A swivel head assembly for a clamshell lathe is provided. The swivel head module includes an adjustable tool head assembly. The adjustable tool head assembly has a tool for machining a work piece, and is capable of being incrementally advanced in a radial direction against the work piece. The swivel head assembly further includes a means for positioning the metal working tool along an x-axis, y-axis, and z-axis. A tripper assembly is operatively coupled to the tool head assembly to incrementally urge the tool head against the work piece upon each revolution of the clamshell ring. The tripper assembly incorporates a solenoid for selectively incrementally advancing the tool against the work piece.