I took this idea from a post on HMEM, and decided to give it a try.  Rather than make my own ring light, I decided to use a suggestion from another poster to get these ring lights.  They come in a variety of sizes, so check your equipment before making a purchase to ensure they will fit.

 

I bought a pair of 60mm ring lights from eBay.  From Hong Kong, chinese made.  Cost me more to ship than the cost of the lights themselves.  I think I paid just over 5 bucks for the pair, shipped.  9 surface mount LEDs, and thay take 12 volts - they're actually made for automotive use.  So, I also purchased a small power supply, again from eBay.  This one is 12 volt, 2.something amps, and was listed as a "LED power supply".  Once again, shipping was more than the item itself, but was less than $7, and it came in last night.  Input voltage is supposed to be from 110V to 208V, but I'll run it on standard household power.

 

I hooked up a power cord, and connected the lights.  Nothing.  Checked polarity, as marked on the lights.  All okay there.  But, I noticed a small connector in the middle.  At that point, they had swapped the red and black wires, so while red was negative where it attached to the light, it was positive at the P/S end.  Switched them around, and presto!  Holy cow, these things are bright!

 

60mm was chosen because it's a perfect fit for the spindle of the mini mill.  Leaves just enough clearance all around to prevent interference, but fits underneath the head of the mill.  They are only slightly thicker than the thickness of the board they're mounted on, about 1/8".  Thin enough to avoid any clearance issues.  The P/S was chosen simply on the basis of the output voltage, and it works great.  Light output is pretty good, and coming from all around the spindle it shines directly on the workpiece.  Next step is to make a mounting plate (P.V.C. for its insulating properties), epoxy the ring to it, and use double-sided tape to attach to the mill head.  I will tap power off the emergency stop switch wires, so that the P/S is energised only while the mill is (and the fan is running) so that I don't forget and leave it on.  Next, I will put a switch in the light wire, so I can turn the light off while cutting if I wish.  Since it's 12V, any cheap switch from the auto parts store will work.  I plan on mounting the P/S to the column, about where the counterbalance spring was.  It's case is fairly open, having lots of holes for ventilation, but being mounted so high and off to the side, I'm not too worried about swarf causing problems.  I'll cover the outside with plastic window screen just to be on the safe side.

 

Light output is good; bright enough that you don't want to stare at the thing.  Probably slightly better than those cheap HF flashlights they give away all the time, but less focused.  Since there is no lens, the light is not diffused, and at 1 foot, shines a ring about 6" in diameter.  Makes picking up center punch marks much easier.  For less than 15 bucks, you can have a custom light right where you need it.