Touch Off Light for my Probotix CNC

No touch off plate came with or is available for my CNC (yet).   I've been using a piece of paper to set my Z axis origin height. I put the paper under the bit, then slowly (.005") step it down until the paper is trapped. Set Z at 0 there then lift up to remove the paper. Paper is generally from .0025 to .005" thick so I know I'm within .005 using this process.

A Probotix.com User Forum member KLWestfall posted his idea of using a light bulb, thin metal plate, and an alligator clip to signal when Z has touched down.   That idea intrigued me so much I knew I had to make my own version.

I went to my local Radio Shack (which sadly has now closed down thanks to internet competition) and picked up a battery case, 3v green LED bulbs, and some AAA batteries to go with the unused touch puck and alligator clip (Link) I've had for awhile.

I drilled a 5mm hole in the battery case for the LED, then wired the LED in line with the batteries and contacts. Now the LED glows when the plate and alligators touch.

To use I clip to the router bit and place the plate underneath it atop the material I'll be cutting. I jog the bit to within 1/8" or so of the disk, then set my step increment to .005".   I'll then step down until the LED glows. I'll move up one .005" step, then reset my increment to .001".  Now I step until the LED glows again, and I know I'm within.001" of the top of my material.  I've carefully measured my plate.  I enter that value when touching off Z in LinuxCNC.

This solution isn't as automated as it could be if the touch plate was wired into my CNC controller, but so far I haven't found any info on how to go about that.  The LED that glows on contact will be more accurate than using paper was. The led is easy to see from back at the keyboard/PC station and makes it easy for one person working alone to handle setting Z.

The only problem to this point was that my touch puck wasn't consistently thick.  I had my CNC mill a hole to tightly fit the metal disk. then press fit my puck upside down in that level (to the CNC) hole. I had the CNC mill a flat plane across the plastic bottom of the puck to make it uniformly thick.  The photos here show a simple bar of aluminum used for the touch plate,  The numbers on the battery box remind me how thick the plate is.

If you don't have or want to buy the puck I linked to, then any small chunk of conductive metal and an alligator clip lead can be used. You'll need a way to hook one wire from the battery case/LED  to the plate, then solder the other wire to the alligator clip.  I drilled a 5/32" hole in my plate and used a speaker plug inserted into it to connect from the battery. I was inspired by how the touch plate for my CNC Shark connects.

4D