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In the field of gear processing, gear tooth surface according to the hardness can be divided into three kinds of soft tooth surface, medium hard tooth surface and hard tooth surface. Among them, the soft tooth surface hardness of 280HBW or less, medium hard tooth surface hardness in 280~400HBW, hard tooth surface hardness in 45~64HRC.
Increasing the hardness of gear tooth surfaces can significantly increase the load carrying capacity of gears. When the surface hardness of the gear tooth form is increased to carburizing and quenching hardness (hardened tooth face), the torque transmitted can be increased by about three times compared with the general tempered gear (medium hardened tooth face). The mass of the gear pair can be reduced to 40% of the original when the transmitted torque remains unchanged. Increasing the hardness of the gear tooth surface also significantly improves the wear resistance of the tooth surface.
Hardened internal gear ring process program EDM is in a certain liquid medium, the use of pulsed discharge on the conductive material of the phenomenon of galvanic corrosion to erode the material, so that the size, shape and surface quality of the parts to meet the predetermined technical requirements of a machining method. It is characterized by the machining process is not affected by the hardness of the workpiece. EDM wire cutting is the use of moving fine metal wire as a tool electrode, according to a predetermined trajectoryfor pulse discharge cutting. According to the size of the wire electrode moving speed is divided into high speed wire walking and low speed wire walking wire cutting. The use of EDM wire cutting in the quenching of the tooth surface processing, the processing process route for the forging → normalizing → rough turning → semi-finish turning → insertion of teeth → carburizing quenching → finish turning → wire cutting, in the wire cutting process on the inner ring teeth finishing. The difficulty of this technology program mainly lies in the process margin of wire cutting, and the process margin of wire cutting is confirmed from the following three aspects:
(1) Hardened layer depth. Drawing of the hardened layer depth requirements for the tooth face 2 ~ 3m m, the root of the hardened layer depth of 1.5 ~ 2.5m m. In the company's existing heat treatment level, the depth of the hardened layer of the tooth face can be up to the deepest (4 ± 0.3) mm, the root of the tooth can be up to (2.3 ± 0.2) mm. in order to ensure that the process of cutting continuity of the wire cutting process, the tooth face and the root should be cut at the same time to the root with a smaller margin to determine the margin of wire cutting process margin should not be more than 0.6mm (tooth face one-side M value converted to the cross-bar distance M value margin of about 3.1mm. In order to ensure the cutting continuity in the wire cutting process, the tooth face and tooth root should be cut at the same time, with the smaller margin of the tooth root to determine the margin of the wire cutting process, the maximum should not be more than 0.6mm (one side of the tooth face), which is converted into a cross-bar distance of the M-value of the margin of about 3.1mm.
(2) Heat treatment deformation. In order to ensure that wire cutting can completely cut out the tooth surface, wire cutting process allowance should be greater than the heat treatment deformation. From the scene of any three pieces of the ring, after quenching of the ring M value were measured, the data are shown in the attached figure. From the measured data, it can be seen that the maximum shrinkage of M value after quenching is 0.61mm, the minimum shrinkage is 0.28mm, the maximum change of M value of the ring after quenching itself is 0.6mm, in order to ensure that all the rings can be completely cut out of the tooth surface to the ring after quenching M value of the ring itself as a reference for the maximum amount of change, based on experience, add 0.6mm, the minimum shrinkage of 0.28mm in the data is set aside! Before quenching, it is determined that the M-value margin should be ≥ 0.92mm.
change in M-value of gear ring after quenching
(3) Diameter of molybdenum wire for wire cutting. In order to ensure that the wire cutting molybdenum wire can be completely into the metal inside, tooth surface unilateral margin should not be less than the diameter of the molybdenum wire, the company now uses the molybdenum wire diameter is generally 0.18mm. thus determine the wire cutting process margin, the minimum should be > 0.18mm (tooth surface unilateral), converted to M value margin of about 0.7mm. according to the above 3 aspects of the analysis of the data, to determine the M value of evaluation of the minimum margin of the wire cutting as follows 0.92mm. After processing the toothed ring by EDM, the precision of the toothed ring can reach level 7 of the national standard, and the roughness value of the tooth surface can reach Ra=3.2μ m, which exceeds the requirement of the drawing.
4. Concluding remarks
After the implementation of this process program, the hardened internal gear ring tooth surface processing problems have been solved. The program is especially suitable for single-piece trial production and small-lot trial production in the new product development stage. After the successful implementation of the EDM hardened internal gear ring process, we extended it to the machining of hardened internal splines, which was also successful.
