1600 rpm just doesn't cut it for CNC. Small radius cutters and soft materials quickly bring the required feeds and speeds up by an order of magnitude. The cnc (cheap n cheerful) motor and gear head combination in my mill is to a real VMC spindle as Arnold Schwarzenegger is to Robin Williams on a chemically enhanced day. All this to say that it was time to build a high speed spindle.
After a little research I discovered that one possibility for an inexpensive alternate spindle motor is a Bosch Colt trim router - one of which I happen to own. The main limit to usability for this kind of setup seems to be the TIR of the bearing and collet for small i.e. <1/32" ΓΈ cutters. If the TIR is above a certain percentage of the bit diameter (I forget the number right this second) it will fatigue and fail within seconds. Although there are apparently some QC issues with the Bosch, if you are lucky the bearing runout in combination with a high quality collet (sold separately and cost as much as the router(!)) will be small enough to allow micro-machining. That is beyond my needs right now and so I haven't purchased a collet or measured the TIR.
All that I was missing to increase my available RPM by an order of magnitude was a suitable bracket. Borrowing heavily from Tormach's design I drew one up in CAD and threw a chunk of MIC6 tooling plate on the mill.
My design uses two 3/8-16 bolts to tighten a pair of split rings sized to the spindle diameter of the mill and the router.
Interior bore with the shop vac running to keep the chips from spreading too far through the shop.
Hole patterns in CNC are your friend.
Reclamping from inside the bore (tightening down before removing the for exterior clamps to maintain the positioning).
I made a small mistake in the design which I discovered when I tried to tap the hole in the first bracket: the hole is too deep for the standard tapping bit. Solved with a small design modification and some creative tool-holding (that is a cheap brazed carbide boring bit).
The slot is sized just over the width of the hex nut so the split ring can be tightened with a single allen key.
Other half of the bracket with the bore sized for the router.
Cutting the splits on the table saw with a non-ferrous metal blade.
Finished! About two days from drawing to done.
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