Broaching of internal splines
Internal splines are a key structure for achieving precise connection between shafts and hubs in mechanical transmissions. The quality of their CNC machining directly impacts the smoothness and reliability of the transmission. Broaching, with its advantages of high efficiency and high precision, has become one of the primary CNC machining methods for internal splines, particularly suitable for mass production. Internal spline broaching utilizes a series of progressively larger teeth on a broach to sequentially remove excess material from the workpiece’s inner bore, ultimately forming a spline tooth profile that meets precision requirements. This process features excellent surface quality, high dimensional consistency, and high production efficiency, and is widely used in automotive transmissions, machine tool spindle boxes, and other fields.
The structural design of an internal spline broach is central to the broaching process, and its rationality directly determines the spline CNC machining accuracy and tool life. A broach consists of a shank, neck, transition cone, leading section, cutting section, calibrating section, and trailing section, with the cutting and calibrating sections being the key functional components. The cutting section includes roughing, transition, and finishing teeth. The roughing teeth perform the primary cutting task, with a large tooth lift (typically 0.03-0.1mm) to quickly remove CNC machining stock. The transition teeth have a gradually decreasing tooth lift, acting as a buffer. The finishing teeth have a tooth lift of 0-0.02mm and are used to refine the tooth profile and improve accuracy. The calibrating section, with no tooth lift, primarily ensures the final dimensional accuracy and surface quality of the spline, while also serving as a guide and stabilizing the broaching process. The tooth shape of the broach must be exactly the same as that of the spline. For rectangular splines, the two sides of the teeth must maintain strict parallelism and symmetry, and the pitch error must be controlled within 0.01-0.02mm to ensure the fitting accuracy of the spline after processing.
Process preparation for broaching internal splines is crucial to CNC machining quality, requiring strict control in areas such as workpiece pretreatment, broaching tool selection, and equipment adjustment. The workpiece must be drilled or bored before broaching, and a reasonable broaching allowance must be reserved. Too little allowance will result in insufficient dimensional accuracy of the finished spline, while too much allowance will increase broaching tool load and exacerbate wear. Generally, the broaching allowance for the minor diameter of a spline is 0.3-0.8mm, and for the key width is 0.15-0.4mm. The specific values depend on the spline’s precision grade and size. The workpiece material hardness must be within an appropriate range. For steel workpieces, a hardness of 180-220 HBW is appropriate. Excessive hardness increases broaching resistance and can lead to broaching tool chipping, while low hardness can easily cause tool sticking, compromising surface quality. Before broaching, the workpiece bore must be cleaned to remove burrs, iron filings, and other impurities. Apply broaching oil to the contact surface between the broaching tool and the workpiece to reduce friction and provide cooling and lubrication.
Process control for internal spline broaching requires careful attention to parameter settings and operating procedures to ensure CNC machining stability and consistency. The broaching speed should be determined based on the workpiece material and broach type. High-speed steel broaches typically have a broaching speed of 3-8 m/min, while carbide broaches can reach speeds of 10-15 m/min. Excessively high speeds can lead to broach overheating and wear, while excessively low speeds can reduce production efficiency. The feed rate during broaching is determined by the broach tooth lift and requires no additional adjustment. However, sufficient broaching force must be maintained to avoid broach jamming or workpiece deformation due to insufficient force. During broaching, close attention should be paid to sound and vibration. Normal broaching should be smooth and low-noise. Any sharp noise or severe vibration may indicate broach wear, uneven workpiece hardness, or excessive stock, requiring immediate machine stop and inspection. For spline holes with a large aspect ratio, segmented broaching can be used to minimize broach bending and deformation and ensure CNC machining accuracy.
Quality inspection and tool maintenance for internal spline broaching are critical steps in ensuring CNC machining quality. After CNC machining, splines must be inspected for dimensional accuracy of the minor diameter, major diameter, and key width, as well as for profile error, tooth profile error, and surface roughness. The minor diameter is typically inspected with a smoothing limit gauge, the key width with a spline plug gauge, and the tooth profile and tooth profile errors are measured with a gear measuring instrument. Surface roughness Ra should be controlled within 1.6-3.2μm. Broach maintenance includes post-use cleaning, edge grinding, and storage management. After use, broaches should be cleaned with kerosene to remove residual chips and broaching oil. The teeth should be inspected for defects such as chipping and wear. Minor wear can be repaired by hand grinding, while severe wear or chipping requires re-grinding. Broaches should be stored in dedicated tool holders to avoid collisions. During long-term storage, they should be coated with anti-rust oil to prevent rust. Strict quality inspection and standardized tool maintenance can effectively improve internal spline CNC machining quality, improve broach life, and reduce production costs.
