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.
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. Groves and slots - you can mill groves and slots to diferent widths and depths using diferent size end mills T groves - you can mill a T shaped grove in something using a t cutter like the groves on a milling table 12. .206 slot, you can use a 3/16 end mill & then move it. 13/64 would give you.203. To make the slots, I had to create a New Geometry ‘layer’ and add simple straight lines dead center of where I wanted the slots milled. FlatCAM would then just mill one pass, centered over that line. This required a third milling file – one for the bottom Cu, one for drilling and the third for the four slots. Came out P E R F E C T!
That formula for calculating spindle speed is wrong too. This is the real formula
(PI * DIA X RPM) / 12 = SFPM
How To Mill Slots In Mild Steel
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
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.