Selection of hob precision
Hob accuracy is a core factor affecting gear machining quality, directly related to the gear’s tooth profile accuracy, pitch error, and surface roughness, which in turn affects the smoothness, noise level, and service life of the gear transmission. The selection of hob accuracy grade should be determined based on the gear’s accuracy level, application, and production batch. Current Chinese standards classify hob accuracy into AA, A, and B grades (corresponding to the international standard ISO 3685). Hobs of different accuracy grades have strict regulations on manufacturing errors (such as tooth profile error, lead error, and pitch cumulative error). Properly selecting hob accuracy ensures gear quality while controlling production costs, and is a crucial step in gear machining process design.
Matching the hob’s accuracy grade to the gear’s accuracy grade is a fundamental principle in gear selection. Generally, the hob’s accuracy should be one to two grades higher than the gear being machined. For example, for machining a grade 7 gear (GB/T 10095), a grade AA hob should be used (grade AA hobs are suitable for machining grade 6-7 gears); for machining grade 8 gears, a grade A hob should be used (grade A hobs are suitable for machining grade 7-8 gears); and for machining grade 9 and below, a grade B hob is sufficient. This matching relationship is based on the fact that some hob manufacturing tolerances are transferred to the gear. Excessively high hob accuracy increases costs, while too low a hob’s accuracy can’t guarantee gear accuracy. For example, using a grade B hob to machine a grade 7 gear can result in a tooth profile error exceeding 0.015mm, affecting transmission smoothness. Using an AA hob to machine a grade 9 gear, on the other hand, results in wasted tool costs and increased production investment.
The application of gears has a significant impact on the selection of hob precision, and the precision requirements for gears in different application scenarios vary significantly. Gears used in high-speed transmissions (such as automotive transmission gears and machine tool spindle gears) typically have speeds exceeding 3000r/min and are sensitive to cumulative pitch error and tooth profile error. Therefore, AA-grade hobs are required to ensure smooth transmission and low noise (noise value ≤85dB). Gears used for low-speed and heavy loads (such as crane reducer gears) have lower requirements for smoothness but must ensure uniform tooth contact. Therefore, A-grade hobs should be used, with tooth profile error controlled to ≤0.01mm/100mm. For lightly loaded, low-speed, non-critical gears (such as agricultural machinery gears), B-grade hobs can be used to reduce manufacturing costs. In addition, for gears that require subsequent finishing processes such as shaving and grinding, the hob precision can be appropriately reduced (for example, a Class A hob can be used for gears before shaving), as some errors can be corrected in subsequent processes.
The type and material of the hob also influence the accuracy selection, with different types of hobs having different applicable accuracy ranges. Solid high-speed steel hobs can achieve accuracy up to AA, making them suitable for machining high-precision gears with small and medium modules (m ≤ 6). Their accuracy is easily maintained after sharpening, but their wear resistance is relatively low, making them suitable for small and medium-volume production. Carbide hobs with inserts generally achieve accuracy of A, making them suitable for machining gears with large modules (m ≥ 8) and high-strength steel gears. They offer high wear resistance and are suitable for mass production, but their manufacturing accuracy is slightly lower than that of solid high-speed steel hobs. Coated hobs (such as TiAlN coatings) have the same accuracy as base hobs, but the coating improves wear resistance and extends service life. Therefore, the accuracy selection can be the same as for uncoated hobs, but the effect of coating thickness on accuracy should be considered (coating thickness ≤ 5μm to avoid affecting tooth profile accuracy).
Verification and maintenance of hob accuracy are crucial to ensuring long-term accuracy. Before new hobs are stored, they must be inspected according to standard accuracy specifications: AA-grade hobs have a tooth profile error of ≤0.005mm and a lead error of ≤0.01mm/100mm; A-grade hobs have a tooth profile error of ≤0.01mm and a lead error of ≤0.02mm/100mm. Hobs should be regularly inspected for wear during use. If tooth wear exceeds 0.02mm, they must be resharpened. After sharpening, the hob must be retested to ensure it meets accuracy requirements. Hobs should also be stored and installed with care. They should be placed horizontally on a dedicated tool holder to avoid collisions. During installation, a dial indicator should be used to calibrate the hob’s radial runout and axial play, keeping the errors within 0.01mm and 0.005mm, respectively. Failure to do so will exacerbate hob wear and gear machining errors. Through scientific selection, precise testing, and standardized maintenance, the hob’s precision performance can be fully utilized, ensuring consistent gear machining quality.
