We have made thousands of handlebar clamps for motorcycles. We CNC
mill the bar hole 7/8, 1" or 1-1/4" with a 3/4" end mill. Then we
slice it in half with a 0.050" thick saw blade in a horizontal mill.
If you don't have a horizontal mill, set up the saw blade in an arbor
and slice it off that way. I think the saw blades are called Jeweler's
blades. They have very fine teeth, are HSS and only cost a few bucks
each at MSC.

Dave