Diameter of measuring ball (with allowance)

For external gears, the greatest outer dimension, measured using two measuring elements. These measuring elements can be balls or cylinders. Their size is specified as the measuring element diameter D_M. For internal gears, the smallest inner measurement between the measuring elements is used.

The diametral measurement M_d with measuring element diameter D_M applies to the finished gear (actual test dimension). These values are also used to determine the section of the involute in which the flank lies. Therefore, they can be given as an alternative to the addendum modification coefficient or other test dimensions.