The method of precise modification of tractor gear profile

1. Shaving before heat

Practice shows that concave phenomenon often occurs in the direction of tooth height during shaving. There is no unified and complete explanation about the cause of concave tooth shape in the theoretical circle, which has become a major problem in the field of shaving. Theoretically speaking, to ensure that there is no “tooth concave”, it is necessary to ensure that the instantaneous cutting force, cutting speed and cutting allowance of each contact point are the same when the shaving cutter and the shaved gear are meshed, but in fact, this can not be achieved, so corresponding technological measures can only be taken in production to eliminate or reduce it. Shaving cutter modification can not only carry out “K-shaped teeth”, but also is one of the most important technological measures to avoid “tooth concave”.

As shown in the figure, it is a common accurate modified tooth profile. The solid line is the theoretical involute tooth profile without modification, and the dotted line is the modified tooth profile.

Shaving profile modification principle: according to the meshing principle, calculate the meshing point of the shaving cutter tooth surface corresponding to the profile modification point on the tooth surface, and lower it when grinding the shaving cutter. In this way, when shaving, the required tooth profile can be machined.

2. Profile modification after hot grinding

The hard tooth surface after heat treatment can be precisely reshaped by using worm wheel gear grinding machine and forming gear grinding machine. Gear grinding is realized by dressing the shape of grinding wheel.

The grinding wheel of worm wheel gear grinder is finished with diamond wheel. Therefore, when using worm wheel gear grinder for tooth profile modification, the profile of diamond wheel needs to be made into a bulging arc-shaped surface, and the tooth surface of worm wheel is a concave surface, so as to grind the required K-shaped teeth.

At present, the forming grinding machine is generally CNC grinding machine. As long as the modification parameters of the gear are input into the CNC system of the machine tool, the system will automatically calculate the path of the grinding wheel dressing mechanism, repair the correct shape of the grinding wheel, and then grind out the K-shaped teeth that meet the requirements.

3. Electrochemical modification

Some manufacturers use electrochemical methods for gear shaping. Electrochemical machining is a kind of machining technology based on the principle of electrolysis. When machining, the cutter is connected with the cathode of DC power supply as cathode, and the workpiece is connected with the anode of power supply as anode. In the electrolyte, there is charge exchange between the cathode and the workpiece, and the anode workpiece is dissolved. In this way, the workpiece can be fixed-point processed without contacting the workpiece, and different profiles of the workpiece can be precisely manufactured. The electrolytic workpiece material precipitates in the electrolyte to form metal hydroxide.

When the external gear is modified by electrochemical method, the external gear is placed in the containing part (the modified internal gear) and put into the electrolytic cell together. The containing part is connected with the cathode and the gear is connected with the anode. After inputting current for a period of time, the modified tooth surface can be processed. Compared with the mechanical modification method, the electrochemical modification method is not limited by the hardness of the tooth surface, has no cutting trace, has no machining deformation and residual stress, and has a smooth transition of the tooth surface, but it needs to manufacture the containing parts, the gears are easy to be corroded by solution, and needs to be cleaned and rustproof after electrochemical processing. When the internal gear is modified by electrochemical method, the modified external gear is placed in the internal gear as the cathode, and the other processes are the same as the external gear.

At present, the advanced electrochemical method doesn’t need to put the whole parts into the electrolytic cell. It can pass the electrolyte directly to the modified tooth surface through the specially designed cathode, and return directly. More environmentally friendly than traditional methods.

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