Scraping can be defined as such a process, which is to remove the small machining allowance on the hardened tooth surface through the cutter. This is a method of finishing or semi finishing the tooth surface. Compared with gear grinding, scraping can not only improve machining quality, reduce surface roughness and noise, but also have the following characteristics:
(1) The efficiency of scraping is higher than that of grinding
In general, the cutting depth used in scraping of hard tooth surface is larger than that of grinding, and the metal removal rate can be higher than that of grinding. The metal removal rate of scraping is often 3-4 times higher than that of grinding. For hardened gears with ordinary accuracy, scraping can be directly carried out to obtain the required accuracy, so as to ensure the manufacturing accuracy of gears with the lowest processing cost. For high-precision gears requiring gear grinding, scraping with high efficiency can be used to replace the rough grinding process, remove the heat treatment deformation of the gear, and leave a small and uniform allowance for fine grinding, so as to greatly shorten the grinding working hours and improve the production efficiency.
(2) The range of scraping is wider than that of gear grinding
Thegrinding machine tool is expensive and the processing cost is high, which is difficult for many enterprises to bear, which limits the application range of spiral grinding. Moreover, there is no spiral bevel gear grinding machine for grinding gears with a diameter greater than 737mm and a modulus greater than 16.9mm in the world. However, by scraping the hard tooth surface, high-quality large spiral bevel gears with a diameter of 2000mm and a modulus of 42.3mm can be processed. This is of great significance to the increasing number of spiral bevel gears with large diameter, large modulus and high precision in modern heavy machinery industry.
(3) Good surface physical layer can be obtained by scraping
The physical and mechanical properties of the surface layer of the workpiece include surface layer hardness, surface layer structure and surface layer residual stress. The cutting specific power of grinding itself is very high. During grinding, a large amount of heat energy is generated, the temperature of grinding area is very high, and a large amount of heat energy is transmitted into the workpiece, and only a small part is taken away by chips. A large amount of heat energy reduces the metallurgical properties of the workpiece, forms an obvious grinding thermal effect, and has an adverse impact on the physical and mechanical properties of the workpiece surface layer. The scraping process generally consumes only 1 / 5 of the heat of grinding, and most of the heat is taken away by chips, so it can obtain better results
Physical and mechanical properties of the surface layer.
The surface hardness is formed by the joint action of work hardening and work softening. The surface layer of workpiece material is affected by cutting force, resulting in plastic deformation, lattice distortion, grain elongation, fibrosis and even fragmentation. These increase the hardness of the surface layer and produce work hardening. At the same time, due to the generation of cutting heat, the plastic deformation is restored and recrystallized, and the work hardening is lost. This phenomenon is called work softening. When grinding the hard tooth surface with soil, a large amount of grinding heat should be generated to make the machining softening more serious, which greatly weakens the work hardening and can not significantly improve the hardness of the tooth surface. The scraping plus the extrusion force on the tooth surface caused by the scraping layer is very thin and the large negative rake angle tool, the surface grain is broken and the microstructure is refined, which strengthens the gear surface: the surface metal of the gear is deformed under the extrusion of external force. Due to the obstruction of grain boundary, the grains in the crystal squeeze each other, the dislocation density increases rapidly, and a plug group is generated, The work hardening is obvious. At the same time, the heat generated during scraping is less than that during grinding, and more is taken away, and the machining softening is relatively weak. The joint action of the two can significantly improve the surface hardness of the gear during scraping.