The 3-2-1 constraint principle has been widely applied as the boundary conditions for the finite element method (FEM) to simulate machining deformation of aerospace structure components. However, this principle is inconsistent with the actual contact surface between workpieces and worktables because it provides only three constraint points. These points have rarely been studied in terms of positions and distances. In addition, the applicability of the principle is limited for the workpiece with geometric centers difficult to find or centers without nodes. Therefore, this study proposed a new boundary condition method, low-stiffness spring element constraint method (SECM), drawing on FEM theories and related mechanic theories. With the method proposed this study established the FE model of machining deformation, and then compared the simulation results with both the analytical results and the experimental results of milling and deformation measurements. Good agreement is found between the three results. Finally, this study examined the effect of the three constraint points on simulation of the 3-2-1 principle in terms of point positions and distances. A comparison between SECM and the 3-2-1 principle revealed that SECM is closer to the actual working conditions and more reliable with wider application, which suggests that SECM can replace the 3-2-1 principle as the boundary conditions for the workpiece.

3-2-1约束原理已被广泛用作有限元方法（FEM）的边界条件，以模拟航空航天结构部件的加工变形。但是，该原理与工件和工作台之间的实际接触表面不一致，因为它仅提供了三个约束点。这些点很少在位置和距离方面进行研究。另外，该原理的适用性限于具有难以找到的几何中心或没有节点的中心的工件。因此，本研究借鉴有限元理论和相关的力学理论，提出了一种新的边界条件方法，即低刚度弹簧单元约束方法（SECM）。通过提出的方法，本研究建立了加工变形的有限元模型，然后将模拟结果与分析结果以及铣削和变形测量的实验结果进行比较。在这三个结果之间找到了很好的一致性。最后，本研究从点位置和距离的角度考察了三个约束点对3-2-1原理模拟的影响。SECM与3-2-1原理的比较表明SECM更接近于实际工作条件，并且在更广泛的应用中更可靠，这表明SECM可以代替3-2-1原理作为工件的边界条件。这项研究从点位置和距离的角度考察了三个约束点对3-2-1原理模拟的影响。SECM与3-2-1原理的比较表明SECM更接近于实际工作条件，并且在更广泛的应用中更可靠，这表明SECM可以代替3-2-1原理作为工件的边界条件。这项研究从点位置和距离的角度考察了三个约束点对3-2-1原理模拟的影响。SECM与3-2-1原理的比较表明SECM更接近实际工作条件，并且在更广泛的应用中更可靠，这表明SECM可以代替3-2-1原理作为工件的边界条件。