Introduction
Here is a question that stumps most students: how do you machine a square hole?
A drill makes round holes. A milling cutter cannot reach into a hole. A grinding wheel cannot produce sharp internal corners. Yet square holes, hexagonal holes, internal splines and keyways exist in enormous numbers inside gearboxes and machine assemblies everywhere.
Broaching is the answer, and it works by an idea unlike anything else in machining. Instead of moving a tool back and forth to gradually remove material, the entire sequence of cuts is built into the tool itself.
Each tooth stands slightly taller than the one before it, so a single straight stroke performs roughing, semi-finishing and finishing in one continuous pass. The tool does not just cut the shape it is the shape.
This guide explains how broaching works, the anatomy of a broach, the formulas you will be examined on, every machine type, and why this process is used by the million in industry but almost never for one-off parts. Written in plain language for mechanical and production engineering students.
What Is the Broaching Process?
Broaching is a machining process in which a multi-tooth cutting tool called a broach is pushed or pulled across or through a workpiece in a single stroke, with each successive tooth removing a small additional layer of material.
The defining feature is that there is no feed motion. In turning, milling and drilling, the depth of cut is set by the machine operator or the program. In broaching, the depth of cut is ground permanently into the tool as the height difference between consecutive teeth, called the rise per tooth. The machine supplies only one motion: a straight stroke.
That single design choice explains everything about broaching, good and bad.
It is extremely fast. One stroke of a few seconds completes an operation that would take minutes of milling. There is no repositioning, no multiple passes, no tool changes.
It is extremely accurate and repeatable. Because the finished dimension is ground into the last few teeth of the tool, every part comes out identical. The machine cannot introduce variation because the machine is barely doing anything.
It is extremely inflexible. A broach cuts one shape and one size only. Change the keyway width by half a millimetre and you need an entirely new tool costing thousands. This is why broaching is a mass-production process almost exclusively the tool cost has to be divided across a very large number of parts before it makes sense.
Broach Tool Nomenclature
A broach is a long bar with progressively taller teeth, and its sections each have a job.
| Part | Function |
|---|---|
| Pull end (shank) | Gripped by the machine to pull the broach through |
| Front pilot | Centres and aligns the broach in the starting hole before cutting begins |
| Roughing teeth | Remove the bulk of the material; largest rise per tooth |
| Semi-finishing teeth | Reduce the remaining stock with a smaller rise |
| Finishing teeth | All the same height, set at the final dimension |
| Burnishing teeth | Rounded, non-cutting teeth that polish the surface (optional) |
| Rear pilot | Supports the broach as the last teeth exit, maintaining alignment |
| Follower end | Supported by the machine to prevent the broach drooping |
Notice that the finishing teeth are all identical in height. This is deliberate. If the last tooth alone set the size, any wear on it would immediately change the part dimension. With several identical finishing teeth, the tool can be resharpened repeatedly and the size holds.
Broach Tooth Geometry
| Term | Meaning |
|---|---|
| Pitch | Distance between two consecutive teeth |
| Rise per tooth (RPT) | Height increase from one tooth to the next this is the depth of cut |
| Gullet | The curved space ahead of each tooth where the chip curls up and is stored |
| Land | The flat top surface of the tooth behind the cutting edge |
| Hook angle (rake) | Equivalent to rake angle; 15–20° for steel, 6–8° for cast iron |
| Back-off angle (clearance) | Small clearance behind the edge, typically 1–3° |
The clearance angle is remarkably small often just 1.5°. This is not an oversight. Every resharpening removes material from the face, and a large clearance angle would cause the tooth height to drop rapidly with each regrind, changing the part size. A small clearance angle preserves tooth height through many resharpenings.
The gullet deserves more attention than students usually give it. The chip has nowhere to go during the stroke it must sit in the gullet until the tooth exits the workpiece. If the gullet is too small for the chip, the chip packs solid and the broach breaks. Gullet capacity, not cutting force, is often what limits broach design.
Broaching Formulas You Should Know
Pitch: p = 1.76 √L, where L is the length of cut in mm.
This empirical formula balances two competing needs. Wider spacing gives bigger gullets for chip storage; closer spacing keeps more teeth engaged for smooth cutting.
Number of teeth in contact: n = (L / p) + 1, rounded down to a whole number.
At least two teeth must always be in contact, or the broach will drift sideways and chatter. This is a hard design rule.
Total cutting force: F = f × w × n, where f is the specific cutting force per mm of edge length, w is the width of cut per tooth, and n is the number of teeth in contact simultaneously.
Typical rise per tooth values:
| Section | Steel | Cast iron |
|---|---|---|
| Roughing | 0.05–0.10 mm | 0.06–0.12 mm |
| Semi-finishing | 0.02–0.04 mm | 0.03–0.05 mm |
| Finishing | 0.01–0.02 mm | 0.01–0.03 mm |
Example. A keyway 60 mm long is to be broached. Pitch = 1.76 × √60 = 1.76 × 7.75 = 13.6 mm Teeth in contact = (60 / 13.6) + 1 = 5 teeth
If a total depth of 6 mm is required with an average rise of 0.06 mm, the broach needs roughly 100 cutting teeth which at 13.6 mm pitch makes the tool over 1.3 metres long. This is why broaches are the largest single-purpose cutting tools in most workshops.
Types of Broaching Operations
Broaching splits into two families based on where the cut happens.
Internal Broaching
The broach passes through an existing hole, enlarging it into the required shape. A starting hole is always required a broach cannot begin a hole in solid material, because there would be nowhere for the front pilot to enter.
Keyway broaching – cutting the slot in a hub or gear bore that receives a key. Uses a horn or guide inserted into the bore to support the narrow broach.
Spline broaching – producing internal splines that transmit torque between a shaft and hub.
Square and hexagonal hole broaching – the classic answer to the question at the top of this article.
Round hole broaching – sizing and finishing a hole to a very accurate diameter, achieving better results than reaming.
Serration and rifling – including the spiral grooves inside gun barrels, cut with a broach that rotates as it travels.
External (Surface) Broaching
The broach passes across an outside surface. In production this is often the reverse – the workpiece moves past a fixed broach.
Flat surface broaching – machining flat faces faster than milling can.
Slot broaching – cutting multiple slots simultaneously with a gang of broaches.
Fir-tree root broaching – producing the complex fir-tree profile on turbine blade roots, one of the most demanding broaching applications in existence.
Contour broaching – cutting curved external profiles such as the bearing caps on connecting rods.
Types of Broaching Machines
Broaching machines are classified by direction of motion and by whether the broach is pushed or pulled.
Horizontal broaching machine – the broach is pulled horizontally through the work. It handles very long broaches because there is no height restriction, making it the standard for internal broaching. Its drawback is the large floor area it occupies.
Vertical pull-down machine – the broach is pulled downward through the workpiece. Compact floor space, easy chip fall-away, widely used in production for internal work.
Vertical pull-up machine – the broach is pulled upward. Common for surface broaching, where finished parts drop clear automatically.
Vertical push machine — a short, stubby broach is pushed through. Because a pushed tool can buckle, the broach length is limited to roughly 15 times its diameter. A simple arbor press with a push broach is how keyways are cut in small workshops.
Surface broaching machine – the workpiece is carried past fixed broaches on a moving slide. Extremely fast and used heavily in automotive production.
Continuous chain broaching machine – workpieces are loaded into fixtures on an endless chain that carries them past a stationary broach without stopping. The fastest configuration available, used for very high volumes of small parts.
Rotary broaching (wobble broaching) – a tool tilted about 1° from the spindle axis rocks around, cutting one small section at a time to produce hexagonal or square holes on a lathe or mill. Technically a different mechanism, but commonly grouped here.
Pull Broaching vs Push Broaching
| Factor | Pull broaching | Push broaching |
|---|---|---|
| Force applied | Tension | Compression |
| Buckling risk | None | High; limits length |
| Broach length | Can be very long | Limited to about 15× diameter |
| Depth of cut possible | Large | Small |
| Typical use | Production internal broaching | Small workshop keyways, sizing |
The reason is simple mechanics. A bar in tension can be any length you like. A bar in compression buckles once it becomes slender, which puts a hard ceiling on how much material a push broach can remove in one pass.
Broaching vs Shaping vs Milling
These three can all produce a keyway, so comparing them shows exactly where broaching earns its place.
| Factor | Broaching | Shaping | Milling |
|---|---|---|---|
| Number of passes | One stroke | Many reciprocating strokes | Multiple passes |
| Cycle time | Seconds | Many minutes | Minutes |
| Tool cost | Very high | Very low | Low |
| Accuracy | Excellent | Moderate | Good |
| Surface finish | Excellent | Poor | Good |
| Flexibility | One shape only | Fully flexible | Fully flexible |
| Economical volume | Very high | One-offs and repairs | Low to medium |
| Operator skill needed | Low | High | Medium |
The pattern is clear. Broaching trades away all flexibility in exchange for speed, accuracy and consistency. For a single prototype keyway, use a shaper or a mill. For fifty thousand identical gear hubs, broaching is not just better it is the only economical option.
Broach Materials and Coatings
High Speed Steel (HSS) is the dominant broach material, usually M2 for general work and M42 where higher hot hardness is needed. HSS is chosen for its toughness, because a broach represents a large investment and a brittle tool that chips is a very expensive failure.
Cemented carbide appears as inserted teeth on surface broaches for cast iron and abrasive materials, where the higher wear resistance is worth the brittleness.
TiN coating is common, extending tool life substantially and reducing built-up edge. Because a broach is resharpened on the tooth face rather than the top, coated broaches are recoated after regrinding.
Cutting fluid in broaching serves mainly to flush chips from the gullets and to lubricate the very large contact area. Straight sulphurised oils are preferred over emulsions, since lubrication matters more than cooling at broaching’s relatively low cutting speeds.
Common Broaching Defects and Their Causes
| Defect | What it looks like | Main cause |
|---|---|---|
| Chatter | Regular wavy marks on the cut surface | Fewer than two teeth in contact, or equal pitch causing resonance |
| Chip packing | Sudden force rise, broken teeth | Gullet too small for the chip volume |
| Drift or wander | Cut runs off true position | Worn pilots, inadequate workpiece support |
| Broach breakage | Snapped tool | Chip packing, overload, or a hardened workpiece |
| Poor surface finish | Torn or scored surface | Dull teeth, wrong hook angle, insufficient cutting fluid |
| Tapered cut | Dimension varies along the length | Tool deflection, or worn roughing teeth |
| Burr at exit | Raised lip where teeth leave the work | Dull finishing teeth, no exit chamfer on the part |
| Tooth chipping | Broken cutting edges | Hard spots in the material, or excessive rise per tooth |
Two design details prevent most chatter. Chip breakers are small notches ground into the roughing teeth, staggered so that consecutive teeth break the chip at different points this splits one wide chip into several narrow ones that curl into the gullet more easily.
And unequal pitch on some broaches deliberately varies the spacing so the teeth do not all enter at a regular frequency, breaking up the vibration before it builds.
Applications of Broaching
- Automotive – internal splines in transmission hubs, connecting rod cap faces, engine block main bearing bores, steering rack teeth
- Aerospace – fir-tree roots on turbine discs, one of the highest-value broaching applications in industry
- Gear manufacturing – keyways and splines in gear bores, produced by the million
- Firearms – rifling grooves inside barrels
- Hand tools – square and hexagonal drive holes in sockets and wrenches
- General engineering – internal keyways in pulleys, sprockets and couplings
Advantages and Limitations of Broaching
Advantages
- Roughing, semi-finishing and finishing complete in one stroke
- Extremely short cycle times, ideal for mass production
- Excellent accuracy and surface finish, often eliminating later operations
- Very high repeatability, since the tool defines the dimension
- Low operator skill needed; the tool does the thinking
- Produces internal shapes that no other process can reach
Limitations
- Very high tool cost, viable only at high production volumes
- One broach cuts exactly one shape and size
- Long lead time to design and manufacture a new broach
- Requires a pre-existing hole for internal work
- Workpiece must be rigid enough to withstand large stroke forces
- Blind holes cannot be broached, since the tool must pass right through
Frequently Asked Questions (FAQ)
1. What is the broaching process in simple words?
Broaching is a machining process where a long multi-tooth tool is pushed or pulled through or across a workpiece in one stroke. Each tooth is slightly taller than the previous one, so roughing, semi-finishing and finishing are all completed in a single pass.
2. What is rise per tooth in broaching?
Rise per tooth is the height difference between consecutive teeth on the broach, and it acts as the depth of cut. It is typically 0.05–0.10 mm on roughing teeth and 0.01–0.02 mm on finishing teeth. Because it is ground into the tool, no feed motion is needed from the machine.
3. What are the main types of broaching machines?
The main types are horizontal, vertical pull-down, vertical pull-up, vertical push, surface broaching, continuous chain broaching, and rotary broaching machines.
4. What is the difference between push and pull broaching?
Push broaching applies compressive force, so the tool can buckle and its length is limited to roughly 15 times its diameter. Pull broaching applies tension, so the broach can be very long and remove much more material in one stroke.
5. Why must at least two teeth be in contact during broaching?
With only one tooth engaged, the tool loses lateral guidance and can drift sideways or vibrate. Keeping at least two teeth in the cut maintains alignment and prevents chatter, which is why the pitch formula matters.
6. Can broaching be used for blind holes?
No. The broach must pass completely through the workpiece so the teeth can exit and the chips can be released. Blind features require shaping, slotting or EDM instead.
7. Why is the clearance angle on a broach so small?
A small clearance angle of 1–3° keeps the tooth height from dropping significantly when the face is reground. A large clearance angle would reduce the tooth height with every resharpening, quickly taking the finished dimension out of tolerance.
8. Why is broaching only used for mass production?
Because a broach is a single-purpose tool that cuts one shape and size, costs a great deal to design and manufacture, and takes weeks to obtain. That cost only becomes economical when spread across thousands of identical parts.
Conclusion
Broaching is the process that answers the impossible question how to machine a square hole, an internal spline, or a fir-tree slot that no rotating tool could ever reach. It does it by moving the intelligence out of the machine and into the tool.
Once you accept that idea, everything else follows. There is no feed motion because the depth of cut is ground into the tooth height. The finishing teeth are all identical so the size survives resharpening. The clearance angle is tiny so the tooth height survives it too.
The gullet must be big enough to hold the whole chip, because the chip has nowhere else to go until the stroke ends. And the tool cost is enormous because all of that precision must be manufactured once, in advance, into a metre-long bar of hardened steel.
Pull apart an old gearbox sometime and look at the internal splines in a hub. Those perfectly parallel grooves, all identical, with sharp internal corners no cutter could have produced that was one stroke of one tool, over in about four seconds. Understanding that trade-off, enormous tool cost against near-zero cycle time, is really what this whole process is about.