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Root grinding problem and solution
With the rapid development of the manufacturing industry, gear processing has become increasingly important. As demands for heavy load, quiet operation, and long service life grow, the precision and durability of tooth processing have reached unprecedented levels. Traditional soft and medium-hard tooth surfaces are gradually being replaced by carburized and hardened ones. The fine scraping process using carbide hobs is also being replaced by grinding. Common methods for gear grinding include the forming method, the Magna method, the worm method, and others. Among these, the forming method has gained widespread popularity in recent years, especially for large and medium-sized modular gears and gear shafts. Wind power and engineering machinery have driven its development, with major companies investing heavily in forming grinding equipment.
Our company was one of the first to introduce forming grinding technology, with over a decade of application experience. We have been purchasing new equipment every two years, witnessing the evolution of this technology firsthand. Forming grinding offers wide adaptability, high efficiency, and simple equipment. It can process various tooth profiles, including involute, rectangular, and arc shapes, as well as straight, helical, and drum teeth. It can also handle internal and external teeth, face gears, and more—just by adjusting programs and replacing accessories.
Despite its advantages, forming grinding faces a critical challenge: during the grinding of involute tooth profiles, the root of the tooth is prone to burning, especially when the root pressure angle is small. This issue becomes more severe as the pressure angle decreases. The problem stems from the principle of involute generation and the nature of forming grinding. The involute is generated from the base circle, where the pressure angle starts at 0° and increases toward the tip. In forming grinding, the grinding wheel is shaped to match the target tooth groove, and radial feed is used to remove material. However, due to the varying pressure angles, the grinding wheel contacts the tooth root first, leading to premature wear and potential burning.
This early contact not only reduces processing efficiency but also increases the machining allowance, as the grinding must start from a shallower groove. Additionally, the tip of the grinding wheel tends to blunt quickly, creating a hidden risk of burning at the tooth root. Radial dressing, which is commonly used, exacerbates the issue because the tip receives less trimming than the root, especially when the pressure angle is near 0°. As a result, the grinding wheel becomes ineffective faster, leading to continuous deterioration during the process.
To address these challenges, several strategies can be employed:
1. Perform roughing before quenching to control tooth shape and minimize the impact on grinding.
2. Ensure uniform carburizing and quenching to reduce deformation and improve the quality of the grinding blank.
3. Carefully align the workpiece using the equipment's probe, adjusting for deformation patterns to optimize the grinding position.
4. Adjust grinding parameters, such as reducing feed rate, increasing dressing amount, and shortening dressing intervals for gears with small root pressure angles.
5. Improve cooling by increasing oil pressure and flow, and use flushing pipes to clear chips from the grinding wheel, preventing heat buildup.
6. Calculate the theoretical involute starting points of paired gears to determine optimal grinding depth, avoiding unnecessary interference and ensuring smoother processing.
By controlling the starting point of the involute line, the influence of small pressure angles can be minimized, resulting in a more uniform feed and better grinding conditions. For some gears prone to burning, the effective tooth surface depth is typically set between 1.5 to 1.7 times the module. This approach has proven effective in our trials.
Another solution involves rotating the tooth surface to a certain angle, allowing the small pressure angle to be processed at a larger angle, thus reducing the difference in feed rates. Using two symmetrically arranged grinding wheels can also reduce the thickness of the grinding wheel, saving material and improving efficiency. While this method requires specialized equipment, it can significantly reduce grinding time and avoid burning at the tooth root.
In conclusion, while forming grinding has inherent limitations in processing involute tooth profiles, careful control of the starting point and optimized grinding techniques can greatly enhance performance. These improvements ensure higher quality, longer tool life, and reduced processing time, making forming grinding a viable and efficient option for modern gear manufacturing.