The selection of gear machining datum is often different due to the different structure and shape of gears. The gear with shaft is mainly positioned by vertex hole; For hollow shaft, after the central inner hole is drilled, locate it with the bevel of the holes at both ends; If the hole diameter is large, use a cone plug. The precision of vertex positioning is high, and datum coincidence and unification can be achieved. The following two positioning and clamping methods are often used for machining the gear with holes on the tooth surface.

　　1. Positioning of inner hole and end face

　　This positioning method uses the inner hole of the workpiece to locate, determine the positioning position, and then use the end face as the axial positioning reference, and clamp against the end face. In this way, the positioning datum, design datum, assembly datum and measurement datum can coincide, and the positioning accuracy is high, which is suitable for batch production. However, the manufacturing accuracy of the fixture is required to be high.

　　2. Outer circle and end face positioning

　　When the fit clearance between the workpiece and the aggravating spindle is large, use a dial indicator to correct the excircle to determine the position of the center, and use the end face for axial positioning, and clamp from the other end face. This positioning mode has low productivity because each workpiece needs to be calibrated; At the same time, the requirements for the inner and outer circles of gear blanks are high, while the requirements for fixture accuracy are not high, so it is suitable for single piece and small batch production.