An Electronic Edge Finder is a device used in machining metal that lets the user know the precise position of a reference surface relative to the center of rotation of a cutter. Typically this is done either via a very precise switch or by having a probe that is insulated from the rest of the machine. The device presented here is similar to the insulated probe approach but does not require it to be insulated. Tests demonstrate that it has a repeatability error of less than +/- 0.0001 inches yet costs less than $10. Most of the complexity is in the software. Furthermore, this device is compatible with Computer Numerical Control’s automatic touchdown function.
Here are all of the details: http://rick.sparber.org/SDEF.pdf
Rick