Abstract: In the universal tool microscope, there would be tilted at an angle for the column of the central microscope by using image method to measuring the parameters (for example: pitch, half of thread angle etc.) of conical outer thread. And the difference from measuring the common cylindrical outer thread is that the angle called the mean spiral angle is a mean value. The three kinds of calculation formulas with their derivation processes for the mean helix angle are introduced.

Key words: Taper thread, Spiral angle, Calculation method, Mean value