lobes we get very different results for the analytical stability lobes for y ¼ 01 and 451 reconfiguration positions (Fig。  9)。

These results would be relevant if the spindle–tool–tool holder is stiffer。 A similar variation in stability lobe diagram would also be observed if the structure of the machine tool is made more compliant。 Since an RMT is designed around a part family, a consistent dynamic performance of the machine across a part family is also desired。 The reconfiguration of an RMT will affect the dynamic performance significantly if the structural fre- quencies are dominant enough to cause chatter near one of the structural frequencies。 These results could have been verified by cutting experiments at lower spindle speeds (o1000 RPM)。 However, the machine does not  have enough power for such cutting tests。 The arch-type RMT is designed for end milling which is often carried out at high spindle speeds and, therefore, does not has enough power at low spindle speeds。

4。 Concluding remarks

The arch-type RMT was designed to achieve customized flexibility for a family of parts: cylinder heads for V6 and V8 automotive engines。 It is a 3-axis CNC machine tool that has an additional passive rotational degree-of-free- dom。 This paper describes the dynamic characteristics of the arch-type RMT as it is changed from one    reconfigura-

tion position (y ¼ 01) to another (y ¼ 451)。 In this case  the

performance of the arch-type RMT does not change much when the machine tool is reconfigured from one position to another。  This  is  because  chatter  occurs  at  640 Hz,       a

frequency that arises due to the spindle–tool–tool holder assembly。 A useful insight gained for designing RMT is to have a structure that is stiff enough at all reconfigurable positions such that the dominant frequency of the structure in the FRF is due to the spindle–tool–tool holder assembly。 This ensures a uniform performance of the machine tool across the part  family。

Various RMT design alternatives that were considered before selecting the arch-type RMT configuration were discussed in Section 2。1 (see Fig。 2)。 Those alternates were not selected because of the ease of use of the arch-type configuration and considerations apart from dynamics。 While in this particular case, we do not observe significant changes in dynamic performance as machine is changed from one reconfiguration position to another, if the spindle–tool–tool holder assembly was made stiffer, the arch-type RMT performance can vary significantly as the arch plate is moved relative to the Y-column。 Therefore, it is important to recognize the trade-offs between dynamic capabilities and other design considerations in the design of RMTs。

Acknowledgments

The authors are pleased to acknowledge the financial support of the NSF Engineering Research Center for Reconfigurable Manufacturing Systems (NSF Grant # EEC-9529125), and the assistance of Mr。 Steve Erskine with the machining experiments。

摘要在现代制造系统中由于产品种类和产品需求的快速变化可重构制造系统(RMS)面临诸多挑战。本文认为一个拱门型可重构机床(RMT)已经被建成来证明RMT设计的基本概念。拱门型RMT旨在实现定制的灵活性和包括一个被动自由度，它可以重新配置机器的零件。机器的运动和动态功能的介绍，包括不同配置机器的实验频率响应函数(FRFs)和稳定性的计算。拱门型RMT的频率响应函数和稳定性的比较揭示了不同的重新配置的位置几乎相似的动态特性。这些类似的特征出现是因为主导模式，颤动的发生是由于主轴刀架的装配。因此，为了确保一致的动态特性而不是重新配置，一个理想的对于RMTs的功能的动态设计是机器的结构频率,不如主轴的结构频率，占主导地位的工具和工具架。