If you've got an Arduino of any variety, you might have noticed some of the pins on the board have a tilde mark or PWM printed next to them. What is PWM?

[Runtimemicro] has the answer, at least if you are writing PWM code for the Arduino. Their free application lets you set a few parameters, visually see the results, and then generates code for you.

Lastly on that connector is the spindle output and, just like the 36V fan output, the connections are: +ve directly connected to the 36V power input and -ve modulated to 0V from the cnc domain by ...

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.