need to remove 1/2 of that on either side of the center line, or.0685' in one direction, and.0685' in the other direction. So, I lower the end mill into the slot. Groove or slot milling is an operation in which side and face milling is often preferred to end milling. Slots or grooves can be short or long, closed or open, straight or non-straight, deep or shallow, wide or narrow Tool selection is normally determined by the width and depth of the groove and, to some extent, length.
.206 slot, you can use a 3/16 end mill & then move it. 13/64 would give you.203.
