The "hex-key" Z-auto probe sucks.... at least mine did. I designed this to use with an 8mm diameter x 3mm magnet. I attached a small metal tab on the bottom of my effector for this to mount to when I want to "calibrate" the flatness of my Kossel Mini bed. It uses the standard micro switch only.... no other moving parts. I get great prints using this. Make sure to install the switch with the roller tip under the magnet, to prevent any wobble/tipping. The magnet is epoxied in. You will also have to make your own steel tab to bolt to the bottom side of the effector - in the pics you can see I left it a tad too long (I wasnt sure if I was going to add a second magnet). UPDATE: This is a magnetic mounting point for my Magnetic Z-probe semi-automatic. My original design used a steel tab for the magnet to attach to; Not everyone will want to or have the means to fabricate the metal tab the way I did, so this is a printable part that gets an 8mm diameter X 3mm magnet epoxied in..... for the "Magnetic Z-probe" to attach to. If you print and use this, let me know how it works out, as I have not put it through the rounds..... should work fine though.