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 on 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.
Interestingly, due to the effectiveness of their original concept and design, today's broaching machines and processes have remained mostly the same since the start of the Industrial Revolution. No job is too large or too small for a broach. Numerous materials such as both ferrous and nonferrous metals as well as many types of plastics are suitable for broaching.
Broaching tools fall under the classification of multiple-point cutting tools as they have at least two cutting edges. Broaches can be custom made from blueprints, but stock broaches are often readily available in numerous lengths and sizes. Common broach shapes include square, round, oval, keyway, serration, D, spline, and pot broaches. Typically, broaches are made from top quality tool steel such as PM-M4.
Besides the broach cutting tool itself, fixtures that support the broach are also needed in broaching operations. For example, in keyway broaching, a fixture called a broach horn supports the broach in a shared circular hole.
All broaching operations require proper alignment of the broach and its supportive tooling. Improper alignment will result in cuts that are not perfectly straight. Misalignment can even cause broach breakage. Lubrication is often used in broaching in order to reduce friction, either by applying cutting oil to the material to be cut, or by lubricating the backs of the cutting broaches, depending on the types of broaches and the broaching materials being used.