Broaching Fundamentals

In a single stroke, a broach can transform a pilot hole into a finished involute spline or a precision keyway to tight tolerances. That combination of speed and accuracy is why broaching remains one of the most enduring metal-cutting processes in gear manufacturing. 

This article covers the essentials: how broaching works, the types of broaches and cutting methods in use today, the materials and coatings that determine tool life, and practical guidance on maintenance and troubleshooting.

How Broaching Works
At its core, broaching is a linear machining process. A broach tool consists of a series of teeth arranged in sequence, with each successive tooth standing slightly taller or wider than the one before it. As the tool passes through or across the workpiece, each tooth removes a small, predetermined amount of material. The final teeth in the sequence—the finishing teeth—are all the same size and produce the finished surface.

This architecture means that roughing and finishing happen in a single stroke, which is a key reason broaching delivers such high productivity. Unlike milling, shaping, or grinding, there is no need for multiple passes or tool changes to go from rough stock to finished geometry.

Internal vs. Surface Broaching
Broaching falls into two broad categories based on where the cutting happens.

Internal broaching shapes the inside surfaces of a workpiece. The broach is inserted through a pre-drilled or pre-bored pilot hole, then pulled (or sometimes pushed) through to create the desired geometry. Internal broaches are the workhorses of gear and drivetrain manufacturing, producing round holes, spline holes, serration profiles, square and polygonal bores, keyways, and a wide range of special internal shapes. The pilot hole itself guides the broach, so the accuracy of that starting bore directly influences the quality of the finished feature.

Surface broaching machines the external surfaces of a workpiece. Here, the broach passes across the outside of the part rather than through it. Surface broaching excels at producing flat surfaces, contoured profiles, external grooves, and complex features like turbine blade root forms (fir tree and dovetail shapes). Because roughing and finishing occur simultaneously in the same stroke, surface broaching is dramatically more productive than conventional milling or planing for these applications.