When machining precision spindle parts, the most typical workpiece positioning method is the positioning of the top hole at both ends. This positioning method conforms to the principle of reference coincidence and the principle of uniformity of the reference. In order to ensure the accuracy of the top positioning, it is generally necessary to re-process the top hole before the final machining of the main outer surface of the main shaft. The commonly used repair methods are as follows: 1. Grinding the top hole with a stone or rubber wheel. Clamp the cylindrical oil stone or rubber grinding wheel on the lathe chuck and trim it into a 60 ° cone with a diamond pen; clamp the top of the workpiece between the oil stone and the top of the lathe, and add a small amount of diesel or light oil to lubricate. The lathe is turned on to rotate the oil stone at high speed for grinding, and the operator holds the workpiece intermittently. 2 Use a cast iron tip to grind the top hole. The principle of the method is the same as above, except that the tip of the cast iron is used instead of the top of the oil stone, and the abrasive should be added during the research, and the top speed is slightly lower. 3 Use a hard alloy tip to grind the top hole. The method has high processing efficiency, but the quality of the research is slightly poor, and is mostly used for roughing the ordinary shaft top hole or as a precision shaft top hole. 4 Grind the tip hole with a tip hole grinder. The machining accuracy of the machine tool is high, the surface roughness of the top hole can reach Ra0.32μm, and the roundness can reach 0.8μm.

When machining the outer circumference of the precision spindle, the spindle tip hole is supported on the fixed front and rear tips. At this time, the rotation axis of the workpiece is determined by two fixed tips. In theory, the axis of rotation is fixed and a high-precision outer circle can be machined. However, when there are shape errors (such as roundness, angular error, etc.) or concentricity errors in the two pointed holes, the finite point contact state between the tip and the tip hole on the positioning section is non-peripheral contact (as shown in the figure). 1)), which will result in insufficient support stiffness and cause the workpiece axis to change, resulting in roundness error of the machined surface. In the final outer circle of the precision spindle shown in Fig. 2, the roundness tolerance is extremely small, even if the two top holes are carefully studied, the two top holes have a very low surface roughness. Degree, high shape accuracy and coaxiality, but the contact area is still difficult to reach more than 85%, the workpiece axis change is still obvious, still has a significant impact on the roundness of the workpiece, the outer circle roundness is out of tolerance happens sometimes.

(a) Top positioning diagram

(b) Positioning section limited point contact state diagram

Figure 1 Double cone tip positioning state diagram

Figure 2 Schematic diagram of high-precision grinding wheel spindle

2 Refinement method for top positioning accuracy

In order to improve the positioning accuracy of the top of the precision spindle and ensure the roundness of the final machining of the outer circumference of the spindle, the following top positioning accuracy refinement measures can be taken.

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