The geometric angle of the turning tool is a key factor in determining the quality, efficiency and tool life of turning processing. Its reasonable selection requires comprehensive consideration of multiple factors such as workpiece material, tool material, processing technology and processing requirements. The geometric angles of the turning tool mainly include the rake angle, clearance angle, main rake angle, secondary rake angle and cutting edge inclination angle, and each angle has its specific role and influence. The rake angle affects the cutting deformation and cutting force, the clearance angle affects the friction between the tool and the workpiece, the main rake angle and secondary rake angle affect the distribution of cutting force and the surface quality of the workpiece, and the cutting edge inclination angle affects the chip discharge direction and the strength of the tool. In actual production, only by optimizing the turning tool geometric angle according to the specific processing conditions can efficient and high-quality turning processing be achieved.
The selection of the rake angle should primarily consider the plasticity and hardness of the workpiece material. For materials with high plasticity, such as mild steel and nonferrous metals, a larger rake angle (generally 15°-25°) should be used to minimize cutting deformation and cutting forces, lower cutting temperatures, facilitate chip removal, and improve surface quality. This is because plastic materials are prone to significant plastic deformation during cutting. A larger rake angle reduces the force exerted by the tool on the workpiece, thereby reducing deformation. For materials with higher hardness and greater brittleness, such as high-carbon steel and cast iron, a smaller rake angle (generally 5°-15°), or even a negative rake angle, should be used to enhance tool strength and prevent edge chipping. For example, when turning cast iron, due to its brittleness, chips are prone to chipping during cutting. A smaller rake angle can improve the impact resistance of the tool edge and extend tool life. Furthermore, the toughness of the tool material should be considered when selecting the rake angle. High-speed steel tools are more tough and therefore require a larger rake angle, while carbide tools are more brittle and therefore require a smaller rake angle.
The primary function of the clearance angle is to reduce friction between the tool’s flank and the machined workpiece surface. Its value depends on the cutting thickness and workpiece hardness. During finish machining, the cutting thickness is small, and the contact area between the tool and the workpiece is small. To reduce friction and improve surface quality, a larger clearance angle (typically 8°-12°) is recommended. This is because finish machining requires high surface roughness. A larger clearance angle prevents excessive friction between the tool’s flank and the workpiece surface, thereby reducing surface roughness. During rough machining, the cutting thickness is large, and the tool is subject to high cutting forces. To ensure tool strength, a smaller clearance angle (typically 5°-8°) is recommended. For workpieces with higher hardness, the clearance angle should also be appropriately reduced to enhance tool wear resistance. For example, when turning hardened steel, the tool’s flank wears rapidly due to the high hardness of the material. A smaller clearance angle increases the flank’s support area, reducing wear. The clearance angle is also related to the tool’s feed rate. At higher feed rates, the clearance angle should be appropriately reduced to prevent tool vibration during feed.
The choice of lead and secondary rake angles significantly influences cutting force distribution, workpiece surface quality, and tool life. The lead angle primarily influences the ratio of radial to axial cutting forces. A smaller lead angle increases radial cutting forces and decreases axial cutting forces. Conversely, a larger lead angle decreases radial cutting forces and increases axial cutting forces. When turning workpieces with poor rigidity (such as slender shafts), a larger lead angle (typically 75°-90°) is recommended to reduce radial cutting forces and prevent workpiece bending and deformation. For example, when turning slender shafts, if the lead angle is too small, radial cutting forces will be excessive, easily causing workpiece bending and affecting machining accuracy. A larger lead angle effectively reduces radial cutting forces and ensures stable machining. The secondary rake angle primarily reduces friction between the tool’s flank face and the machined workpiece surface and also influences the residual area height on the workpiece surface. A smaller lead angle reduces the residual area height and lowers the surface roughness. However, a too small lead angle increases friction between the tool and the workpiece, accelerating tool wear. Therefore, during finishing, in order to improve the surface quality, a smaller secondary deflection angle (generally 5°-10°) should be selected; during roughing, a larger secondary deflection angle (generally 10°-15°) can be selected to reduce tool wear.
The choice of rake angle primarily affects the direction of chip discharge and tool strength. A positive rake angle ejects chips toward the workpiece surface to be machined, preventing scratches on the machined surface and making it suitable for finishing. A negative rake angle ejects chips toward the machined surface, potentially scratching it, but increasing tool strength and making it suitable for roughing and interrupted cutting. For example, when turning castings, where surface defects such as pinholes and pores may exist, a negative rake angle can improve the tool’s impact resistance and prevent edge breakage. The larger the absolute value of the rake angle, the more inclined the chip discharge direction. When turning slender shafts or thin-walled workpieces, a larger positive rake angle can be used to prevent chips from wrapping around the workpiece and ensuring smooth chip discharge. Furthermore, the rake angle affects tool sharpness and cutting force distribution. A positive rake angle increases tool sharpness and reduces cutting forces, but reduces tool strength. A negative rake angle reduces tool sharpness and increases cutting forces, but improves tool strength. Therefore, the selection of the cutting edge angle needs to find a balance between ensuring the strength of the tool and the processing quality.
