Broaching machining is a process that has evolved from traditional planing to deliver precision and a smooth surface finish, all in one swift motion. This article is designed to be a complete guide for engineers to understand this process.
What is Broaching Machining?
Broaching can be considered a further development of the planing. It uses a multi-toothed cutting tool, called a broach, to sequentially remove thin layers of metal from the workpiece surface with each tooth, achieving high precision and a fine surface finish. If the cutting force applied to the tool is compressive instead of tensile, the process is called push broaching, and the tool is a push broach. The machine tool for pull broaching is known as a broaching machine, while push broaching is often performed on a hydraulic press. For broaching large surface areas, a progressive-style broach can be utilized to reduce the total broaching force.
Broaching Machining Process
Careful preparation is necessary before the actual broaching operation begins. A fixture on the broaching machine securely holds the workpiece in position. Proper alignment is critical to ensure the accuracy of the final product.
Selecting the appropriate broach is essential. Broaches vary widely in shape and size, and the correct choice depends on the required geometric features and the workpiece material. The broach is then carefully guided through or across the workpiece, removing material with each movement. This operation is completed in a single pass. The cutting motion can be linear or rotary, depending on the broaching equipment and the type of broach used.
Following the broaching process, the workpiece is thoroughly inspected to ensure all requirements and dimensional tolerances are met. Any necessary modifications would be executed at this time.
Types of Broaching Machining
Broaching is a machining operation where the broach serves as the cutting tool. As the broach moves linearly relative to the workpiece, the machining allowance is successively removed by the incrementally sized cutting teeth. Because the desired form is usually achieved in a single working stroke, broaching is a highly efficient finishing method. However, due to the complex structure, high manufacturing cost, and dedicated nature of the broach, this process is primarily used for mass production.
Broaching is classified based on the surface feature being machined: Internal broaching and external broaching.
Internal Broaching
This method is used to machine through-holes and internal features of various cross-sections, such as round, square, polygonal, and spline holes, keyways, and internal gears. A pre-machined pilot hole is required for the broach to be inserted. The typical hole diameter range is 8 to 125mm with the depth usually not exceeding five times the diameter. In special cases, the diameter can range from 3 to 300mm, with depths up to 10m.
External Broaching
This is used to machine non-enclosed surfaces, such as flat surfaces, contoured surfaces, grooves, dovetails, turbine blade root forms, and external gears. It is particularly suitable for high-volume production of large flat and complex contoured surfaces. Broached surfaces can achieve dimensional accuracy from tolerance grades IT8 to IT6 and surface roughness from 2.5 to 0.04μm.
The types of machining allowance from the workpiece include:
Forming. Achieves high accuracy and low surface roughness but has lower efficiency. It requires a longer broach and is mainly used for small-to-medium-sized round holes and high-precision contoured surfaces.
Shear/Progressive. Suitable for rough broaching of complex features like square, polygonal, and spline holes. The broaches are easier to manufacture, but the resulting surface quality is inferior.
Ring-Cut/Overlap. Features high cutting efficiency, which reduces the broach length, but yields poor surface quality. It is primarily used for larger holes with generous machining allowances and lower precision requirements.
Integrated ring. Utilizes the contour-cut method for roughing and the form-cut method for finishing, combining the advantages of both. It is widely applied in round hole broaching.
Advantages of Broaching Machining
High productivity. While the cutting speed in broaching is generally low, the multi-toothed nature of the broach ensures many teeth and a long cutting edge are engaged simultaneously. Furthermore, the entire operation, such as roughing, semi-finishing, and finishing, is completed in a single pass, significantly reducing the basic process time and auxiliary time.
High precision and fine surface finish. The broach includes a sizing section that calibrates the dimension and smooths the surface. These sizing teeth have a minimal cutting load, only removing the material’s elastic recovery. The low, stable cutting speed also helps prevent the formation of built-up edges.
Simple machine structure and operation. Broaching involves a single primary motion, the linear travel of the broach. The feed motion is built into the tool by the incremental height difference between adjacent teeth, known as the rise per tooth.
Disadvantages of Broaching Machining
High broach tooling cost. Due to the complex structure, shape, and high accuracy/surface quality requirements, broaches have a high manufacturing cost. However, the low cutting speed results in slow tool wear and long tool life, a single broach can be re-sharpened multiple times and process thousands of parts per sharpening. The high volume of production results in a low tool cost per piece.
Cannot correct positional errors. Broaching cannot correct positional errors, such as the axis misalignment of the pre-machined hole.
Small-batch production limitation. The high cost and long lead time for broach manufacturing make the process uneconomical for small-batch production, requiring careful consideration of cost and delivery time.
Blind hole machining limitation. The progressively increasing size of the broach teeth requires a certain length to achieve the final shape, and the tool must pass entirely through the workpiece. Therefore, it is not suitable for blind hole machining. Slotting cutters or push broaches are typically used for blind holes.
Applications of Broaching Machining
Although an internal broach is a fixed-size tool dedicated to a specific internal surface dimension and shape, different internal broaches can process various through-hole geometries, including round, square, polygonal, and spline holes, as well as internal gears. They are also used for various groove shapes, such as keyways, T-slots, dovetail grooves, and fir tree slots on turbine disks. External broaching can machine flat surfaces, contoured surfaces, external gears, and blade root forms.
The pre-machined hole does not require a precise finish, and the part does not need complex clamping; it is supported only by its end face. This necessitates a perpendicularity requirement between the original hole axis and the end face. If the hole axis is not perpendicular to the end face, the part’s end face should be seated on a spherical washer. Under the broaching force, the part and the washer can slightly rotate, automatically adjusting the hole’s axis to align with the broach’s axis.
Broaching is mainly applicable to batch and mass production, being especially suitable for machining large, complex profiles in high volumes, such as engine cylinder blocks. However, it is used in single-piece or small-batch production for certain high precision, specially shaped surfaces that are difficult to machine by other methods.
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Considerations for Broaching Machining
Material Hardness
The suitable material hardness for broaching is typically 170 to 240 HB. Harder materials generally yield better surface quality than softer ones, as soft materials are prone to galling onto the cutting edges of the broach, causing scratching. Conversely, overly hard materials lead to rapid broach wear and shortened life.
Pilot Hole
The size and quality of the pilot hole for a round broach significantly impact the processed surface quality, tool life, and accuracy. Therefore, the pilot hole must be correctly machined:
Perpendicularity: The pilot hole must be perpendicular to the mounting reference face. Non-perpendicularity can lead to broach bending and an inability to achieve high accuracy and surface quality.
Sizing: If the pilot hole is too small or bent, the front pilot of the broach cannot enter. If the hole is too large, the broach may contact eccentrically on one side.
Contaminants: The presence of dislodged built-up edges or other hard foreign matter in the pilot hole is a factor that reduces broach life.
Cutting Speed
Cutting speed affects the processed surface quality, machining accuracy, and broach life. It is crucial to select the speed based on the material’s machinability. Broaching speed generally ranges from 2 to 8 m/min.
Wall Thickness
The wall thickness of the workpiece subtly influences the processing accuracy of the hole diameter and roundness. During broaching, the workpiece is subjected to radial force, causing expansion due to elastic and plastic deformation. After the cut, elastic recovery largely restores the original shape. If the deformation during machining is excessive, some plastic deformation will remain, meaning the elastic recovery varies considerably with the workpiece wall thickness.
About Getzshape
When your project demands the high accuracy and efficiency of broaching, choosing a reliable partner will play an important role. Getzshape delivers expert broaching services for complex internal and external features, ensuring tight tolerances and good surface finish. Contact us to discuss your broaching specifications.
