Broaching Fundamentals

In a single stroke, a broach can transform a pilot hole into a finished involute spline or a precision keyway to tight tolerances. That combination of speed and accuracy is why broaching remains one of the most enduring metal-cutting processes in gear manufacturing. 

This article covers the essentials: how broaching works, the types of broaches and cutting methods in use today, the materials and coatings that determine tool life, and practical guidance on maintenance and troubleshooting.

How Broaching Works

At its core, broaching is a linear machining process. A broach tool consists of a series of teeth arranged in sequence, with each successive tooth standing slightly taller or wider than the one before it. As the tool passes through or across the workpiece, each tooth removes a small, predetermined amount of material. The final teeth in the sequence—the finishing teeth—are all the same size and produce the finished surface.

This architecture means that roughing and finishing happen in a single stroke, which is a key reason broaching delivers such high productivity. Unlike milling, shaping, or grinding, there is no need for multiple passes or tool changes to go from rough stock to finished geometry.

Internal vs. Surface Broaching

Broaching falls into two broad categories based on where the cutting happens.

Internal broaching shapes the inside surfaces of a workpiece. The broach is inserted through a pre-drilled or pre-bored pilot hole, then pulled (or sometimes pushed) through to create the desired geometry. Internal broaches are the workhorses of gear and drivetrain manufacturing, producing round holes, spline holes, serration profiles, square and polygonal bores, keyways, and a wide range of special internal shapes. The pilot hole itself guides the broach, so the accuracy of that starting bore directly influences the quality of the finished feature.

Surface broaching machines the external surfaces of a workpiece. Here, the broach passes across the outside of the part rather than through it. Surface broaching excels at producing flat surfaces, contoured profiles, external grooves, and complex features like turbine blade root forms (fir tree and dovetail shapes). Because roughing and finishing occur simultaneously in the same stroke, surface broaching is dramatically more productive than conventional milling or planing for these applications.

Cutting Methods

Not all broach teeth are designed to cut the same way. The choice of cutting method affects accuracy, surface finish, tool life, and the types of workpieces that can be handled successfully.

Radial (or upward) cutting is the most common approach. Each tooth's groove in the workpiece serves as a guide for the next tooth, which is positioned slightly outward or upward. This method yields excellent profile accuracy and has the added benefit that tooth dimensions do not change after resharpening. However, because the side rake angles tend to be small, chips can adhere to the blade more easily.

Double-cut (roughing then finishing) methods separate the process into two distinct stages. Roughing teeth remove the bulk of the material, and a separate set of finishing teeth handles final sizing. Because the finishing teeth take only a light cut, cutting forces during the final stage are low. This reduces workpiece distortion and produces more consistent dimensions across a batch, making it the preferred method for thin-walled parts, fir tree profiles, and any application demanding tight tolerances.

Back taper relief is a specialized approach where the tooth thickness decreases slightly from front to rear, and side relief is applied to each blade. This is particularly effective for materials prone to adhesion on the cutting edges, such as aluminum, or for workpieces with long cut lengths where chip evacuation becomes challenging.

Progressive form cutting uses roughing teeth that incrementally build up the profile shape, followed by finishing teeth ground to the exact final geometry of the workpiece. This method is especially well-suited for gear broaches and high-precision spline applications where the finished tooth profile must meet demanding accuracy requirements.

Blade Arrangement in Spline Broaches

For internal spline broaching, the placement of the round (minor diameter) teeth relative to the spline-cutting teeth is a critical design decision that affects concentricity, tolerance control, and overall accuracy.

In a front round blade arrangement, the round teeth come first. They true up the bore diameter before spline cutting begins, which is advantageous when the pre-broaching hole has a wide dimensional tolerance.

A rear round blade configuration places the round teeth after the spline teeth. This is the more common general-purpose arrangement and works well when the primary concern is controlling the minor diameter tolerance of the finished spline.

Combination (alternating) arrangements interleave round and spline teeth in the finishing section. The spline teeth act as guides while the round teeth are cutting, which significantly reduces eccentricity between the spline form and the minor diameter.

Some designs combine all of these strategies—front round teeth for initial bore sizing, followed by alternating spline and round teeth in the finishing section—to achieve the highest possible concentricity.

Tool Materials and Coatings

The choice of tool material has a direct and measurable effect on broach life, especially as workpiece hardness increases or as manufacturers push for higher cutting speeds.

Conventional high-speed steel (HSS), such as SKH55 (equivalent to AISI M35), has long been the standard broach material. It offers a good balance of toughness and wear resistance for general-purpose applications on carbon and alloy steels within the typical broaching hardness range.

Powder metallurgy (PM) high-speed steels represent a significant step up. PM grades offer finer, more uniform carbide distribution, which translates to better wear resistance and edge retention. Within PM grades, there is a range of options: some are optimized for general high-wear applications, while others are formulated specifically for difficult-to-machine materials like nickel-based superalloys found in aerospace turbine components.

On the coatings side, the industry has moved well beyond basic nitriding. Physical vapor deposition (PVD) coatings, particularly aluminum-based nano-structured formulations, can dramatically extend tool life. Advanced PVD coatings can double the number of parts produced between resharpenings compared to conventional titanium nitride (TiN) coatings, while also improving the surface finish of the broached workpiece by reducing tooth surface roughness to extremely low levels (on the order of Ra 0.03 µm). This smoother tooth surface also reduces flaking on the finished part—a meaningful quality improvement for gear and spline applications.

Workpiece Hardness and Cutting Speed

Getting the best results from a broach requires matching process parameters to the workpiece material.

The ideal workpiece hardness range for broaching is generally HB 200 to 230 (approximately HRC 14 to 21), though materials up to about HB 300 (HRC 32) are commonly broached without issue. Material that is too soft can cause adhesion on the tooth flanks and lands, resulting in poor surface finish with tearing or flaking. Material that is too hard will accelerate broach wear and shorten tool life.

Cutting speed is another critical variable, influencing both dimensional accuracy and tool longevity. Recommended speeds vary by workpiece material—ferrous alloys, cast irons, and non-ferrous metals each have their own optimal ranges. Staying within the recommended speed window for a given material is one of the simplest ways to maximize broach life and part quality.

Resharpening: Protecting Your Investment

Broach tools represent a significant capital investment, and proper resharpening is essential to getting the most out of that investment. Knowing when to resharpen—and doing it correctly—can mean the difference between consistent, high-quality parts and costly scrap.

Signs that resharpening is needed include a visible whitish wear land on the cutting edges, abnormal wear or chipping, chips packing into the gullets, undersized parts (the go-gauge no longer passes), degraded surface finish, excessive heat at the end of the stroke, or abnormally high cutting forces on the broaching machine.

Best practices for resharpening start with preparation. Remove any material adhered to the cutting faces, check and correct any warpage in the tool, and mount the broach securely in the sharpening machine. Use a sharp CBN grinding wheel—as large a diameter as practical—and dress it regularly. Grind the same amount from each tooth to uniformly remove the worn zone. For internal broaches, pay careful attention to the grinding wheel angle relative to the broach to avoid interference between the wheel and adjacent teeth. The correct wheel angle depends on the relationship between the broach diameter, the wheel diameter, and the cutting edge rake angle.

After resharpening, verify that all wear and chipping have been completely removed, that there are no grinding burns, that the resharpened surface finish is Ra 3.2 µm or better, that the tool has been degaussed, and that the gullet surfaces are smooth and free of irregularities that could impede chip flow.

Troubleshooting Eccentricity in Internal Broaching

One of the most common quality problems in internal broaching is eccentricity—the broached feature is not concentric with the pilot hole. This occurs because the broach, supported only by the pulling mechanism during cutting, can be pushed off-center by unbalanced cutting forces.

The symptoms show up in several ways: unprocessed material remaining when the outer diameter is subsequently turned, a go-gauge that won't pass, improperly formed tooth profiles, excessive variation in between-pin measurements, or partial uncut surfaces on the minor diameter.

Diagnosing the root cause requires a systematic approach. A straightforward first step is to mark both the workpiece and the broach so their angular positions are tracked, then broach several parts while rotating the broach mounting position in 90-degree increments. If the direction of maximum runout follows the workpiece orientation, the machine is the likely culprit. If it tracks with the broach position, the broach itself is suspect. If the eccentric direction varies randomly, the workpiece is the most probable source.

Machine-related causes include an out-of-level faceplate (on vertical machines), misalignment between the puller and the travel axis, worn or damaged slides, excessive clearance, or uneven coolant flushing that creates asymmetric forces on the broach.

Broach-related causes include uneven wear or chipping around the circumference of the tooth tips, inconsistencies in chip pocket geometry (often introduced during resharpening), or a bent tool. Runout can be checked by measuring the tooth tip diameter at several circumferential positions along the length of the tool using a dial indicator referenced to the center holes.

Workpiece-related causes include poor roundness, cylindricity, or perpendicularity of the pilot hole, excessive clearance between the broach's front guide and the pilot bore (ideally within 0.03 mm), or uneven material hardness across the cross-section of the part.

The Role of Broaching in Modern Gear Manufacturing

Even as processes like power skiving continue to gain ground, broaching holds a critical position in gear and drivetrain manufacturing. Its unmatched ability to produce complex internal profiles in a single, highly repeatable stroke makes it indispensable for high-volume production of splined hubs, synchronizer rings, planetary carriers, and countless other components. Advances in tool materials, coatings, and broach design continue to expand the process's capabilities, enabling faster cycle times, tighter tolerances, and a wider range of workpiece materials.

For manufacturers looking to optimize their broaching operations, the fundamentals remain the same: start with the right tool design and material for the application, maintain proper cutting parameters, resharpen on schedule and to specification, and approach quality issues with a systematic diagnostic mindset.

Resource credit: Technical reference material courtesy of Nidec Machine Tool Corporation (nidec.com/en/machine-tool/).