Int J Adv Manuf Technol (2000) 16:23–31  2000 Springer-Verlag London Limited CAD-Based Measurement Path Planning for Free-Form Shapes Using Contact Probes I. Ainsworth, M. Ristic and D. Brujic Mechanical Engineering Department, Imperial College of Science, Technology and Medicine, London, UK Dimensional inspection of engineering components comprising free-form surfaces demands accurate measurement of a large number of discrete points, such that the actual shape may be fully characterised. This paper presents a methodology for CAD-based measurement of such components using a coordi- nate measuring machine equipped with a touch-trigger probe. The main shortcomings of the conventional methodology have been identified to be in relation to registration and probe radius compensation. The proposed measurement process involves the following main steps: registration, definition of measurement points, probe path generation, path optimisation and verifi- cation, measurement and probe radius compensation. By employing the CAD model at every step, the implemented methodology maximises the measurement accuracy and this is verified through a detailed simulation study. In addition, the implemented tools for CMM programming achieve accurate control of the overall measurement process and provide a high level of confidence when dealing with complex component geometry. Keywords: CAD; CMM; Free-form shape; Measurement; NURBS; Registration 1. Introduction This paper attempts to demonstrate an efficient, yet flexible, measurement planning methodology for components that consist partly or solely of free-form surfaces. The motivation for this work has been provided by the needs of modern manufacturing industry, particularly in the aerospace and the automotive sec- tors. Inspection of free-form components such as aero-engine turbine blades and car body panels involves the measurement of a sufficiently large number of points, with an appropriate point distribution, to enable a subsequent comparison to be made of the actual shape in relation to its nominal model. As Correspondence and offprint requests to: I. Ainsworth, Imperial College of Science, Technology and Medicine, Mechanical Engineering Department, Exhibition Road, London SW7 2BX, UK. E-mail: i.ainsworthic.ac.uk part of this process, it is also becoming increasingly important to reconstruct the actual shape in a required CAD format [1] and to analyse the component performance using the available aerodynamic, structural and other computational analysis tools. Computer controlled coordinate measuring machines (CMM), equipped with touch-trigger probes, represent the standard mea- suring instrument for dimensional inspection. In spite of a wide range of non-contact measuring systems becoming widely available, the CMM continues to be the first-choice solution for the metrology practitioners, owing to its high accuracy and the widely available operator skills [2]. Measurement accuracies of the order of 3–5 m are readily achievable using modern CMMs. Measurement of free-form shapes using a CMM and a contact probe poses considerable difficulties [3,4]. In order to capture the shape, it is necessary to perform dense measure- ments, where the required local point distribution is dictated by the local curvature, tolerances, and other factors. The com- monly used “teach-by-showing” method for CMM program- ming is inadequate in such situations and it is necessary to employ CAD-based techniques for probe path generation, verification and collision avoidance [1,5]. Indeed, there are many commercially available packages for CAD-based CMM programming, such as CAMEO, SILMA, ICAMP, Origin and others. However, many of these packages are limited to per- forming “teach-by-showing” off-line, with the tools for auto- matic generation of measurement positions being limited in their ability to control adequately the local point density and to capture adequately the surface shape. The accuracy achieved in performing measurements on free- form shapes using current methods and tools also causes concern. The root cause of these problems is the related issue of probe radius compensation and registration. Since the contact probe tip is a sphere of a given radius, the raw measurement data, represented by the ball centre positions, must be compen- sated by introducing a correct offset in the direction of the surface normal at the point of contact. Current methods for probe radius compensation are based on the assumption that the probe makes contact with the surface at exactly the pre- scribed point, so that the surface normal vector is known. However this assumption is generally not true in the presence