Which component is not part of the headstock?

The headstock of a machine tool, particularly in lathes, typically contains several critical components that facilitate the operation of the machine. The back gear, spindle, and motor are all integral parts of the headstock's function.

The back gear serves to adjust the speed and torque of the spindle, allowing for versatile operations depending on the material being worked on. The spindle itself is the main rotating component that holds and drives the cutting tool or workpiece. The motor provides the necessary power to drive the entire assembly, thereby influencing the spindle's rotation speed.

In contrast, an anvil is not a component of the headstock. Instead, it is usually associated with a forging or blacksmithing environment, where it serves as a surface for shaping metal. Therefore, while the back gear, spindle, and motor are essential for the headstock's operation and performance, the anvil does not play any role in the headstock assembly, making it the correct choice to identify as the component that does not belong.