M3 turns on the ______.

The correct answer is that M3 turns on the spindle. In CNC programming, the M codes are typically used to control various auxiliary functions of the machine. The M3 command specifically instructs the machine to start the spindle in a clockwise rotation. This command is crucial for operations that require the spindle to spin in order to cut into a workpiece using the inserted tool.

Understanding the significance of controlling the spindle is vital in CNC machining because the rotation speed and direction directly influence the machining process and can affect tool wear, surface finish, and overall machining efficiency. Knowing how to properly utilize commands like M3 is essential for effective programming and operation of CNC machines, ensuring that the spindle functions optimally for the intended cutting operations.