A gearbox includes a housing having therein a receiving chamber, a transmission gear wheel mounted in the receiving chamber for rotation driven by an external driving force, an eccentric shaft assembly mounted in the receiving chamber and coupled to the transmission gear wheel for rotation together with the transmission gear wheel and driving action of a cutter, and a lubricating blade mounted at a bottom side of the receiving chamber, rotatably connected with the eccentric shaft assembly for rotation together with the eccentric shaft assembly, and having a plurality of arched grooves for delivering the lubricating oil located at the bottom side of the receiving chamber to the transmission gear wheel and the eccentric shaft assembly by a centrifugal force generated by the rotation.