Awesomely unsafe at any speed setting.

I learned two things tonight:

1. When connecting a LEGO TECHNIC™ building set to a Dremel rotary tool, no matter how favorable a gearing is used, a drive shaft spinning at 15,000 RPM’s will generate enough friction to melt plastic.
2. Despite its excellent performance in loosening tight mechanisms, WD-40 is in fact a cleaner, not a lubricant, and is highly flammable. It should not be used to combat friction in a gearbox.

I could probably have arrived at both of these discoveries via Socratic reasoning had I stopped to think through the implications of my actions beforehand.

…but where’s the fun in that? (Video of attempt #3 below the cut.)

It’s just a Lego gearbox, running so fast that it would melt if it wasn’t being liquid-cooled. :)
(No application in mind for this; I had a few free hours, was tired of coding PHP, and there was my Dremel saying “Play with me!”)

I imagine you could come up with some nasty applications involving cordless Dremels in cars. (Could you rig a working Torque Converter out of Lego? With Mindstorms controlling the throttle?)

Note: This gearbox wasn’t fully enclosed because I wanted to be able to lubricate it. Big mistake. You can’t see on the video, but it’s spraying stuff EVERYWHERE. A sealed unit with a reservoir would be the way to go for any practical application.

Note #2: Attention to detail matters– I dremel’d down the drive shaft to fit the tools’ head… but the result wasn’t perfectly even, which means it’s vibrating at a frequency of ~15,000 cycles/minute. Holding the box steady reallyhurt– my hand was acting as a shock absorber/vibration sink. Without a balanced drive shaft, any finished project will shake itself apart. (I suppose you could just try find a 4-point screwdriver head that’s compatible with the TECHNIC… but that seems like a bad situation, torque-wise.)