This article takes an in depth look at Broaching.
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Broaching is an exacting machining process utilizing a hardened tool with serrated edges to systematically and consistently remove material from a workpiece. This technique employs a tool embedded with teeth of different sizes, systematically shaving away a specified quantity of material during each pass over the workpiece.
In broaching, the action involves the multi-toothed tool either moving while the workpiece stays stationary or vice versa. In certain scenarios, there could be relative rotational movement between the tool and the workpiece. Broaching operations can occur internally through the center of the workpiece or externally along its surface.
External or surface broaching incorporates designs like slab, slot, contour, pot, and straddle broaching. On the other hand, internal broaching encompasses approaches such as keyway, concentricity, and cut recut. The specific broaching technique applied depends on the segment of the workpiece being machined or cut.
Broaching is a widely used machining technique designed to precisely remove material from a workpiece to create specific contours, shapes, and profiles. As a specialized metalworking process within subtractive manufacturing, broaching is recognized for its efficiency in producing intricate internal and external geometries in high-precision industries like automotive, aerospace, and manufacturing. Unlike other machining processes such as milling, turning, or drilling, broaching utilizes purpose-built tools—broaches—with a series of progressively sized cutting teeth to achieve accurate transformations in a single pass.
The primary function of a broach is to press, pull, or push against the workpiece in order to form precise holes, slots, teeth, or patterns. This allows manufacturers to achieve unique internal broaching (e.g., keyways, splines, and polygonal holes) and external broaching (e.g., surfaces, slots, or flat features) that are often unattainable or less efficient when using milling or grinding machines. Broaching stands out for its speed, repeatability, and cost-effectiveness in mass production, as well as the superior surface finish it can deliver.
A broaching machine is a straightforward CNC or manually operated industrial machine tool that comprises a broaching tool, a holding fixture, a drive mechanism (usually hydraulic or electromechanical), and a robust support frame. There are two main types of broaching machines: horizontal and vertical. Broaching machines may feature one or more rams—linear actuators that propel the broach—depending on specific applications. For high-volume production, multiple rams allow for simultaneous cutting and return strokes, improving operational efficiency and throughput.
Broaching machines perform operations by either pulling or pushing the broach through or past the stationary or moving workpiece. Power is typically supplied by hydraulic systems, which deliver consistent force and precise control for both internal broaching (creating holes, splines, and keyways) and surface broaching (producing flat or contoured profiles). The machine setup determines the type of broaching—push, pull, surface, or rotary—ensuring precision for every component produced.
Horizontal broaching machines employ the pull-type broaching method, ideal for internal broaching as well as certain external applications. These machines are commonly used for manufacturing keyways, splines, slots, round holes, and complex internal shapes. The configuration features an elongated bed, a broach pilot to align and guide the broaching tool, and a powerful drive mechanism.
This setup is optimal for machining long broaches and handling heavy workpieces—requirements often found in automotive or heavy equipment manufacturing. However, horizontal broaching machines require significant floor space in the production facility. They deliver high accuracy, repeatability, and allow quick changeovers for multiple part geometries.
A vertical broaching machine operates by pushing or, in some types, pulling the broaching tool vertically relative to the workpiece. These machines offer great versatility, enabling both internal and surface broaching. With push broaching as the most common operation, vertical broaching machines are suited for mass production of parts requiring accurate slots, holes, and other geometric features.
There are three primary types of vertical broaching machines: push down, pull up, and pull down—each with different capabilities for workpiece handling and broaching force. Pull up machines are especially prevalent in high-production manufacturing for their efficiency and quick loading/unloading of workpieces. Vertical broaching machines may be powered by hydraulic systems or electro-mechanical drives, with hydraulic models offering superior force control and reduced maintenance costs.
A surface broaching machine is designed to remove excess material from the flat or contoured surfaces of a workpiece. The broaching tool, typically mounted on a reciprocating ram, traverses the workpiece, cutting multiple profiles or features in a single pass. Surface broaching processes are highly efficient for producing complex contours, slots, or dovetails on components like turbine blades, pistons, and connecting rods. These machines can be configured for horizontal or vertical operation, and duplex broaching machines (with two rams) increase production throughput for batch manufacturing and high-volume runs.
In continuous broaching (also known as chain broaching or production broaching), the workpiece moves steadily while the broach remains stationary. This process leverages a conveyor or chain-driven fixture to transport multiple workpieces in sequence through the cutting zone. Continuous broaching is ideal for machining numerous identical parts such as automotive transmission components, small gears, or fasteners, maximizing productivity and part consistency in large-scale manufacturing environments. The process can involve linear, horizontal, or circular movement, depending on the design and intended application.
Rotary broaching—also known as wobble broaching—is a precision method for producing intricate internal and external polygonal shapes. In this technique, often performed on CNC lathes or milling machines, the specialized rotary broach is set at a 1° angle to the workpiece centerline, creating a unique cutting action that produces shapes such as hexagons, squares, serrations, involute splines, spur gears, and even custom forms like numbers and letters, all with tight tolerances and superior surface finishes.
Rotary broaching is highly valued in industries requiring complex non-circular features—such as medical device manufacturing, aerospace, and fastener production—due to its ability to generate finished forms in one quick pass. The rotary broaching tool holder is engineered for stability, feeding the broach into a pre-drilled pilot hole and completing the full shape efficiently. A back taper on the broach minimizes thrust load, extending tool life and protecting the workpiece integrity.
Turn broaching machines are utilized for circular, linear, and spiral cutting applications, where a high-quality surface finish and tight dimensional tolerances are essential. The machine uses a specialized tool fitted with multiple cutting inserts to remove material as the workpiece, such as a crankshaft, is rotated between centers. This process supports both roughing and finishing operations—selecting segmental roughing inserts for aggressive material removal and adjustable finishing cartridges for producing precision components with demanding specifications.
Turn broaching is preferred for complex machining operations in automotive engine manufacturing, aerospace turbine construction, and high-precision industrial part production—delivering consistent part geometry, enhanced cycle times, and superior surface integrity.
Advancements in Computer Numerical Control (CNC) technology have transformed the broaching process, enabling simultaneous integration of both rotary and linear broaching operations with advanced automation and repeatability. CNC broaching machines are equipped with programmable tool paths, indexable insert tools, and custom-designed holders that interface seamlessly with modern CNC lathes and mills. This allows manufacturers to increase productivity, minimize setup time, and achieve consistent results across job lots and custom production runs.
CNC broaching systems support a diverse range of operations—including rotary (wobble), punch, keyway, spline, polygon, and serration broaching—delivering exceptional versatility for producing parts such as transmission gears, medical implants, precision aerospace fasteners, and tooling inserts. By utilizing advanced coolant systems and rigid fixturing, CNC broaching machines enhance surface finish quality and tool life, reducing both cycle times and production costs. This makes CNC broaching an optimal choice for industries demanding high-precision, complex component manufacturing under tight tolerances.
In summary, the broaching process encompasses a broad spectrum of techniques, machine types, and applications—ranging from conventional internal and surface broaching to sophisticated CNC and rotary broaching systems. Understanding the capabilities, advantages, and industry applications of each machine type enables manufacturers, engineers, and machinists to make informed choices for part design, process optimization, and high-volume metalworking production. For those evaluating broaching as a solution, consulting with specialized broaching machine manufacturers and tooling suppliers will ensure optimal results in efficiency, cost, and product quality.
Broaching demands a high level of precision, and several manufacturers have mastered this process. Here are five renowned brands known for producing broaching machines used in the United States and Canada, including specific models and their distinctive capabilities, features, and characteristics:
Description: American Broach & Machine Co. is a leading name in broaching machine manufacturing. Their AB-2500H model is engineered for high-volume production and comes with advanced features including high-speed operation, precise process control, and versatility for handling different broach sizes. This model is celebrated for its reliability, robustness, and consistent accuracy in producing broached parts.
Description: Colonial Tool Group is known for its expertise in broaching machine manufacturing. The G-Series model is a versatile machine suitable for both internal and surface broaching tasks. It features multiple broach stations, sophisticated control systems, and high rigidity, which contribute to superior precision and productivity. The Colonial G-Series is recognized for its adaptability, accuracy, and capability to meet complex broaching needs.
Description: Ohio Broach & Machine Co. produces a variety of vertical broaching machines, including the OB Series, which is engineered for efficient vertical broaching operations. This series boasts a compact design, intuitive controls, and customizable features tailored to specific broaching requirements. The OB Series is praised for its reliability, user-friendliness, and its capability to deliver high-quality, precise broached parts.
Description: Broaching Machine Specialties (BMS) is a prominent manufacturer of broaching machines. Their horizontal broaching machine is engineered for precision broaching tasks, featuring robust construction, advanced control systems, and efficient chip removal mechanisms. The BMS horizontal broaching machine is recognized for its rigidity, accuracy, and versatility in handling different broach sizes and materials.
Description: Nachi America Inc. specializes in gear cutting broach machines designed for precision gear manufacturing. These machines feature high-speed operation, accurate gear cutting capabilities, and advanced tooling systems. Nachi's gear cutting broach machines are renowned for their reliability, efficiency, and ability to produce high-quality gears with stringent tolerances.
Please note that the availability and specifications of specific models may change over time. For the most current and detailed information on the models and features available from these manufacturers in the United States or Canada, I recommend contacting the manufacturers directly or consulting their product catalogs and specifications.
The process of broaching produces parts with exceptional surface finishes and dimensional accuracy. A broaching tool has rough, semi-finish, and finish cutting teeth capable of finishing the machining of a rough surface in one stroke.
For internal broaching, the tool can either be pulled or pushed through the workpiece, or the workpiece can be moved across the stationary tool. This process requires a starting hole to insert the broach into the workpiece.
A broach is a cutting tool with multiple teeth that functions similarly to a chisel, but with a flat-ended sharp edge. In some instances, the broach teeth are arranged diagonally to perform shearing cuts, resulting in sliding or skewing deformations.
A broach is a cutting tool featuring multiple rows of teeth, each progressively larger than the last. This design allows a broach to create both simple and intricate cuts quickly and with precision. Throughout the broaching process, each pass of the broach removes a shallow layer of material along the length of the workpiece.
Among the different broaching techniques, linear broaching is the most widely used. In this process, the tool moves in a straight line across the surface of the workpiece, making cuts as it progresses to the end. The diagram illustrates an example of linear broaching.
In pull broaching, the broach is drawn through a stationary workpiece. This method uses long broaches equipped with a specialized head, and the tension generated during the pulling action enhances the effectiveness of the broach. While pull broaching is primarily used for internal or interior broaching, it can also be applied to surface broaching. These broaches feature numerous teeth, allowing for extended cutting operations.
Pull broaching can be performed vertically (up or down) or horizontally, either through or across the workpiece. It is commonly used to achieve flat surfaces on automobile cylinder blocks and heads.
A push broaching tool is shorter to withstand the compressive forces exerted during the broaching process. It typically has fewer teeth, which are more prone to bending or breaking under these forces. Push broaching is suited for creating shorter cuts, making it ideal for producing smaller parts.
Push broaching tools are often used for surface broaching, and when employed for this purpose, they are specifically referred to as surface broaching tools.
Pot broaching is used with cylindrical workpieces and has a pot shaped tool holder. In pot broaching, the tool remains stationary as the workpiece is pushed or pulled through it. Pot broaching produces a cut inverse that of interior broaching and is used with other machining or independently. The tool for pot broaching is extremely complex and requires a special fixture; this makes pot broaching a seldomly used process.
Spline broaching, also referred to as round broaching, is used to create round, plain, or irregular splines and spiral tooth forms. It is commonly employed to manufacture shafts, gears, and other mechanical components that mesh together. This prevalent form of broaching involves using a broach to produce a series of ridges or teeth around the circumference of a pre-cut hole. Spline broaching is an efficient method for creating serrated, straight, and involute splines.
In the automotive industry, spline broaching is essential for producing transmission and driveshaft components. The grooves formed in the spline enable effective torque transfer by interlocking separate parts.
Blind broaching refers to the broaching process where the tool does not pass completely through the part, meaning the tool machines into a blind cavity. The process of blind broaching is used on parts that are designed to not have a pass through. In the case of keyways, they can be designed to be a blind broaching when the tool stops in the middle of the bore.
With internal broaching, material is removed from the interior or internal surface of the workpiece; this is known as hole broaching. Prior to beginning the internal broaching process, a starter hole is drilled into the workpiece; this is referred to as blank prepping. Internal broaching is used to enlarge, widen, and resize a hole.
The internal broaching tool features teeth that progressively increase in size, ensuring a high-quality finish on the workpiece. Internal broaching is typically performed using automated machinery, such as CNC machines, which offer rapid and efficient hole production. This method can be executed using either the push or pull broaching process. Internal broaches are categorized into types such as round and spline.
External broaching, also known as surface broaching, involves removing material from the exterior of the workpiece. Similar to internal broaching tools, external broaching tools feature teeth that gradually increase in size. This process employs a guided ram with a broach holding fixture, with the cutting force transmitted to the ram along the length of the broach.
External broaches are used for creating flat and contoured surfaces. In flat surface broaching, the tool moves across the workpiece to achieve a specific surface finish. Form surface broaching, on the other hand, is used to create various contours and shapes, including serrated edges, angular splines, gear profiles, fir tree slots, compressor slots, and keyways.
The effectiveness of broaching largely depends on the design of the broaching tool. A well-designed tool minimizes issues such as drift, deflection, and breakage while enhancing cutting efficiency. Designing broach tools is a specialized field within manufacturing engineering that encompasses the analysis, planning, design, production, and application of these tools.
A crucial aspect of a broach is the "rise per tooth," which indicates the amount of material each tooth removes with each pass. Broaches are typically divided into sections for roughing, semi-finishing, and finishing, and can be used for various types of shaping, including internal, external, surface, or exterior broaching.
Slab broaching is one of the most straightforward broaching methods, used primarily to cut flat surfaces. This process is often employed to create square ends, which can then serve as a foundation for subsequent broaching operations.
Slot broaching is used to create slots of varying depths and widths in the workpiece. It is often preferred in high-production environments due to its efficiency and cost-effectiveness compared to milling.
Nibbling or egress broaches feature narrow roughing teeth set at an angle to the centerline, along with full-width teeth designed for semi-finishing cuts.
Contour broaches are used to cut concave, convex, cam-shaped, and irregular surfaces with high precision and exceptional tolerances.
Dovetail or pine tree broaches are used to create forms in compressor wheels for securing blades within a turbine disc. This type of broaching typically requires multiple passes due to the complexity of the workpiece and the detailed nature of the forms.
Pot broaches are designed to cut external shapes like spur gears, splines, slots, and other tooth profiles. Their design enables the creation of these forms in a single pass. Pot broaching has largely replaced hobbing for manufacturing external gears due to its efficiency.
Straddle broaching employs two slab broaches to simultaneously cut identical parallel surfaces on opposite sides of the workpiece in a single pass. This method ensures a precise dimensional relationship between the two surfaces.
Internal broaching comes in two main types: rotary and vertical. Both are designed to enlarge holes within a workpiece. All internal broaching processes require a preliminary starter hole, known as blank prepping. Vertical broaching can be performed using either the push or pull method, while rotary broaching is ideal for creating asymmetrical holes in a single pass.
Internal broaches are mounted on their shank and secured with a set screw clamp that features a whistle notch for stability.
Shell broaches are versatile tools used for semi-finishing, roughing, and finishing operations. Sections of a shell broach can be replaced or sharpened as needed. For finishing tasks, shell broaches offer superior accuracy compared to longer broach tools.
Round hole broaching is one of the most straightforward forms of internal broaching. This tool features a series of teeth arranged around its circumference, specifically designed to cut the outer edge of the hole. When working with soft, ductile metals, round hole broaches may require a chipbreaker to manage chip removal.
The keyway broach is a widely used type of internal broach. It is designed to cut a slot within the bore of the workpiece and is guided by a fixture known as a horn, which ensures precise positioning of the keyway.
An internal gear broach meticulously removes material from the internal surface of the workpiece to create a gear profile.
A gun barrel broach is exceptionally long, designed to traverse the entire length of the barrel. It features very fine teeth, as the grooves in the barrel are only a few thousandths of an inch deep. The broach is pulled through the barrel while rotating, creating the spiral rifling pattern.
For spline broaching, maintaining concentricity of the pitch diameter is crucial. A concentric broach is designed as a comprehensive finishing tool, featuring both round and spline teeth that refine the diameter and spline profile.
Broaching splines in thin-walled components can be challenging because the walls may expand during the broaching process and then return to their original shape. To address this, a cut and recut broach includes a relief area behind it to avoid wall deformation. The initial tool performs the spline cut, and a subsequent tool refines the cut area.
Square broaches are employed to modify, enlarge, and finalize irregular openings. They are designed to convert a round hole into a square shape using a rotary broach holder that positions the broach so only one corner is cut at a time. These broaches create square holes for accommodating square shafts, and the holes can be either blind or through cuts.
Broaching tools are used to remove material from a workpiece using a multi-toothed cutting instrument, where each row of teeth is progressively higher than the previous one. Broaching tools generally have three primary sections: roughing, semi-finishing, and finishing.
The pull end attaches the broach to the pulling mechanism on the broaching machine.
The shank refers to the portion from the bridge to the root diameter, which is held and drawn into the machine. The length from the pull end to the root diameter is termed as the shank length.
The front pilot, as the name suggests, ensures the broach is centered within the hole being broached.
Located at the front of the broach, these teeth make initial contact with the workpiece.
Following the roughing teeth, the semi-finishing teeth are smaller and remove less material from the workpiece.
The finishing teeth are uniform in size and are responsible for refining the workpiece to the final dimensions and shape required.
The rear pilot stabilizes and aligns the broach during operation.
The follower end serves a function similar to the rear pilot, providing support to the tool's components.
The push type broaching tool is designed to be shorter and endures compressive forces as it advances through the workpiece. The teeth of a push broaching tool are smaller and less prone to breaking or bending compared to those of a pull-type broaching tool.
The components of a push broaching tool mirror those of the pull broaching tool.
External or surface broaching involves removing material from the outer surface of the workpiece. Similar to pull-type and push-type broaches, the teeth of external broaching tools increase progressively.
The land is the area at the base of the teeth, providing support to the cutting edge.
During the broaching process, the rake helps direct the chips created by the cutting action.
The pitch refers to the spacing between adjacent cutting edges on the broach teeth.
The clearance angle is the angle formed between the land and the horizontal axis, designed to minimize friction between the teeth and the workpiece. This ensures that only the cutting edges engage with the workpiece.
The depth denotes the vertical measurement of each tooth.
The gullet radius is the curved space between two teeth, allowing the chips to flow out after being formed.
Chipbreakers are notched features designed to prevent chip buildup and facilitate chip removal. They are ground into the roughing and semi-finishing teeth parallel to the tool's axis, arranged in a staggered pattern to follow the cutting teeth. Chipbreakers are crucial in round broaching tools for preventing the formation of ring-shaped chips.
When broaching slots, the tool is confined by the slot and must carry chips along its length. A side relief angle is applied to the teeth to reduce wear from rubbing against the slot walls and to maintain a small cutting edge area.
While broaching is an age-old machining technique, it remains a crucial element in modern industrial processes. Today, broaching is integral to the manufacturing of precision components such as nuts, bolts, and gears used in aerospace and automotive sectors, thanks to its ability to produce highly accurate parts with tight tolerances.
Broaching is predominantly utilized for high-volume production where precise, repetitive, and intricate cuts are required. It accommodates a diverse range of materials, with metals ideally suited for broaching typically having a Rockwell C hardness rating between 26 and 28.
Different broaching techniques are tailored to meet specific application needs. Broaching excels in applications that demand parallel and multiple surface cuts, rapid processing, automation, large-scale production, and working with tough materials.
Broaching can replace various metal shaping processes, offering time efficiency and reduced tool usage. It is effective on a broad spectrum of materials, including soft metals like aluminum, copper alloys, brass, some plastics, wood, and midrange steel. Although broaching is possible for very hard materials like titanium, the process tends to quickly dull the broaching tool due to the metal's hardness.
Examples of components produced using broaching for automotive transmissions include:
A gear used for positioning clutch plates:
Here is an example of a transmission component manufactured using push broaching, where the splines and teeth are cut in a single pass.
The component shown below exemplifies pot broaching for a drive mechanism, where meticulous control is essential to achieve accurate tolerances.
Below is an example of internal broaching used for creating transmission gears in various sizes and involute splines.
While broaching is versatile and effective for various machining tasks, several considerations must be taken into account before selecting this process.
Firstly, the workpiece surface needs to be aligned parallel to the movement of the broaching tool. Any surface imperfections, obstructions, or irregularities can hinder the broaching process, as it relies on a continuous, unidirectional cut.
Broaching is typically not suitable for creating multiple contours, curves, or angles across different planes in a single pass, with the exception of specialized applications like helical gear teeth.
Additionally, the material being broached should be capable of withstanding the forces exerted during the process. Components with thin walls or delicate cross-sections may not be suitable for broaching due to their potential fragility.
Broaching works best with materials that are softer, more ductile, and flexible. Although some high tensile strength steels and other hard metals can be broached, the suitability of a material depends on the equipment, tools, and methods used in the process.
Steel is the most frequently broached material due to its versatility across various applications. The specific grade of steel plays a significant role in determining its suitability for broaching.
Here is a brief overview of materials frequently used in broaching. Various types of plastics, copper alloys, and other materials are also processed using this technique. Broaching tools can be specially designed and engineered to accommodate the specific requirements of different materials, ensuring they meet the demands of any application.
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