DUOADD: a software to detect and export damages of 3D scanned objects

One of the most challenging problems in repairing a damaged object using a hybrid additive machineis to detect the missing material and place it in the precise position in the working area. This workuses octrees to discretize both the original CAD model and the 3D scanned geometry and to comparethem in the process of identifying the mismatching volumes. Firstly, the presented software recursivelysubdivides the bounding box of the nominal solid model into eight octants at every iteration, dividingevery edge in two parts, recursively building an octree representation. For each depth level of the octree,the program checks whether the nodes intersect at least one triangle of the scanned model and only inthis case the node is marked as part of the model. This process proceeds until the desired resolution isreached. After this first step, the result is a compound of cubes that represents the shell of the scannedmodel. Secondly, the software deals with the scanned surface, which has to be converted in an octree too.To compute a boolean difference between the two octrees it is necessary that they are fully filled, so theone representing the scan must be expanded into the inside of the component. For this reason, DUOADDuses a ray–triangle intersecting algorithm to classify the leaf nodes of the octrees that are inside the shelland those outside the shell. If the number of intersections between the ray — traced from the node center— and the mesh is odd, then the node belongs to the volume inside the meshed surface. Otherwise, itbelongs to the surrounding volume. The data structures of the two octrees need to be consistent — i.e.they must have a coherent number of levels and leaves. An iterative algorithm has been developed inorder to uniform the octrees data structures. To perform the boolean comparison, DUOADD scans overthe data structure of both octrees at the same time and checks — with a NOR operation — whether thenodes are marked as “inside node” in both the octrees or not. The result of this comparison representsthe damaged spots. Once the analysis result is obtained, DUOADD joins all the nodes representing thedamage into a single solid body that can be exported in various file format as STEP, IGES or VTK.DUOADD allows to approach the repair problem from a new point of view and provides a powerful toolto obtain a solid model used to add the new material. Authors successfully employed DUOADD to repairdie injection molds and there are many other applications that could benefit from the use of this software.Many improvements are expected in the near future such as the binary encoding of the octree structureand the smoothing of the exported solid body shape.

The recent appearance of hybrid CNC machines – which can both add and remove material to an object – has facilitated a new type of approach to the problem of repairing damaged components. Up to now, repair operations have been carried out manually and for this reason they are errors prone, costly and time consuming. The use of precise and repeatable CNC machines in this field is therefore very attractive for the sake of reliability and repeatability. One of the biggest obstacles on the automation of the repairing process is that the CAM software requires a solid CAD model to create the toolpaths needed to perform additive operations. The present work proposes the use of octrees to detect the damaged spot starting from the 3D scan of the damaged object. A software named DUOADD has been developed to convert these informations into a CAD model suitable to be used by the CAM software. The new workflow designed to perform a complete repair operation is here described. DUOADD allows to approach the repairing problem from a new point of view opening the doors to save both time and money. The successful application of the entire process to repair a damaged die injection mold is also here reported.§ DUOADD = DUOADD Uses Octrees As Damage Detector