Requirements for tool geometry when turning slender shafts
Turning slender shafts is a particularly challenging process in machining. These parts typically have an aspect ratio greater than 20. Due to their poor rigidity and susceptibility to bending and deformation, they place special demands on the tool geometry. Tool geometry directly influences the magnitude and distribution of cutting forces, the direction of chip removal, and the surface quality of the workpiece. Proper angle design can effectively reduce radial cutting forces, prevent bending and vibration of slender shafts, and ensure machining accuracy. When turning slender shafts, targeted optimization of multiple angle parameters, including the lead angle, rake angle, clearance angle, and inclination angle, is necessary to accommodate the machining characteristics of slender shafts.
The selection of the lead angle is crucial for reducing radial cutting forces. Slender shafts have weak rigidity, and excessive radial cutting forces can cause workpiece bending and vibration. Therefore, the lead angle of the turning tool should be as large as possible, generally between 75° and 93°. Increasing the lead angle converts radial cutting forces into axial cutting forces, reducing radial loads on the workpiece. For example, a turning tool with a 90° lead angle converts most cutting forces into axial forces, which are borne by the lathe spindle and tailstock, minimizing workpiece bending. For ultra-slender shafts with an aspect ratio exceeding 30, the lead angle can be increased to 93° to further reduce the impact of radial forces. However, it should be noted that an excessively large lead angle reduces the tool’s heat dissipation area, necessitating a good cooling system. In actual machining, the lead angle can be adjusted based on the diameter and length of the slender shaft. The smaller the diameter and the longer the length, the larger the lead angle should be to ensure cutting stability.
A properly designed rake angle can reduce cutting deformation and cutting forces, preserving the rigidity of slender shafts. When turning slender shafts, a rake angle of 15°-20° is generally recommended. A larger rake angle ensures a sharper cutting edge and reduces cutting resistance, making it particularly suitable for machining slender shafts made of plastic materials (such as 45 steel and alloy steel). For slender shafts made of high-strength alloys, the rake angle can be reduced to 10°-15° to enhance cutting edge strength and prevent chipping. The rake angle should also be adjusted in conjunction with cutting speed: a slightly larger rake angle is recommended for high-speed cutting and a slightly smaller angle for low-speed cutting to ensure smooth chip evacuation and prevent entanglement in the workpiece. Furthermore, the rake face should be ground with a defined chamfer or chip groove. The width and depth of the chip groove should be adjusted according to the feed rate, typically 3-5mm in width and 0.5-1mm in depth, to ensure that chips are discharged in a short spiral pattern, avoiding scratches on the machined surface or vibration in the workpiece.
The setting of the back angle must take into account both reducing friction and ensuring tool rigidity. When turning slender shafts, the back angle is usually 6°-10°, with a larger value for fine machining and a smaller value for rough machining. A larger back angle can reduce the friction between the back face of the turning tool and the machined surface of the workpiece, reduce cutting temperature, and help improve surface quality; but an excessively large back angle will weaken the rigidity of the tool and easily cause blade breakage, so a balance must be found between friction and rigidity. For high-speed cutting of slender shafts, the back angle can be appropriately reduced by 1°-2° to resist cutting vibration by increasing the rigidity of the tool. At the same time, the secondary back angle should be 2°-4°, which can not only reduce the friction between the secondary back face and the workpiece, but also ensure the overall rigidity of the tool, and avoid a decrease in tool strength due to an excessively large secondary back angle.
The rake angle plays a crucial role in controlling chip flow and regulating cutting forces. When turning slender shafts, a positive rake angle (3°-10°) is generally used to direct chips toward the surface being machined, preventing them from entanglement or scratching the machined surface. A positive rake angle also increases the tool’s effective rake angle, reducing cutting forces while also altering the distribution of cutting forces, further minimizing the impact of radial forces on slender shafts. When finish turning slender shafts, the rake angle can be increased to 5°-10° for superior surface quality. For rough turning, the rake angle can be adjusted to 3°-5°, ensuring smooth chip evacuation while enhancing cutting edge strength. For slender shafts requiring machining of steps or end faces, the rake angle can be adjusted based on the specific situation. For step machining, the rake angle should be 0°-3° to avoid burrs on the step; for end faces, the rake angle should be -3°–5° to allow chips to be discharged outward.
The selection of the secondary rake angle must balance surface roughness and tool strength. A too small secondary rake angle increases friction between the secondary flank and the workpiece, resulting in increased surface roughness; an excessively large secondary rake angle weakens the tool tip and can easily induce vibration. When turning slender shafts, the secondary rake angle is generally 1°-3°. A smaller secondary rake angle increases the tool-workpiece contact length, reduces cutting vibration, and lowers surface roughness. For slender shafts with higher precision requirements (such as IT6 grade), the secondary rake angle can be reduced to 1°-2°, and a transition radius of 0.2-0.5mm is ground at the tool tip to enhance tool tip strength and prevent built-up edge. Furthermore, the secondary rake angle must be coordinated with the feed rate. When the feed rate is high (0.2-0.3mm/r), the secondary rake angle is 2°-3° to prevent chip accumulation. When the feed rate is low (0.1-0.15mm/r), the secondary rake angle is 1°-2° to ensure surface quality. Through the precise design of the geometric angles of the turning tool, the cutting force and vibration can be effectively controlled to achieve high-precision machining of slender shafts.
