测量小于半圆的圆弧半径时,经常采用弦高法。这里介绍一种按弦高法原理制成的大尺寸R尺。该尺由圆柱、连杆、轴及螺钉、螺帽构成(图1),各零件尺寸可根据所测工件尺寸范围而定。测量时,使三圆柱与所测之圓弧相切后固紧滚轮螺钉4,取下该尺,并测出L、H值(图2),由下式可以算出R尺寸: When measuring the radius of a circle less than a semicircle, the chord height method is often used. Here introduced a method according to the principle of chord made of large-size R ruler. The rule by the cylinder, connecting rod, shaft and screw, nut (Figure 1), the size of each part can be measured according to the workpiece size range. When measuring, tighten the roller screw 4 after tangential to the measured circular arc of the three cylinders, remove the ruler and measure the L and H values ​​(Figure 2). The R dimension can be calculated as follows: