Having looked at some designs for 3D touch probes to use with CNC machines, I thought maybe I could make one.
I found a design that apparently is based on an expired patent, probably this one: https://patents.google.com/patent/US5146691A/en
So, I got busy making my own design in 3D. I got some ball bearings from an old spinner, but I guess any bicycle or skateboard bearing will provide the same balls. I measured their diameter and looked at what else I had laying around. I found some 4mm brass solid rod which I would use as the contacts between the metal balls. And I also found some M6 threaded rod which I thought I could use to make the probe tip.
This is the design I came up with:
I was reasonably happy with the result. It would be nice in the future to make a ball tip, maybe with some epoxy to protect the objects being probed.
I 3D printed the parts in FormFutura ApolloX filament on my Wanhao Duplicator 4S. I cut the brass rod into 3 8mm long pieces. Then I first tried to just lay some pieces of wire and hope that the metal balls would make a good connection. That didn't work. I then tried to solder wire directly to the metal balls, and I had almost no hope this would work, but actually worked really well. I soldered all the balls to their respective wires.
I then glued the wires and the balls to the base with superglue. As you see when the hub with the brass rods rests in between the metal balls, it closes the circuit, and as soon as the probe tip moves, it will open that circuit. The spring shown ended up being too short to force a decent connection, so I had to replace it with a longer one. The springs I got them from a box I bought from Aldi. They sometimes have these plastic boxes of springs, o-rings, rivets, etc. I recommend you grab some of those when they come around, they're very handy (and cheap).
The first time I glued the 3 rods in place and didn't immediately check if they sat correctly between the metal balls. The result was that there was a slight misalignment and the circuit didn't close properly. I had to break the hub to remove the rods and print a new one. This time I first made sure all was aligned and glued them in place afterwards.
I applied some lubricant on the metal parts and while checking the continuity I closed the assembly. I modeled the probe to match the length of some M3 screws I had, and made it so the nuts would be captive. This makes it very easy to close the assembly and looks really clean.
After that I could check if it all worked correctly.
Parts needed: 3x M3x30 screws 3x M3 nuts 3x 8mm segments of 4mm brass solid rod 1x 40mm length of M6 threaded rod or any other solid metal rod that has at least 5mm in diameter to shape into the probe tip. 6x ball bearings from skateboard bearing. 1x Spring, mine was 9mm in diameter and 35mm long, spring wire is 0.75mm thick.
If you know how to 3D model, this should be easy enough to make. But in case you want to make mine, here's where you can get it, for a small price.