Broaching of keyway in inner hole of long shaft
Broaching the keyway in the inner bore of a long shaft is an efficient and high-precision keyway processing method suitable for machining keyways in the inner bores of long shaft parts. It offers advantages such as high machining efficiency, high keyway precision, and excellent surface quality, making it widely used in the mechanical manufacturing field. Broaching the keyway in the inner bore of a long shaft requires the use of a dedicated broaching tool and broaching equipment. The linear motion of the broaching tool and the sequential cutting of the teeth achieve the keyway forming process. Due to the long length of the long shaft, broaching the inner bore keyway presents some special requirements and technical difficulties, requiring appropriate process measures to ensure machining quality.
The key to broaching keyways in long shafts lies in the design and selection of the broach. The broach’s structure and dimensions must match the keyway dimensions, the shaft’s internal bore dimensions, and the material properties. A broach consists of a shank, neck, transition section, working section, and tail section. The working section, the primary component of the broach, includes roughing teeth, finishing teeth, and calibration teeth. The roughing teeth are used to remove the majority of machining allowances, the finishing teeth are used to improve the keyway’s accuracy and surface quality, and the calibration teeth are used to ensure the final dimensions of the keyway. Key broach parameters include keyway width, keyway depth, tooth lift, pitch, and tooth material. Keyway width and depth must be determined according to design requirements. Tooth lift refers to the height difference between adjacent teeth. Roughing teeth have a larger tooth lift (0.03-0.1mm), while finishing teeth have a smaller tooth lift (0.01-0.03mm). Tooth lift should be selected based on the hardness and toughness of the workpiece material. High hardness and high toughness require a smaller tooth lift. Tooth pitch refers to the distance between adjacent teeth. Too large or too small a pitch will affect the smoothness of the broaching process. Generally, the pitch is 1/3-1/5 of the keyway length. Broaches are typically made of high-speed steel (such as W18Cr4V) or carbide. High-speed steel broaches are suitable for machining general materials, while carbide broaches are suitable for machining high-strength, high-hardness materials. The teeth of broaches must be hardened to a hardness of HRC60-65 to ensure wear resistance and longevity.
The equipment used for broaching keyways on long shafts depends on the shaft’s length, bore diameter, and keyway dimensions. Common broaching equipment includes horizontal broaching machines, vertical broaching machines, and specialized broaching machines. Horizontal broaching machines are suitable for broaching keyways on long shafts. Their worktable can move longitudinally, with the broaching tool fixed to the bed and the workpiece mounted on the table, allowing broaching to occur with the table. Vertical broaching machines are suitable for shafts with larger bore diameters and relatively short lengths. The broaching tool moves vertically, with the workpiece fixed to the table. Dedicated broaching machines are designed and manufactured to meet the specific structural and machining requirements of long shafts, offering greater specificity and efficiency. The accuracy of the broaching equipment directly impacts the machining precision of the keyway. The straightness of the guide rails must not exceed 0.01 mm/m, and the broaching tool’s motion accuracy (parallelism and perpendicularity) must meet requirements to avoid keyway skew or dimensional errors.
Process preparation for broaching keyways on long shafts is crucial, including workpiece pretreatment, broaching tool installation and adjustment, and determining broaching parameters. Workpiece pretreatment includes machining and cleaning the inner hole. The inner hole must be bored or reamed to ensure diameter accuracy, roundness, and surface roughness. The inner hole diameter should match the broaching tool shank size, leaving appropriate clearance. The surface roughness of the inner hole should be below Ra 3.2μm to avoid scratching the broach during broaching or affecting the keyway quality. Furthermore, the inner hole must be cleaned of impurities such as oil and iron filings to prevent them from entering between the broaching tool and the workpiece during broaching, potentially damaging the broach or degrading the keyway surface quality. During installation, the broaching tool must be aligned with the workpiece inner hole axis. During installation, use an alignment tool (such as a dial indicator) to check the radial runout of the broaching tool; the error should be within 0.01mm. The broaching parameters mainly include broaching speed and broaching feed. The broaching speed is generally 2-8m/min, and a lower value is taken when the material has high hardness and toughness. The broaching feed is determined by the tooth lift of the broach and does not require additional adjustment.
Process control during broaching of keyways in long shafts is crucial for ensuring machining quality. Attention must be paid to stress, vibration, and cooling and lubrication during broaching. Due to the length of the shaft, the workpiece is prone to bending and deformation during broaching, which can increase keyway straightness errors. Therefore, auxiliary supports are required during broaching. The placement of these supports should be determined based on the length and rigidity of the shaft. Typically, supports are placed every 1-2 meters, with appropriate clearance between the supports and the shaft to prevent axial movement. Severe vibration during broaching can cause ripples on the keyway surface or chipping of the broaching tool. Check the broaching tool’s mounting accuracy, workpiece clamping, and broaching parameters, and make timely adjustments to eliminate vibration. Cooling and lubrication are crucial aspects of the broaching process. High cutting heat and friction during broaching necessitate the use of extreme-pressure cutting oil as a cooling lubricant. The cutting oil should have excellent lubricity, cooling properties, and rust resistance. A high-pressure oil pump should spray the cutting oil into the broaching area at a flow rate of at least 10 L/min to reduce friction between the broaching tool and the workpiece, lower cutting temperatures, and improve tool life and keyway surface quality.
After broaching the keyway in the inner hole of a long shaft, quality inspection and subsequent processing must be carried out in strict accordance with requirements. Quality inspection includes the keyway’s width, depth, length, symmetry, parallelism, and surface roughness. Keyway width and depth can be measured with a vernier caliper, micrometer, or specialized gauge, while symmetry and parallelism can be measured with a dial indicator or a coordinate measuring machine. Surface roughness must meet Ra 1.6μm or higher. If keyway dimensions are out of tolerance or surface quality is substandard, the cause must be analyzed and remedial measures implemented. For example, undersized keyways can be trimmed with a slotting tool; keyways with excessive surface roughness can be honed or polished. After broaching, the inner hole of the long shaft must be cleaned to remove residual chips and cutting oil. Rust-proof treatment should be applied if necessary to prevent rust on the workpiece. After use, the broach must be cleaned, inspected, and maintained. Chips and oil stains must be removed from the teeth, and the teeth must be inspected for wear or chipping. Worn teeth must be sharpened to ensure reusability. By strictly controlling each link of the broaching process, the broaching of the keyway in the inner hole of the long shaft can achieve high processing quality and meet the requirements of mechanical transmission.
