Name and definition of cutting part of tool
A cutting tool is the instrument directly involved in cutting during machining. The structure and geometry of its cutting element have a decisive influence on cutting performance, machining quality, and tool life. The cutting portion of a tool is composed of multiple surfaces and cutting edges, each of which has a specific name and definition. Understanding these names and definitions is fundamental to understanding the working principles of cutting tools and their proper selection and use.
The main surfaces of a tool’s cutting portion include the rake face, flank face, and secondary flank face. The rake face is the surface on the tool that directly contacts the chip and controls its outflow direction. Its shape and angle affect chip formation, curling, and evacuation. For example, in a turning tool, the rake face is typically a flat or curved surface. By adjusting the angle of the rake face (i.e., the rake angle), the magnitude and direction of the cutting force can be altered. Increasing the rake angle reduces cutting forces and facilitates chip evacuation, but the tool’s strength decreases. The flank face is the surface of the tool that faces the workpiece surface. During cutting, friction occurs between the flank face and the workpiece’s machined surface, so the flank face needs to have a certain degree of wear resistance. The angle between the flank face and the cutting plane is called the clearance angle. The size of the clearance angle directly affects the degree of friction between the flank face and the workpiece. Increasing the clearance angle reduces friction and extends tool life, but reduces tool rigidity. The secondary back face refers to the surface on the tool opposite to the transition surface of the workpiece. It is connected to the secondary cutting edge. Its main function is to reduce the friction between the tool and the transition surface of the workpiece. The angle between the secondary back face and the secondary cutting plane is called the secondary back angle. Its function is similar to the back angle, but the value of the secondary back angle is usually smaller.
The cutting edge of a tool consists of the primary cutting edge, secondary cutting edge, and tip. The primary cutting edge is the intersection of the rake and flank faces. It performs the primary cutting work, directly removing excess metal from the workpiece. Its length and shape vary depending on the machining method and workpiece material. For example, the primary cutting edge of a turning tool is a straight or curved line, while the primary cutting edge of a milling cutter is located on a cylindrical or conical surface. The angular parameters of the primary cutting edge, such as the lead angle and rake angle, have a significant impact on the distribution of cutting forces, tool wear, and machined surface quality. The secondary cutting edge is the intersection of the rake and flank faces. It assists the primary cutting edge in completing the cutting process, primarily used to smooth the transition surface of the workpiece and reduce surface roughness. The angle between the secondary cutting edge and the feed direction is called the secondary rake angle. A smaller secondary rake angle reduces the residual area on the workpiece surface and reduces surface roughness. However, a too small secondary rake angle increases friction between the tool and the workpiece and can easily cause vibration. The tip is the junction between the primary and secondary cutting edges. It can be sharp or have a rounded or chamfered edge. A sharp tool tip has low cutting resistance, but low strength and is easy to wear; a rounded tool tip (called a tool tip arc) can improve the strength and wear resistance of the tool tip and reduce the waviness of the machined surface.
The angle of the cutting part of the tool is an important parameter that describes the tool geometry, including the rake angle, clearance angle, main deflection angle, secondary deflection angle and cutting edge inclination angle. These angles are collectively referred to as the tool’s marked angles. The rake angle is the angle between the front cutting edge and the base plane measured in the orthogonal plane. It mainly affects the cutting force, cutting temperature and chip formation. The size of the rake angle should be selected according to the workpiece material and the tool material. The rake angle can be appropriately increased when processing plastic materials, and the rake angle should be appropriately reduced when processing brittle materials. The clearance angle is the angle between the rear cutting edge and the cutting plane measured in the orthogonal plane. It mainly affects the friction between the rear cutting edge and the machined surface of the workpiece and the wear of the tool. The size of the clearance angle is usually selected according to the cutting thickness. When the cutting thickness is small, the clearance angle should be increased to reduce friction; when the cutting thickness is large, the clearance angle should be reduced to ensure the strength of the tool. The lead angle is the angle between the projection of the main cutting edge on the base plane and the feed direction, measured within the base plane. It primarily affects the distribution of cutting forces and tool life. A smaller lead angle increases radial cutting forces, which can easily cause vibration, but extends tool life. An increased lead angle decreases radial cutting forces and shortens tool life. The secondary angle is the angle between the projection of the secondary cutting edge on the base plane and the direction opposite to the feed direction, measured within the base plane. It primarily affects the surface roughness of the workpiece and the strength of the tool. A smaller lead angle reduces surface roughness but increases tool strength. The rake angle is the angle between the main cutting edge and the base plane, measured within the cutting plane. It primarily affects the direction of chip outflow and the strength of the tool tip. When the rake angle is positive, chips flow toward the workpiece surface to be machined and will not scratch the machined surface. When the rake angle is negative, chips flow toward the machined surface, potentially scratching it, but the tool tip is stronger.
The structural parameters of the cutting part of the tool also include the cutting edge length, the tool tip arc radius, etc. The cutting edge length refers to the effective working length of the main cutting edge. It should be selected according to the cutting depth and feed rate to ensure that the cutting edge can fully participate in the cutting work and avoid local wear. For example, when turning the outer circle, the cutting edge length should be greater than the cutting depth to ensure the smoothness of the cutting. The tool tip arc radius refers to the arc radius at the tool tip, which has an important influence on the strength of the tool tip, the quality of the machined surface and the cutting force. As the tool tip arc radius increases, the tool tip strength increases and the machined surface roughness decreases, but the radial cutting force increases, which is prone to cause vibration. Therefore, when processing workpieces with poor rigidity, a smaller tool tip arc radius should be selected.
The material properties of the cutting portion of a tool are also key factors affecting its cutting performance. The tool material should have high hardness, high wear resistance, sufficient strength and toughness, and good heat resistance. Commonly used tool materials include high-speed steel, cemented carbide, ceramics, and cubic boron nitride. Different tool materials are suitable for different processing applications. For example, high-speed steel tools have high strength and toughness, suitable for low-speed cutting and the manufacture of complex tools; cemented carbide tools have high hardness and wear resistance, suitable for high-speed cutting and processing hard materials. The name, definition, and geometric parameters of the cutting portion of a tool form an organic whole, which together determine the cutting performance of the tool. In actual application, it is necessary to rationally select the type and geometric parameters of the tool based on the processing requirements, workpiece material, and equipment conditions to achieve the best processing effect.
