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Precise positioning method for precision spindle machining
In machining precision spindle components, the positioning of the top hole at both ends is a commonly used method. This approach follows the principles of reference reunification and standardization, ensuring accurate alignment. To achieve high-precision positioning, it's essential to rework the top holes before the final machining of the main cylindrical surfaces. Several methods are typically employed for this purpose.
One common technique involves using a whetstone or rubber wheel. The tool is mounted on the lathe’s chuck, and a diamond pen is used to shape it into a 60° cone. The workpiece is then placed between the whetstone and the lathe center, with a small amount of lubricant like diesel oil applied. As the lathe rotates at high speed, the operator manually holds the workpiece intermittently to ensure even contact.
Another method uses a cast iron tip instead of the whetstone. The principle remains similar, but the grinding agent is added during the process, and the rotational speed is slightly reduced. A third option is to drill the top hole using a carbide tip, which offers higher efficiency but may result in lower surface quality. It is often used for rough machining of ordinary or precision spindles.
Finally, a top hole grinder can be used, offering the highest level of precision. This machine can achieve a surface roughness of Ra 0.32μm and roundness of 0.8μm, making it ideal for high-accuracy applications.
When machining the outer diameter of a precision spindle, the top holes are supported by fixed front and rear centers. Ideally, this setup ensures a stable axis of rotation. However, if there are errors in the shape or coaxiality of the two top holes, the contact area may become limited, leading to instability and increased roundness error. Even after careful preparation, achieving full contact (over 85%) is challenging, especially in high-precision grinding, where even minor deviations can cause significant issues.
To address these challenges, several refinement methods can be implemented. One effective solution is to use a spherical ball tip for the final cylindrical grinding. This method ensures continuous line contact between the sphere and the conical bore, reducing stress concentration and improving contact stiffness. The result is better positional stability and significantly improved roundness of the finished spindle.
Another key step is to carefully rework the top holes before each grinding operation. This includes multiple steps such as rough grinding, semi-finishing, finish grinding, and superfinishing. Each stage requires the top hole to meet specific standards, including surface roughness and contact area requirements. For example, before coarse grinding, the surface roughness should be Ra 0.63μm or less, with a minimum contact area of 60%. As the process progresses, the standards become stricter, reaching up to 95% contact area before the final grinding.
Maintaining consistent temperature conditions (20 ± 1°C) is also crucial to ensure dimensional stability. These refined measures not only improve the accuracy of the spindle’s outer diameter but also reduce the dispersion of roundness errors, stabilizing them within 0.005 mm.
In conclusion, using spherical tips and refining the top hole positioning process significantly enhances the precision of spindle machining. These techniques are practical, cost-effective, and contribute to higher production yields and better product quality.