Industrial robots are widely used for part manufacturing besides simple task (welding, assembly). A parallel kinematic machine (PKM) with extending axes have been utilized in large volume machining because of their adequate stiffness and agility. Parallelism error in the PKM with an extending axis causes deterioration of dimensional accuracy of machined parts. This paper proposes a technique for compensating the parallelism error through measurement of the squareness error between the PKM with its extending axes using a laser interferometer. The four squareness errors are estimated to reduce the parallelism errors. The squareness error is calculated by measuring linearity of the extending axis and the PKM moving axis, and through the measurement of diagonal displacement error and position dependent geometric errors. Compensation of the parallelism error was done by transforming the basic coordinate system of the PKM. The parallelism error was significantly reduced from 0.735 to 0.022 mm and further verified experimentally.

The parallel kinematic machine (PKM), which is applied Exechon mechanism, is efficiently used for manufacturing industry due to its agile movement, flexibility and high rigidity. On-Machine measurement (OMM) in high-dof manufacturing machines such as the PKM and five-axis machine tools has importantly used for processed part measurement, coordinate system set and machine performance evaluation. In this study, measurement and compensation of touch probe offset, which occurs measurement error of the OMM, are carried out for the PKM. A dependent rotational motion is occurred due to kinematic constraint, and causes non-constant offset of a touch probe. The dependent rotational motion is calculated via inverse kinematics analysis. The probe offset is accurately measured using a master ring with considering the analyzed dependent rotational motion angle. In addition, measurement procedure to eliminate the offset induced measurement error is presented. To verify the proposed technique, circular tests using a master ring and commercial touch probe on the PKM were performed. Circularity measurement deviation of a master was reduced 65% without the PKM’s kinematic error calibration.