Here is the mold being made for the back plate on the self-tuning headstock. The master, or “plug”, is a hi-res 3D-printed plastic model of the part. You can see from all the layers of plywood that this mold was built to last. Someone had fun making it so perfect.

A basic mold has two parts – the base and the lid. The base starts out as an empty box with .260 bullets glued into the edge to serve as taper pins for the lid, and is then filled in with artist’s clay (pale yellow stuff) to about an inch below the top edge of the box. The plug is embedded in the clay, care being taken not to bend it so that it rests in the clay without stress in its final proper shape. The clay is smoothed and brought up very carefully to a desired “parting line” around the edge of the part. The picture was taken at this point. The next step is to put on the mold’s lid and pour silicone in through a hole to fill up the space between the clay and the underside of the lid. When that is hardened, the mold is opened, the plug is fit into its impression on the underside of the lid, and the clay is removed.  Silicone is poured into the base to precisely fill the space where the clay was. When it is hardened, the mold is opened, the plug is removed, and you are ready for business.