I am purchasing a high speed spindle for engraving and using very small endmills, and need a way to attach it securely to the mill. The spindle itself is out as there is not a lock for it, plus the possible danger of the motor starting up. I've decided to use the ring at the bottom in some way. The spindle itself comes with a clamping mount that is 76mm, but the ring is 100mm. If I can machine the clamp to fit the 100mm diameter I'll do that. Otherwise I need to fabricate a mount.
I removed the ring, whose purpose is to seal the spindle cartridge keeping swarf out and oil in. It's attached with 6 screws. After measuring the hole separations, I derived the following . The outer three screws are M4-0.7x16 on a 90mm bolt circle, and the inner three are M5-0.8x16 on a 72mm bolt circle. One option will be to permanently attach a bracket to the ring using these screws. An advantage of this solution is that the distance from the mill spindle to the high speed spindle will be constant, so that after zeroing X and Y using the mill spindle, zeroing the high speed spindle is a matter of changing the DROs.
Should be an interesting project.