每个定位块-工件接触对在z方向的总偏差为?Zi?(?zi)L?(?zi)W。将定位块-工件接触对(?Z1,?Z4)和(?Z2,?Z3)的连线的中心高度定义为?Z1?4和?Z2?3，分别有：?Z1?4?0.5(?Z1??Z4)和?Z2?3?0.5(?Z2??Z3)，如图23所示。显然，处于更高位置的定位块-工件接触对实际与工件主定位面始终保持接触。如果；如果?Z1?4??Z2?3，则实际工件与定位块的接触对为?(L1,W1),L(4,W?4)，则实际工件与定位块的接触对为?(L2,W2),(?Z1?4??Z2L3,W3)?，而如果?3?Z1?4??Z2?3，则工件实际与四个定位块都保持接触。
图23 定位块-工件接触对的确定
显然，倘若?Z1?4??Z2?3，则工件的定位方式仍然不确定（还有2种可能），例如，若?(L1,W1),(L4,W4)?始终保持接触，则还有?(L1,W1),(L2,W2),(L4,W4)?和?(L1,W1),(L3,W3),(L4,W4)?两种可能的定位方式。接下来，利用最小势能原理（the principle of minimum potential energy）可以确定工件稳定平衡的状态。利用公式66，计算剩下的两种可能定位方式定位的变形和夹紧力，分别计算出其各自的势能：
1???Fi?zii?124(69)
通过比较最终可以得到工件在4-2-1定位下实际的定位情况，并最终演化为一个3-2-1的定位方案。因此，可以利用此前关于3-2-1定位的偏差分析方法分析此时装夹的偏差。而对于?Z1?4??Z2?3，即工件与四个定位块均都保持接触的情况，可以任意在主定位面的4个定位块中选择三个，组成3-2-1定位进行偏差分析，唯一的区别在于，在计算柔性偏差时（公式66）需把边界条件设置为4个定位块同时与工件保持接触。综上所述，得到一种4-2-1定位下的刚柔综合的装夹偏差分析模型。
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