Hi John,
I have only bench tested the spindles but I loaned one to a mate and he has had very impressive results. So much so he kept it and brought me a replacement.
Here is his comment on the spindle and the material was aluminum.
The first test was a 20mm circular pocket, 2mm deep, 0.25 depth of cut, 40% stepover, 3 flute, 1/8” carbide cutter. I started at 600mm a min and ended up limited by the acceleration on the router, I saw the feed rate get up to 3.1m/min. It cut really well. Finish was spot on. On the cuts faster than 2m/min, I noticed that the circular pockets became a little oval shaped. Not sure if this was cutter flex (I was using stupid long cutters) or Mach’s constant velocity code.
I would never have run a 1/8th cutter at 3m/min for fear of snapping it off so the results are very impressive by my standards. The speed he ran the spindle was 24k. It was a water cooled 800 watt spindle. If the X2 head with a belt kit was not so quiet to run i would skip it and just run a spindle on this mill. I cut jobs late at night so stealth mode is necessary for me.
I haven't started this project yet as I am finishing off a gantry router - wrong gantry spindle.
Maybe in a coupla three weeks I might get going on the mill. Build for router is here
http://www.cnczone.com/forums/showthread.php?t=71219