A broach tool is a specialized cutting tool lined with rows of progressively larger teeth, designed to remove material in a single pass. Where a drill makes round holes and a saw cuts straight lines, a broach creates complex internal and external shapes (keyways, splines, gear profiles, square holes) with exceptional precision, often holding tolerances tighter than 5 micrometers on critical surfaces. It’s one of the few metalworking tools that can produce finished, ready-to-use profiles without requiring a second operation.
How a Broach Tool Works
The key to a broach is its tooth design. Each tooth along the length of the tool is slightly taller than the one before it. As the broach passes through or across a workpiece, each tooth shaves off a thin layer of material. The first teeth do the roughing work, the middle teeth refine the shape, and the final teeth produce the finished surface. The amount each tooth removes is called the “rise per tooth,” and it’s typically very small, around 0.06 mm per tooth in many applications.
This progressive cutting design means the entire machining operation happens in one stroke. That single-pass capability is what makes broaching so fast compared to milling or grinding the same shape, which may require multiple passes and tool changes.
Linear vs. Rotary Broaching
Linear broaching is the more common method. The broach moves in a straight line against the workpiece, either pushed or pulled through the material. This requires a dedicated broaching machine, which is essentially a press that drives the tool with enough force and alignment to make a clean cut.
Rotary broaching works differently. The broach is mounted in a special tool holder that tilts its axis about 1 degree off from the rotation axis of the workpiece. This slight misalignment causes only one edge of the broach to contact the material at a time, creating a rotating cutting action that gradually forms the desired shape. The big advantage of rotary broaching is that it doesn’t need a dedicated machine. You can use it on a lathe, milling machine, or screw machine, making it far more accessible for shops that can’t justify a standalone broaching setup. Rotary broaches also work in blind holes (holes that don’t go all the way through), which push and pull broaches cannot do. The tradeoff is slightly less dimensional accuracy compared to linear methods.
Push Broaches vs. Pull Broaches
Within linear broaching, there are two main approaches. A push broach is pressed through or across the workpiece by a ram or press. These tools must be kept relatively short because the compressive force can cause a long, thin tool to buckle or snap under pressure. Push broaching is common for shorter cuts and simpler profiles.
A pull broach works in the opposite direction. The leading end of the tool passes through the workpiece first, then a machine grips it and pulls it through. Because the tool is under tension rather than compression, pull broaches can be much longer, with more teeth and a more gradual cutting progression. This makes them well suited for deeper cuts and tighter tolerances. Pull broaches can operate either horizontally or vertically.
Internal and External Broaching
Internal broaching removes material from inside a hole or opening. It’s the go-to method for shapes that are difficult or impossible to produce with conventional drilling or boring. Common internal broaching applications include:
- Keyways: the rectangular slots inside gears and pulleys that lock them onto a shaft
- Splines: evenly spaced ridges inside a bore, both straight and helical, used for torque transmission
- Internal gear profiles: teeth cut on the inside surface of a ring gear
- Square and hexagonal holes: a rotary broach can convert a round drilled hole into a square or hex shape by cutting one corner at a time
- Gun barrel rifling: the spiral grooves inside a firearm barrel that spin a bullet for accuracy
External (or surface) broaching shapes the outside of a workpiece. This includes cutting flat surfaces, slots, contours, serrated edges, angular splines, gear profiles, and specialized shapes like fir tree slots used to hold turbine blades in jet engines. External broaching replaces what might otherwise require multiple milling or grinding operations.
What Broach Tools Are Made Of
Most broach tools are made from high speed steel (HSS). This might seem surprising given that carbide tooling dominates so many other areas of machining, but broaches have a specific problem that favors HSS: length. A broach can be quite long, with dozens or even hundreds of teeth along its body. Carbide is harder than HSS but also more brittle, making it prone to chipping or fracturing in a tool this size. HSS offers a combination of high working hardness and toughness that resists brittle failure, which matters when the tool is under significant cutting forces across its full length.
Carbide-tipped or coated broaches do exist for high-volume production or when cutting particularly hard materials, but HSS remains the standard for most applications because of its durability and lower cost.
Precision and Surface Finish
Broaching produces some of the tightest tolerances in metalworking. In demanding applications like fir tree slots for turbine components, tolerances below 5 micrometers are standard. The surface finish quality depends primarily on the tool’s rake angle (the angle of each tooth’s cutting face) and the use of coolant during the cut. Cutting speed, interestingly, has almost no measurable effect on surface roughness, which means shops can optimize speed for productivity without sacrificing finish quality.
This combination of precision and speed is why broaching dominates in high-volume manufacturing. A single stroke that takes seconds can produce a finished shape that would require minutes of careful milling to achieve otherwise.
Maintenance and Sharpening
Because a broach’s accuracy depends entirely on the geometry of its teeth, maintenance is critical. Worn teeth produce telltale signs: rough surface finish on parts, increased cutting resistance, vibration during the stroke, and inconsistent dimensions on the finished workpiece. If cutting generates too much heat, it can cause burrs and dimensional errors in the parts being produced.
Sharpening a broach requires specialized grinding equipment designed to maintain the precise tooth geometry and cutting angles. Removing too much material during sharpening shortens the tool’s effective lifespan, so each sharpening cycle is a balancing act. After every sharpening, the tool’s dimensions need to be verified to confirm it still meets tolerance requirements. A well-maintained broach can produce thousands of parts before needing replacement, but a poorly sharpened one can drift out of spec and produce scrap.