Turning of large diameter outer spherical surface of cast iron
Cast iron large-diameter outer spherical surfaces are a special structure in mechanical parts, commonly found in valves, pump bodies, machine tool worktables, and other components. Their function is to achieve rotational motion or sealing fit of the parts, and they place high demands on the roundness, surface roughness, and dimensional accuracy of the spherical surface. Cast iron material has moderate hardness, good wear resistance, and excellent casting properties, but it is also quite brittle and prone to chipping during turning, which places high demands on the tool’s wear resistance and impact toughness. The key to turning large-diameter cast iron outer spherical surfaces is to ensure the spherical surface’s geometric accuracy and surface quality, which requires the use of appropriate clamping methods, tools, and processing methods.
Clamping large-diameter outer spherical surfaces in cast iron is one of the difficulties in turning. Due to the large diameter and heavy weight of the workpiece, the stability and positioning accuracy of the clamping directly affect the processing quality. There are two common clamping methods: one is to use a disc clamping method, in which the workpiece is fixed to the disc with a pressure plate and bolts. The disc is installed on the lathe spindle. By adjusting the position of the workpiece, the center of the sphere coincides with the rotation axis of the lathe spindle. This method is suitable for large workpieces with irregular shapes, but it requires the installation of a balancing block on the disc to eliminate unbalanced forces and avoid lathe vibration. The other is to use a special clamping fixture for clamping. For mass-produced workpieces, a special clamping fixture can be designed and manufactured. The fixture is equipped with a positioning reference surface and clamping device. It can quickly and accurately position the workpiece, ensure that the center of the sphere coincides with the spindle axis, and improve clamping efficiency and accuracy. When clamping, it is necessary to pay attention to applying the clamping force evenly to avoid deformation of the workpiece due to uneven force. For thin-walled castings, auxiliary support should be added to prevent cracks during clamping.
When selecting a tool for turning large-diameter cast iron external spherical surfaces, the material properties of the cast iron and the specific characteristics of surface machining must be considered. Since cast iron generally has a hardness between 150 and 300 HB and contains graphite, turning can easily lead to abrasive wear on the tool. Therefore, wear-resistant carbide tools, such as YG6 and YG8, should be selected. These tools, based on tungsten carbide and a cobalt binder, offer high wear resistance and impact toughness, making them suitable for machining brittle materials like cast iron. Tool geometry should be determined based on the radius of curvature of the spherical surface and the machining requirements. The rake angle is generally set between 0° and 5°. A smaller rake angle increases tool strength and prevents chipping. The clearance angle is set between 5° and 8° to reduce friction between the tool and the workpiece. The primary and secondary rake angles are adjusted based on the radius of curvature of the spherical surface, generally between 45° and 60°, to ensure a good contact between the tool’s cutting edge and the spherical surface. The tool’s cutting edge should be sharp and, if necessary, negative chamfering can be applied to enhance edge strength and reduce chipping. In addition, the tool holder should be rigid enough to prevent vibration during turning that affects the surface quality.
There are two methods for turning large-diameter external spherical surfaces in cast iron: the forming tool method and the tracking method. The forming tool method uses a forming turning tool with the same radius of curvature as the sphere. The tool’s cutting edge is arc-shaped, allowing the spherical surface to be formed in a single pass. This method is suitable for spheres with smaller radii and lower precision requirements. During turning, the forming turning tool is mounted on a tool holder, and the tool height is adjusted so that the center of the arc of the cutting edge coincides with the center of the sphere. The tool holder is then slowly rotated and fed to produce the spherical surface. This method is simple to operate and highly efficient, but the forming tool is more difficult to manufacture, and the machining accuracy is significantly affected by the tool’s accuracy. The tracking method is suitable for large-diameter external spherical surfaces with higher precision requirements. A tracking device is installed on the lathe, with the curved surface on the tracking tool having the same radius of curvature as the sphere to be machined. The turning tool contacts the tracking tool via a slider. As the large carriage advances, the slider slides along the tracking tool, driving the turning tool in an arc motion, thus producing a precise spherical surface. The machining accuracy of the template method is high. The roundness error of the sphere can be controlled within 0.01 mm, and the surface roughness can reach Ra1.6μm. However, a special template needs to be made and it is suitable for mass production.
When turning large-diameter cast iron external spherical surfaces, the following key process points must be considered: First, appropriate cutting parameters should be selected. Due to the large workpiece diameter, the spindle speed should not be too high, generally 30-60 rpm, to reduce centrifugal force and vibration. The feed rate should be 0.1-0.2 mm/rev, and the back-cut should be 1-2 mm per pass. Multiple passes should be used to remove machining allowances. Second, regarding cooling and lubrication, while cutting fluid is generally not required when turning cast iron, kerosene or specialized cutting fluids can be used for spherical surfaces requiring higher precision to reduce tool wear and improve surface quality. During the turning process, broken chips should be removed promptly to prevent them from scratching the machined surface or causing tool edge breakage. After machining, the spherical surface must be precisely measured. Commonly used measuring tools include spherical gauges, templates, and dial indicators. The contact area of the spherical gauge is checked by coloring, and the contact area should be greater than 80% to ensure the accuracy of the spherical surface. For spherical surfaces with extremely high precision requirements, grinding is also required, using cast iron grinding discs and grinding paste. This grinding process removes minor surface errors and further improves the roundness and surface quality of the spherical surface. Through reasonable clamping, correct tool selection, and machining methods, it is possible to turn a large-diameter cast iron outer spherical surface that meets the requirements and meets the use needs of mechanical parts.
