Broaching vs Milling: Internal Features Compared
Short answer: broaching is the fastest way to cut a straight internal feature such as a keyway or spline, holding roughly ±0.01–0.025 mm with a good finish, but the tooling is expensive and pays back only at volume. Milling is flexible and needs no special tool, but internal features are limited by how deep and narrow the cutter can reach. The modern middle ground is rotary broaching on a CNC lathe, which cuts hexes, squares and splines in one setup at modest volume. Choose broaching for volume splines, milling for shallow or odd features, and rotary broaching for turned parts with small internal forms.
Internal features are the awkward jobs in machining. A boring bar or end mill has to reach inside the part, and the deeper and narrower the feature, the more the tool deflects and the worse the finish. Broaching was invented to sidestep that problem: instead of a single small cutter reaching in, a rigid multi-tooth broach is pushed or pulled straight through the part and cuts the whole profile in one stroke.
What Is Broaching?
A broach is a long bar carrying a series of teeth, each slightly taller than the last. As the broach passes through the workpiece, each tooth removes a thin slice of material, so the final tooth forms the finished profile. Because the cutting action is a single straight stroke and the broach is fully supported, the feature comes out with excellent straightness and a fine finish. Linear broaching is the classic version, used for keyways, splines and internal profiles; it needs a broaching machine and a dedicated broach for each profile.
Broaches come in several forms, and the form follows the feature. Internal pull broaches are drawn through the part and are the standard for keyways and splines. Push broaches are shorter and used where the stroke is limited. Surface broaches cut an external profile in one pass. Keyway broaches are sold in sets with shims, so a single bushing and broach can cut a range of keyway widths by stacking shims behind the broach. That modularity is why keyway broaching is so widely available even in smaller shops.
What Milling Does for Internal Features
Milling cuts internal features with a rotating cutter moved along a toolpath. It needs no special tool, which makes it the default for one-offs and odd geometry. But an internal feature is limited by the cutter's reach: a long, small-diameter end mill deflects, chatters, and cuts a tapered or out-of-tolerance wall. Milling also needs an entry path wide enough for the cutter, so a keyway closed at both ends is impossible to mill with a standard tool without a special approach.
Interpolation is what makes milling so versatile. By moving the cutter in a programmed path, a mill can produce a bore, a slot or an irregular pocket with the same tool, and it can blend a feature into the surrounding geometry in ways a broach never could. The price is time: where a broach finishes in one stroke, a mill may take many passes, and each pass adds a small chance of deflection or error. For one-offs that trade is fine; for thousands of identical splines it is not.
Head-to-Head Comparison
| Attribute | Broaching | Milling | Rotary broaching |
|---|---|---|---|
| Tool cost | High, per profile | Low, standard cutters | Moderate, one tool per form |
| Speed for keyways/splines | Very fast | Slow, cutter-dependent | Fast |
| Typical tolerance | ±0.01–0.025 mm | ±0.02–0.05 mm | ±0.02 mm |
| Surface finish | Good to excellent | Fair to good | Good |
| Blind features | Difficult | Easier | Good, on a lathe |
| Best volume | High | Any | Low to mid |
| Setup | Dedicated machine | Standard mill | Standard CNC lathe |
The table hides one important column: flexibility. Broaching is unbeatable at the one feature it was tooled for, and useless for anything else. Milling is slower but can make almost anything. That single difference drives most real decisions.
There is also a third option worth naming: EDM. Sinking and wire EDM can cut internal profiles that neither a broach nor a mill can reach, including blind and closed features and hardened material, because the tool never touches the part. The trade-off is speed — EDM is slow — so it is reserved for shapes that are otherwise impossible, or for small quantities of a difficult profile. In practice, internal-feature work often is not a two-way choice but a three-way one, and the best quote names all three before recommending one.
When Broaching Wins
Broaching wins when the same internal feature repeats many times. A keyway in a pulley hub, a spline in a coupling, or a square bore in a gear — if the profile is fixed and the quantity is high, a broach pays for itself through cycle time and consistency. Broaching also wins on straightness and finish in deep features, where a milled cutter would deflect. For a spline that must transmit torque accurately, broached straightness is a real functional advantage, not just a cosmetic one.
When Milling Wins
Milling wins on flexibility, on low volume, and on features that are not a straight through-profile. A shallow internal keyway, a non-standard pocket, or a one-off with an awkward blend is a mill job. Milling also wins when the feature is not a constant cross-section, because a broach only cuts one profile and cannot vary along the length.
Rotary Broaching: The Middle Ground
Rotary broaching — sometimes called wobble broaching — mounts a special holder on a CNC lathe turret and wobbles a profiled tool into the rotating part, cutting a hex, square, Torx or spline in one pass. It is the most practical option for turned parts with small internal forms, because the feature is cut in the same setup as the turning, so the internal form and the external diameter stay concentric. Tolerances land around ±0.02 mm, which suits most fasteners, sockets and couplings. For a turned part with a hex bore, rotary broaching is usually cheaper than a second broaching operation and far faster than milling the form.
The limits of rotary broaching are size and depth. It suits small forms rather than large splines, and depth is restricted by tool rigidity. Beyond those limits, linear broaching or EDM takes over.
Burrs and secondary work also differ by process. Broaching tends to leave a clean, low burr because each tooth takes a light chip, though the exit face may still need a deburr. Milling can leave a heavier burr on the trailing edge of a slot, especially in ductile materials, and that burr matters on any part with a sliding or mating joint. Whichever process is chosen, ask how the supplier handles the burr, because a functionally perfect keyway with a razor edge will still fail a cleanliness or assembly requirement. Deburring and edge quality are part of the internal-feature specification, not an afterthought.
| Feature | Recommended process | Reason |
|---|---|---|
| Through keyway, high volume | Linear broaching | Fastest, tightest, consistent |
| Through keyway, low volume | Milling or EDM | No broach investment |
| Small hex/square in a turned part | Rotary broaching | One setup, concentric |
| Internal spline, high volume | Linear broaching | Straightness and torque accuracy |
| Internal spline, one-off | Milling or EDM | Flexibility without tooling cost |
| Closed or blind profile | EDM | Broach and mill cannot reach |
Design Rules for Internal Features
Give the feature a practical entry. Broaching needs the broach to pass straight through, and rotary broaching needs clearance for the holder, so a feature that is buried deep inside a part usually moves to EDM. Specify the tolerance the application needs rather than the tightest you can imagine, because broach tolerances are cheap only when they are the natural output of the process. State whether the feature is blind or through, because that single fact can decide the process. And if the part is turned, ask whether the internal form can be cut in the same setup by rotary broaching, because one setup is almost always cheaper and more accurate than two.
Frequently Asked Questions
Q: Can a keyway be milled instead of broached?
A: Yes, and for low volume it usually should be. Milling a through keyway is slower and less accurate than broaching, and a long keyway invites cutter deflection, but it avoids the cost of a broach. For high volume, broaching wins on cycle time and consistency.
Q: What tolerance does broaching hold?
A: Typically ±0.01–0.025 mm on the feature, with good straightness and finish, because the broach is fully supported during its single cutting stroke. That is tighter and more repeatable than a milled internal feature of the same depth, which is why torque-carrying splines are often broached.
Q: What is rotary broaching used for?
A: Cutting small internal forms — hexes, squares, Torx and small splines — on a CNC lathe in the same setup as the turning. It suits turned parts such as inserts, sockets and fittings, where keeping the internal form concentric with the outside diameter matters and a second operation would add cost and error.
Q: Is broaching more expensive than milling?
A: The tooling is more expensive, especially for a dedicated linear broach, so at low volume milling or EDM is usually cheaper. At high volume the broach amortises and the cycle time advantage makes broaching the cheapest route per part. The crossover depends on the feature and the quantity.
Q: How do I decide between broaching and milling for my part?
A: Tell your supplier the feature, whether it is blind or through, the tolerance, the material and the quantity. A source factory should recommend the cheapest process that meets the drawing rather than the one that fills its own machines. Send the drawing to sc@bquq.com and we will quote the realistic route in about 12 working hours.
Related Resources
- CNC turning vs milling: choosing a process — the decision framework behind most internal-feature jobs.
- CNC machining services — turning, milling and precision internal-feature work from a Dongguan source factory.
- About BQUQ — an ISO9001 source factory running CNC, stamping, spring and heat sink lines under one roof.
- Contact us — send your drawing for a quote within 12 working hours.
Authored by the BQUQ Engineering Team. BQUQ is an ISO9001-certified source factory in Dongguan, China, running CNC machining, metal stamping, custom springs, heat sink and collet lines under one roof. Send drawings to sc@bquq.com or WhatsApp +86 13713157787 for a quote within 12 working hours. www.bquq.com
