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Lesson 5.4CutChapter 5 · Machining

Broaching

A long toothed bar is pulled through or across in one stroke

Broaching cuts a whole shape in a single stroke: a long bar covered in teeth, the broach, is pulled or pushed past the workpiece. Each tooth stands a little taller than the one before, so each takes a thin slice. The keyway (a slot that locks a pulley to its shaft) in a bicycle sprocket or a car gear is typically broached.

Broaching: a real photo
Photo: Franklin Day Jones et al., Public domain, via Wikimedia Commons
A long bar of ever-taller teeth cuts the whole shape in one pullInternal broaching of a keyway: cut-away side view, a close-up of the teeth, and the finished holeMid-pull, cut through the middleBroach (the tool)Finishing teethRoughing teeth, each a little tallerFront pilotWorkpieceFaceplatePullerPulled through in one strokeClose-up: each tooth slices off a thin layerWorkpieceRise per tooth (exaggerated)ToothChip curled in the gulletBroach moves this wayFinished hole, end viewKeyway123456Workpiece and chipsBroach and machineMotion
Broaching a keyway · side view, tooth close-up and finished hole, 6 stepsCircled numbers match the steps below. The tooth heights in the close-up are greatly exaggerated; in reality each step is thinner than a sheet of paper.

How it works

  1. Start with a pilot hole. The workpiece already has a plain hole, drilled or bored. It sits against the faceplate (a fixed plate on the machine) that takes the pulling force.

  2. Thread the broach. The front pilot (a smooth guiding section) slides through the hole, and the puller grips the broach's end.

  3. Pull in one stroke. The machine pulls the broach through at a slow, steady speed, typically a few meters per minute.

  4. Each tooth slices a thin layer. The rise per tooth (how much taller each tooth is) is typically a few hundredths of a millimeter. Chips curl into the gullets (the spaces between teeth) and fall out after the stroke.

  5. Finishing teeth size it. The last few teeth all have the same height. They clean up the surface and set the final size.

  6. The shape is done. After one stroke the keyway, spline (a ring of grooves) or hexagon is finished, often in seconds.

Key facts

Materials
Steels, cast iron, aluminum, brass, nickel alloys
Typical parts
Keyways, splined hubs, square and hexagon holes, gear teeth, slots in turbine disks
Part size
Holes from a few millimeters to about 300 mm; surface broaches can be over 1 m long
Precision
Typically ±0.01–0.025 mm
Surface finish
Typically Ra 0.8–3.2 µm (Ra = average roughness; 1 µm is a thousandth of a millimeter)
Batch size it suits
Medium to very high volume; standard keyway broaches suit small shops too
Tooling and machine cost
A custom broach costs thousands to tens of thousands of dollars; machines cost tens to hundreds of thousands. A hand keyway set with an arbor press costs a few hundred
Speed
Seconds per part for a complete internal shape

Strengths and limits

Strengths

  • Very fast: roughing and finishing in one stroke
  • Makes shapes that are hard any other way, such as internal splines
  • Very consistent from part to part
  • Good finish and accuracy without skilled operators

Limits

  • Each broach makes only one shape and size
  • Custom broaches are expensive and slow to make
  • The cut must run straight through, with no obstacle at the end
  • The part must be strong enough to take large pulling forces

Where it's used

  • AutomotiveSplined hubs in gearboxes, steering rack teeth, keyways in gears
  • AerospaceFir-tree slots that hold the blades in turbine disks
  • MachineryKeyways in pulleys, sprockets and couplings
  • FirearmsRifling grooves inside barrels (traditional method)
  • Fasteners and toolsHexagon and star sockets, made by rotary broaching (a small wobbling broach)

How to recognize it

  • A slot, spline or polygon inside a hole, with perfectly straight, parallel walls running end to end.
  • Fine, straight lines along the direction of the stroke, never circular marks.
  • On a polygon socket made by rotary broaching, a slightly rounded bottom where the cut stops.

Compare with

Milling5.2can cut a keyway too, but slower, and only on outside surfaces or open ends.Gear cutting5.5hobbing and shaping generate gear teeth; broaching copies an internal gear shape in one stroke.Wire EDM6.5cuts any internal shape without custom tooling, but takes minutes to hours per part.

Watch it

ChrisandJimCIM · A classroom explanation of the broaching process

mrpete222 · Pushing a keyway broach through a pulley with an arbor press

Tool Room Engineers Faridabad · An industrial surface broaching machine at work on a flange

Polygon Solutions Inc. · Rotary broaching cutting a spline inside a socket

See real photos

Remember

  • A broach is a whole sequence of cuts frozen into one tool.
  • Each tooth is slightly taller than the last, so the shape grows tooth by tooth in one stroke.
  • Expensive tools, very cheap parts: broaching pays off in volume.

More in Machining

All 100 processes
5.1ShortTurning (lathe)The part spins; a single-point tool cuts itOpen the lesson5.2ShortMillingA spinning multi-edge cutter moves over a fixed partOpen the lesson5.3ShortDrillingA spinning twist drill plunges into the partOpen the lesson5.5Gear cutting (hobbing, shaping)Hobs or shaper cutters generate gear teethOpen the lesson5.6KnurlingHardened wheels press a grip pattern into a turning partOpen the lesson5.7GrindingA fast wheel of hard grains shaves the surfaceOpen the lesson5.8HoningStones turn and stroke inside a boreOpen the lesson5.9Superfinishing and polishingFine stones or buffs remove the last microscopic peaksOpen the lesson

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