Which operation is NOT a cold-working operation?

Cold-working operations refer to processes that shape materials at room temperature, leading to improvements in strength through strain hardening while typically avoiding significant thermal effects.

Milling, grinding, and drilling are all examples of processes that can be categorized under cold-working because they involve removing material to shape a part without applying heat. These processes operate at ambient temperature and primarily focus on altering the surface and dimensions of the workpiece.

Piercing, on the other hand, is a metal forming process where a tool creates a hole in a material, often involving deformation. However, piercing can sometimes involve heating the material to make the deformation easier, especially in thicker or tougher materials. Therefore, it is not classified strictly as a cold-working operation in many contexts.

Understanding the nature of the processes and their temperature applications underscores why piercing is distinct from the other operations listed, making it the correct answer in this context.