Shandong Xinsuju Steel Co.,Ltd. , https://www.sawlpipe.com The surface of the cast iron worm wheel blank has a hard spot on its surface and inside due to the presence of impurities. Its presence affects the service life of the hob and the transmission stability of the product. At present, many factories only use visual inspection and hearing to find hard spots, and the effect is not good. In practice, we explored the hard spots that were found without any equipment and instruments, only by hand to feel the vibration, and using the ear to identify the noise to find the worm gear liquid teeth. The accuracy is 100%, convenient and practical. This method can be used for car and milling. The specific practices are as follows.