[Joezx10r]

Would a variable speed router motor work to drive the spindle on a mini mill assuming you could gear it down with timing pulleys to a slower speed for metal and alu?

[RomanLini]

Yes, but with some problems. You would need more gearing as the router's operate at higher RPM, their motor is designed to make its powerband from about 20000 RPM to 35000 RPM.

And apart from the extra gearing they make a ton of noise when operating in their powerband, so that would be unpleasant right next to your head when you are milling.

It's probably best to stick with a heavy DC motor or some motor already optimised to produce good low speed torque, quietly, and use less gearing. Treadmill motors are available cheap on ebay, and are designed for both high torque and low noise.

[Johann ohnesorg]

DC motor or AC motor with VFD. Those are the options, both have pros and cons. Whatever you do, a larger motor asks for a belt drive. Otherwise, you´ll wreck one pair of gears after the other. A KBLC 120 controller may be a good start for a threadmill motor. This one goes up to three horsepowers with forced cooling.
As stated above, the router is far too fast for the milling cutters and the spindle bearings. When you need the speed, the router should be mounted on a mounting plate parallel to the heads spindle. Servesboth for wood as it would serve for engraving or high speed work in aluminum.
In case you need to rig the belt drive yourself get a second set of castings, bearings and a spindle and built the new head so you can simply switch it when you´re finished with your mod. The parts for the mill are dirt cheap. And having a mill at hand for the mod is really pretty neat. Stay away from cheap asian bearings, get SKF, NTN or Timken. They are worth the little extra money. There are belt mods with readily availiable parts that cost 50 bucks or less, just use the forum search.

Cheers,
Johann