Correction of thread turning tool angle
The accuracy of the thread turning tool angle directly determines the profile accuracy of the thread. Due to tool manufacturing errors, grinding deviations, or differences in the properties of the processed material, the thread turning tool angle must be adjusted in practice to compensate for processing errors. For example, when machining a standard M20×2 thread, if the tool rake angle is not 0°, the actual profile angle will be less than the theoretical angle (60°), resulting in tooth profile error. When machining plastic materials, a too small tool clearance angle will increase friction and lead to rough thread surfaces. Therefore, the correction of the thread turning tool angle must be based on the thread type, material properties, and processing requirements. By adjusting parameters such as the rake angle, clearance angle, and profile angle, the machined thread meets the design standards.
Correction of the rake angle should be based on the workpiece material and thread accuracy requirements. The theoretical rake angle of a standard thread turning tool is 0° to ensure accurate thread angle. However, when machining plastic materials (such as mild steel and aluminum alloys), the rake angle should be corrected to 5°-15° to reduce cutting forces and friction and avoid built-up edge. Increasing the rake angle decreases the actual thread angle (Δα ≈ γo × sin (α/2), where γo is the rake angle and α is the thread angle). Therefore, the tool’s ground thread angle should be increased by Δα to compensate. For example, when machining a 60° standard thread, if the rake angle is 10°, the ground thread angle should be corrected to 60° + 10° × sin 30° = 65°, ensuring a 60° thread angle after machining. When machining brittle materials (such as cast iron), the rake angle should be corrected to 0° to -5° to enhance cutting edge strength. In this case, the ground thread angle can be equal to the theoretical angle. When processing high-precision threads (such as screw threads), the front angle must be strictly controlled at 0°±1°, and verified by trial cutting method, and the thread profile fit must be checked with a thread gauge until the error is ≤0.01mm.
Correction of the clearance angle must take into account the influence of the lead angle to avoid friction between the flank and the thread surface. The lead angle φ (tanφ = lead/πd, where d is the major diameter of the thread) reduces the actual clearance angle between the thread flank and the tool flank. Therefore, the tool’s grinding clearance angle should be corrected to: αp = 6° – 8° + φ in the feed direction, and αf = 6° – 8° – φ in the back-to-back direction, ensuring the actual working clearance angle is ≥ 3°. For example, when machining an M48×5 thread (φ ≈ 1.91°), the clearance angle in the feed direction should be 8° + 1.91° ≈ 10°, and in the back-to-back direction, 8° – 1.91° ≈ 6°. When machining multi-start threads or threads with a large lead (φ > 5°), the clearance angle correction should be increased accordingly to avoid interference between the flank and the machined surface. In actual operation, the back angle can be corrected by rotating the tool angle on the tool holder (equal to the thread lead angle), and the corrected angle deviation can be checked with a template to ensure that it is ≤0.5°.
Correction of the thread angle requires compensation for tool wear and system errors. Newly sharpened thread turning tools must be calibrated using an angle template. The thread angle error for a 60° standard thread turning tool should be ≤±15′, and the error for a 55° Whitworth thread turning tool should be ≤±20′. During machining, tool wear can cause the thread angle to decrease (typically, this should be checked every 50-100 parts). This requires correction through sharpening: the actual thread angle is measured using an optical goniometer. If the wear is 1°, the thread angle is increased by 1° during sharpening until the theoretical angle is restored. For mass production, a “test cut-measure-correct” cycle can be employed: after machining the first part, the thread angle is measured using a tool microscope, the correction value (Δα = theoretical angle – actual angle) is calculated, and the tool is then sharpened to compensate. Precise thread angle correction on an automotive bolt production line has increased the pass rate of thread go/no-go gauges from 90% to 99.5%, effectively reducing assembly issues caused by tooth profile errors.
Correction of the tool tip radius and rake angle must balance surface quality and tool strength. The tool tip radius of a thread turning tool should be ≤ 1/6 of the thread root groove width. For standard threads (M10 and above), the tool tip radius is typically 0.1-0.3mm. A radius too large will result in an overly rounded root and reduced strength; a smaller radius will weaken the tool edge and easily chip. When machining high-strength bolt threads, the tool tip radius can be adjusted to 0.2-0.4mm to enhance blade strength while ensuring a smooth transition to the root radius. The rake angle is typically 0°, but when machining slender threaded parts, the rake angle can be adjusted to a positive value of 5°-10° to direct chips toward the surface being machined and avoid scratching the existing threads. The corrected tool tip radius and rake angle must be inspected with a tool microscope and compared to the root contour of a test part to ensure compliance with the drawing requirements (typically, the root radius error is ≤ 0.02mm).
Angle corrections for special thread turning tools must adhere to specific standards. When machining trapezoidal threads (30° flank angle), the larger lead (typically 4-12mm) significantly impacts the thread lead angle, requiring a 2°-3° larger back angle correction than for standard threads. Furthermore, to ensure the accuracy of the trapezoidal thread’s working surface (30°), the rake angle should be controlled between 0° and 5°, requiring calibration using a dedicated trapezoidal template during sharpening. When machining serrated threads (3°/30° flank angle), the 3° working surface is the load-bearing surface, and its angular error must be ≤±10°. Therefore, more precise angle corrections on this side of the tool are required. This can be achieved by positioning the tool on a tool grinder using a dividing head and inspecting with a projector. For pipe threads (such as G1/2″ cylindrical pipe threads with a 55° thread angle), the taper (1:16) requires a gradient correction of the tool’s clearance angle along the taper, increasing toward the larger end and decreasing toward the smaller end to ensure a uniform clearance angle along the entire length. Using a special thread angle correction technique, a hydraulic pipe fittings manufacturer has increased the sealing performance qualification rate of pipe threads from 85% to 98% and reduced leakage by 90%.
