CNC Machining of large piston rods
Large piston rods are crucial components in large hydraulic equipment and construction machinery. They are typically long (often exceeding 1 meter) and have a large diameter (usually greater than 50 mm). They are primarily used to transmit force and achieve linear motion. CNC Machining large piston rods faces challenges such as poor rigidity, deformation, and difficulty ensuring machining accuracy. Therefore, optimal clamping methods, tool selection, cutting parameters, and process measures are essential to ensure machining quality and efficiency.
The clamping method for large piston rods directly affects machining accuracy and stability. Common clamping methods include double-ejector clamping, one-end chuck and one-end ejector clamping, and steady rest-assisted clamping. Double-ejector clamping is suitable for piston rods longer than the original length and smaller in diameter. Supporting the workpiece with front and rear ejectors ensures the rod’s coaxiality. During clamping, center holes must be drilled at both ends of the rod. The center hole accuracy must meet the required standards (generally Grade A or B) to avoid machining accuracy errors caused by center hole errors. Single-end chuck and one-end ejector clamping is suitable for piston rods with larger lengths and diameters. The chuck clamps one end of the rod, while the ejector supports the other end. This clamping method provides superior rigidity and can withstand high cutting forces, making it suitable for rough and semi-finishing operations. For particularly long piston rods (e.g., over 3 meters in length), a steady rest is also required. The steady rest is mounted on the lathe guide rails and supports the center of the rod to enhance rigidity and reduce vibration and deformation during cutting. When using a center stand, you need to first machine a support surface on the piston rod. The surface roughness of the support surface should be small to ensure the support accuracy of the center stand. At the same time, you need to adjust the tightness of the three support claws of the center stand. If it is too tight, it will damage the surface of the workpiece, and if it is too loose, it will not play a supporting role.
Tool selection depends on the material and machining requirements for the large piston rod. Common materials include 45 steel, 40Cr, and 27SiMn. These materials offer high strength and toughness, necessitating the selection of appropriate tool materials and geometry for turning. For rough turning of ordinary carbon steels like 45 steel, carbide tools (such as YT15 and YG8) can be used. YT15 tools offer excellent red hardness and are suitable for high-speed cutting; YG8 tools offer excellent toughness and are suitable for interrupted cutting. High-speed steel tools (such as W18Cr4V) can be used for finish turning. High-speed steel tools, when sharpened, can produce excellent surface finishes (Ra 1.6-3.2μm). For structural alloy steels like 40Cr, due to their greater tendency to work harden, carbide tools with high wear resistance (such as YW1 and YW2) should be selected. YW-type tools offer excellent versatility and can process a variety of materials, including steel and cast iron. The geometric parameters of the tool also need to be designed reasonably. The rake angle is generally 10°-15° to reduce cutting force; the back angle is 6°-8° to reduce friction between the tool and the workpiece; the main deflection angle is 45°-90°. For large piston rods with poor rigidity, the main deflection angle is 90°, which can reduce radial cutting force, vibration and deformation.
Cutting parameters should be selected based on the rigidity, material properties, and machining requirements of the large piston rod to improve machining efficiency and quality. The choice of cutting speed is dependent on both the tool and workpiece materials. When using carbide tools for rough turning of 45 steel, the cutting speed is generally 80-120 m/min; for finish turning, it is 100-150 m/min. When using high-speed steel tools for finish turning, the cutting speed is 30-50 m/min. The feed rate should be determined based on the surface quality requirements. For rough turning, the feed rate is generally 0.2-0.3 mm/r; for finish turning, it is 0.1-0.15 mm/r to ensure that the surface roughness meets the requirements. The back-cut depth should be determined based on the stock size and workpiece rigidity. A larger back-cut depth (2-5 mm) can be used for rough turning to quickly remove stock; for finish turning, a back-cut depth of 0.1-0.5 mm is used to ensure machining accuracy. Since the large piston rod has poor rigidity, excessive cutting force should be avoided when selecting cutting parameters. Multiple passes can be used to gradually remove the allowance and reduce deformation.
The turning process for large piston rods should be rationally arranged based on the required precision and structural characteristics. It generally includes rough turning, semi-finishing turning, quenching and tempering, and finish turning. Rough turning primarily removes most of the stock and establishes the approximate shape and dimensions, such as the outer diameter and end faces, to prepare for subsequent processes. After rough turning, the piston rod should be tempered. This treatment improves the overall mechanical properties of the piston rod, imparting higher strength and toughness while also eliminating internal stresses generated during rough turning. Semi-finishing turning, after quenching and tempering, further enhances the dimensional accuracy and surface quality of the piston rod, preparing it for finish turning. During semi-finishing turning, the machining allowance must be carefully controlled, typically leaving a 0.5-1mm allowance for finish turning. Finish turning is crucial for ensuring the final precision of large piston rods. Features such as the outer diameter, steps, and threads must be machined according to the drawing. High-precision tools and gauges should be used during finish turning to ensure that dimensional and form tolerances meet the required standards. For large piston rods with higher precision requirements, grinding is also required after finish turning to further improve surface roughness and dimensional accuracy.
Controlling deformation during the turning process of large piston rods is crucial for ensuring machining quality. Due to their large length and poor rigidity, they are susceptible to bending deformation due to cutting forces, clamping forces, and thermal stress. To minimize deformation, the first step is to choose a suitable clamping method, such as using a center stand for auxiliary support to increase workpiece rigidity. For thin-walled large piston rods, axial clamping can be used to avoid deformation caused by excessive radial clamping forces. Secondly, during the cutting process, cutting parameters should be carefully controlled to avoid excessive cutting forces and heat. A symmetrical cutting method should be adopted to ensure uniform force distribution on the workpiece and minimize deformation. Furthermore, attention should be paid to cooling and lubrication, using sufficient cooling and lubricating fluid to reduce cutting temperatures and thermal deformation. During machining, the straightness and roundness of the piston rod should be regularly measured. If deformation exceeds tolerance, it should be straightened promptly. For large piston rods with high precision requirements, aging treatment is required after finish turning to eliminate internal stresses generated during machining and stabilize dimensional accuracy. These measures can effectively control deformation of large piston rods and ensure machining quality.
