Roundness error and there are four main methods.

roundness measurement error and there are four main methods.
① minimum zone method: the outline of an inclusive circle radius difference measured radius of the smallest difference of two concentric circles as roundness error.
② method of least squares the circle: circular contour corresponding to the measured points to the square of the distance and the circumference of the smallest circle center of a circle, made inclusive measured radius of two concentric circular contour difference is the roundness error.
③ Minimum circumcircle method: apply only to the outer. Inclusive test circle contour and a minimum radius of circumcircle center of a circle, a circle made inclusive measured two concentric contour radius difference is roundness error.
④ maximum inscribed circle method: apply only to the inner circle. Connected to the test within a radius of two concentric circular contour and circular contour difference between the measured radius of the largest inscribed circle center of a circle, simulation testing the roundness error is made inclusive.