Machining of slender shafts
Slender shafts, defined as shaft parts with a length-to-diameter ratio (A/D ratio) greater than 20, are widely used in machine tools, automotive, aerospace, and other fields. Due to their poor rigidity and low strength, slender shafts are prone to bending, vibration, and thermal deformation during turning, resulting in reduced machining accuracy and surface quality. Therefore, turning slender shafts is a challenging task in machining, requiring optimal tool selection, optimized clamping methods, controlled cutting parameters, and vibration control measures to ensure machining quality.
The main difficulty in turning slender shafts lies in the deformation problem during the machining process. Slender shafts have a large aspect ratio and poor rigidity. They are prone to bending and deformation under the action of cutting force, gravity and centrifugal force, resulting in cylindricity errors in the workpiece. At the same time, the cutting heat generated during the turning process will cause the workpiece to elongate due to heat. If the two ends are rigidly fixed, large thermal stress will be generated, causing the workpiece to bend, deform or even break. In addition, when the slender shaft rotates at high speed, centrifugal force will be generated, which will intensify vibration, affect the quality of the machined surface, and in severe cases, cause tool chipping. To solve these problems, measures must be taken from multiple aspects such as tools, clamping, and cutting parameters to reduce deformation and vibration.
Tool selection and geometric parameters significantly impact the turning quality of slender shafts. To reduce cutting forces and heat, sharp tools should be selected with large rake and relief angles. A rake angle of 15°-20° is generally recommended to minimize chip deformation and cutting forces, while a relief angle of 6°-10° reduces friction between the tool and the workpiece. The lead angle should be as large as possible, generally 75°-90°, to reduce radial cutting forces, which are the primary cause of bending and deformation in slender shafts. A rake angle of 0°-5° ensures chip flow toward the surface being machined, avoiding scratches on the machined surface. Regarding tool material, high-speed steel tools (such as W18Cr4V) are suitable for low-speed turning when machining medium-carbon steels such as 45 steel. Carbide tools (such as YT15 and YG8) are recommended for machining alloy steels and stainless steels to increase cutting speed and efficiency. In addition, the tool tip radius should be small (generally 0.2-0.5mm) to reduce cutting force and vibration.
The clamping method for slender shafts is key to controlling deformation. Traditional two-end center clamping methods can cause slender shafts to bend due to cutting forces and thermal stresses, necessitating an improved clamping method. Common clamping methods include single-clamp and top clamping and double-center clamping with a steady rest. In single-clamp and top clamping, the chuck clamps one end of the workpiece (with a clamping length of 1-1.5 times the workpiece diameter) and the center supports the other end. This method reduces workpiece bending deformation, but requires an axial locating device at the chuck end to prevent axial movement of the workpiece. The double-center clamping with a steady rest is the most common method for turning slender shafts. The steady rest is mounted on the lathe slide and moves with the slide, supporting the workpiece and increasing its rigidity. The steady rest has two support jaws, located above and below (or on either side) the workpiece. A clearance of 0.05-0.1mm should be maintained between the support jaws and the workpiece to avoid scratching the workpiece surface due to overtightening. When using a tool holder, a reference journal must be machined at the end of the workpiece for positioning and guiding the tool holder. The diameter of the reference journal should be consistent with the design diameter of the workpiece, and the surface roughness should reach Ra1.6μm or above.
Optimizing cutting parameters and strengthening cooling and lubrication measures can effectively reduce the deformation and vibration of slender shafts. When turning slender shafts, a lower cutting speed should be used (80-120m/min for high-speed steel tools and 150-250m/min for carbide tools) to reduce the generation of cutting heat. The feed rate should be moderate (0.1-0.3mm/r). Too little feed will increase the friction between the tool and the workpiece, while too much feed will increase the cutting force and cause deformation of the workpiece. The cutting depth should be reasonably allocated according to the machining allowance. It can be 2-4mm for rough turning and 0.1-0.5mm for fine turning. In order to reduce the impact of cutting heat on the workpiece, cooling and lubrication need to be strengthened. Use high-pressure and high-flow cutting fluid (such as emulsion, concentration 5%-8%) and spray the cutting fluid directly into the cutting area to promptly remove the cutting heat and reduce the workpiece temperature. In addition, the reverse feed turning method can be used, that is, the tool is fed from the tail end of the workpiece to the chuck end, so that the workpiece produces tensile deformation under the action of cutting force instead of bending deformation, which is beneficial to improve the processing accuracy.
