In order to solve the problems of DOF and DOF number which needs to be constrained when the workpiece is located, and position-element can constrain, this paper analyzes the cases of typical position and position-elements. It indicates: DOF and DOF number which needs to be constrained is determined by the geometric features of location datum, DOF and DOF number which the position-element can constrain is determined by the geometric features of essence contact surface between the position elements and the location datum, the DOF number is three, two and one when the datum and contact surface is surface line and point.%为解决工件在定位时需要约束的自由度数和自由度以及定位元件约束工件自由度数和自由度的问题，应用六点定位原理，对各种典型定位实例和定位元件进行了分析。分析表明，工件需要约束的自由度数和自由度由所选择的定位基准几何特征决定，而定位元件约束的自由度数和自由度也由定位元件与定位基准实质接触面几何特征来决定，且其个数都遵循“面三、线二、点一”的规律。
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