Tolerance Graph for Tolerance Control in Process Planning

这项拨款为在数控（NC）加工中离散机械零件设置规划的自动公差控制的理论方案的发展提供资金。提出了基于图论的公差控制方案。零件的特征用顶点表示，特征之间的参数公差和几何公差用边表示。零件的整个设计规格由超图或公差图表示。因此，将最优设置规划问题转化为图搜索问题。根据各自的设置方法，在分析最小设置误差的基础上，开发了优化算法。数控加工的特点被纳入公差图。最后，建议的研究将被实施到一个计算机程序，并将与计算机辅助设计系统集成。如果这项研究取得成功，将产生两大贡献：(1)为工艺规划中的公差控制奠定了坚实的理论基础和科学严谨的基础；(2)将发展出一种公差绘图技术，最终取代有限公差绘图技术。借助公差图，可以有效地解决与规划相关的公差控制问题。因此，可以以较低的成本生产精密零件。提出的公差控制算法为工艺设计和分析现有工艺方案提供了一种实用的方法。具体来说，通过使用公差图，可以(1)选择合适的设置、定位基准和操作顺序，以最小化公差叠加；(2)计算加工切割的操作尺寸和公差；(3)同时处理所有轴的公差。作为额外的好处，本研究采用了基于科学的方法来解决公差控制问题。基于科学的方法更容易实现计算机化，以促进计算机化公差的实现。

This grant provides funding for the development of a theoretical scheme for automated tolerance control of discrete mechanical part setup planning in numerically controlled (NC) machining. The proposed tolerance control scheme is based on graph theory. The features of a part are represented by means of vertices while the parametric and geometrical tolerances assigned among the features are represented by edges. The entire design specifications of a part is represented by a hyper graph or tolerance graph. Hence, an optimal setup planning problem is transformed into a graph search problem. An optimization algorithm will be developed based on analysis of the minimum setup errors according to the respective setup methods. Characteristics of NC machining are incorporated within the tolerance graph. Finally, the proposed research will be implemented into a computer program and will be integrated with a computer aided design system. If successful, this research will yield two major contributions: (1) It will establish a theoretically sound foundation and scientifically rigorous base for tolerance control in process planning, and (2) It will develop a tolerance graphing technique that will eventually replace the limited tolerance charting technique. With the help of a tolerance graph, planning related tolerance control problems can be solved effectively. As a result, precision parts could be produced at reduced cost. The proposed tolerance control algorithm would provide a practical approach for process design and analyzing currently existing process plans. Specifically, by using a tolerance graph, one can (1) select appropriate setups, locating datums, and operation sequences to minimize tolerance stackup, (2) calculate operational dimensions and tolerances for machining cuts, and (3) simultaneously deal with tolerances along all axes. As an additional benefit, this research adopts a science-based approach to solving tolerance control problems. Science-based approaches are much more easily computerized to facilitate the realization of computerized tolerancing.