通常,用三针法测量外螺纹单一中径,用钢球法测量内螺纹单一中径。用直径相同的钢球和三针来推导测量外螺纹单一中径的精确公式,尽管思路、推导过程及所得公式不同,但其实际结果相同。由于用钢球推导比用三针推导简单,且可对外、内螺纹同时进行公式推导(而用三针只适于外螺纹的公式推导),所以本文用钢球测量进行推导. Normally, a single intermediate diameter of the male thread is measured by the three-needle method and a single intermediate diameter of the female thread is measured by the steel ball method. The exact formula for measuring the single diameter of the male thread is deduced from the same diameter steel ball and three needles. The actual results are the same, although the ideas, the derivation process and the formulas obtained are different. Because the derivation of steel ball is simpler than that of three-pin derivation, and the formula derivation can be carried out simultaneously with the external thread and the female thread, but the derivation is based on the ball measurement.