Precise positioning method for precision spindle machining

1 General technical measures to ensure the positioning precision of the top hole

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 reunification and the principle of unified standards. In order to ensure the top positioning accuracy, it is generally necessary to rework the top hole prior to the final machining of the major cylindrical surface of the spindle. Commonly used methods for repairs include: 1 Repairing the top hole with a whetstone or rubber wheel. The cylindrical whetstone or rubber grinding wheel is clamped on the chuck of the lathe, and the diamond pen is used to trim it into a 60° cone; the workpiece is sandwiched between the oilstone and the top of the lathe, and a small amount of diesel oil or light oil is added to lubricate. The lathe was turned on to make Whetstone rotate at high speed for study, while the operator held the workpiece intermittently. 2 Repair the top hole with a cast iron tip. The principle of this method is the same as above except that the top of cast iron is used instead of the top of the oilstone. The grinding agent should be added during the study, and the top speed is slightly lower. 3 Carefully drill the top hole with a carbide tip. This method has high processing efficiency, but the quality of the research is slightly poor. It is mostly used for studying the rough machining of the ordinary shaft top hole or the precision shaft top hole. 4 Grind the top hole with a top hole grinder. The machining precision of this machine is high. The surface roughness of the top hole can reach Ra0.32μm and the roundness can reach 0.8μm.

When machining the precision main shaft outer circle, the main shaft's top hole is supported on the fixed front and rear top tips. At this time, the rotation axis of the work piece is determined by two fixed top tips. Theoretically, the axis of rotation is fixed and can be machined to a high-precision outer circle. However, when there are shape errors (such as roundness, angle errors, etc.) or coaxiality errors in the two top holes, the contact between the top and top holes on the positioning section will be non-circular line contact (see figure). (1), which will lead to the lack of support stiffness and cause the workpiece axis to fluctuate, resulting in roundness error of the machined surface. In the final grinding (superfinishing) main outer circle of the precision spindle shown in Fig. 2, due to the extremely small roundness tolerance, 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 more obvious, it will still have a significant impact on the roundness of the workpiece outer circle, circular round of excess happens sometimes.

(a) Top positioning diagram

(b) Positioning section limited contact state diagram

Figure 1 double tapered top positioning state diagram

Fig. 2 Sketch of high-precision grinding wheel spindle

2 Top positioning precision refinement method

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

• Use spherical ball tip for final cylindrical outer grinding

Using the spherical cross section of any sphere and the contact between the sphere and the cone bore with a circumferential line, the special spherical tip of the final cylindrical outer crown is used to position the workpiece (as shown in Fig. 3). The spherical tip can ensure that the top and the top hole always maintain the circumferential line contact state in the positioning section from the geometrical principle. Even if there are obvious taper errors and coaxiality errors in the two top holes, the circumferential line contact state will not be changed. When using spherical top positioning, the contact stress of the positioning round section is evenly distributed, the contact stiffness is significantly improved, and the axial displacement caused by contact deformation of the positioning surface of the ground shaft part is substantially eliminated, so that the positional stability of the spindle rotation axis can be significantly improved, and The roundness of the main shaft after final grinding is greatly improved.

(a) Top positioning diagram

(b) Positioning section contact state diagram

Figure 3 spherical top positioning state diagram

• Multiple holes in the top hole

The major cylindrical surfaces of the precision spindle components (such as the spindle journal surface) are generally processed through multiple steps such as rough grinding, semi-finishing, finish grinding, and final grinding (superfinishing), and must be processed before each grinding process. The top hole is carefully studied to gradually reduce the surface roughness value of the top hole and improve the shape accuracy and coaxiality of the two top holes to achieve the purpose of improving the contact accuracy between the top hole and the top hole.

The precision of top holes before each grinding process shall meet the following requirements: 1 Accuracy before coarse grinding and grinding of the outer circle: The surface roughness is Ra 0.63μm or less, and with the standard top coloring inspection, the contact area is not less than 60%; Pre-refining accuracy of cylindrical grinding: Surface roughness Ra0.32μm, inspection with standard tip coloring, contact area not less than 65%; 3 Pre-refining accuracy of fine grinding outer circle: Surface roughness Ra0.32μm, working with grinder Coloring inspection, contact area of ​​not less than 75%, and in the near end contact. 4 Finishing grinding before the final round of precision: surface roughness Ra0.32μm, in the final grinding machine with a special final grinding ball grinding grinding and coloring the top and inspection, the contact area of ​​not less than 95%; At the same time should ensure constant temperature processing conditions (20 ± 1 °C).

3 Conclusion

The process test shows that by using the top hole for a number of years and using spherical top grinding before the final grinding of the outer ring, the quality of the top hole can be significantly improved, and the precision of the top position can be refined. The spherical top is easy to manufacture, use and maintain. With high grinding quality, the contact area can be more than 95% stable, and easy to ensure contact at the near end. By adopting the refinement measures of the top positioning accuracy, the precision of the outer circle of the precision spindle part is significantly improved, and the dispersion range of the roundness error is reduced (can be stabilized within 0.005 mm), ensuring the processing yield of the precision spindle parts.

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