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CNC Height Probe – Understanding GRBL

cnc height probe

Make sure that your CNC Height Probe is working properly with the very simple approach I describe in this article. With the proper response, you know you are good to go ahead with milling your  PCB.

So, the other day I went to mill my first printed circuit board using my 3018 CNC machine. Because the layer of copper on a PCB is so very thin, you only need to mill (carve) away a tiny bit of copper (less than one tenth of a millimetre) to get an isolated trace. You will find that the entire board is only 1.5 mm thick. So, exactness counts.

What makes this type of CNC milling a challenge is that very few things in life are perfectly level or completely flat. So, you need to adjust your milling pattern to compensate for variations in the surface of a raw circuit board. You accomplish this by creating a surface map of the PCB, then automatically adjusting your CNC bit to track the actual surface, blemishes and all.

Most cheaper CNC machines run GRBL motion control software. Both CNC and 3D Printer machines use G-Code to translate CAD designs into Cartesian (x,y,z) motion. GRBL takes in your G-Code and coverts it into exact tool motion. If you have previously created a height map for the surface of your raw material, GRBL makes tiny adjustments on the fly to compensate for variations in surface flatness.

You can see how this whole process of surface mapping and GRBL milling a PCB works in this video.

But, when I went to create my surface map, I found the height probe did not work. It turns out I had a bad solder connection which I needed to fix.

CNC Height Probe – Testing To Make Sure It Is Working

My GRBL CNC Height Probe works by sending an electric current through the cutting tool and copper surface. You connect the voltage between the tool and copper, as shown above. When the tool touches the copper, the circuit completes and you have probed and measured the exact height at a certain point. Normally, you probe say fifty or more times across the area you intend to cut and GRBL records the results in a height map.

If you want to test your probe to make sure it is working, here is what you do in GRBL Version1.1f.

  • Connect a serial port terminal to your GRBL controller, set for 115200 baud.
  • Change the bit map for status reporting $10=19.
  • Request a status report by entering “?”. If the probe is working and shorted, you will get a Pn=P (probe positive) response. If the probe circuit is open, there is no Pn response.

Now that I have fixed the bad solder joint, everything is good to go. You can read more about GRBL configuration settings here.

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