圆跳动——同轴度 （Runout - vs Concentricity）
http://www.dimcax.com/gdt_web/june-97.htm
圆跳动由圆度和同心度两项公差要求组成，如图A所示，被测特征若要求是圆的且是同心的，优先选用圆跳动。
同轴度仅仅是给出被测特征的中心轴线相对于基准轴线的误差变动范围，如图A、B、C、D所示，被测特征可以是具备对称性的椭圆、正方形、六边形等。
Avoid specifying concentricity. Concentricity requires deriving the median line of a feature. All of the features shown below are concentric. Usually, designs require that a feature be round as well as concentric like example A. A better geometric control is usually circular runout. Circular runout controls circularity (roundness) as well as concentricity.