The three ring drive mainly adopts the internal meshing angle modified gear, so the construction method of modified gear is needed to establish the model of internal gear plate and external gear.
In Cartesian coordinate system, the parameter equation of profile curve of modified gear is as follows：
In the formula, DB is the diameter of base circle; Theta is the pressure angle from 0 to 45 °。 The tooth profile curve of the first modified gear is established, and the tooth profile curve of the second modified gear is generated by creating datum plane, then mirror image, and finally stretch entity. For the outer gear and the inner gear plate, the tooth is different. The inner gear plate adopts the cutting method to construct the tooth.