precise positioning/milling a hole

You need a "wiggler". It is a needle point shaft on a flexible ball joint on the end of a 3/8" shank. You chuck the shaft in your mill spindle and run the spindle at about 1000 RPM (speed is not critical) Use a wooden or plastic stick to bump the spinning needle shaft until the point "freezes" or appears to go stationary. It is now coaxial with the spindle and you can use the it to locate your hole center. Wigglers are available at any machinist tool supplier.

Randy

Although it sounds awfully crude it's often more convenient (and just as accurate) to use a "sticky pin". This is just the broken off end of a darning needle stuck into a lump of plasticene (modelling clay). The clay lump is simply stuck against the end of the end mill that you are using and the needle point centred in the same way as with a wiggler.

I use the pointy end of my edge finder for this. A sharpish cone that you can sight down from, to your mark. I think that's what it's for, anyway...:)

Brian

Make a "spud". This is just a piece of drill rod with a tapered point turned on one end. I made mine with a 15 degree included angle. The drill rod stock should be large enough to be stiff - at least 1/4". Used carefully with the help of a good magnifing glass, you can easily get within a few thou.

Ted