Selection of tool geometry for thin-wall turning
When turning thin-walled workpieces, due to their thin walls and poor rigidity, deformation easily occurs under the effects of cutting forces, cutting heat, and clamping forces, affecting machining accuracy and surface quality. Therefore, properly selecting tool geometry is crucial to ensuring the machining quality of thin-walled workpieces. Tool geometry includes rake angle, relief angle, primary deflection angle, secondary deflection angle, and cutting edge inclination angle. The size of these angles directly affects the magnitude of cutting forces, cutting temperatures, and the tool’s wear resistance and service life. When selecting tool geometry, various factors should be comprehensively considered based on the material, structural characteristics, and machining requirements of the thin-walled workpiece to achieve the best cutting effect.
The rake angle is the angle between the front cutting edge of the tool and the base surface, and it has a significant impact on the cutting force, cutting temperature, and tool sharpness. When turning thin-walled workpieces, a larger rake angle should be selected to reduce cutting forces and avoid deformation of the workpiece. A larger rake angle can make the tool edge sharper, cut more easily, and reduce metal deformation, thereby reducing cutting forces and cutting power. For plastic materials such as aluminum alloys and copper alloys, the rake angle can be 15° to 25°; for brittle materials such as gray cast iron and cast steel, the rake angle can be 5° to 15°. However, the rake angle should not be too large, otherwise it will reduce the strength and heat dissipation performance of the tool, resulting in accelerated tool wear. In addition, a chip breaker groove on the rake face can facilitate chip discharge, prevent chips from wrapping around the workpiece or scratching the machined surface, and help reduce cutting forces and prevent workpiece deformation.
The clearance angle is the angle between the tool’s flank face and the cutting plane. Its primary function is to reduce friction between the flank face and the workpiece surface, thereby improving the tool’s wear resistance. When turning thin-walled workpieces, vibration is likely to occur due to the workpiece’s poor rigidity, so a larger clearance angle is recommended. A larger clearance angle reduces the contact area between the flank face and the workpiece, lowering frictional resistance and, consequently, vibration and workpiece deformation. Generally speaking, the clearance angle can be between 6° and 12°. For thin-walled workpieces requiring higher precision, the clearance angle can be increased. However, an excessively large clearance angle can also reduce tool strength and predispose the tool to chipping. Therefore, when selecting the clearance angle, a balance should be considered between ensuring tool sharpness and reducing friction, while also considering tool strength.
The lead angle is the angle between the projection of the main cutting edge on the base surface and the feed direction. It has a significant impact on the distribution of cutting forces and deformation of the workpiece. When turning thin-walled workpieces, a larger lead angle should be selected to reduce radial cutting forces and avoid bending deformation of the workpiece. The larger the lead angle, the smaller the radial cutting force and the greater the axial cutting force, which helps reduce deformation of the workpiece. Generally speaking, the lead angle can be 75° to 90°. For thin-walled workpieces with particularly poor rigidity, the lead angle can be 90° to 95°. At the same time, an increase in the lead angle can also reduce the tool tip angle and reduce the strength of the tool tip. Therefore, when selecting the lead angle, it should be reasonably adjusted according to the strength of the tool material and the processing requirements of the workpiece.
The secondary rake angle is the angle between the projection of the secondary cutting edge on the base surface and the opposite direction of the feed. Its main function is to reduce the friction between the secondary flank and the machined surface of the workpiece and prevent the tool from scratching the machined surface. When turning thin-walled workpieces, in order to reduce the surface roughness value of the workpiece and improve the quality of the machined surface, a smaller secondary rake angle should be selected. A smaller secondary rake angle can increase the finishing effect of the tool and make the machined surface smoother and flatter. Generally speaking, the secondary rake angle can be 5°~10°. For thin-walled workpieces with higher precision requirements, the secondary rake angle can be 2°~5°. However, if the secondary rake angle is too small, it will increase the friction between the secondary flank and the workpiece, and it is easy to generate vibration. Therefore, when selecting the secondary rake angle, friction and vibration should be minimized while ensuring the quality of the machined surface. In addition, the choice of the rake angle cannot be ignored. The rake angle mainly affects the chip discharge direction and the tool tip strength. When turning thin-walled workpieces, a rake angle of 0°~5° is generally used to facilitate the upward discharge of chips and avoid scratching the machined surface. Cutting