Geometric parameters of thread turning tools
Thread turning tools are the primary tool for thread CNC machining. Their geometric parameters directly impact thread CNC machining accuracy, surface quality, and tool life. These parameters include angles, dimensions, and cutting edge parameters. The optimal selection of these parameters depends on the thread type (e.g., triangular, trapezoidal, rectangular), material, and CNC machining requirements. Understanding the design principles and selection methods for thread turning tool geometric parameters is crucial for ensuring thread CNC machining quality and improving production efficiency.
The angular parameters of a thread turning tool are crucial to determining thread profile accuracy. These primarily include the profile angle, rake angle, clearance angle, and tool tip angle. The profile angle is the angle between the two cutting edges of the thread turning tool projected onto the base surface. It must match the profile angle of the thread being machined to ensure profile accuracy. For example, the profile angle of a standard triangular thread is 60°, that of a trapezoidal thread is 30°, and that of a rectangular thread is 0°. Therefore, the profile angle of the thread turning tool must be ground to these angles. The accuracy of the profile angle directly affects the fit of the thread. Generally, an error of no more than ±30° is required, with high-precision threads requiring a tolerance of within ±10°. The rake angle, measured in an orthogonal plane, is the angle between the rake face and the base surface. The rake angle affects cutting forces and chip evacuation. When CNC machining plastic materials (such as steel), a rake angle of 5°-15° is generally used to reduce cutting forces and facilitate chip evacuation. When CNC machining brittle materials (such as cast iron), a rake angle of 0°-5° is generally used to enhance tool edge strength. It should be noted that the rake angle will affect the actual tooth profile angle. When the rake angle is not 0°, the actual tooth profile angle of the cutting edge will be smaller than the tooth profile angle of the grinding, so it needs to be corrected. The correction amount can be determined by calculation or experiment.
The clearance angle is the angle between the flank face and the cutting plane, measured in an orthogonal plane. Its function is to reduce friction between the flank face and the machined workpiece surface. The size of the clearance angle depends on the thread pitch and CNC machining method. Because the cutting edge of a thread turning tool moves along a helical trajectory during CNC machining, friction occurs between the flank face and the machined workpiece surface. Therefore, the clearance angle of a thread turning tool is divided into the feed direction clearance angle and the perpendicular feed direction clearance angle. The feed direction clearance angle (i.e., the clearance angle on the side closest to the major diameter of the thread) is generally 6°-10°, while the perpendicular feed direction clearance angle (i.e., the clearance angle on the side closest to the minor diameter of the thread) is generally 3°-5° to minimize friction between the flank face and the workpiece surface throughout the cutting process. For threads with a higher pitch (e.g., pitch greater than 4mm), the clearance angle needs to be increased to prevent interference between the flank face and the workpiece due to the larger helix angle. For precision threads, the clearance angle can be reduced to increase tool rigidity and ensure CNC machining accuracy. In addition, the main flank surface and secondary flank surface of the thread turning tool need to be relief ground to form a certain back angle to ensure smooth cutting.
The tool tip angle and tool tip radius of a thread turning tool significantly impact thread CNC machining quality and tool life. The tool tip angle is the angle between the two secondary cutting edges of a thread turning tool. For triangular thread turning tools, the tool tip angle should be equal to the thread profile angle. For trapezoidal and rectangular thread turning tools, the tool tip angle should be determined based on the thread root shape, typically ranging from 30° to 60°. The tool tip radius refers to the fillet radius at the tool tip of the thread turning tool. It must be smaller than the minimum radius of the thread root to ensure the root shape accuracy. For example, the root radius of a common triangular thread is relatively small, typically 0.12-0.15mm. Trapezoidal threads have a larger root radius, and the tool tip radius can be increased to 0.2-0.5mm. An appropriate tool tip radius can increase tool tip strength, reduce tool wear, and improve the surface quality of the thread root. However, an excessively large radius can result in an excessively wide thread root, affecting thread strength and fit.
The shank size and rigidity of a thread turning tool are also important parameters affecting CNC machining quality. The shank’s cross-sectional dimensions must be determined based on the thread diameter and pitch being machined to ensure sufficient rigidity and avoid vibration during CNC machining. The shank’s length should be as short as possible, generally 3-5 times its cross-sectional height, to minimize bending and deformation. For turning tools processing large-diameter or deep threads, the shank’s cross-sectional dimensions should be increased accordingly. If necessary, a square or round shank can be used to increase rigidity. The shank is typically made of 45 steel or tool steel, quenched and tempered to a hardness of 25-30 HRC to ensure sufficient strength and toughness. Furthermore, the mounting angle of the thread turning tool must match the geometric parameters. The tool axis should be perpendicular to the workpiece axis to ensure symmetry of the thread profile angle. The tool tip should be flush with the workpiece center to prevent thread profile skew.
The selection of thread turning tool geometric parameters requires comprehensive consideration of multiple factors and optimized design. When CNC machining plastic materials, larger rake and clearance angles should be selected to reduce cutting forces and friction and improve surface quality. When CNC machining brittle materials, smaller rake angles and greater edge strength should be selected to prevent chipping. When CNC machining high-precision threads, the accuracy of the tooth profile angle, rake angle, and clearance angle must be strictly controlled to reduce the impact of tool wear on thread accuracy. When CNC machining high-pitch threads, the tool tip radius and toolholder rigidity should be increased to improve tool life and process stability. Furthermore, the characteristics of the tool material must be considered. High-speed steel thread turning tools have greater toughness and can use a larger rake angle. Carbide thread turning tools are harder but more brittle, so smaller rake angles and greater edge strength should be used. By rationally selecting the geometric parameters of thread turning tools, thread CNC machining accuracy and surface quality can be effectively improved, tool life can be extended, production costs can be reduced, and the requirements of different thread CNC machining processes can be met.
