[Qbranch]

While it's true you can't really make a very good keyway on a mill, you may be able to make a hex or D-shaft bore on the mill. 1/8" end mills are available up to about 3/4" in length, so you should be able to mill a hex shaft bore out of the block.

And, now you're all about to type 'what about the rounds on the corners from the round cutter?'. Well, personally, whenever things really need to be square I do one of two things: 1, before the milling operation, drill holes at all of the points the fillet would be left from the end mill, thus removing this excess material. Or 2, If you want to be trickier and more efficient, program over-travels the length of your tool's radius away from the center of the bore you're making, also eliminating the corner radius. With a hex shaft, you should still have plenty of surface area left on all of the sides of the shaft... you should only lose 1/8" of each side.

If you're mass producing these, you can always cut your bore times by starting with a larger tool and only finishing the corners with the 1/8" tool.

-q