My smallest is a .023 three flute cutter for fret slots. I also use a spindle and worked out the best feed/speed with Ron from Precise Bits/Tinker and Tinker on the phone. For my particular setup doing 5 passes to get to 2mm depth, the math worked out to 18K RPM and 24 ipm. It's worked flawlessly. That particular piece of tooling has a .125" shank and I use a .125" collet. (I regularly use 1mm and 1.5mm tooling for some details on resin inlay toolpaths on charcuterie boards I cut for a friend to get corner details)