Commercial (e.g., Tormach, Novakon, or Sherline) or retrofitted (e.g., Rong Fu or Flashcut) home shop CNC mills are generally driven by software with DROs that display the spindle speed setpoint but don't monitor actual spindle speed. Mach3 is typical of control software; it has a digital spindle speed readout, which shows the spindle speed set-point, but does not monitor actual spindle speed. Some manufacturers (for example, Tormach) offer an analog load meter that monitors the VFD drive, but the load meter does not show spindle speed. A direct reading spindle speed monitor will confirm actual spindle speed and can also serve as an overload monitor since rpm will drop as the spindle power limit is reached.
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