Broaching Machining: Process, Types, Tools and CNC Applications

Broaching is a machining process used to produce precise keyways, splines, slots, polygonal holes, internal profiles, and external shapes. Unlike conventional milling, where a cutting tool normally removes material through multiple passes, a broach contains a series of progressively sized cutting teeth. As the tool travels through or across the workpiece, each tooth removes a small additional amount of material until the final profile is produced.

This makes broaching extremely productive when the same feature must be manufactured repeatedly. However, broaching is not automatically the best process for every keyway, spline, or square hole. Tooling cost, production quantity, material, feature depth, whether the feature is blind or through, and the available machining equipment can all change the most economical manufacturing method.

For custom components, engineers should therefore consider broaching during the design stage rather than simply specifying an internal profile and leaving the manufacturing method undefined. This guide explains how broaching works, where it is most useful, its design limitations, and how Tuofa CNC germany evaluates broaching alongside CNC milling, turning, slotting, and EDM.

What Is Broaching?

Broaching is a subtractive manufacturing process in which a multi-tooth cutting tool called a broach progressively removes material from a workpiece. Each cutting tooth is normally slightly larger or higher than the tooth before it. This progressive increase is known as the rise per tooth.

The first teeth remove most of the material, intermediate teeth progressively establish the required geometry, and the final teeth control the finished dimension and surface condition. As a result, roughing and finishing can effectively occur during one continuous cutting stroke.

Broaching can produce both internal and external features. Internal broaching is particularly common for keyways, splines, square holes, and other non-circular bores. External broaching can machine flats, slots, serrations, and complex outside profiles.

How Does the Broaching Process Work?

A conventional internal broaching operation generally begins with a pre-machined starting hole. The diameter and geometry of this pilot hole are important because they position the broach and determine how much material the cutting teeth must remove.

The broach is then aligned with the workpiece and pushed or pulled through the hole. Rather than removing the full depth with one cutting edge, dozens of teeth may share the total material removal. Each tooth cuts only a small amount.

Chip gullets located between the teeth provide space for chips created during the cutting stroke. Once the finishing section of the broach passes through the workpiece, the required internal profile is complete.

This differs significantly from milling a similar geometry. A CNC mill may require repeated tool paths, indexing, repositioning, or a small-diameter end mill to machine corners. A correctly designed broach produces the entire form directly from the geometry built into the tool.

What Are the Main Types of Broaching?

Internal Broaching

Internal broaching produces features inside a pre-existing bore. Typical parts include gears, pulleys, couplings, hubs, bushings, and transmission components.

Common internal broached features include:

• Sleuven voor sleutels
• Internal splines
• Square holes
• Hexagonal holes
• Polygonal bores
• Internal serrations
• Special internal profiles

Because the broach must normally enter an existing opening, pilot-hole size and accessibility should be reviewed before the component design is finalized.

External Broaching

External broaching removes material from an outside surface rather than passing through a bore. It can create flats, grooves, serrations, slots, and contoured surfaces.

External broaching can be highly productive in mass production because an entire surface profile can be generated during one machine stroke instead of several separate milling operations.

Pull Broaching

With pull broaching, the tool is pulled through the workpiece. The broach operates primarily in tension, allowing relatively long tools to be used while reducing the risk of buckling.

This method is widely associated with conventional internal production broaching.

Push Broaching

Push broaching forces the tool through the workpiece in compression. Because long slender tools can buckle under compressive loading, push broaches are generally shorter than comparable pull broaches.

Keyway broaching using a press is a familiar example of this concept.

Rotary Broaching

Rotary broaching, sometimes called wobble broaching, is particularly useful for producing small polygonal forms such as hexagonal and square sockets on CNC lathes, mills, and machining centers.

The broach is held at a slight angular relationship to the workpiece. During operation, only a limited portion of the cutting edge is heavily engaged at any moment, progressively forming the required shape.

One major advantage is that a hexagonal or square internal feature may be produced on the same machine used to turn the rest of the component. This can eliminate a secondary milling setup.

However, rotary broaching is not an unlimited substitute for conventional broaching. Feature depth, material strength, required corner geometry, chip clearance, broach size, and machine capacity all affect whether the process is practical.

What Parts and Features Are Commonly Broached?

Internal Keyways

Keyways are among the most common broached features. They provide a mechanical connection between a shaft and components such as gears, pulleys, sprockets, and couplings.

For a conventional through keyway, a properly sized bore is usually produced first. A guide bushing supports and positions the keyway broach while the tool passes through the component.

Internal Splines

Internal splines contain multiple teeth around an internal circumference. Broaching is attractive for high-volume spline production because all spline teeth can be generated from a fixed tool profile without individually indexing every tooth.

Parts such as transmission hubs, coupling components, drive-system parts, and precision mechanical assemblies commonly use internal splines.

Square and Hexagonal Holes

Non-round internal holes are difficult to produce using conventional drilling because rotating drills naturally create circular holes. Broaching allows a pre-drilled round hole to be converted into a square, hexagonal, or other polygonal profile.

Rotary broaching is particularly relevant when a small female hex or square feature needs to be produced directly on a turned component.

Can Broaching Machine Blind Keyways?

Yes, certain broaching methods can produce blind internal keyways, but blind features require more careful design than through keyways.

The primary problem is chip evacuation. During internal broaching, the tool pushes or carries chips along the direction of travel. In a through hole, the tool and chips can exit the opposite side. In a blind feature, there may be no natural exit.

If the cutting tool reaches a solid shoulder while chips accumulate ahead of the tool, cutting forces can increase sharply. The result can be poor surface quality, tool damage, or broach breakage.

A practical blind-keyway design may therefore require:

• An internal relief groove at the end of the keyway
• A radial relief hole
• Additional clearance beyond the functional keyway length
• Enough space for accumulated chips
• A controlled retract-and-clear machining strategy

This is an important DFM consideration. A drawing may show a perfectly functional keyway from an assembly perspective while leaving almost no room for the cutting tool to exit or for chips to escape.

Why Is End Relief Important for Blind Broaching?

An end-relief feature gives the cutting edge somewhere to travel after completing the functional portion of the keyway and provides space for chips.

Without sufficient relief, the tool may be forced to stop cutting directly against an internal shoulder. Each additional stroke can then compress previously generated chips against the end wall.

For this reason, a small design change such as adding an internal groove can sometimes make a blind keyway significantly easier and safer to manufacture.

When an internal groove is not acceptable for functional reasons, alternative processes such as slotting, CNC shaping, sinker EDM, or wire EDM may be considered depending on whether the geometry is accessible.

Can Broaching Be Done on a CNC Lathe?

Yes. CNC lathes can perform certain broaching operations, particularly internal keyways and rotary-broached polygonal holes.

For a straight keyway, a stationary cutting tool can sometimes be moved repeatedly along the Z axis while the spindle is held in a fixed orientation. Each stroke removes a small additional depth until the required keyway is generated.

This approach behaves more like a small shaping operation than conventional production pull broaching.

Its main advantage is setup reduction. A turned shaft, sleeve, hub, or coupling may receive its bore, outside diameter, grooves, threads, and internal keyway without being transferred to another machine.

However, the cutting forces act through the machine structure, spindle-locking system, turret, and axis drive components. The suitability of the method therefore depends on keyway width, material, cutting depth, tool projection, machine rigidity, and machine-tool manufacturer recommendations.

Can Broaching Be Done on a CNC Mill?

A CNC milling machine can also perform certain slotting or broaching-style operations using the machine axis to drive a stationary cutting tool.

This can be useful for relatively small quantities where purchasing a dedicated broaching machine would make little economic sense.

However, just because the machine can generate the required motion does not mean every broaching load is suitable for its spindle bearings or feed-axis components. Large keyways, deep slots, and difficult materials can create substantial cutting force.

For custom low-volume work, Tuofa CNC germany evaluates machine load and geometry before selecting this method.

What Materials Can Be Broached?

Broaching can be applied to many metals, including carbon steel, alloy steel, stainless steel, cast iron, aluminum, copper alloys, and other machinable materials.

Material properties influence cutting force, tool material, tooth geometry, lubrication, chip formation, tool wear, and achievable production rate.

Aluminum and many free-machining steels are relatively straightforward to broach. Tough stainless steels, hardened alloys, high-strength tool steels, and work-hardening materials can be significantly more demanding.

For difficult materials, simply applying more press force is not necessarily an effective solution. Tool wear, tooth fracture, chip packing, and part deformation may become limiting factors.

How Accurate Is Broaching?

Broaching is capable of producing high dimensional consistency because the finished geometry is controlled directly by the final teeth of the broach rather than being generated through many separately programmed tool paths.

Under suitable production conditions, broaching can hold tight tolerances and produce excellent repeatability across large production quantities.

However, achievable tolerance should never be specified independently of feature size, material, broach condition, tool guidance, part rigidity, machine condition, and inspection method.

Tool wear is also important. Because the broach geometry defines the part geometry, progressive wear can gradually influence finished dimensions. High-volume production therefore requires tool-life monitoring and periodic inspection.

What Surface Finish Can Broaching Produce?

The finishing teeth of a broach remove relatively small amounts of material and can create a smooth functional surface without requiring a separate finishing operation in many applications.

Actual surface roughness depends on the workpiece material, cutting fluid, broach sharpness, tooth geometry, machine rigidity, chip control, and cutting conditions.

Critical sealing, sliding, or bearing surfaces should still be evaluated separately rather than assuming that every broached surface automatically satisfies a very low Ra requirement.

How Does Part Design Affect Broaching?

Pilot Hole Size

Internal broaching normally begins with a machined hole. The starting diameter must provide sufficient material for the broach to generate the profile while also correctly supporting and locating the tool.

An incorrectly sized starting hole can increase cutting load, weaken the final feature, or prevent proper broach guidance.

Feature Depth

Deeper internal features increase tool engagement, chip volume, cutting force, and tool-deflection risk. Very deep keyways or polygonal holes may require a different machining strategy.

Tool Access

A designer should consider how the broach enters and exits the component. A geometry that is accessible in CAD may still be inaccessible to a physical broach, holder, bushing, or machine ram.

Runout Space

Blind features should provide suitable runout wherever possible. Eliminating a few millimeters of internal clearance can make manufacturing substantially more difficult without providing any functional benefit.

Corner Requirements

One reason engineers consider broaching is the ability to produce internal profiles with well-defined corners. However, specifying perfectly sharp internal corners should still be avoided unless the mating component genuinely requires them.

The allowable corner radius affects whether milling can become a lower-cost alternative.

Broaching vs Milling: Which Should You Choose?

Milling offers exceptional flexibility because standard cutting tools can produce many different geometries through CNC programming. This makes milling particularly attractive for prototypes and low-volume production.

Broaching uses tooling that more directly represents the required geometry. The tooling investment can therefore be higher, but each subsequent machining cycle may be much faster.

If a prototype design may still change, CNC milling or another flexible machining process may be more economical. Once geometry is fixed and quantity increases, broaching becomes much more attractive.

Broaching vs EDM

EDM is another important alternative for internal keyways, splines, and unusual profiles, particularly when conventional broach access is limited.

Wire EDM provides excellent accuracy but requires the cutting wire to pass through the geometry, making accessibility important. Sinker EDM can create blind internal shapes using a specially manufactured electrode.

EDM is especially useful for hardened or difficult-to-machine materials because material removal does not depend on conventional cutting force.

Its disadvantages include relatively slow cycle times and higher process cost compared with a mature high-volume broaching operation.

Therefore, EDM can be attractive for low quantities, difficult blind features, hardened components, or geometries for which dedicated broach tooling is not economically justified.

When Is Broaching Economical?

Broaching economics depend heavily on quantity.

A standard keyway broach may be economical even for relatively small batches if the required size is already available. By contrast, a special spline or proprietary internal profile may require a custom-designed broach.

The cost of this custom tool must be distributed across the production quantity. If only five components are required, milling, slotting, or EDM may be cheaper despite having a longer machining cycle.

If 20,000 identical components are required, investing in specialized broach tooling may dramatically reduce unit cost.

For this reason, a request for quotation should include estimated annual quantity rather than only the geometry of the component.

When Is a Custom Broach Required?

Custom tooling is normally considered when the required form cannot be produced using standard keyway, hexagonal, square, or spline tooling.

Typical situations include:

• Non-standard spline geometry
• Proprietary coupling profiles
• Special polygonal holes
• Unusual keyway dimensions
• Combination profiles
• High-volume components requiring optimized cycle time

A custom broach can provide excellent production efficiency, but future design changes become more expensive because changing the finished profile may also require modifying or replacing the broach.

Common Broaching Problems

Broach Deflection

Long tools, deep features, excessive cutting load, and insufficient support can cause the broach or broaching bar to deflect. The resulting slot may become tapered, angled, oversized, or dimensionally inconsistent.

Tool Breakage

Excessive rise per tooth, hard material, inadequate lubrication, misalignment, chip packing, or insufficient end relief can overload cutting teeth.

Chip Packing

Chip packing is particularly important for deep or blind internal features. The feature and tool should provide enough volume for chips to leave the cutting area.

Poor Keyway Alignment

A functional keyway requires correct width, depth, straightness, and angular location relative to other part features. Proper tool guidance and workholding are therefore as important as nominal slot width.

Burr Formation

Broaching may leave burrs at the tool exit or along feature edges. Deburring should be included in the manufacturing plan when burrs could interfere with assembly, sealing elements, or operator handling.

How to Choose the Right Process for an Internal Keyway

There is no single best machining process for every internal keyway.

For a standard through keyway and repeated production, conventional broaching can be extremely efficient.

For one prototype, manual or CNC slotting may avoid dedicated tooling.

For a long blind keyway, a CNC shaping-type process or EDM may provide better access.

For a small female hex in the end of a turned component, rotary broaching may eliminate an additional setup.

For a hardened component with an unusual internal profile, EDM may offer lower technical risk.

Manufacturing decisions should therefore be based on geometry, material, quantity, tolerance, accessibility, and tooling cost rather than choosing a process based solely on the feature name.

DFM Guidelines for Broached Parts

Design for manufacturing can reduce both tooling risk and machining cost before production begins.

When designing a component that may require broaching:

• Use standard keyway or spline sizes whenever function allows.
• Provide adequate entry clearance for the tool and holder.
• Provide exit or chip-relief space for blind features.
• Avoid unnecessarily deep internal profiles.
• Specify realistic internal corner requirements.
• Identify the functional keyway or spline tolerances rather than applying tight tolerance to every dimension.
• Consider annual production quantity before specifying a proprietary internal form.
• Discuss material hardness before the manufacturing process is selected.

A small design modification can sometimes convert a difficult blind feature into a straightforward machining operation without changing how the finished component functions.

Custom Broaching and CNC Machining at Tuofa CNC germany

Broaching is most valuable when it is treated as part of the overall machining strategy rather than as an isolated operation. A component may require CNC turning for the outside diameter and bore, CNC milling for mounting features, broaching for an internal keyway, heat treatment, deburring, surface finishing, and final inspection.

Tuofa CNC germany supports custom CNC machining projects involving shafts, gears, hubs, couplings, bushings, transmission components, mechanical assemblies, and other precision parts. Depending on geometry and quantity, our engineering team can evaluate conventional broaching, CNC broaching, rotary broaching, CNC milling, turning, slotting, and EDM alternatives.

For low-volume components, avoiding unnecessary special tooling can reduce initial manufacturing cost. For repeat production, investing in dedicated broach tooling may substantially reduce cycle time and unit price.

Send Tuofa CNC germany your 2D drawing or 3D CAD model together with the material, quantity, tolerance, heat-treatment condition, and annual demand. A DFM review can determine whether broaching is the most suitable process and identify issues such as insufficient tool access, blind-keyway chip clearance, excessive feature depth, or unnecessary custom tooling before production begins.

FAQs About Broaching

Is Broaching a CNC Machining Process?

Broaching is a machining process that can be performed on dedicated broaching machines or adapted to CNC equipment. CNC lathes and machining centers can perform certain linear and rotary broaching operations, depending on machine configuration and cutting forces.

Can You Broach a Blind Hole?

Yes, but blind broaching requires sufficient clearance for the tool and chips. Internal relief grooves or other runout features are often useful. Some blind geometries may be better suited to CNC slotting or EDM.

Can You Broach Stainless Steel?

Yes. Stainless steel can be broached, although cutting force and tool wear are generally greater than with easy-machining materials. Grade, hardness, tool geometry, lubrication, and feature size must be considered.

Is Broaching Good for Low-Volume Production?

It depends on tooling. A standard broach can be economical for relatively small quantities. A custom broach for only a few parts may cost more than milling, slotting, or EDM.

Why Is Broaching Used for Keyways?

A broach can generate the required keyway width and depth accurately with relatively few machine motions, making it fast and repeatable for recurring production.

What Is the Difference Between Broaching and Rotary Broaching?

Conventional broaching normally drives a progressively toothed tool linearly through or across the part. Rotary broaching uses an angled rotating relationship between the tool and workpiece to progressively generate shapes such as hexagonal and square holes.

Should I Use Broaching or EDM for an Internal Keyway?

Use broaching when the geometry is accessible, material is suitable, and production quantity justifies the tooling. EDM becomes attractive for difficult blind geometries, hardened materials, special profiles, or low quantities where dedicated broaching tooling would be expensive.

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