This is my machine, application and method:
The machine is a Joe type router table powered by a 2.25 Bosch variable speed router.
The application. Principally making violin family instruments. For those of you not familiar with violins, think of them as if you were starting with a 1-inch thick wooden board and cutting out a 4mm thick shallow dish (both inside and out). Instrument tops are made from spruce, the backs are made from Maple.
The method is horizontal milling with a flat mill leaving, DOC about 0.125, leaving 0.100 of material, then parallel finishing with a ball end mill.
Now for the question. HSS, m-42 cobalt or solid carbide?
My thoughts: The ball end router bits I typically see are 2 parallel flutes. I’m trying to stay away from these because I have come to the opinion that spiral cut bits will give a smoother finish with less tearout. I shouldn’t imagine left or right hand spirals probably don't matter much for this application.
A machinist I have worked with in the past told me that, HSS and cobalt take a better edge than carbide and will ultimately give a smoother finish in wood. This may not be a deciding factor for the inside of the bowl because I still have to voice the instrument, which will require taking another .5mm off the surfaces by hand. But it would be nice to get as close to a finish cut on the outside as I can. Of course a solid carbide mill, taking off small bites of about 0.125 DOC will probably last forever.
The flat mills will likely be solid carbide since they are readily available (although I do have 3 cobalt 4 flute flat mills comming from ebay and will likely start with these for a while). However, for ball end spiral cut mills I think I will have to turn to the metal working supply houses (enco, msc, etc.). If it matters the flat and ball mills are all going to be 0.500 dia with 0.500 shank center cut. Number of flutes is still an open question too, but I am leaning towards 4 flute mills.
So again, this brings me to the choice of materials for the ball mills.
If you cut it to small you can always nail another piece on the end, but if you cut it to big... then what the hell you gonna do?