Abstract: A jaw assembly (17) in combination with a master chuck jaw (18) of a chuck (16), the assembly (17) having a base (29) that includes a sleeve insert (48) that is splined (49) to mate with a spline (50) in a bore (44) in the base (29); with an annular chamfer (46) on the bottom of the base (29) to which a flared-out portion (52) of the insert (48) keys. A slug (26) adapted to hold a workpiece (30) includes splines (36) that mount to the splines (49) of the splined insert (48) that extends upwardly of the base (29). Above the splines (36) in the slug's bore (32), an unsplined portion of the bore (32) provides for a shoulder (38) that seats a threaded bolt (40) that threads to internal threads (54) provided in the insert (48), for securing the slug (26) to its base (29).