## 1 Introduction  
For the ultra-precision cutting process, in order to obtain high accuracy of the shape and size of the parts and an ultra-smooth machining surface, it is necessary to have ultra-precision machine tools, high-resolution testing instruments, and ultra-stable processing environmental conditions. It is also necessary to have high-precision cutting tools. Especially for difficult-to-cut composite materials, engineering ceramic materials, hard and brittle non-metallic materials, and so on, there is an urgent need for ultra-precision machining tools to replace traditional machining tools. Single-crystal diamond has the characteristics of extremely high hardness, good wear resistance and thermal conductivity, and a low friction coefficient and thermal expansion coefficient. Because of these characteristics, single-crystal diamond tools can obtain nanometer sharpness of the cutting edge; therefore, single-crystal diamond tools are known as one of the most promising tools to improve productivity. However, single-crystal diamond has the characteristics of high hardness, wear resistance, difficult welding, anisotropy, and so on; these characteristics bring great difficulties to machining tools. At present, the degree of automation for ultra-precisiongrinding technology of single-crystal diamond cutting tools is low, and it is urgent to solve the \"bottleneck\" problem in mechanical grinding, such as online orientation technology, detecting technology, and control technology. Breakthroughs in these technologies are very important to enhance the automation level of single-crystal diamond machine tools. [PERSON] and [PERSON] of Cavendish Laboratory [1], University of Cambridge, UK, describe the grinding process as follows: \"When grinding in the soft direction, the grinding temperature is higher, the machine vibration is small, and accompanied by low-frequency sound, when grinding in the hard direction, it will be harsh sound, smaller grinding temperature, larger machine vibration, The grinding disc will generate significant damage that is difficult to repair\". In addition, both the EWAG machine tool and the Coborn machine tool use acoustoelectric sensors as standard machine tool accessories to monitor whether the tool is in contact with the grinding wheel during tool alignment operation. It is more important to use the sensor to find the hard point position of the cutting edge of the tool in order to set the dwell time at the hard point during the rotary grinding of the circular blade tool [2; 3]. The front-line operators of Shanghai Shubhardt Tools Co., Ltd. (Shanghai, China), Shenzhen Jingtian Jinke Venture Co., Ltd. (Shenzhen, China), and Shanxi Litian Century Cutting Tools Co., Ltd. (Yuncheng, China) also believe that acoustic emission signals have a good auxiliary role in tool grinding operations. [PERSON] of the China Academy of Engineering Physics conducted experiments on the grinding efficiency of diamond tools in each crystal direction and analyzed the power spectral density and effective value of vibration signals. The research results showed that the effective value could be used as an index of grinding efficiency to preliminarily determine the grinding direction of diamond tools. This study provides a new idea for the crystal orientation method of diamond cutting tools [4]. [PERSON] from the Harbin Institute of Technology analyzed the mapping relationship between the statistical characteristics of AE signals and the crystal direction of the grinding surface in the grinding process of diamond tools and used the self-organized neural network method to perform cluster analysis on the signals to determine the difficult grinding direction of the grinding surface in real-time through the characteristics of acoustic emission signals. These studies provide a basis for further improving the online identification accuracy of the grinding direction of single-crystal diamond tools and for studying the optimization method of the grinding direction of the tools [5].