What is Manual Pulse Generator on a CNC machine！

The simple answer is
Handwheel on a CNC machines is called a Manual Pulse Generator.

Or you might say,
Handwheels are manual pulse generators used primarily for positioning of axes on CNC machines.

Technically,
A manual pulse generator (MPG) is a device normally associated with computer numerically controlled machinery or other devices involved in positioning. It usually consists of a rotating knob that generates electrical pulses that are sent to an equipment controller. The controller will then move the piece of equipment a predetermined distance for each pulse.

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