Posting this because someone should have thought of it already!
A touch-probe made out of a recycled gel pen and its innards.
Like many of you, I've dreamed about building one of those Renishaw-like 3-point rods-on-balls probes, but it just seemed like too much machining. I wanted something simpler, and I noticed that my clicker pen seemed to have a lot of the necessaries.
Here's what I came up with: <trumpet fanfare><searchlights><fireworks><announcer>"The 2014 edition of the Penticle 5000!!!"<da-da-da-da-da-DA!>
(Pen Tickle)(5000 because its like 5000 times better than just a 1)
What you do: 1. break off a carbide tool to use as a collet mount (choose a diam. that closely fits as a replacement for the clicker in the upper barrel), 2. disassemble a gel pen (ball point might work, but gel has a nice fat cartridge to receive a folded paper clip for half the switch), 3. shorten top and bottom of the barrel and the cartridge (use an exhausted pen to reduce ink mess), 4. glue the pieces back to make a (very) shorty pen (contact cement works well), 5. slot the upper half of the cartridge (dremel: clamp in pliers!), 6. fold up a paper clip for the lower contact and stick it inside the cartridge to ride in the slot, 7. epoxy the shank into the top "at appropriate depth" (kneadable solid epoxy "clay" or "rope" is much easier than liquids), 8. assemble the pieces with the spring in the top next to the shank, 9. fold and wrap a second, stouter paper clip around the upper barrel to form the upper contact (use another pen as a mandrel and needle-nosed pliers as a vise), 10. epoxy upper contact to the upper barrel, 11. assemble with "small gap" between paper clips (hint: they bend), 12. attach "the right type of electrical probe connector" to the free ends of the two separated paper clips, 13. run a probing program (probe output is high until paper-clips contact, pulling it low, so parallel port input is ACTIVE LOW).
As assembled neither paper clip connects to anything so either can be ground or connected to +5V through a pull-up resistor. This means there's no current flowing (no waste) until contact is made, when the parport senses the high voltage is pulled low. (I already made up a probe cable like this with 2 alligator clips for different types of ACTIVE-LOW probes, one clip to ground and one to +5 through a pull-up; the first probe was a copper-pipe center- and Z-finder which also has a place to receive an alligator clip. I'd like to "vacuum-wrap" aluminum foil over the surface of something to be probed and use that as a touchplate too. Anyone ever see this done? 30 years or more ago industry was doing "magneforming.")
It does work, electrically.
How well? I don't know.
Accuracy? Precision? I don't know.
I have to write the LinuxCNC G-code (with its nifty IF-THENs and GOSUBs to allow clever probing), but I'm posting this now, so those of you for whom this prompts a better idea can get on with it!
Hope someone's inspired.