The original rigid tap feature was introduced on the Fadal machine back in 1979 with the first machine, the VMC45. A very unique feature at the time, it was possible thanks to a large DC permanent magnet servo motor was used for the main spindle motor, programmable just like the XYZ axes.
In 1983, the VMC40 and VMC4020 models were first introduced without rigid tapping. For higher RPM and as a cost reduction, a common AC induction motor was powered by a PWM Inverter. Unfortunately, the inverters could not control the spindle rotation with the servo precision required for rigid tap. Machines at the time used tapping heads to accommodate for the synchronized mismatch of the Z axis and the spindle during non-rigid tapping.
Then in 1991, the introduction of PWM Vector Technology made it possible to control an AC induction motor in ways the inverter never could. With the Vector Technology, it also made it possible for Fadal engineers to revive the old VMC45 code for Rigid Tapping and add the feature to all the 10,000 RPM machines.