Hi Russell,
The bearing limit is for the big bearings I have supporting the chuck. I decided to use a standard ER16 collet chuck and this came with a 20mm ground hollow shaft. I've used fairly big bearings to support this, but the highest speed ones I could find in this size (without going to insanely expensive ceramic ones) were limited to 20,000 rpm.
I opted not to use the motor shaft, mainly because it's only 5mm diameter and probably not up to taking much in the way of side loads. The standard bearings in these motors are supposedly able to take very high rpm, although there seems to be a fair bit of chat on some of the RC model forums about replacing the cheap Chinese bearings they are usually fitted with and going for something a bit better. Apparently bearing failure is reasonably common in big models that put a lot of load on the motor.
I've got around the potential bearing runout problem on the motor by coupling it to the collet chuck using a rubber dog coupling. This was easy enough to do and I can be sure that the motor bearings aren't seeing any side loads. You can just see the coupling inside the slot in the case.
If you were to go for one of the really big RC motors, then you get the benefit of a bigger shaft that you might well be able to use directly. I bought a couple of 2.8kW RC motors that have 8mm shafts, they were pretty cheap for the power, around $50 each. Probably overkill for a small spindle, but they can always be under-run for reliability.
Using an off-the-shelf ER type collet chuck means I don't have to worry about concentricity, plus I get a good range of collet sizes, from around 1mm diameter to maybe 10mm diameter. I'm currently probably only going to use the 3-4mm collet and perhaps the 6 to 7mm collet, as these will hold 1/8" and 1/4" cutters, which is all I really need.
The spindle came from a local tool shop, but they are often on eBay. Do a search on ER16 collet chuck and you should find a few. They aren't too pricy, either, I think mine was around £20 (maybe $30 US or so).
Jeremy  |