Tip width of thread turning tool
The tip width of a thread turning tool is a crucial parameter in thread machining, directly impacting the thread profile accuracy, surface quality, and fit. Tip width refers to the horizontal width of the tool tip, and its size must match the thread root width. If the tip width is too large, the thread root will be too wide, weakening the thread strength. If it is too small, the fillet radius at the root cannot be guaranteed, affecting the thread’s load-bearing capacity and fatigue life. The tip width of a thread turning tool must be precisely calculated and sharpened based on the thread type (e.g., triangular, trapezoidal, rectangular), pitch, and precision grade to ensure the machined thread meets design requirements.
The most common calculation for the tip width of a triangular thread turning tool is based on the thread pitch and profile angle. For common triangular threads (such as metric threads), which have a profile angle of 60°, the tip width (i.e., root width) is calculated as: w = 0.125P + (0.05-0.1) mm, where P is the pitch and 0.05-0.1 mm is a correction factor to compensate for grinding errors and ensure adequate root radius. For example, for a metric thread with a pitch of P = 2 mm, the tip width w = 0.125 × 2 + 0.07 = 0.32 mm. For imperial triangular threads, which have a profile angle of 55°, the tip width is calculated as: w = 0.25P + 0.02 mm (P is the pitch in threads per inch). The tip width of a triangular thread turning tool must be precisely controlled. Too much or too little will affect the thread fit. For example, for 6g threads, the tip width tolerance is ±0.03mm. During sharpening, use a thread template or a specialized measuring tool to check the tip width to ensure it meets the requirements.
The tool tip width of a trapezoidal thread turning tool is related to the thread pitch and crest clearance. For a trapezoidal thread with a 30° pitch angle, the tool tip width (root width) is calculated as: w = 0.366P – 0.536ac, where P is the pitch and ac is the crest clearance. According to national standards , when P ≤ 4mm, ac = 0.25mm; when 4mm < P ≤ 12mm, ac = 0.5mm; and when P > 12mm, ac = 1mm. For example, for a trapezoidal thread with a pitch of P = 6mm, ac = 0.5mm, and the tool tip width w = 0.366 × 6 – 0.536 × 0.5 ≈ 2.196 – 0.268 = 1.928mm. The tool tip width of a trapezoidal thread turning tool must ensure sufficient root strength while avoiding interference with the crest of the lead screw nut. Therefore, during sharpening, the tool tip width must be calculated strictly according to the formula and measured using a caliper or a dedicated template. For trapezoidal threads with high precision requirements (such as screw threads), the tolerance of the tool tip width must be controlled within ±0.02mm to ensure the transmission accuracy of the thread.
The tool tip width of a rectangular thread turning tool depends on the thread pitch and tooth height. Rectangular threads have a rectangular profile with a tooth angle of 0°. The formula for calculating the tool tip width (root width) is: w = P – 2h, where P is the pitch and h is the tooth height (h = 0.5P + ac, where ac is the tooth top clearance, generally 0.25-0.5mm). For example, for a rectangular thread with a pitch of P = 10mm, ac = 0.5mm, and h = 0.5 × 10 + 0.5 = 5.5mm, the tool tip width w = 10 – 2 × 5.5 = -1mm (clearly unreasonable). This indicates that the tool tip width calculation for rectangular threads needs to be adjusted based on actual conditions. Typically, the root width of a rectangular thread is 0.5P – 0.5mm. For example, for a P = 10mm, w = 5 – 0.5 = 4.5mm. The tip width of the rectangular thread turning tool must ensure the strength of the thread root and at the same time make the thread have good force transmission performance. During grinding, the actual thread root width can be measured and adjusted to ensure that it meets the design requirements.
Sharpening and measuring the tip width of thread turning tools are critical to ensuring dimensional accuracy. Sharpening requires a grinding wheel and specialized tools. When sharpening triangular thread turning tools, use an angled template to ensure accurate tooth angles. Then, use a diamond pen to trim the end face of the grinding wheel to align with the tip width. The desired tip width is then achieved by crossfeeding. When sharpening trapezoidal and rectangular thread turning tools, a universal tool grinder is required to precisely control the position and feed of the grinding wheel to ensure accurate tip width. For fine-pitch thread turning tools (P ≤ 3mm), a tool microscope can be used to measure tip width, achieving an accuracy of up to 0.001mm. For large-pitch thread turning tools (P > 3mm), a vernier caliper or micrometer can be used, maintaining an accuracy of within 0.01mm. After sharpening, the tool tip should be passivated to remove burrs and sharp edges to prevent chipping during cutting. The surface roughness of the thread root should also be maintained at Ra ≤ 1.6μm.
The optimal selection of a thread turning tool’s tip width also requires consideration of cutting conditions and thread requirements. For rough threading, the tool tip width can be appropriately increased (0.05-0.1mm larger than the calculated value) to improve efficiency and expedite stock removal. For finish threading, the tool tip width must be strictly ground to the calculated value to ensure thread accuracy. For threads subject to impact loads (such as those used in lifting machinery), the tool tip width can be appropriately increased to improve root strength and fatigue resistance. For precision transmission threads (such as machine tool lead screw threads), the tool tip width must be strictly controlled to ensure transmission accuracy and flexibility. Furthermore, the tool tip width of a thread turning tool must match the thread’s tolerance grade. For example, the tool tip width tolerance for grade 6 threads is smaller than for grade 8 threads to ensure higher fit accuracy. Through proper calculation, precise grinding, and rigorous measurement, the tool tip width of a thread turning tool can be guaranteed to meet the required dimensions, resulting in high-quality threads.
