after trying designs for digital probes I modified others' ideas to make one that will work repeatedly using common hardware and not require a lot of precision during assembly. unique ideas for this probe are - 1. screw tips rest in shallow pockets in bolt heads, 2. foam replaces spring mechanisms typically used. the foam also helps the triangular probe table from getting rocked off-center. this probe is simply a single pole switch. any movement of the probe tip disrupts the circuit that passes thru the three screws. mach3 software and controller hardware [like gecko 540] use this to record the current position at interruption. the probe tip starts out as a carriage bolt. I ground the tip down on a basic grinding wheel. be cautious while grinding not to get the bolt jammed between the wheel and the support table. wear gloves and hold the bolt with locking pliers. it will also get very hot. quench in water occasionally. experiment with different thicknesses and densities of foam to adjust the sensitivity. too loose and the tip won't re-center itself 100% of the time. cnc machine probing speed also effects results. I mount the probe under the portion of the router mount that typically holds the vac hose. this way I can use it and not have to remove router bits or accidently turn the router on with the probe installed! I used solid thermostat wire because that 's what was on my shelf. stranded wire would be best. the pockets were made by scribe drilling a very shallow hole in each bolt head after it was installed. a center hole is shown in the drawings. originally I was using an AL tube and closet door spring for tension, but that proved unreliable. I also realized I didn't have a 1/2" collet for the router to mount it. parts drawing - top row - left:shows alignment of all parts. middle - bottom piece. hex shape routed out to allow bolt head to fit flush. center hole for probe tip. right - make three of these pieces. bottom row - left - probe table. 3 screws pass thru corners and protrude slightly on backside. lockwashers help electrical connection. center hole for carriage bolt/probe tip held with nut. right - top piece. center hole may/not be necessary depending on how you intend to mount to machine. tiny hole is for wire.