式中A——大、小三针的直径之差,mm B——大、小三针测量的读数之差,mm α——螺纹牙型角,(°) 测量时选用大、小两套三针,或用两套三根直径相等的优质钢丝(或新钻头柄替代)。为获得较高的测量精度,大、小三针的直径差应尽量大一 A - large and small three-needle diameter difference, mm B - large and small three-point difference between the measured reading, mm α - thread angle, (°) measurement of large and small two sets of three-pin Or replace with two sets of three high-quality steel wires (or new drill shanks) of equal diameter. In order to obtain higher measurement accuracy, large and small three-needle diameter difference should be as large as possible