Forgot to mention that, angle heads and speeders are other possibilities with the secondary locator. I'm hoping to build myself a decent speeder since they are pretty expensive. One thing I'm giving up with this bt30 upgrade is the 24k rpm I had with the Chinese spindle. Very useful when doing decent sized carbon fiber plate jobs.

Hopefully the extra constraining isn't an issue. Like you said, I can lock in the main taper, then raise the secondary taper into place with some pressure and clamp it in. Easy enough to dial in that aspect. For matching up center to center distance between tapers on both spindle and holders, I could build in an adjustment but id rather not. I think if I come up with a good machining strategy, i can make it match up pretty well. The part that clamps onto the spindle that has the female taper needs to be machined in the same location of my table as the holder plates so that the center to center distance comes out exactly equal regardless of any ballscrew error I have. Also need to have a specific temperature to keep the machine constant at while running those mating pieces. Another variable is the concentricity of the ground OD on those f1 boring head shank I'm using. My custom plates will reference to that od. I'm really hoping they grind it same time as the taper. If there's any runout there, forget about designing it without center to center distance adjustment. I have 5 of them coming with the spindle so I'll find out soon.