Commonly used cutting parameters for turning slender shafts
When turning slender shafts, the choice of cutting parameters directly impacts machining accuracy, surface quality, and production efficiency. Due to their poor rigidity, propensity for vibration, and poor heat dissipation, slender shafts require different cutting parameters than conventional shaft parts. The key is to minimize the effects of cutting forces and heat on the workpiece by properly controlling cutting speed, feed rate, and depth of cut. The larger the aspect ratio of the slender shaft, the stricter the cutting parameters. While ensuring machining efficiency, workpiece deformation and vibration must be minimized. Therefore, cutting parameters must be set according to the principle of “low cutting forces, low vibration, and high efficiency,” achieving stable machining through multi-parameter collaborative optimization.
The choice of cutting speed must balance vibration suppression and machining efficiency. Slender shafts are prone to self-excited vibration during cutting, and cutting speed is a key factor influencing this vibration. Generally, avoid the medium-speed range (60-100 m/min), which is prone to resonance, and opt for low or high speeds. Low-speed cutting (30-50 m/min) produces low and stable cutting forces, making it suitable for machining ultra-slender shafts with extremely poor rigidity (aspect ratio > 50). However, this reduces efficiency, and high-speed steel tools are typically used with adequate cooling to prevent excessive tool wear. High-speed cutting (100-150 m/min) reduces cutting forces slightly as speed increases, and chips are rapidly evacuated, reducing friction and compression on the workpiece. This makes it suitable for slender shafts with an aspect ratio of 20-50. In this case, carbide tools are required to withstand higher cutting temperatures. For example, when turning slender shafts made of 45 steel (aspect ratio 30), a cutting speed of 120 m/min can both avoid resonance and ensure high machining efficiency.
The feed rate should be set to minimize radial cutting forces and ensure surface quality. Slender shafts are sensitive to radial forces. Excessive feed rates increase radial cutting forces, causing workpiece bending and deformation. Excessive feed rates prolong cutting time, increase tool-workpiece friction, and increase surface roughness. When rough turning slender shafts, a feed rate of 0.2-0.3 mm/r is generally used. This allows for rapid stock removal with low radial forces, resulting in short, spiral-shaped chips for easy evacuation. For finish turning, the feed rate should be reduced to 0.08-0.15 mm/r to minimize tool pressure on the workpiece surface and achieve a surface roughness below Ra1.6 μm. For slender shafts requiring multiple passes, the feed rate should be gradually reduced with each pass, for example, 0.3 mm/r for the first pass, 0.2 mm/r for the second pass, and 0.1 mm/r for the final finish pass. This incremental cutting approach reduces cumulative workpiece deformation. In addition, the feed rate needs to match the main deflection angle. A turning tool with a main deflection angle of 90° can use a larger feed rate, while a turning tool with a main deflection angle of 75° needs to reduce the feed rate appropriately to balance the cutting force distribution.
The depth of cut should be adjusted based on the rigidity of the slender shaft and the stage of machining. The primary task of the rough turning stage is to remove the majority of the stock. A depth of cut of 1-3mm is recommended, but this should be determined based on the workpiece diameter. The smaller the diameter, the smaller the depth of cut. For example, for a slender shaft with a diameter of 10mm, the depth of cut should not exceed 1mm to prevent workpiece bending; for a slender shaft with a diameter of 30mm, the depth of cut can be 2-3mm. The depth of cut during the finish turning stage is generally 0.1-0.5mm to remove surface errors left by roughing and ensure dimensional accuracy. For slender shafts with an aspect ratio exceeding 40, even during the rough turning stage, the depth of cut should be controlled within 1-2mm. Multiple passes should be used, with the workpiece straightness checked after each pass. Any bending should be straightened promptly. The appropriate depth of cut also needs to consider tool durability. Carbide tools can withstand greater cutting depths, while high-speed steel tools require a reduced depth of cut to avoid excessive wear.
The coordination of cutting parameters and cooling and lubrication conditions is crucial for machining slender shafts. Because slender shafts have a small heat dissipation area, cutting heat easily accumulates on the workpiece and tool, leading to workpiece thermal deformation and tool wear. Therefore, adequate cooling measures are necessary. Typically, an emulsion ( 8%-10%) is used, with a high-pressure coolant pump (pressure ≥3 MPa) spraying the cutting fluid directly into the cutting zone at a flow rate of at least 20 L/min. Under good cooling conditions, cutting speeds can be increased by 20%-30%. For example, a speed of 100 m/min can be increased to 120-130 m/min. For high-precision slender shafts (such as machine tool lead screws), oil mist lubrication can be used during finish turning to reduce friction and prevent contamination of the workpiece by the cutting fluid. In this case, the cutting speed should be reduced by 10%-15%, while the feed rate remains unchanged to ensure surface quality. Furthermore, the cleanliness of the cutting fluid must be controlled, with regular filtration to remove impurities and prevent scratches on the workpiece surface.
Cutting parameters for slender shafts made of different materials vary and require targeted adjustments. When turning slender shafts of medium-carbon steels such as 45 steel, the cutting speed is 80-120 m/min, the feed rate is 0.2-0.3 mm/r, and the cutting depth is 1-3 mm. When turning slender shafts of stainless steel (such as 304), due to its high plasticity and severe work hardening, the cutting speed needs to be reduced to 60-100 m/min, the feed rate is 0.15-0.25 mm/r, and the cutting depth is 1-2 mm. Cooling intensity should also be increased. When turning slender shafts of non-ferrous metals (such as brass), the cutting speed can be increased to 100-150 m/min, the feed rate is 0.2-0.4 mm/r, and the cutting depth is 1-3 mm. Because non-ferrous metals have lower cutting forces, the feed rate and cutting depth can be appropriately increased to improve efficiency. By adjusting the cutting amount according to the material properties and combining it with auxiliary support devices such as the tool rest and center rest, the machining deformation of the slender shaft can be effectively controlled, ensuring that its straightness and cylindricity errors are within 0.05mm/m, meeting the design requirements.
