A CNC Compatible Electronic Edge Finder

An essential part of setting up a machine tool is detecting when a probe or cutter comes in contact with a reference surface. My circuit performs this task in conjunction with a Computer Numerical Control (CNC) system with the same accuracy as the best edge finders. The cost of the components is under $3.

The Electronic Edge Finder uses five resistors, one capacitor, and an LM393B comparator. I chose to use four connectors to make it easier to remove the circuit board from its enclosure. It is hand-wired. This picture shows two transistors that are no longer used.

Using an ohm meter, measure the resistance between spindle and machine body. It must be at least 1 ohm for my circuit to work. I do have an option for working down to 1/2 ohm that is explained in the full article.

Wire up the circuit on a perf board. See the supporting file for a suggested layout.

The spindle wires independently connect to the spindle of the machine. The machine body wires connect to the metal enclosure which rests on the machine body. For a mill, it is the table. For a lathe, it is the ways. Connect power to a USB wall wart and touchdown and probe detect to the CNC system. When a probe or cutter contacts the machine's body, the system detects touchdown and the LED lights. Here is a demonstration using an older version: https://www.youtube.com/watch?v=3Fj7y2lxvy4&t=18s