Cutting a blind keyway

[TomG]

I would never trust an end mill to mill a slot the same size, even if was roughed out first with a smaller one. For a 1/4" keyway, I would use a 3/16" end mill to rough it to within .005 of finished depth, take a climb cut on each side an equal distance each side of center, check the width using gauge blocks then finish mill it to width and depth, taking spring passes until the key fits. Climbing is always a good idea when milling tight tolerance slots or grooves because any cutter flex will be to the center of the slot, not to the outside. You want the keyway to be snug enough on the key that it needs to be tapped in place.

Using a key seat cutter as Rob suggested also works well. Unlike an end mill, they will hold a slot to the proper width because they don't cut on the sides.

Tom