CNC Machining of steel stepped shafts
Steel stepped shafts are common components in mechanical transmissions. Composed of multiple cylindrical segments of varying diameters, they are used to mount gears, pulleys, bearings, and other components, transmitting torque and power. CNC Machining steel stepped shafts is a typical lathe operation, requiring guaranteed diameter and length accuracy of each step, as well as coaxiality between the cylindrical segments, to ensure proper assembly and performance. The turning process for steel stepped shafts involves selecting the blank, clamping method, tool selection, determining cutting parameters, and arranging the machining sequence, each requiring meticulous design and execution.
The blank selection for a steel stepped shaft should be determined based on the part’s material, dimensions, and precision requirements. Common blanks include round steel and forgings. For stepped shafts with smaller diameters and lower precision requirements, round steel can be used as the blank. Round steel offers higher dimensional accuracy, uniform tolerances, and lower processing costs. For stepped shafts with larger diameters and greater loads, forgings should be used. Forging refines the grain size and improves the material’s mechanical properties, particularly strength and toughness, to meet the torque transmission requirements of the stepped shaft. For example, a 45 steel stepped shaft for general transmission applications can be made from round steel, while for heavy-duty applications, a forged blank should be used. The blank’s diameter and length should be determined based on the maximum diameter and total length of the stepped shaft, plus an appropriate machining allowance. The size of the machining allowance should be determined based on the blank type and precision requirements. Round steel has a smaller machining allowance, typically 3-5mm, while forgings have a larger machining allowance, typically 5-10mm.
The clamping method of the steel stepped shaft is the key to ensuring processing accuracy, and the appropriate clamping method should be selected according to the length and diameter of the stepped shaft. For short and thick stepped shafts, a three-jaw self-centering chuck can be used for clamping. The three-jaw self-centering chuck can automatically center, and the clamping is convenient and quick. The centering accuracy is generally 0.05-0.1mm, which is suitable for processing stepped shafts with smaller diameters and shorter lengths; for long and thin stepped shafts, a one-clamp-one-top clamping method should be adopted, that is, one end is clamped with a three-jaw self-centering chuck, and the other end is supported by the tailstock top. This clamping method can enhance the rigidity of the workpiece and reduce vibration and deformation during the cutting process. It is suitable for processing stepped shafts with longer lengths and smaller diameters. For stepped shafts with higher precision requirements, a center stand or a tool holder is required for auxiliary support. The center stand is used to support the middle part of the stepped shaft, and the tool holder is used to follow the movement of the tool and is supported near the tool to further improve the rigidity of the workpiece. When clamping, attention should be paid to the size of the clamping force. Too large a clamping force may easily cause deformation of the workpiece, while too small a clamping force may cause displacement of the workpiece during the cutting process, affecting the machining accuracy.
The choice of tools for turning steel stepped shafts should be determined according to the material and processing requirements. Commonly used tools include external cylindrical turning tools, end-face turning tools, and cut-off tools. External cylindrical turning tools are used to turn the outer cylindrical surface of stepped shafts. Depending on the different processing stages, they can be divided into rough turning tools and fine turning tools. Rough turning tools should have high strength and wear resistance to quickly remove excess. Usually, larger main deflection angles (45°-75°) and rake angles (10°-15°) are selected. Fine turning tools should have high sharpness and precision to ensure the quality of the machined surface. Usually, smaller main deflection angles (90°) and rake angles (5°-10°) are selected, and a smaller tool tip arc radius (0.2-0.5mm) is used. End-face turning tools are used to turn the end face of the stepped shaft. The main cutting edge of the tool is required to be straight to ensure that the end face is perpendicular to the axis and to avoid convexity or tilt of the end face. Cut-off tools are used to cut off longer blanks or create undercuts on stepped shafts. The cutter head width should be selected based on the undercut width, and the head strength should be high to prevent breakage during cutting. The choice of tool material is also crucial. When machining common carbon steels like 45 steel, carbide tools (such as YT15) can be used for rough turning, while high-speed steel tools (such as W18Cr4V) can be used for fine turning. When machining structural alloy steels, carbide tools with improved wear resistance (such as YW1) should be selected.
The turning sequence for steel stepped shafts should follow the principle of “roughing first, finishing second, farthest first, nearth, and outermost first, innermost” to ensure machining accuracy and efficiency. Roughing first, finishing second means rough turning to remove most of the stock, followed by finish turning to achieve final dimensional accuracy and surface quality. Roughing allows for a larger cutting amount to quickly remove the stock, leaving a 0.5-1mm finishing allowance for the stepped shaft after rough turning. Finish turning uses a smaller cutting amount to ensure machining accuracy, aiming for a surface roughness of Ra1.6-3.2μm after finish turning. Farthest first, finishing second means machining the step farther from the chuck first, followed by the step closer. This minimizes the impact of workpiece rigidity variations on machining accuracy and avoids vibration when machining the step closer to the end. The outermost first, finishing second means machining the outer surface first, followed by the inner hole (if any). However, for stepped shafts, machining the outer surface is typically the primary focus. During the processing, attention should be paid to the control of the length of each step. The lathe dial or vernier caliper can be used for measurement. For step lengths with higher precision requirements, a dial indicator can be used for precise control.
Precision control during the turning process of steel stepped shafts is key to ensuring part quality. The precision that needs to be controlled includes dimensional accuracy, form and position tolerances, and surface roughness. Dimensional accuracy primarily refers to the diameter and length accuracy of each step. Diameter accuracy is generally required to reach IT7-IT8 levels and can be measured with a micrometer. Length accuracy is generally required to reach ±0.1-±0.2mm and can be measured with a vernier caliper or depth gauge. Form and position tolerances primarily include the roundness and cylindricity of each step’s outer diameter, as well as the coaxiality between steps. Roundness and cylindricity errors are generally required to not exceed 0.01-0.03mm and can be measured with a dial indicator or roundness gauge. Coaxiality error is generally required to not exceed 0.02-0.05mm and can be measured using the dial gauge method. This involves mounting the workpiece between two centers and using a dial indicator to measure the radial runout of each step’s outer diameter. This runout is the coaxiality error. Surface roughness is primarily controlled by selecting appropriate tools and cutting parameters. During finish turning, using a smaller feed rate (0.1-0.15 mm/r) and a higher cutting speed (80-120 m/min) can achieve lower surface roughness values. During machining, the workpiece’s dimensions and form and position errors should be regularly measured, and cutting parameters and tool position should be adjusted promptly to ensure that machining quality meets requirements.
