Apparatus and methods for performing and controlling the sequence of steps in a multi-step machining process utilizing a plurality of drilling units, where each drilling unit is configured to perform at least one step of the multi-step machining process. Air pressure sensors in each drilling unit measure air pressures at the surface of a workpiece where the cutting tool of the drilling unit is engaged, which measured air pressures indicate air flow at the surface. These air flows in turn indicate the state of the machining at the surface of the workpiece, and based on the state of the machining, a machine control system will determine whether a drilling unit can perform its particular machining operation in the proper sequence on the workpiece.