Chad have you [or did/do you] ever had any vibration with that super long z axis? I've got a machine w/ a similar setup, only have ~24" on the Z [which is a 6x6 .250 post] and I'm getting all kinds of vibrations from my spindle. I can't get it past ~9k rpm with a tool in it, before it starts vibrating the crapp outta everything.. [Which is no easy thing to do as my Gantry is a couple thousand pounds..] I'm thinking it has to do with harmonic's of my tube axis but I'm not entirely sure.. it could be my spindle, my tooling or my structure...
Curious..
Jerry