Abstract: A workpiece holder assembly for vacuum-holding an aircraft wing skin or other workpiece for machining includes a base plate formed preferably of metal and adapted to be installed in a well area of a mill bed having vacuum passages therein, and an insert tool formed preferably of polymer material and adapted to be removably installed in a recess formed in the base plate. The base plate and insert tool have vacuum holes extending therethrough for communication with the vacuum passages in the mill bed. The surface of the insert tool facing away from the base plate includes one or more seal strips retained in grooves for sealing against a surface of a wing skin or other workpiece, and also includes one or more depressed regions for accommodating one or more protruding features that project from the workpiece surface, such as padups, steps, or taper planes on the inboard end of a wing skin panel.