Abstract: Top jaw assemblies combined with a chuck. Each of these top jaw assemblies comprises a body having a base projecting radially inwardly when mounted to the chuck, a cylindrical wall in the body which with the base provides a cutout portion and in which a workable rotatable workpiece gripping member or "slug" is mounted and secured to the base. The slug's periphery or annular wall conforms to and engages the cylindrical wall and includes an opening having a counterbored hole in which a screw mounts and extends below the slug to thread to a threaded aperture in the base. The slugs are machinable to provide different gripping surfaces contoured to the configuration of different workpieces, and are rotatable to provide the necessary reset position for such gripping surfaces in relation to the different sized workpieces. The base can be slightly inclined so as to force the slug's diameter against the matching radius of the cylindrical wall when the slug is torqued to the base.