Design and analysis of a tripod machine tool with an integrated Cartesian guiding and metrology mechanism

This paper introduces a novel parallel kinematic mechanism (PKM) machine called Cartesian-Guided Tripod (CGT) developed for high-speed machining of precise dies and molds. CGT has two kinds of functional independent legs, the driving functional leg and the integrated Cartesian guiding/metrology functional leg, parallel-linked to the platform. The guiding/metrology functional leg constraints and measures the platform motions in Cartesian coordinates. Because of the separation of metrology function from the drive mechanisms, the metrology loop of CGT is immunized to the geometric-, thermal-, and force-induced errors of the drive mechanisms. Another feature of this CGT is that the kinematic parameters can be auto-calibrated on line. CGT has explicit solutions of the inverse and forward kinematic analysis. This allows system specifications, such as the footprint/workspace ratio, maximum Cartesian speed/acceleration and even stiffness, be easily related to the machine component specifications. The CGT drive mechanism can be divided into two basic types: sliding leg and telescoping leg. The telescoping leg CGT has the higher edges in the kinematic categories than the sliding leg CGT. Conversely, the sliding leg CGT has better static rigidity and dynamic stiffness than the telescoping leg CGT.