The National Physical Laboratory (NPL), the UK’s National Measurement Institute is offering the UK’s first service the verification of Articulated Arm Co-ordinate Measurement Machines (CMMs) to the ASME B89 standard used by arm manufacturers.
Articulated arm (or portable) CMMs can achieve uncertainties of a few tens of micrometres and are used in industry for inspecting parts and reverse engineering. Unlike traditional inspection equipment they allow the measurement system to come to the part and are becoming an increasingly common sight next to assembly lines in high technology factories in industries such as aerospace, automotive and motorsport. NPL and industry recognised that there was a lack of UK-based support for these devices and has addressed this need through a new service. This will be the first service in the UK offering verification of articulated arm CMMs to the recognised and frequently used ASME B89 4.22 standard. This standard is the basis of the manufacturers’ certification. An application has been made for UKAS accreditation of the service.
NPL’s Engineering Measurement team designed a specialised calibration facility for these devices. It comprises a unique and highly stable precision-engineered granite structure, weighing around four tonnes. The granite supports both the instrument being tested and a pair of calibrated length artefacts made from carbon-fibre tubes, selected for their lightness, stiffness and low coefficient of thermal expansion. These artefacts are 2.4 m long and sit on kinematic mounts bolted to the granite structure. Each artefact has 19 magnetic nests defining all possible measurement positions for articulated arms that can range in size from about 1 m to 4 m in length. NPL manufactured and assembled the artefacts, including bonding the nests to the tubes. Following calibration by CMM and laser tracker, each nest was found to have been positioned to better than 100 ìm.
As articulated arm CMMs are used on the shop floor, the service has to be fast to avoid production downtime. So NPL produced specially written software for guiding the operator through the test process, recording and analysing the measurements and then automatically generating the certificate.
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