Broaching is here to stay

I started working for my father in 1993, buying and selling machined parts. Not long afterward, we visited a small machine shop. It was my first trip to a shop, and I was hooked.

The shop had several Bridgeports, Monarchs and LeBlonds, all of which were manufactured in the 1940s and ’50s. A few graybeard craftsmen with scales in their pockets turned dials, read indicators and manufactured parts without computers. Since then, I have been enamored with craftsmanship and have admired the skill of well-seasoned machinists and toolmakers.

Advances in machine tools, cutting tools and manufacturing processes are inevitable and necessary to ensure we remain productive, but they cannot replace the skill of craftspeople or replace processes fundamental to manufacturing. That is why I become frustrated reading articles that talk about dying crafts or predict obsolescence of old-school methods.

I recently read a story about a small broach manufacturer whose owner was concerned that other technologies eventually would make broaching obsolete. Impossible. Broaching is one of the most productive metal removal processes available to manufacturers regardless of size.

Automotive manufacturing relies heavily on broaching technologies to deliver low-cost, accurate parts like gear racks, internal splines and other complex internal shapes. Gas turbines and aerospace engine components depend on broaching for complicated internal shapes with close tolerances and fine surface finishes. Firearms makers utilize broaching technology to generate barrel rifling. Even small tool shops use inexpensive broach tools to create internal keyways and hexagonal and square holes in parts.

Broaching has four advantages that protect it from obsolescence: speed, stability, accuracy and versatility.

The cutting speed of traditional broaching operations is around 762 mm/min. (30 ipm), whereas the surface speed of other processes, such as milling, can be several hundred inches per minute, making broaching appear slower. When judged on surface speed alone, broaching is slower. However, when compared with milling, for example, broach tooling does not need separate roughing and finishing passes. Accurately milling complex geometries often involves changing from a roughing tool to a finishing tool and making finish passes with low chip loads to achieve size and finish requirements. A broach tool is made so its finishing portion has the optimum number of cutting edges and spacing to produce the required finish. No tool change is necessary.

Milling tools also require constant adjustment to maintain desired geometry. And when they are sharpened, the diameter changes, which forces adjustments. Broach tools are capable of repeated sharpening before modification is needed to maintain geometric requirements. Broaching also spreads work over a larger number of cutting edges, which greatly increases the number of parts that can be machined before the tools must be changed.