making a digitizer probe

I'd like to try and make myself a digitizer probe now that my mill is
up and running but am not sure of how to go about it. So far I have
come up with the idea of having a pointed probe running on a little
precision slide that will block a slotted opto sensor when the tip is
depressed.

I'm not sure of the best way to get repeatability but I already have
a tiny little THK slide that might form a basis of the unit. Having
said that it would not mount easily into the tool holder.

Anyone done this?

Cheers,

Graham Stabler

www.indoor.flyer.co.uk