What is a CNC probe?

A CNC probe is a device you connect to a CNC machine to aid you in zeroing the Z axis for depth cutting.

This wasn’t something that I was too bothered about to be honest when I first started building my first CNC machine. They aren’t essential as you can lower the spindle or router manually and move it in smaller and smaller increments and use a piece of paper under the router bit to lower it to the work surface. This is how I used to do it but I wanted something that was quicker after I started using my machine and once you’ve used a CNC probe you won’t want to go back to the method above.

DIY CNC probe

You can make your own probe and that’s what I did when I started experimenting. You need something that is metal so it’s electrically conductive and you need to know how thick it is. You need to be spot on with how thick it is as this will affect all your measurements. I used a micrometre like the one which is described in the “tools and bits you’ll find you need”

You also need to connect a wire to it and the usual method is to use a crocodile clip to connect the wire to the router bit.

How does a CNC probe work?

Basically as you’ve probably already guessed the whole probe thing works by electrical conductivity and lowers automatically until a circuit is completed between the metal probe base and the metal router bit, once the circuit is complete it stops the lowering as it has found the correct depth setting.

It is always a bit scary watching the router and bit automatically lowering itself towards the probe base and table. The probe only lowers slowly so on its first attempt I lift up the base to touch the descending router bit to check it stops. If it doesn’t you’ve still got time to hit the emergency stop button. You did fit an emergency stop button didn’t you?

I definitely advise you to fit an emergency stop button if you haven’t already. You can read why and how to do this in the article here.

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OK I admit in all the initial excitement of getting my first experimental CNC machine built, tested and working I didn’t bother with an emergency stop button. I was going to fit one after the first tests were successful but then one thing lead to another and I was excited to make some stuff so the emergency stop button idea never happened.

So there I was with my probe getting ready to cut some parts out of a sheet of material and the probe lowered but didn’t stop. Then router bit crushed the probe and made a mess of the router bit as it managed to get about 8mm into the probe base before I had yanked the CNC mains plug out of the wall to stop the machine. Things could have been much worse and I would seriously insist you wire an emergency stop button.

Anyway back to the probe, you need to wire it up. As I used the popular Arduino Uno board it was wired to the A5 connection and earth, then to two sockets on the CNC machine so I can unplug it after its set the probe.

You can see the two sockets in the picture above, the probe can be unpluged and put out of the way when it’s not needed.

One connection goes to the probe base and one connection goes to a crocodile clip that you clip to the router bit.

The software that I use is the OpenBuilds CONTROL and if you switch to the troubleshooting page it gives you a real time display read out of the status of the end stop switches and the probe under the “Inputs / End stop” list. So if you lift the probe base up to touch the router bit and alligator clip, you can see the circuit connect and the status goes red and says “on” and then back to “off” when the probe base is moved away. You can also use this page to see the limit switches. If you can reach and access one press it and you should see the switch status change on screen.

This shows you that the probe circuit connectivity is working so when you select probe on the OpenBuilds CONTROL software you get asked to make sure the probe is in right place before you go ahead. If you want to know more about CNC software I have an article for you to read here.

So with all this checking how did I manage to get the router bit to crush my probe?

Well after checking the troubleshooting page and touching the probe base to the router bit I realised the bit wasn’t quite over the material so I manually jogged to bit along a few millimetres and in doing this I didn’t realise the crocodile clip had come off the router bit so there was no circuit to complete and the probe got crushed.

So now I always lift up the probe base to touch the router bit on its first run to double check it stops before I then reset it and let it complete its proper setting. I still get a sense of relief though when it actually stops.