Broaching is the machining process of cutting a shape by moving a broach cutting tool, usually just called a broach, over material such as metals or plastics. The broach’s rows of teeth, or chisels, progressively increase in size. Each tooth removes the excess material gradually and the desired shape is complete only after the final broach tooth has passed through the material. The shape found in an internal keyway in a pulley or gear is the most common shape produced by broaching since broaching is the simplest method of cutting internal forms known as splines in gears, sprockets, and hubs. Polygons such as squares are also commonly and easily produced by a broach, especially when a round hole needs to be enlarged into a square or other non-circular shape. Sometimes, broaches are also used to cut external shapes such as slots.