Existing methods of tool depth control experience instabilities when applied to dual-headed welding or parts lacking sufficient support around the nugget. With the increasing use of Friction Stir WeldingFriction stir welding in the assembly of long aluminumAluminum extrusions, there is a desire for better control in this scenario. A new method of tool depth control is presented which can control the relative engagement of a pin tool with the part, regardless of part deflection. This control methodControl method is based on a relationship between the tool engagement and the in-plane forces on the tool, and can accommodate live disturbances to the part position due to part deflection, loss of containment, or movement introduced by an opposing weld tool in the case of dual-sided welding. The patent-pending control algorithm is presented along with supporting data showing the degree of allowable disturbances. A theory on the underlying mechanics enabling this method of control is also presented.