Reflective RPM sensor mount for Taig mill

This is an experimental RPM sensor mount that is meant to friction-fit behind a Taig mill's headstock without modification, and allow an optical sensor to point at the pulley and trigger on the passing of the set screw.

To date I have printed it, mounted a sensor and hand-tested with a simple power supply and LED to verify basic operation, but have not tested at speed on a running mill (and due to other obligations am unlikely to have a chance to do so in the near future).