Originally Posted by
thebodger@roger
We had early generation NC machines the used servo valves and rotary hydraulic actuators ( motors) for positioning. They still had to have ballscrews to move the axis. The machines were made by Giddings and Lewis U.S.A, and Marvin of England. The reason they used hydraulics, was because the power electronics needed for electric servo motors, was not yet developed. The servo valve controllers ( amplifiers) were only needed to supply low voltage and current to the servo valve, so it was do-able with what was at hand at the time. I remember that the surface finish on the machined parts was not very good. The hydraulic motors were hard to control at low cutting speeds, and at stand still. These systems were quickly abandoned, once high power transistors came of age. Everything switched over to electric drives, the benefits were obvious.