Low Cost Reverse Engineering Techniques for 3d Modelling of Propellers

In aeronautics and shipbuilding fields quality control of propellers is an obligatory step, a hard task up to some years ago: the shape of each blade has to be measured with high accuracy and referred to a global frame. Specific CMMs were used in the past to perform such complex analyses. When propellers were mounted on ships on dry dock, inspections became nearly impossible to be performed. In this work, both photogrammetric and laser scanning low cost methodologies for 3D reverse modelling of complex objects with high level of detail are proposed. Three photogrammetric software for both commercial and research uses, have been tested in this study for generating high density point clouds. The feasibility of using a low cost triangulation scanner for high accuracy purposes has been studied in the case of small screw propellers. The problem of bringing together all the measurements in a single reference frame with high accuracy is solved by means of colour information. The study shows that a hybrid approach that integrates both photogrammetry and laser scanning methods is necessary. Screws of different sizes, surveyed in laboratory and on the field, such as in dry docks, are modelled and inspected. Different procedures for determining geometrical parameters are shown. Geometrical characteristics derived by 3D models of screws, are compared both with those expected by original plans and CMM measurements.