A while back I picked up a PeakMeter PM6208B non contact digital tachometer to measure the rpm of my spindle.  I happened to get it on a great sale, but it turns out to be just an ok meter. This one uses a LED to shine a light on a shaft that has been marked with a bright or dark stripe and then reads the reflected light to detect a pulse when the shaft rotates. The problem is that it is not very sensitive, using the included foil tape I could not get it to reliably read my spindle at all, I had to paint half my fan black and the other half white to get a reliable signal.  I suspect the tachometers that use a low power laser for a light source probably work better since the light is more directed and hopefully more will bounce back to the detector.

Anyway paint seems to have helped a lot, I can get a reliable signal now, but I have to hold the meter very close to the source, about half an inch.  Now that I have it working I can begin to investigate the capacity of my spindle.  I have a  Machifit ER11 Chuck CNC 500W Spindle from Bangood that is rated for 3,000 to 12,000 rpm.  For starters the spindle won't start turning before about 2,000 rpm, but once started I can then turn it down to 500 rpm however I doubt I could mill even these speeds without stalling out the motor.  The motor is peaking out at around 11,800 rpm, so not to far off from the rated speed.  

I tried milling some plywood at full RPM and saw no perceivable dip in speed while milling.  I need to rerun that same test at 3,000 rpm to see if the motor torque drops with speed (I'm sure it does) and to determine what the minimal RPM I could mill at would be.

On a side note I think at $70 this is a very good deal for a spindle.  You can get a stronger spindle with a more advanced controller but you probably can't beat the price and it seems well built and runs quiet compared to my Dremel tool.  The brushless spindles run even quieter and may last longer, but they cost twice as much for the same relative performance.  If your going to go down that road you probably want to invest in a larger machine and a larger spindle that can handle higher speeds and torques as well.

A word of caution, before painting my spindle I tried wrapping it in tape to create a contrast in color.  That worked great at low speeds but once I got up to around 9,000 rpm the tape exploded off the spindle making for a very exiting moment. These spindles move fast, so don't mess around too much!