Thank you for the information it is very helpful.

I was thinking of design for balance having smooth operation at higher speeds. I had in mind that a tool holder for RH turning was weight balanced differently than a LH tool holder. I seems that the diagram you posted states the design is mostly for convenience not balance.

I modified a 4" fly cutter I have to accept a 3/4" square CBN/PCD round cutter I made and it has a vibration at 600 rpm which is quite noticeable. I do not want to use it like that so I decided to have a go at making a fly cutter.

I have a 3" x 3" chunk of round bar and wanted to make a tool holder for my vertical mill to finish intake and exhaust mounting surfaces on cylinder heads among other things smaller in scope. I will use this with an R8 to 5/8 x 11 adapter. My thoughts are to face off both ends of the bar stock, drill and tap one end then mill a 3/4" slot at approximately 15 degrees of angle with set screws to finish.

My intention was to mill the slot on center but after looking at the diagram you posted that may not produce good results. Any suggestions here would be appreciated.

I see some successful fly cutters are milled on center, or at least they look like it. The PCD/CBN cutter holders I made are milled for about 7 degrees of negative rake.

I want to use a standard carbide from time to time for milling an intake mounting surface which has an over hang or ledge for valve cover mounting. There are certain applications where I do not want to get into the lip with a round cutter. I believe the carbide holder is typically at a positive rake. I want the fly cutter to work well with either positive or negative rake cutters in it.