Expanding Jaw Tailstock Grip and Controlled Torsional Forming: A Retrofit Assembly for Standard Lathes

This paper proposes two connected mechanical innovations for standard lathe platforms. The first is an expanding jaw tailstock grip: a shaft-mounted radially-expanding jaw mechanism that inserts into the open bore of a hollow workpiece or a sacrificially-bored solid workpiece end, expands outward against the inner surface, and transmits torque bidirectionally — enabling the tailstock to hold, centre, and resist rotation of workpieces that cannot be gripped from outside. The second is a controlled torsional forming process enabled by the first: a braking element applied to the tailstock spindle creates a controlled rotational resistance while the headstock drives at full speed, causing the workpiece to experience deliberate, adjustable torsional stress along its length simultaneously with machining or forming operations. Combined with variable headstock-tailstock centre distance, the assembly additionally applies controlled axial tension or compression. Together these constitute a retrofit kit — two additions to any existing lathe — that converts a standard rotational machining platform into a multi-axis simultaneous stress-state forming and machining system without drive system modification or electronic synchronisation.