The Arduino Mega's PWM pins are digital output pins located ... If your board has no printed labels, check out its pinout diagram documentation (found on the board's Arduino Store page).

and went on to wiring a grbl-fueled Arduino into the controller box. They reverse-engineered the motor driver connections – those go through a 74HC245 buffer between the original CPU and the ...

The machine uses a ‘grbl’ controller, which has an Arduino-based circuit that includes multiple outputs, including a powerful 24V pwm signal that is generally used to drive a 60-80W spindle motor.