Quick check error verification of coordinate measuring machines

Since CMM’s first became available 30 years ago, they have become the dominant measuring instruments in the field of dimensional metrology. Both for manufacturers and users there is a requirement to comprehensively evaluate their performance. These machines must be checked on installation and periodically during their operation. An increasing number of CMM’s are being placed in production line environments where there are potential threats to the machines accuracy such as dust, heat fluctuations, physical contact; hence the requirement for a daily quick check arises. Quick check procedures involve the use of mechanical artifacts, as the use of optical methods (laser interferometry) is not feasible due to the complexity and time of the set-up. There are many general artifacts for CMM checking on the market but for large CMM’s in excess of 1 m3 working volume there are few practical artifacts available for this purpose due to the inevitable trade off between artifact size and accuracy. This paper describes a quick check method in which CMM performance may be evaluated, involving the use of a telescoping ball-bar. This is a development of the principles used ball-bar technologies currently used for checking numerical controlled machines by extending the measuring range of the sensor and applying this to the CMM. The ball-bar system consists of two spheres where the centre distance can be adjusted, where the centre distance is being measured by a transducer or by interferometry. Due to the system consisting of an adjustable artefact, the system is aimed at checking a wider range of machine sizes, from small to medium sized CMMs. The system evaluates errors on the machine from the circular plots that were generated to simulate coordinate points taken on a circular sweep of a telescoping ball-bar. This paper describes methods in which squareness errors are quantified from the circular plot taken from the machine.