Geometric parameters of turning tools for turning slender shafts
Machining slender shafts is one of the most challenging aspects of machining. Due to their large aspect ratio (generally greater than 20) and poor rigidity, slender shafts are susceptible to bending, deformation, and vibration under the influence of cutting forces, cutting heat, and clamping forces, affecting machining accuracy and surface quality. Therefore, the proper selection of the turning tool’s geometric parameters is crucial to ensuring the machining quality of slender shafts. The turning tool’s geometric parameters include the rake angle, clearance angle, primary deflection angle, secondary deflection angle, cutting edge inclination angle, and tool tip radius. These parameters directly affect the distribution of cutting forces, cutting temperatures, and the tool’s wear resistance and service life. When selecting the geometric parameters of a slender shaft turning tool, the characteristics of the slender shaft should be fully considered, with the goal of reducing cutting forces, lowering vibration, and improving machining accuracy.
The rake angle is one of the most important tool geometry parameters affecting cutting forces. For turning slender shafts, a larger rake angle is recommended. A larger rake angle ensures a sharper cutting edge, smoother cutting, and minimizes metal deformation, thereby reducing cutting forces, especially radial cutting forces. Radial cutting forces are the primary cause of bending and deformation in slender shafts. Reducing radial cutting forces can effectively prevent this. Generally speaking, when machining plastic materials (such as 45 steel and aluminum alloy), a rake angle of 15° to 25° is recommended; when machining brittle materials (such as cast iron), a rake angle of 5° to 15° is recommended. Furthermore, a chipbreaker groove on the rake face facilitates chip evacuation, preventing chips from entanglement in the workpiece or scratching the machined surface. This also helps reduce cutting forces. The shape and size of the chipbreaker groove should be selected based on the cutting load and workpiece material. Linear or circular chipbreakers are typically used, with a moderate width and depth to ensure effective chip breaking.
The primary function of the clearance angle is to reduce friction between the tool flank and the workpiece surface, thereby improving tool wear resistance. When turning slender shafts, 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 tool flank and the workpiece, lowering frictional resistance and, consequently, vibration and workpiece deformation. Generally, a clearance angle of 6° to 12° is recommended. For slender shafts requiring higher precision, the clearance angle can be increased, but not excessively, as this will reduce tool strength and lead to chipping. The secondary clearance angle should also be appropriately increased to reduce friction between the secondary clearance and the workpiece surface, thereby improving surface quality. The tool material properties should also be considered when selecting the clearance angle. High-speed steel turning tools can have a slightly larger clearance angle than carbide turning tools because high-speed steel has greater toughness and can withstand the strength loss associated with a larger clearance angle.
The lead angle significantly influences the distribution of cutting forces. When turning slender shafts, a larger lead angle should be selected. A larger lead angle reduces radial cutting forces and increases axial cutting forces, which helps minimize bending deformation of slender shafts. Generally speaking, the lead angle can range from 75° to 93°, with 90° to 93° being the most widely used because this angle minimizes radial cutting forces and minimizes deformation of slender shafts. Furthermore, a larger lead angle can reduce the tool’s nose angle, reducing tip strength. Therefore, when selecting the lead angle, a larger nose radius or chamfering should be used to enhance tip strength and wear resistance. The nose radius is generally 0.5–1mm, the chamfer width is 0.5–1mm, and the chamfer angle is -5° to 0°. This enhances the tool tip’s impact resistance and reduces tool wear.
The primary function of the secondary rake angle is to reduce friction between the secondary flank and the workpiece surface, preventing the tool from scratching the machined surface. It also affects the workpiece’s surface roughness. When turning slender shafts, a smaller secondary rake angle is recommended to minimize surface roughness and improve surface quality. A smaller secondary rake angle enhances the tool’s finishing effect, resulting in a smoother and flatter machined surface. Generally, the secondary rake angle is between 1° and 3°. For slender shafts requiring higher precision, a secondary rake angle of 0° to 1° is acceptable. However, a smaller secondary rake angle increases friction between the secondary flank and the workpiece, potentially causing vibration. Therefore, the secondary rake angle should be selected to minimize friction and vibration while ensuring surface quality. The rake angle also affects chip evacuation and tool tip strength. When turning slender shafts, a rake angle of 0° to 5° is generally recommended to facilitate upward chip evacuation, preventing scratches on the machined surface. This also increases tool tip strength and reduces the risk of tool chipping.
