The open-closed principle also named the law of closure.
Every piece cannot be completely equal when a circum is divided into a number of equal pieces. it may have random error. But the sum of internal error from nose to tail is zero (0°and 360°is always coincide). Therefore, if we can satisfy the closed condition during measurement, the measurement error will be zero. In the condition which is absence of Cylindricity Measuring Instrumenct, we can achieve the goal of high precision measurement by “self-check”. For example, we can use this method when we measure the divided circle , cylindrical gear, cubic box, square gauge. The open-closed principle and the abbe principle are both important principles for length measurement.
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