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 . the workpiece can subsequently be returned to the first interface plate for fine turning of the trunnion . also , the various interface plates can be exchanged for other plates providing differently oriented indexing surfaces to machine parts for other bits of other sizes or designs . a relatively small part of the leg work is ultimately removed before incorporating the base in a bit .