Requirements for the geometric shape of triangular thread turning tools
Triangular thread turning tools are specialized tools used to cut triangular threads. The rationality of their geometry directly impacts thread machining accuracy, surface quality, and efficiency. Triangular threads offer advantages such as good self-locking properties, high connection strength, and ease of manufacturing, making them widely used in mechanical manufacturing for applications such as conventional and pipe threads. The geometric requirements for triangular thread turning tools primarily include the rational design of parameters such as the tooth angle, rake angle, clearance angle, tool tip angle, and tool head width. These parameters must be determined based on factors such as thread type, precision requirements, workpiece material, and machining conditions to ensure the cut thread meets design requirements.
The tooth angle is the most basic geometric parameter of a triangular thread turning tool and must be consistent with the tooth angle specified in the thread standard; otherwise, the thread fit accuracy will be reduced. The tooth angle of an ordinary triangular thread is 60°, the tooth angle of an imperial triangular thread is 55°, and the tooth angle of a pipe thread also has corresponding standard regulations. The tooth angle of a turning tool is formed by sharpening the rake and flank faces. During sharpening, it must be ensured that the angle between the two main cutting edges is equal to the tooth angle of the thread, and the cutting edges should be straight and sharp, without defects such as chipping and notches. In order to ensure the accuracy of the tooth angle, an angle template can be used for inspection during sharpening to ensure that the tooth angle error of the turning tool is within the allowable range (generally ±15′). For threads with higher precision requirements, precision instruments such as optical goniometers are also required for measurement and calibration to ensure the accuracy of the tooth angle.
The rake angle is a critical geometric parameter of triangular thread turning tools, influencing cutting forces, cutting temperatures, and thread surface quality. The rake angle should be selected based on the properties of the workpiece material and the required thread precision. When machining plastic materials (such as mild steel and non-ferrous metals), a larger rake angle (generally 5°-15°) is recommended to minimize cutting deformation, cutting forces, and lower cutting temperatures. When machining brittle materials (such as cast iron and high-carbon steel), a smaller rake angle (generally 0°-5°), or even a negative rake angle, is recommended to enhance tool strength and prevent edge chipping. For threads requiring high precision, the rake angle should be appropriately reduced to minimize the impact of tool wear on thread profile accuracy. It is important to note that the rake angle affects the tool’s actual thread angle. When the rake angle is not 0°, there will be a discrepancy between the tool’s marked thread angle (measured within the base plane) and the actual thread angle (measured within the cutting plane). Therefore, corrections must be made during sharpening to ensure the thread angle meets the required requirements.
The function of the clearance angle is to reduce friction between the tool’s flank and the machined surface of the thread, thereby reducing tool wear and improving the thread’s surface quality. The clearance angles of triangular thread turning tools are divided into primary and secondary clearance angles. The primary clearance angle is generally 6°-10°, while the secondary clearance angle is generally 4°-8°. Because the relative motion of the tool and workpiece during turning forms a helical trajectory, friction between the flank and the thread surface increases with increasing lead angle. Therefore, for threads with a larger pitch (larger lead angle), the clearance angle should be appropriately increased to reduce friction. Furthermore, to ensure thread profile accuracy, the clearance angles on both sides of the tool should be symmetrical to avoid skew due to asymmetric clearance angles. The clearance angle should also be adjusted according to the processing stage. A smaller clearance angle can be used for roughing to enhance tool strength, while a larger clearance angle should be used for finishing to improve surface quality.
The tool tip angle refers to the angle formed at the tip of a triangular thread turning tool between the two main cutting edges. Its size should be equal to the thread profile angle minus the angle corresponding to the two tool tip radii. To avoid overcutting or burrs at the thread crest, the tool tip should be ground with an appropriate tool tip radius or chamfer. The tool tip radius should be determined based on the thread pitch and precision requirements, generally ranging from 0.12-0.25mm. For coarse threads, the tool tip radius can be increased; for fine threads, the tool tip radius should be decreased to ensure the shape accuracy of the thread root. The symmetry of the tool tip angle is also important. The two main cutting edges must be symmetrically distributed at the tool tip; otherwise, the thread profile will be asymmetrical, affecting the thread’s fit. During sharpening, the symmetry and accuracy of the tool tip angle can be checked by measuring the fit of the two main cutting edges against the template.
The cutter head width refers to the radial dimension of the triangular thread turning tool head. Its size should be determined according to the thread pitch and machining requirements. A cutter head width that is too small will result in insufficient tool strength, easily causing vibration and chipping. A cutter head width that is too large will increase friction between the tool and the workpiece, affecting cutting performance and surface quality. The cutter head width is calculated as: cutter head width = (0.8-0.9) × pitch. The specific value can be adjusted according to actual conditions. For roughing thread turning tools, the cutter head width can be increased to improve tool strength and durability. For finishing thread turning tools, the cutter head width should be reduced to reduce cutting forces and friction, thereby improving surface quality. Furthermore, the cutter head length should be sufficient to ensure smooth penetration to the full depth of the thread during thread turning. The cutter head length is generally 1.5-2 times the thread pitch .
