Electrochemical machining method, tool assembly, and monitoring method

In an electrochemical machining tool assembly having at least one electrode arranged across a gap from a workpiece, the electrode being energized by application of a potential difference DeltaV between the electrode and the workpiece, a method of monitoring machining includes exciting at least one ultrasonic sensor to direct an ultrasonic wave toward a surface of the electrode and receiving a reflected ultrasonic wave from the surface of the electrode using the ultrasonic sensor. The reflected ultrasonic wave includes a number of reflected waves from the surface of the electrode and from a surface of the workpiece. The method further includes delaying the excitation of the ultrasonic sensor a dwell time Td after a reduction of the potential difference DeltaV across the electrode and the workpiece occurs.