Gear finishing allowance
As the core component of mechanical transmission, the accuracy of gears directly affects the transmission efficiency, noise level and service life of the entire mechanical system. The reasonable determination of gear finishing allowance is a key link to ensure the quality and economy of gear processing. Finishing allowance refers to the thickness of the metal layer retained on the gear after roughing and before finishing. This part of the allowance needs to be removed during the finishing process to eliminate the shape error, position error and surface defects left by roughing, so as to achieve the accuracy and surface quality required by the design. If the allowance is too large, the workload of finishing will increase, production efficiency will be reduced, and materials and energy will be wasted; if the allowance is too small, the roughing defects cannot be completely eliminated, resulting in the gear accuracy not meeting the requirements. Therefore, scientifically and reasonably determining the gear finishing allowance is of great significance to the production of gears.
Gear material and heat treatment methods are important factors influencing finishing allowance. Different materials have different mechanical properties and processing characteristics, and therefore require different finishing allowances. For example, after forging, forged steel gears have a denser internal structure, but may have oxide scale and decarburization layers on the surface, necessitating a larger finishing allowance to remove these defects. Cast steel gears, on the other hand, may develop defects such as pores and pinholes during the casting process, so to ensure gear strength and precision, finishing allowances are typically larger than forged steel gears. Furthermore, heat treatment can cause gear deformation. For example, after quenching, gears experience volume expansion and shape changes. Therefore, the effects of heat treatment deformation must be considered when determining finishing allowances. For gears that require quenching, finishing allowances should be appropriately increased to compensate for quenching deformation and ensure the gears achieve the specified accuracy after final processing.
The gear’s accuracy grade is the primary basis for determining the finishing allowance. The higher the gear’s accuracy grade, the more stringent the requirements for form error, position error, and surface roughness, and the larger the required finishing allowance. For example, for a grade 6 gear, the finishing allowance on the tooth surface is generally 0.1-0.3mm, while for a grade 8 gear, the finishing allowance can be reduced to 0.05-0.15mm. This is because high-precision gears require additional metal removal during finishing to correct for various errors introduced during roughing and heat treatment. Furthermore, different gear components (such as the tooth flank, addendum, and root circle) have different accuracy requirements, and their finishing allowances should also differ. The tooth flank, as the working surface of the gear, has the highest accuracy requirements, so its finishing allowance should be the largest. The addendum and root circles, on the other hand, have relatively lower accuracy requirements, and their finishing allowances can be appropriately reduced.
The structure and size of the gear also affect the finishing allowance. Gears with complex structures and poor rigidity, such as thin-walled gears and gears with slender shafts, are prone to deformation during machining, requiring a larger finishing allowance to correct for this deformation. On the other hand, gears with simpler structures and better rigidity, such as solid gears and gears with short teeth, can require a smaller finishing allowance. Furthermore, the larger the gear size, the greater the errors introduced during roughing and heat treatment, requiring a larger finishing allowance. For example, large gears with a diameter greater than 500mm may require a finishing allowance of 0.5-1mm on the tooth surface, while small gears with a diameter less than 100mm typically require a finishing allowance of 0.1-0.3mm. The gear machining process should also be considered when determining the finishing allowance. Different machining methods (such as hobbing, shaping, shaving, and grinding) have different requirements for finishing allowance.
The gear machining process is another important factor in determining the finishing allowance. A complete gear machining process typically includes blank manufacturing, rough machining, heat treatment, semi-finishing, and finishing, each of which affects the final finishing allowance. For example, tempering after rough machining will produce an oxide layer and a certain amount of deformation on the gear surface. Therefore, semi-finishing requires removing this oxide layer and correcting the deformation to leave an appropriate allowance for finishing. If surface quenching is performed after semi-finishing, since quenching will produce a hardened layer and deformation on the tooth surface, finishing (such as grinding) requires removing the errors caused by quenching deformation and defects on the hardened layer surface. Therefore, the finishing allowance should be determined based on the extent of the quenching deformation. When formulating the machining process, the machining allowance for each process should be reasonably arranged so that each process can fully play its role and ultimately ensure the machining quality of the gear.
There are three main methods for determining gear finishing allowance: empirical, calculation, and experimental. The empirical method relies on long-term production experience and references the machining allowances of similar gears. This method is simple and easy to use, but has lower accuracy and is suitable for machining gears with average precision. The calculation method uses theoretical calculations based on factors such as gear precision requirements, material properties, and machining technology to determine the finishing allowance. This method offers higher accuracy but requires mastering a large number of parameters and calculation formulas, making it suitable for machining high-precision gears. The experimental method uses trial cutting and adjusts the finishing allowance based on the test results. This method is the most accurate, but more expensive, and is suitable for trial production of new products or machining special gears. In actual production, these three methods are often combined to determine an appropriate finishing allowance. Furthermore, the finishing allowance should be dynamically adjusted based on actual production conditions to ensure gear machining quality and production efficiency.
