Abstract: Method and apparatus for forming a base defining a trunnion and a portion of a leg for a roller-cone-type rock bit. A workpiece is rough formed to comprise leg work, having a preliminary end surface, and trunnion work extending angularly from the leg work. An interface plate is mounted on a holder of a lathe with an indexing surface thereof facing generally radially inwardly but angled to correspond to a desired angular orientation of a trunnion in a finished bit. The workpiece is removably mounted on the interface plate, with the preliminary end surface abutting the indexing surface, by at least one pin-type connector projecting from one of the surfaces into the other. The trunnion work is then turned on the lathe. Other parts of the machining of the workpiece may be performed by removing the workpiece from the first interface plate and similarly mounting it on plates adapted for other machines, but otherwise similar to the first plate.