# Overview of Tool Wear Monitoring Methods Based on Convolutional Neural Network
## 2 Tool Wear Mechanism  
The phenomenon that a sharp edge is deformed or damaged due to the interaction between the tool and the workpiece during machining is called tool wear. Tool wear is divided into normal wear and abnormal wear [19]. Normal wear refers to the wear that occurs gradually during the cutting process when the design and use of the tool are acceptable. Additionally, the quality of manufacturing and sharpening meets the requirements. Abnormal wear, namely tool damage, is generally an abnormal failure. It is mainly related to improper use. Here, we mainly discuss and analyze normal wear. As for the mechanism of tool wear, Relevant researchers have carried out much research work [20; 21; 22; 23; 24]. Researchers generally believe that tool wear is the result of a mechanical and chemical reaction between tool and workpiece material. Under different working conditions, especially under the condition of high-temperature alloy cutting, the tool wear mechanism mainly includes abrasive wear, adhesive wear, diffusion wear, chemical wear, and so on [25].  
Abrasive wear refers to tool loss or tool material loss during cutting friction caused by the extrusion of peeled, hard particles on the tool surface and movement along the surface. Some researchers also believe that work hardening burrs on the surface also cause abrasive wear when particles flow laterally. This is an important mechanism that is responsible for flank wear, notch wear and nose wear in general [11; 12; 25; 26]. Adhesive wear, also known as bite wear, refers to a form of wear in which metal adhesion occurs locally at the contact surface between the tool and the workpiece; the adhesive part is destroyed in the subsequent relative sliding, and metal chips are pulled down from the tool surface or the tool surface is scratched. Adhesive wear has a great relationship with cutting speed, and the common results are flank wear, notch wear, nose wear, chipping, built-up edge (BUE) and built-up-layer (BUL) formations [24; 25; 27]. Diffusion wear is caused by the diffusion process of atom transfer from a high-concentration region to a low-concentration region under the influence of a high temperature. The decisive factor is the chemical proximity between a high-temperature tool and working material elements. It has a great influence on coated, ceramic, and CBN cutting tools for high-speed cutting [28; 29; 30]. Chemical wear means that at a certain temperature, some elements in the tool material react with some elements in the surrounding medium to form a layer of compound with a low hardness on the surface of the tool, which is taken away by the cutting and workpiece; therefore, tool wear is increased or the tool material is corroded by a medium that causes tool wear. It is not the main wear mechanism [31].  
The amount of tool wear increases with the usage time, and the wear speed primarily depends on the cutting conditions. The experiments demonstrated that the tool wear process under different cutting conditions was similar, including three stages: initial wear, normal wear, and sharp wear [32; 33; 34].  
During the cutting process, tool wear is closely related to changes in the physical parameters of the machine tool operation such as cutting force, vibration, acoustic emission, temperature, current, voltage, power, and resistance. The phenomena or parameter changes related to the machine tool operation can be used as the basis for tool wear monitoring [35]. Therefore, monitoring the changes in the physical parameters can realize tool wear monitoring [36]. For example, the cutting force is a variable of the machining process condition and is very sensitive to the tool wear condition [37]. After the tool is worn, the cutting force and cutting temperature increase, the workpiece machining accuracy and surface quality deteriorate, and vibration and noise occur during machining. This will significantly shorten the tool service life.