A CMM has been developed which operates over a working volume of 50 * 50 * 50 mm, and achieves an uncertainty in 3D probing of ～100 nm. This miniature CMM is based around the concept of a metrology frame, mounted on a host CMM, with a miniature probe system held on the host CMM's ram. The probing system is rigidly connected to 3 orthogonal mirrors, the positions and rotations of which are measured using 3 dual axis interferometers (length, angle) and 3 dual axis angular sensors. Corrections for the mis-alignments of the interferometers, flatness errors of the mirrors and the performance of the miniature probe system are all determined in situ, by reference to the calibrated laser wavelength. This process performs a full error map of the CMM and requires only two artefacts: a precision sphere and a good quality optical cube. The error map is used online to determine the 3D position of the probe tip, based on measurements of the interferometers and angle sensing systems. The CMM is fully programmable and operates as a normal CMM, albeit with considerably improved accuracy. The design, manufacture and calibration of the CMM are described, followed by examples of measurements made with the machine and a determination of the uncertainty sources. This CMM is designed as the first step in bridging the gap between conventional (millimetre scale metrology) and nanometrology.
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