Making a ring light for my Sherline mill. It's based on a cheap Chinese USB light. This is available from www.buyincoins.com (code P003369 or P003370 with a lens) and also from www.dealextreme.com (part numbers 50080 or 53782).
They're dirt cheap ($3-$4) and pretty bright. I made a new backing ring to hold the circuit board. It has eight magnets embedded in it which attach to the underside of the headstock, locating on the two little bearing washer retaining screws. The original clear cover has the little prongs cut off and is attached with silicone adhesive so it can be removed again at some point.
All the 18 LEDs are connected in parallel and driven through one limiting resistor. Not the best way, but simple. There were originally three 33 ohm resistors in parallel, resulting in a current of about 150mA. I went for a resistance of about 8.8 ohms, giving 190mA total, or 10.5mA each, assuming current is shared equally.
To make the LED PCB fit the housing, I had to cut off the little bit where the switch was originally mounted.