A New Kind of Parallel Mechanism Machine Tools with an Integrated Cartesian Guiding and Metrology Mechanism

A new kind of parallel mechanism machine called the CGT (Cartesian-Guided Tripod) for three-axis machining, CGP (Cartesian-Guided Pentapod) for five-axis machining, and CGH (Cartesian-Guided Hexapod) for six-axis positioning is presented on this paper. The CGT/CGP/CGH has two kinds of functional independent legs: the driving functional leg and the integrated Cartesian guiding/metrology functional leg. The platform displacements are guided and measured in Cartesian coordinate system using the integrated Cartesian guiding/metrology leg. A systematic comparison of the CGT’s performance relative to conventional machine tools was conducted. This comparison includes the footprint, transmission ratio, static rigidity and dynamic stiffness, geometric error sensitivity, and servo-controlled contouring accuracy. (key words: parallel kinematic mechanism, tripod, direct metrology loop, Cartesian guiding mechanism) Introduction The basic idea of the CGT/CGP/CGH is to separate the guiding and metrology functions from the driving function for every leg. The CGT/CGP/CGH has two kinds of separated and functional independent legs: the driving function leg and the integrated Cartesian guiding/metrology function leg (Fig.1). The number of driving function legs depends on the number of degrees of freedom (DOF). The integrated Cartesian guiding/metrology leg is purposed to guide and measure the platform motion in the Cartesian coordinate system. The integrated Cartesian guiding/metrology leg has the same DOF as the machine’s Cartesian DOF. The separated driving function legs and integrated Cartesian guiding/metrology leg are parallel-linked to form the workspace for the CGT/CGP/CGH. Figure 2 shows the preliminary design drawings for the integrated Cartesian guiding/metrology leg and the assembled CGT for a three-axis machine tool application. There is no real need for a six-axis machine (for multi-axis machining the sixth axis is redundant with spindle rotation) [1-4]. By adding two more driving legs and a universal joint, the CGT can easily be expanded into a five-axis CGP (Cartesian Guided Pentapod) without the redundant sixth axis (see Fig.3). A six-axis CGH (Cartesian Guided Hexapod) can be made in the same way by adding three more driving legs and a ball joint to the CGT. Footprint Ratios Table 1 lists and compares information on several well-designed high-speed machining centers gathered from the commercial market. Because there are so many axis arrangements for conventional machine tools, the configuration of each machine is also listed. The CGT has a Cartesian-axis-travel to floor-stand-length ratio of 1/5 ~ 1/6. Well-designed conventional machine tools in the commercial market have a ratio between 1/3 ~ 1/7. The reachable space of this CGT is slightly larger than a rectangular box. Therefore, the CGT has the same footprint quality as the conventional machines. Table 1: Comparison of the footprint quality Brand Machine Configuration Workspace (mm) Footprint (mm) Travel/Floor 1 CGT X/Y/ Z(moving column) 400x400x40