Tools and methods for turning slender shafts
Slender shafts are shaft parts with a length-to-diameter ratio greater than 20. Their turning poses a challenge due to their poor rigidity and proneness to deformation. The proper selection of cutting tools and machining methods is crucial for ensuring machining quality. Slender shafts are primarily characterized by low rigidity, making them susceptible to bending, deformation, and vibration under the influence of cutting forces, gravity, and centripetal forces. This can lead to defects such as cylindricity errors, taper, or a drum-like shape in the machined shaft, and surface roughness cannot be guaranteed. Furthermore, slender shafts have poor heat dissipation, and cutting heat can easily cause thermal deformation in the workpiece, further exacerbating machining errors. Therefore, when turning slender shafts, the tool geometry, cutting parameters, and machining methods require special design to reduce cutting forces and vibration and control workpiece deformation.
When selecting tools for turning slender shafts, the goal is to reduce cutting forces, friction, and heat dissipation. The tool material and geometry are particularly important. Tool materials with good wear and heat resistance, such as cemented carbide (YT15, YG6, etc.), should be selected. For applications requiring higher precision, high-speed steel tools can be used for finish turning. Regarding tool geometry, the rake angle should be large (15°–20°) to reduce cutting forces and friction; the relief angle should be 6°–10°, with a ground negative chamfer to enhance tool strength; and the lead angle should be as large as possible (90°–93°) to direct cutting forces axially and minimize bending effects of radial cutting forces on the workpiece. Furthermore, the tool nose radius should be small (0.2–0.5mm) to reduce cutting forces and vibration. The cutting edge should be ground to improve surface finish and minimize scratches on the workpiece.
Adopting appropriate machining methods is crucial for controlling deformation of slender shafts. Commonly used machining methods include backfeed, steady rest, and steady rest. The backfeed method involves rotating the lathe spindle forward while the tool feeds from the headstock toward the tailstock. This axial tension on the workpiece offsets some of the radial cutting force, reducing workpiece bending and deformation while facilitating the entry of cutting fluid into the cutting zone and improving heat dissipation. The steady rest method uses a steady rest to support the free end of the workpiece during turning, increasing its rigidity. The steady rest typically has two or three support jaws that contact the workpiece surface and move with the tool. This is suitable for slender shafts with long lengths and small diameters. When using a steady rest, the pressure of the support jaws must be appropriately adjusted. Too tight can scratch the workpiece surface, while too loose can provide insufficient support. Trial cutting is necessary to ensure reliable support. The steady rest method secures the steady rest to the lathe bed, supporting it in the middle or at a specific location on the workpiece. It is used to support slender shafts with particularly large aspect ratios. The steady rest’s support position should be determined based on the workpiece’s length, generally near its midpoint to maximize rigidity.
The proper selection of cutting parameters significantly impacts the turning quality of slender shafts. The principle of “low cutting speed, small feed rate, and appropriate cutting depth” should be adhered to. Excessively high cutting speeds can increase cutting temperatures, exacerbating workpiece thermal deformation and tool wear. Generally, the cutting speed should be kept between 80 and 120 m/min. For high-speed steel tools, the cutting speed should be even lower (30 to 50 m/min). The feed rate should be kept low (0.1 to 0.2 mm/r) to reduce cutting forces and vibration and improve surface quality. For finish turning, the feed rate can be reduced to 0.05 to 0.1 mm/r. The cutting depth should be appropriately distributed based on the machining allowance. For rough turning, it can be 2 to 4 mm, while for finish turning, it can be 0.5 to 1 mm. This avoids workpiece deformation caused by excessive cutting depth. Furthermore, the choice of cutting fluid is crucial. Use an emulsion or extreme pressure cutting oil with good cooling properties. High-pressure injection of the cutting fluid into the cutting zone effectively reduces cutting temperatures, friction, and tool wear.
Processing measures for turning slender shafts also include workpiece clamping, heat treatment, and auxiliary supports. These measures work together to ensure machining accuracy. When clamping the workpiece, it should be lightly clamped with a chuck at one end (to avoid bending due to overtightening) and supported with a centering tool at the other end. The chuck should leave 10-15mm of clearance from the workpiece end to prevent excessive clamping that would affect the workpiece’s free expansion and contraction. For slender shafts requiring high precision, a double-center clamping system can be used in conjunction with a steady rest to further improve workpiece rigidity. Before machining, slender shafts should be tempered to eliminate internal stresses and reduce deformation during machining. After rough turning, they should be aged to eliminate machining stresses before finishing. Furthermore, reverse cutting and additional auxiliary supports can be employed, such as adding temporary supports in the middle of the workpiece or using flexible centers to allow for free axial expansion and contraction, minimizing the impact of thermal deformation on machining accuracy. By combining appropriate tools, machining methods, and process measures, deformation of slender shafts can be effectively controlled, ensuring machining accuracy and surface quality.
