ZHEJIANG AOSITE AUTO PARTS CO.,LTD , https://www.aositebelt.com
Precise positioning method for precision spindle machining
**General Technical Measures to Ensure the Positioning Accuracy of the Top Hole**
When machining precision spindle parts, one of the most common and effective methods for workpiece positioning is using the top hole at both ends. This approach follows the principles of reference reunification and standardization, ensuring consistent and accurate positioning throughout the manufacturing process. To maintain high precision in the top hole, it is essential to rework the top hole before the final machining of the main cylindrical surface of the spindle.
Common repair methods include:
1. **Reconditioning with an oilstone or rubber wheel**: A cylindrical oilstone or rubber grinding wheel is mounted on the lathe chuck, and a diamond pen is used to shape it into a 60° cone. The workpiece is placed between the oilstone and the lathe center, with a small amount of diesel or light oil added for lubrication. The lathe is then turned on, allowing the oilstone to rotate at high speed while the operator intermittently holds the workpiece.
2. **Using a cast iron tip**: Similar to the oilstone method, but instead of an oilstone, a cast iron tip is used. A grinding agent is applied during the process, and the rotational speed is slightly reduced to avoid damage.
3. **Drilling with a carbide tip**: This method offers high efficiency but may result in lower-quality finishes. It is typically used for rough machining of ordinary or precision shafts.
4. **Grinding with a dedicated top hole grinder**: This machine provides high precision, with surface roughness as low as Ra 0.32μm and roundness up to 0.8μm, making it ideal for critical applications.
During the machining of the precision main shaft’s outer circle, the top holes are supported by fixed front and rear centers. Ideally, this setup allows for precise rotation, but if there are shape errors (such as roundness or angular deviations) or coaxiality issues in the top holes, the contact between the centers and the top holes becomes non-circular, reducing support stiffness and causing fluctuations in the workpiece axis. This can lead to roundness errors on the machined surface.
Even after careful preparation, the contact area between the two top holes may not exceed 85%, which can still affect the roundness of the final product, especially during superfinishing processes. As shown in Figure 2, even with high accuracy and low surface roughness, the limited contact area can cause significant issues in the final roundness of the spindle.
To address these challenges, several refinement methods can be employed:
- **Use of a spherical ball tip for final cylindrical grinding**: This method ensures continuous line contact between the spherical tip and the conical bore, maintaining stability even if the top holes have minor errors. The spherical tip distributes contact stress evenly, improves stiffness, and reduces axial displacement caused by deformation. This significantly enhances the positional stability of the spindle and improves its roundness after final grinding.
- **Multiple-hole top hole processing**: The major cylindrical surfaces of precision spindles are typically processed through multiple steps—rough grinding, semi-finishing, finish grinding, and final superfinishing. Before each step, the top hole must be carefully reworked to gradually reduce surface roughness and improve shape accuracy and coaxiality.
The required precision of the top hole before each grinding process includes:
1. **Before coarse grinding**: Surface roughness ≤ Ra 0.63μm, with a minimum contact area of 60%.
2. **Before semi-finishing**: Surface roughness ≤ Ra 0.32μm, with a minimum contact area of 65%.
3. **Before fine grinding**: Surface roughness ≤ Ra 0.32μm, with a minimum contact area of 75%.
4. **Before final superfinishing**: Surface roughness ≤ Ra 0.32μm, with a minimum contact area of 95%.
Additionally, temperature-controlled conditions (20 ± 1°C) should be maintained to ensure consistency.
In conclusion, the use of spherical top tips before final grinding significantly improves the quality of the top hole and refines the positioning accuracy. This method is easy to implement, cost-effective, and ensures stable contact areas exceeding 95%. By applying these refinement measures, the precision of the outer circle of the spindle is greatly enhanced, and the roundness error dispersion is minimized, stabilizing within 0.005 mm. This leads to improved production yield and better performance of precision spindle components.