Broaching is a high-precision machining process used to remove material in a linear motion, creating complex internal or external shapes in metal parts. The cutting tool, known as a broach, contains a series of teeth that increase in height along its length, cutting progressively deeper with each pass. Broaching can be performed by either pushing or pulling the broach through or over the workpiece, and the choice between push broaching and pull broaching significantly impacts the speed, cost, and quality of the finished part. Understanding the technical differences, equipment requirements, and application-specific advantages of each method is essential for manufacturing engineers seeking to optimize production.

What Is Push Broaching?

Push broaching is a method where the broach is inserted manually or by a press into the workpiece and then forced through it by applying axial compressive force. The broach is pushed from one end, and the cutting action occurs as the tool moves through the workpiece. This process is typically performed using a hydraulic or mechanical press, although some setups use a vertical push broaching machine with a ram that drives the broach downward. The broach must be supported throughout the stroke to prevent buckling, as the tool experiences significant compressive stress.

Push broaching is primarily used for creating internal features such as keyways, splines, square holes, hexagonal holes, and other non-round profiles. It is especially effective for machining parts that are relatively short and rigid, as the workpiece must be able to withstand the axial forces involved. Common parts produced via push broaching include bushings, gears, pulleys, and coupling hubs. The process is also used for cutting external shapes when the workpiece is fed over a stationary push broach, though internal applications dominate.

Equipment for Push Broaching

The most common equipment for push broaching is a vertical hydraulic press with a ram that pushes the broach through the workpiece. The workpiece is mounted on a fixture below the ram, and the broach is guided by a bushing or pilot hole to ensure alignment. The ram's stroke length and force capacity must match the broach length and material being cut. For high-volume production, automated push broaching machines are available with part feeders and broach handling systems.

Push broaches are shorter and stouter than pull broaches because they must resist compressive loads without bending. They are often made from high-speed steel (HSS) or carbide-tipped for longer tool life. The broach design includes a pilot section, roughing teeth, finishing teeth, and a shank. Because the force is applied to the end of the broach, the tool must be carefully hardened and ground to maintain straightness.

Materials Suitable for Push Broaching

Push broaching works well on ductile materials such as low-carbon steel, stainless steel, aluminum, brass, and bronze. Harder materials like tool steel or heat-treated alloys can be push-broached but require slower speeds, more robust equipment, and careful lubrication. The process is less common for brittle materials because the compressive force can cause cracking or chipping.

Advantages of Push Broaching

  • High speed: The broach passes through the workpiece in a single stroke, making push broaching one of the fastest methods for producing internal profiles, especially in high-volume production.
  • Lower equipment cost: Hydraulic presses are relatively inexpensive compared to the specialized machines needed for pull broaching.
  • Simplicity: The setup is straightforward; the operator places the workpiece, the press pushes the broach through, and the part is removed.
  • Good surface finish: The progressive cutting action leaves a smooth surface, often eliminating the need for secondary finishing operations.
  • Short cycle times: For parts that can be produced in a single pass, cycle times are very short, improving throughput.

Limitations of Push Broaching

  • Workpiece length restriction: The part must be long enough to allow the broach to pass through completely, but not so long that the broach buckles. Generally, push broaching is limited to parts with a length-to-diameter ratio less than about 8:1.
  • Force on workpiece: The compressive force can cause thin-walled parts to collapse or deform. Fixturing must be robust to prevent movement.
  • Broach support: The broach must be guided accurately; misalignment can lead to tool breakage or poor tolerances.
  • Limited to simpler shapes: While push broaching can produce various profiles, extremely complex shapes often require pull broaching due to tool length constraints.

What Is Pull Broaching?

Pull broaching is a method where the broach is attached to a pulling mechanism—such as a hydraulic cylinder, chain, or cable—that draws the tool through the workpiece. The broach is placed in front of or through the part, and then pulled under tension, creating the cut. This method avoids the compressive stress on the tool, allowing for longer and more slender broaches that can produce longer strokes and deeper cuts. Pull broaching is typically performed on horizontal or vertical broaching machines designed specifically for this operation.

Pull broaching is widely used for both internal and external surface broaching. Internally, it can produce long spline holes, rifling, and irregular passages. Externally, it is used to create gear teeth, dovetails, and other contoured surfaces on large or irregularly shaped workpieces. The ability to pull the broach through longer distances makes it ideal for parts with substantial length or multiple features along the axis.

Equipment for Pull Broaching

Pull broaching machines are purpose-built and come in several configurations. Horizontal pull broaching machines feature a bed along which the workpiece is clamped; the broach is drawn through by a hydraulic piston located at one end. Vertical pull broaching machines operate similarly but in a vertical orientation, which is more compact and simplifies part loading for certain geometries. Machine capacities range from small units for keyways up to massive industrial machines for large transmission components.

The pulling mechanism must provide consistent force and controlled speed. Modern machines often use servo-controlled hydraulic systems to maintain constant cutting velocity and minimize shock. The broach is typically connected via a shank that engages a puller head. Unlike push broaching, the tool experiences only tensile stress, allowing for longer and thinner broaches. Pull broaches are often made in sections that can be assembled to achieve the required length for the cut.

Materials Suitable for Pull Broaching

Pull broaching can handle the same range of materials as push broaching but is also suitable for harder materials due to the ability to use slower speeds and higher force without tool buckling. It is commonly used on alloy steels, cast iron, powdered metal components, and even superalloys. The gentle pulling action reduces the risk of cracking brittle materials, making it the preferred method for sintered parts and ceramic composites.

Advantages of Pull Broaching

  • Greater stroke length: Since the broach is under tension, it can be very long, enabling cuts over longer distances and deeper features.
  • Gentle on delicate parts: The pulling motion does not compress the workpiece, reducing the risk of deformation for thin-walled or complex shapes.
  • Better chip evacuation: In horizontal pull broaching, chips fall away from the cutting area due to gravity, improving surface finish and tool life.
  • Capability for complex shapes: Pull broaches can be designed with multiple steps and sections to produce intricate profiles in a single pass.
  • Higher precision: The controlled pulling speed and constant force often yield tighter tolerances and better repeatability than push broaching, especially on longer parts.

Limitations of Pull Broaching

  • Higher equipment cost: Pull broaching machines are more expensive than hydraulic presses, both in initial investment and maintenance.
  • Longer setup time: Fixturing and broach alignment take more time because the broach must pass through the workpiece and be secured at both ends.
  • Broach length: Pull broaches are often several feet long, requiring more space for storage and handling, and increasing tool cost.
  • Lower speed for small parts: For short, simple features, pull broaching may be slower than push broaching due to the need to fully retract and reload the broach.

Key Differences Between Push Broaching and Pull Broaching

The fundamental difference lies in how the force is applied—compression vs. tension—but the implications affect every aspect of the process. The following table summarizes the critical distinctions:

Factor Push Broaching Pull Broaching
Direction of force Compressive (push) Tensile (pull)
Tool stress High compressive stress, risk of buckling Tensile stress, no buckling
Broach length Short to moderate (typically under 24 inches) Long (can exceed 8 feet)
Stroke length Limited by press stroke and part length Longer, up to several feet
Common applications Keyways, small splines, square/hex holes Large splines, gear teeth, rifling, external surface contours
Typical part size Small to medium parts Medium to large parts
Equipment type Hydraulic/mechanical press Horizontal or vertical broaching machine
Production speed Very high for simple parts High, but slower for short features
Tool cost Lower (shorter, simpler geometry) Higher (longer, may be sectional)
Precision Good, but limited by tool deflection Excellent, especially for long features
Ideal for delicate parts No (compressive force can distort) Yes (pulling is gentle)

Note: The above table is a general guide. Actual performance depends on machine condition, tool design, and workpiece material.

Factors to Consider When Choosing a Broaching Method

Selecting between push and pull broaching requires evaluating several interconnected factors. No single method is superior in all cases; the decision should be based on part geometry, production volume, material properties, tolerance requirements, and budget constraints.

Part Geometry and Size

Short, rigid parts with simple internal profiles are ideal candidates for push broaching. Conversely, long parts, large components, or parts requiring deep external features are better served by pull broaching. For example, a steel bushing with a small keyway is best push-broached, while a heavy truck transmission shaft with multiple long splines requires a pull broach. The part's wall thickness matters: thin walls cannot withstand the compressive pressure of push broaching without distortion, so pull broaching is safer.

Production Volume

For high-volume production of small parts (thousands per day), push broaching offers the fastest cycle times and lowest per-part tooling cost. The simplicity of a hydraulic press allows for quick changeovers between part families. For medium to large volumes of larger parts, pull broaching machines, though slower per cycle, can achieve similar throughput by machining multiple features in one pass.

Material and Hardness

Softer, ductile materials are well-suited for push broaching. As material hardness increases, the risk of tool breakage under compression rises, making pull broaching more reliable. For hardened steel (above 35 HRC), pull broaching with slower speeds and carbide tooling is standard. Cast iron and powdered metals, which can be brittle, also benefit from the tensile method.

Tolerance and Surface Finish Requirements

Both methods can achieve tolerances of ±0.001 inch or better, but pull broaching generally provides more consistent results across longer parts due to better guided tool travel. Surface finishes typically range from 32 to 63 microinches Ra with proper coolant and sharp tools; push broaching may leave slight marks at the entry or exit if not properly supported. When a mirror finish is required, pull broaching with a finishing section is preferred.

Cost Considerations

Initial investment: A hydraulic press for push broaching is significantly cheaper than a dedicated pull broaching machine. However, for ongoing production, the tool cost for pull broaching is higher due to longer and more complex broaches, but the tool life can be longer if properly maintained. For low-volume jobs, push broaching on an existing press is economical; for dedicated high-volume production of large parts, the efficiency of a pull broaching machine justifies the higher upfront cost.

Broach Tool Design: Push vs. Pull

The broach tool itself differs significantly between the two methods. Push broaches have a shank on one end for the press ram to push against, and the teeth are cut along the body, which must be robust enough to resist buckling. The tooth geometry—rake angle, relief angle, land width—is designed to manage chip formation and reduce cutting forces. Push broaches often have a smaller overall rise per tooth compared to pull broaches to keep the force manageable.

Pull broaches have a pull end with a slot or threaded hole to attach to the pulling head. The body can be slimmer and longer because it is under tension. Many pull broaches are made in sections, allowing replacement of worn segments rather than scrapping the entire tool. This modular design reduces long-term tooling costs. The tooth design in pull broaches can incorporate larger rises per tooth, enabling faster material removal in a single pass, but requiring higher machine power.

Both types rely on proper chip load, chip breaker geometry, and coolant delivery. Efficient chip evacuation is critical; in push broaching, chips are pushed ahead of the tool, so the workpiece must have clearance holes or the broach must have chip gullets sized accordingly. In horizontal pull broaching, chips fall downward, reducing the risk of clogging.

Safety and Maintenance Considerations

Broaching machines, whether push or pull, involve high forces and sharp tools. Safety protocols must include interlocked guarding to prevent access during the stroke, emergency stop buttons, and regular inspection of broach condition. Push broaching presses require careful alignment to avoid tool explosion if the broach buckles. Operators must be trained to handle heavy broaches—especially long pull broaches—using overhead cranes or hoists.

Maintenance focuses on keeping the machine hydraulics clean and free of leaks, checking guide bushings and wear plates, and ensuring the pulling mechanism (chain, cable, or hydraulic cylinder) is in good condition. Broaches should be sharpened periodically; dull teeth increase cutting forces and may damage the workpiece. Coolant filtration systems are important to prevent chip recirculation, which can degrade surface finish and tool life.

Advanced Topics in Broaching

CNC Broaching

Modern CNC machines can perform broaching using specialized attachments or by using the spindle to push or pull a broach, though dedicated machines remain dominant. CNC broaching allows for flexible production with quick changeovers between different profiles, but cycle times are generally longer than with a hydraulic press. This approach is useful for prototyping and low-volume production.

Surface Broaching

Both push and pull methods can be applied to external surface broaching. In push surface broaching, the workpiece is pushed against a stationary broach; in pull surface broaching, the broach is pulled across the workpiece surface. This is common for producing flat surfaces, angled slots, or unusual contours on engine blocks, cylinder heads, and other castings.

Horizontal vs. Vertical Broaching Machines

Horizontal broaching machines are typical for pull broaching because they allow long strokes and easy chip removal. Vertical broaching machines are often used for push broaching because they can handle part loading from the top and use gravity to assist chip fall in some cases. Some vertical pull broaching machines exist for heavy, hard-to-fixture parts. The choice of orientation affects floor space, part handling, and automation compatibility.

Coolant and Lubrication

Broaching generates significant heat due to high material removal rates. Water-soluble coolants with extreme-pressure additives are commonly used to reduce friction and flush chips. In push broaching, coolant is directed into the cutting zone through holes in the broach or from external nozzles. In pull broaching, flood coolant is easier to apply because the tool and workpiece are more accessible. Proper coolant selection can extend tool life by 30% or more.

Real-World Examples

Example 1: Small keyway in a steel bushing. A manufacturer of hydraulic components needs 10,000 bushing parts per month, each requiring a 0.25-inch keyway. Using a vertical hydraulic press with a push broach, the cycle time is about 10 seconds per part, including loading and unloading. The total operation cost is low, and the part quality is consistent. Pull broaching would be overkill and slower for this simple, short part.

Example 2: Large spline in a transmission shaft. An automotive tier-one supplier needs to produce 8-inch-long internal splines on forged steel shafts. The material is 8620 alloy steel, case-hardened to 58 HRC. The required tolerance on the major diameter is ±0.001 inch. A horizontal pull broaching machine with a segmented carbide-tipped broach is used. The broach is pulled through at a speed of 10 inches per minute, completing the spline in less than a minute. The pulling action ensures the spline is concentric and the shaft does not distort. Push broaching would risk buckling the broach and deforming the shaft.

Example 3: Complex external contour on a cast iron component. A manufacturer of large air compressors needs a series of angled dovetail slots on a cylinder head. The part is heavy and irregularly shaped. A vertical pull broaching machine with a custom surface broach is employed. The broach is drawn upward through the fixed workpiece, creating the profile in one stroke. The method allows for accurate positioning of multiple slots without repositioning the part.

Conclusion

The decision between push broaching and pull broaching hinges on the specific demands of the project. Push broaching excels in speed and economy for short, simple parts made from ductile materials, especially when producing internal features like keyways and small splines. Pull broaching offers the versatility, gentler action, and longer stroke required for large, delicate, or complex parts, often achieving tighter tolerances in longer features. By carefully considering part geometry, material, volume, and budget, engineers can select the optimal broaching method to meet their quality, cost, and throughput goals.

For further reading on broaching fundamentals, visit the Society of Manufacturing Engineers' broaching guide. Detailed specifications on broach tool design can be found at American Machinist. For machine selection advice, refer to Modern Machine Shop's article on broaching machine choices.