Broaching

Broaching uses a multi-tooth tool pulled or pushed in one stroke to form precise internal or external profiles quickly at medium to high volumes.

Overview

Broaching is a machining process that uses a long, multi-tooth cutter pulled or pushed through a part to generate accurate internal or external profiles in a single stroke. Each tooth removes a small amount of material, producing features like keyways, splines, and complex internal forms with good surface finish and consistent dimensions.

Broaching shines when you need the same profile repeated in volume: transmission splines, keyways in shafts, or shaped through-holes in production parts. Tooling is expensive and setup takes effort, so it pays off best for medium to high quantities or recurring work. Expect tight, repeatable profiles and short cycle times once the tool is built, but limited flexibility if the geometry changes.

The tradeoff is clear: very low cost per part after tooling, in exchange for higher upfront design and tool investment and the need to align your geometry with a straight stroke and constant cross-section.

Common Materials

  • Alloy steel 4140
  • Carbon steel 1018
  • Stainless steel 304
  • Aluminum 6061
  • Ductile iron
  • Brass

Tolerances

±0.001" to ±0.003"

Applications

  • Internal keyways
  • Spline shafts and hubs
  • Internal gear profiles
  • Hex or polygonal through-holes
  • Surface flats on forgings or castings
  • Turbine disk fir-tree slots

When to Choose Broaching

Choose broaching when you need a repeated internal or surface profile with a straight stroke and largely constant cross-section, especially in medium to high production volumes. It fits parts where keyways, splines, or shaped through-holes must be accurate, consistent, and fast to produce. It also suits hard-to-machine materials once the broach is properly designed and coated.

vs Milling

Pick broaching over milling when you have a repeat internal or external profile that will run in production and justifies dedicated tooling. Broaching will usually give faster cycle times, more consistent form profiles, and better control of multiple teeth or flanks in a single pass.

vs Turning

Choose broaching instead of turning when the feature is non-rotational or cannot be generated by lathe tools, such as internal keyways, splines, and polygons. Use broaching for production shafts and hubs where the OD is turned but the internal profile needs a fast, precise, single-stroke operation.

vs Drilling

Use broaching over drilling when you need a shaped hole or internal form rather than a simple round hole. Often you will drill a pilot hole first, then broach to final profile for keyways, hexes, or splines with better form accuracy than you can achieve by drill-only operations.

vs Grinding

Select broaching instead of grinding when form accuracy and speed on repeated profiles matter more than ultra-fine surface finish or sub-thousandth tolerances. Broaching is better suited for high-volume production of complex internal shapes, while grinding is usually reserved for finishing critical diameters or surfaces after heat treat.

vs Electrical Discharge Machining (EDM)

Choose broaching over EDM when the material is machinable and you want much faster cycle times and lower cost per part at volume. EDM remains useful for very hard materials, extremely intricate shapes, or low-quantity work, but broaching wins on throughput and part cost once you justify the tooling.

Design Considerations

  • Align the broached feature with a straight tool stroke and keep the cross-section as constant as possible along the length
  • Provide adequate stock for the broach to cut uniformly, but avoid excessive material removal that increases tool load and wear
  • Include runout and relief grooves so the broach can exit cleanly without crashing into shoulders or fillets
  • Standardize keyway, spline, and polygon profiles to use existing broach sizes rather than custom tooling where possible
  • Hold tolerances realistic for broaching and call out only truly critical surfaces to avoid unnecessary tool complexity and cost
  • Ensure robust fixturing surfaces and clear datums so the shop can locate the part consistently relative to the broached feature