PSA: Limit/Homing switches, probing, etc... fixing the interference

I’ve been working on my machine, tuning it to be capable of very fine aluminum ‘engraving’ (per se) and the need for using GRBL’s probe functionality arose when my trial cuts revealed that I needed to either mill flat the surface of my aluminum or perform autoleveling. Due to a lack of cutters, and being that my plates are already pretty thin (.05") I decided to go with the autoleveling. I wired in my own little cable to the ground pin and the A5 pin on the arduino (after an hour long nightmare with a soldering iron that wasn’t hot enough, eek!) and followed with some big fat alligator clamps on the opposite ends of the cable.

Initially I was hooking up the A5 pin clamp to the spindle and the aluminum plate to the ground clamp, and was encountering a lot of false touches a long ways before the tool was even close to the metal. I assumed this had something to do with conductivity in the machine’s frame, being that I’m directly mounting the aluminum to the 20mm extrusions that would normally have a wasteboard mounted ontop. I properly insulated the aluminum from the frame but continued having the same issues.

However, I did discover that if I connected the ground clamp to the spindle, and the A5 pin clamp to the aluminum I had much better results. The problem was that there remained a sparse number of intermittent false touches. This occurred only when I had the A5 clamp on the aluminum, and otherwise never managed to cause a false touch when it wasn’t clamped to anything.

I googled for a solution but couldn’t find much on the probing functionality of GRBL, but I did manage to find a lot of information on fixing the limit switches. One fix, in particular, stood out to me due to the fact that it was a different approach than the others I had read. Instead of resorting to shielded cables and adding extra pull up resistors (or are they pull downs?) this instructables explains that the proper fix is the use of capacitors:

Being that the probe pin is virtually the same thing as the limit switch pins on the Arduino, and would naturally suffer the same problems as anything else with a machine/setup that has all the things going on that the X-Carve does, I assumed that whatever fixes worked for the limit switches would work for the A5 probe pin.

He suggests using a .47uf electrolytic capacitor. I only had a 220pf electrolytic as well as a .1uf ceramic disc. I opted to try out the .1uf ceramic disc and it has been working flawlessly. I just thought I’d share this little adventure that sent me off on a crazy work binge for the past few hours, and now that it’s too late (for the wife and babies) to cut I’ll have to save my spindle-squareness test cut for the morning. Happy X-Carving!


I can’t wait to find out how it works this morning. This may be worth making a project or video? I’d love to see the process :wink: