Special tools for turning spherical surfaces
Special tools for turning spherical surfaces are specialized devices used to machine spherical workpieces on lathes. They can effectively ensure the geometric accuracy and surface quality of the sphere, improve machining efficiency, and are widely used in industries such as valve manufacturing, mold processing, and instrumentation. Machining spherical workpieces is relatively difficult, requiring high standards for the roundness, surface roughness, and fit accuracy of the sphere with other parts. Ordinary turning methods are difficult to meet these requirements, so special tools are needed to achieve this. There are many types of special tools for turning spherical surfaces. According to their working principles and structural characteristics, they can be divided into special tools for the profiling method, special tools for CNC turning, special tools for whirlwind milling, etc. Each tool has its own unique scope of application and machining advantages.
The tool used for spherical surface machining using the mastering method is a traditional spherical surface machining tool. Its operating principle is to use the mastering contour to control the feed motion of the cutting tool, allowing the tool to cut along the trajectory of the sphere, thereby producing a spherical surface that meets the requirements. The mastering method tool primarily consists of a mastering tool, a roller, a tool holder, and a transmission mechanism. The mastering tool’s shape matches the contour of the desired spherical surface, and the roller contacts the mastering tool. As the workpiece rotates, the transmission mechanism drives the roller along the mastering tool, thereby controlling the corresponding feed motion of the cutting tool, achieving the desired spherical surface machining. This tool has a simple structure, low manufacturing cost, and easy operation. It is suitable for batch machining of medium- and low-precision spherical workpieces, such as ball valves and ball studs for bearings. However, the mastering tool’s manufacturing accuracy directly affects the machining accuracy of the spherical surface, and changing spherical workpieces with different specifications requires replacing the corresponding mastering tool, which reduces flexibility. Therefore, it is rarely used in single-piece, small-batch production and high-precision spherical surface machining.
CNC spherical turning tools, which emerged with the development of CNC lathes, are efficient, high-precision spherical surface machining tools. They primarily consist of a CNC system, servo motor, ball screw, and dedicated toolholder. CNC spherical turning tools utilize a compiled CNC program to control the tool’s interpolation motion according to the mathematical equations of the sphere, achieving precise machining of the sphere. Compared to tools using a mastering method, CNC spherical turning tools offer the following advantages: First, they offer high machining accuracy. Through precise control by the CNC system, they guarantee a roundness error of no more than 0.005mm and a surface roughness Ra of no more than 0.8μm. Second, they offer high flexibility: simply modifying the CNC program allows for machining spherical workpieces of varying specifications and curvatures without the need to change the mastering tool. Third, they offer high machining efficiency, enabling high-speed cutting and automatic tool changing, significantly reducing auxiliary time. CNC spherical turning tools are suitable for single-piece, small-batch production, and the machining of high-precision spherical workpieces, such as spherical cavities in precision molds and spherical components for aircraft engines.
A dedicated tool for high-speed spherical surface machining (SSM) is a specialized tool designed for high-speed spherical surface machining. It primarily consists of a high-speed milling cutter, a workpiece spindle, and a feed mechanism. During SSM, the cutter rotates at a speed significantly higher than the workpiece (typically 10-50 times the workpiece speed). While rotating at high speed, the tool simultaneously feeds along the workpiece’s axis, producing the spherical surface through the relative motion between the tool and the workpiece. SSM tools offer exceptionally high machining efficiency, 5-10 times that of conventional turning, and produce excellent surface quality, with surface roughness Ra reaching 0.4-1.6μm. This specialized tool is suitable for machining large-diameter, high-volume spherical workpieces, such as spheres for large valves and hemispherical heads for pressure vessels. However, the high cutting forces and heat generated during SSM place high demands on tool material and machine tool rigidity. Carbide or ceramic tools are typically used, and a robust cooling system is required to prevent tool wear and thermal deformation of the workpiece.
Proper use and maintenance of tools specifically designed for spherical turning are crucial for ensuring machining quality and extending tool life. When using tools specifically designed for the spherical turning method, the guide and rollers should be regularly inspected for wear, and severely worn parts should be replaced promptly to ensure the accuracy of the guide. The relative position of the tool and guide should be adjusted to ensure that the tool’s trajectory aligns with the spherical surface contour. When using tools specifically designed for CNC turning, the NC program should be correctly compiled, cutting parameters should be appropriately set, and the servo motor and ball screw should be regularly calibrated to avoid transmission errors that could affect machining accuracy. Furthermore, the CNC system should be kept clean and dry to prevent dust and moisture from entering the system. When using tools specifically designed for cyclonic milling, appropriate tool materials and geometry should be selected to ensure the cutterhead’s balance and accuracy, avoiding vibration during high-speed rotation. Cooling and lubrication should be enhanced, and chips should be removed promptly to prevent chip accumulation that could affect machining quality. Furthermore, regardless of the type of tool used for spherical turning, the tool should be thoroughly inspected before machining. During machining, the machining status should be closely monitored, and any abnormalities should be promptly stopped and addressed to ensure a safe and stable process.
