Internal Broaching

Internal broaching forms precise internal keyways, splines, and polygonal profiles in a single pass, ideal for repeatable features at moderate to high volumes.

Overview

Internal broaching removes material with a toothed tool pulled or pushed through a pre-formed hole to create internal keyways, splines, hexes, and gear-like profiles. Each tooth cuts slightly deeper than the previous, producing an accurate profile in one stroke with excellent repeatability and good surface finish.

This process shines when you need the same internal form in many parts: transmission hubs, couplings, gears, or socket drives. Setup and custom broach tooling are expensive, so it suits moderate to high production runs or standard keyway sizes where off‑the‑shelf broaches exist. Internal broaching delivers tight, consistent feature sizes and fast cycle times but requires a straight tool path, adequate chip clearance, and materials within machinable hardness limits. It is less flexible for low volumes or complex, non-standard profiles, where more general machining methods may be more economical.

Common Materials

  • Steel 1018
  • Steel 4140
  • Aluminum 6061
  • Stainless 304
  • Stainless 416
  • Brass C360

Tolerances

±0.0005" to ±0.002"

Applications

  • Keyways in gears and pulleys
  • Internal splines in shafts and couplings
  • Internal hex or square in sockets and drive tools
  • Internal gear profiles in hubs
  • Rotor and stator bores with key features
  • Firearm bolt and breech internal profiles

When to Choose Internal Broaching

Choose internal broaching when you need repeatable internal keyways, splines, or polygonal shapes in a pre-drilled bore at moderate to high production volumes. It works best for straight-through or adequately relieved blind features in machinable metals where you want tight, consistent profiles with short cycle times.

vs Surface Broaching

Use internal broaching when the feature is inside a bore or enclosed hole, such as keyways or internal splines, and the broach can pass linearly through the part. Surface broaching is better suited to external flats, slots, or profiles on open surfaces rather than inside bores.

vs CNC machining

Choose internal broaching when you need standard keyways or splines in quantity and want faster cycle times and more consistent profiles than milling or slotting can provide. CNC machining is preferable for prototypes, low volumes, or odd, non-standard internal shapes where custom broach tooling cost cannot be justified.

vs Wire EDM

Internal broaching is the better fit when you have moderate tolerances, machinable materials, and enough volume to amortize broach tooling, giving much lower cost per part and faster throughput. Wire EDM makes more sense for very hard materials, intricate or non-broachable shapes, or low-volume work where flexibility outweighs cycle time.

vs Keyseating / Slotting

Pick internal broaching for high-volume, repeatable keyways where you want better accuracy, finish, and dramatically shorter cycle times than traditional keyseating or slotting. Keyseating or slotting remain useful for one-offs, repairs, or features that are difficult to fixture for broaching.

vs Gear shaping

Internal broaching can produce internal gear-like profiles quickly when the geometry matches an available or practical broach design and volumes are high. Gear shaping is more appropriate for larger diameters, special profiles, or lower quantities where custom broach tooling is not economical.

Design Considerations

  • Provide a straight pre-formed pilot hole or bore with enough length and diameter for the broach and required chip clearance
  • Use standard keyway and spline sizes where possible to leverage off-the-shelf broaches and avoid expensive custom tooling
  • Avoid cross-holes, deep key chamfers, or interruptions along the broach path that can chip the tool or trap chips
  • Clearly specify tolerance zones and datum schemes for the internal feature relative to the bore and external geometry to aid fixturing and inspection
  • Keep material hardness within the broach supplier’s recommended limits or plan for roughing and heat treatment before final broaching
  • Allow for approach and exit relief so the broach can enter and leave the cut without impacting shoulders or critical faces