Milling cutters for forming gear milling
Gear profile milling uses a profile milling cutter with the same shape as the gear tooth groove to directly mill the gear tooth profile on a milling machine. This method is simple to operate, has low equipment costs, and is suitable for single-piece, small-batch production and repair work. The selection and use of the milling cutter for profile milling gears has a significant impact on the machining quality and efficiency of the gears. The cutter type, parameters, material, and grinding quality are all key factors to consider.
The types of milling cutters used for forming gear milling are primarily categorized by the gear’s module, number of teeth, and tooth profile angle. Commonly used are disc gear milling cutters and finger gear milling cutters. Disc gear milling cutters are disc-shaped cutters with cutting edges distributed along the end faces. They are suitable for milling spur and helical gears with smaller modules (generally m ≤ 8mm). Disc gear milling cutters are typically produced in sets of 8 or 15 cutters, each corresponding to a specific range of tooth counts. For example, a set of 8 disc milling cutters with a module of 2mm is suitable for machining gears with tooth counts of 12-13, 14-16, and 17-20, respectively. This is because the gear tooth profile varies with the number of teeth, and the radius of curvature of the tooth profile varies with the number of teeth. To ensure tooth profile accuracy, the appropriate cutter must be selected based on the number of teeth. Finger gear milling cutters are cylindrical milling cutters with cutting edges distributed along the sides of the cylinder. They are suitable for milling gears with large modules (generally m > 8mm) and herringbone gears. Their shape resembles a finger, hence their name. Finger milling cutters offer advantages such as high rigidity and the ability to withstand high cutting forces, making them suitable for roughing and semi-finishing. However, due to their relatively low tooth profile accuracy, finishing requires additional machining methods.
The parameter design of the milling cutter directly affects the gear tooth profile accuracy and machining efficiency. Key parameters include module, tooth profile angle, tooth range, and cutting angle. Module is one of the fundamental parameters of a gear. The module of the milling cutter must be equal to that of the gear being machined; otherwise, errors in the tooth thickness and tooth height will occur. The tooth profile angle refers to the angle between the normal at a point on the gear tooth profile and the instantaneous direction of motion at that point. The tooth profile angle of a standard gear is typically 20°, so the tooth profile angle of the milling cutter should also be designed to be 20° to ensure proper tooth meshing. The tooth range is a key parameter for disc gear milling cutters. As mentioned earlier, each milling cutter set corresponds to a specific tooth range. This is because when using a milling cutter with an approximate tooth profile to machine gears with different tooth counts, tooth profile errors, known as “theoretical errors,” will occur. The smaller the tooth range, the smaller the theoretical error and the higher the tooth profile accuracy. Cutting angles include rake angle, back angle, and inclination angle. Increasing the rake angle reduces cutting force and cutting temperature, but also reduces the strength of the milling cutter. Increasing the back angle reduces friction between the milling cutter and the workpiece, extending tool life, but also reduces tool rigidity. Therefore, the cutting angle of the milling cutter should be reasonably designed based on the material and hardness of the gear being machined. For example, when machining hardened gears, a smaller rake angle and a larger back angle should be selected.
The material properties of milling cutters are crucial to ensuring their cutting performance and service life. Commonly used materials include high-speed steel (HSS) and carbide. High-speed steel milling cutters offer high strength and toughness, can withstand heavy impact loads, and have excellent sharpness, resulting in a sharp blade. These cutters are suitable for milling gears with smaller modules and softer materials, such as 45 steel gears. However, HSS cutters have poor heat resistance and low cutting speeds, generally not exceeding 30 m/min, resulting in relatively low machining efficiency. Carbide milling cutters consist of a carbide insert and cutter body. Carbide offers high hardness, wear resistance, and excellent heat resistance, allowing for cutting speeds of 100-300 m/min. These cutters are suitable for milling gears with larger modules and harder materials, such as hardened steel gears. However, carbide cutters are brittle and have poor impact resistance, making them prone to chipping during milling. Therefore, they are suitable for smooth cutting but not for intermittent cutting or applications with heavy impact loads. When selecting a milling cutter material, it is important to consider factors such as the gear material, hardness, module, and production batch size to achieve optimal machining results.
The quality of milling cutter grinding has a direct impact on gear machining quality. After grinding, the cutter should ensure a sharp edge, accurate tooth profile, and low surface roughness. During grinding, the cutter’s tooth profile must be consistent with the standard. Excessive tooth profile deviation will result in inaccurate gear tooth profiles, impacting gear meshing performance. Furthermore, the cutter’s rake and flank faces should be smooth and flat after grinding, free of burrs and cracks. Failure to do so will cause vibration during cutting, leading to ripples or scratches on the gear surface. For disc gear cutters, the radial runout and face runout of each cutting edge must be within acceptable limits during grinding to avoid excessive pitch errors in the resulting gear. Milling cutters are typically sharpened on a tool grinder using a dedicated grinding fixture to ensure accurate grinding. After grinding, the cutter must be inspected using a tooth profile template or an optical projector to ensure the tooth profile meets the required accuracy before use.
The use and precautions of milling cutters used in forming gear milling are also crucial for ensuring machining quality. When installing the cutter, it must be securely clamped to the milling machine’s arbor to prevent loosening or vibration during the cutting process, which could affect tooth profile accuracy. After installation, check the cutter’s radial and face runout. If excessive, reinstall or adjust it. When selecting cutting parameters, the cutting speed, feed rate, and back-cut should be appropriately determined based on the cutter material, gear material, and module. For example, when milling 45 steel gears with a high-speed steel cutter, the cutting speed is generally 15-30 m/min and the feed rate is 0.1-0.3 mm/r. When using a carbide cutter, the cutting speed can be increased to 50-150 m/min and the feed rate to 0.05-0.2 mm/r. During the milling process, adequate cooling and lubrication are essential to reduce cutting temperatures, minimize tool wear, and improve gear surface quality. A suitable cooling lubricant, such as an emulsion or cutting oil, should be used and sprayed directly onto the cutting area through a nozzle. Furthermore, after milling, parameters such as the gear tooth profile, pitch, and tooth thickness must be inspected. If any errors are detected, the milling cutter or cutting parameters should be adjusted promptly to ensure the quality of the subsequent gears.
