Okay, did a few more test cuts to get a feel for the spindle.
Best cut I was able to do in terms of MRR was ~5mm axial, 2.4mm radial, 30k RPM, 600mm/min with a 3-flute, 6mm Hartner endmill. Not very exciting to look at but here’s a clip.
IIRC, that’s over 7x the maximum MRR I was able to get out of the old spindle, so, uhh, looks like the upgrade worked?
The problem now looks like workholding. You can see in the video that the stock had lifted out of the vise. I’ll try again later with clamps on the T-slot bed or threaded table.
I also tried to do a rough stiffness measurement and got a maximum of ~0.1mm of movement when I pulled on a 6mm rod in the spindle, so the Z-axis did give some improvement there.
And finally, damn this spindle results in some nice surface finishes. I need to surface something with it.