A new out-of-machine calibration technique for passive contact analog probes

The necessity of improving the quality of the products together with the pressure for reducing the production cycle has driven the manufacturers of parts and products to look for accurate but faster ways to perform the inspection tasks. Analog probes, also known as scanning contact probes, are being one of the most used solutions for coordinate measuring machines (CMM) users owing to their ability to collect large amounts of points and execute fast measurements. Likewise, the probe is one of the most important sub-systems of a CMM and is one of the main responsible for the accuracy of a measurement and of the flexibility of CMMs. However, most of the studies on scanning CMMs do not separate the performance of the probing system from other error sources of the CMM. Since analog probes are precision equipments themselves, their performance should be studied separately from the rest of the components of the CMM in order to characterise behaviour, independently on the machine where they are working, and, therefore, to improve their accuracy. This paper presents a new out-of-machine calibration technique with sub-micrometer repeatability for passive contact analog probes. The sub-micrometer repeatability is obtained from the use of kinematic couplings which, in this case, have the peculiarity of being indexing (able to be placed and interchanged in different positions). The proposed calibration system has been applied to two analog probes: a serial-kinematics 3-degrees-of-freedom probe and a parallel-kinematics 6-degrees-of-freedom probe. From the results of extensive experiments in and out of the CMM it can be concluded that the out-of-machine calibration technique is useful to determine the static errors of passive analog probes that are not able to be obtained from in-machine scanning tests. Finally, an optimisation process is proposed and applied to the parallel-kinematics probe obtaining a sensible improvement of its accuracy.