OK thank you. I think I get it although one post suggests not using a tool holder for an end mill, other post does. I have a bunch of R8 collets that came with the machine. Before I started asking questions I would simply have used on for holding an end mill and going to work. And I still will, but I'd like to get a consensus on using a tool holder and how that works vs just holding the tool in a collet...
B&S #9, as said, has roughly DOUBLE the grip and HP transfer of R8.
For two mills with B&S 9 spindles, I have one full set of brand-new B&S 9 collets. But only because B&S has gone scarce. They've never left their wrappers. They will probably ALL still be in their wrappers when I die.
I have side-locks. Lots of them. My PDQ-Marlin VS and S are also side-locks.
IF.. I had a 5K, 10K or 20K RPM CNC spindle to tool? THEN I'd want a milling chuck or shrink holder because.. only up at those challenging speeds does the very minor clamping offset of a Weldon side-lock's screw actually matter to a mill. Needless to say, it would not be R8, either. Capto, probably.
Down in the "manual" world, a mill doesn't ordinarily mount a no-flutes item like diamond wheel. Cutters have teeth or flutes. Each one is already a form of "interrupted cut" as it is rotated into the cut on material that is also moving, relative to its axis.
Low/no TIR does bring better tool life. But really.. its a mill. Not a balance staff in a fine Swiss watch. You can't make all the other movements go away and still do useful work, anyhow. There WILL BE imbalanced side-forces at work. Or it is not a mill, it's an "only direction is DOWN!" drill press.
Eschewing side-locks is just not helpful. Those should become your go-to for routine milling, especially if you could "only have one".
Better to HAVE both "available", of course. I do have ER and TG and Gorton collets -even "native" 40-taper collets for the mills.
2CW