Interim Project Report!

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1 Interim Project Report Innovative software for 3D printing Project Website: Student: Zhejiao Chen Supervisor: Prof. Francis Lau Department of Computer Science University of Hong Kong 1 of 11

2 1. Background and Objective 3D printing technology is a developing technology which has been around since 1980s. A more technical name for 3D printing is rapid prototyping(rp). The professional literature in RP contains different ways of classifying RP processes. One representation based on German standard of production processes classifies RP processes according to state of aggregation of their original material. A few important RP processes are Stereolithography (SL), Selective Laser Sintering (SLS), Fused Deposition Modeling (FDM) and Laminated Object Manufacturing (LOM). For example, in SLS, fine polymeric powder is spread on the substrate using a roller; in FDM, a movable nozzle above a substrate deposits thread of molten polymeric material[1]. In this project, we focus on FDM which is most obvious in terms of layer by layer processing. Two important factors to consider are orientation and slicing. Due to slicing, the staircase effect which we want to eliminate is inevitable; however, some orientation can minimize the shortcoming which is caused by slicing either directly or indirectly. For example, part deposition orientation can affect surface quality and build time. A cube printed standing on one of its six surfaces will have no staircase effect on the bottom face at all; however, if printed with an angle between the one of its six faces and the XY plane, then the staircase effect can be prominent on all faces[2]. This is shown in Figure 1.1. Since the build time is dominated by the number of slices needed, build time reduction is achieved by considering an orientation that offers minimum number of slices among few pre-selected orientations. 2 of 11

3 As for the development environment, Google SketchUp is a 3D modeling tool which understands the Ruby language. To extend the functionality of SketchUp, we use SketchUp Ruby API to create a Ruby script to run on SketchUp. The objective of this project is to implement a Google SketchUp plugin that addresses the two important factors mentioned above orientation and slicing. This plugin can slice a model in any orientation under a certain slice thickness. However, this step can only help the user to visualize the staircase effect. In order to further help the user to compare the surface quality and build time of different orientations, a report is generated to help quantify the differences among different combinations of orientation and slice thickness. Figure 1.1 Orientation and Staircase Effect 3 of 11

4 2.Methodology (Theory and Approach) 2.1 Related work Massod and co-workers [3] [4] [5] used a volumetric error based approach to determine suitable part deposition orientation. In this methodology, difference in the volume of the part deposited using uniform slices and the CAD model of the part is minimized for selecting a suitable orientation. As shown in Figure 2.1, the black area is the difference in volume of CAD modle and LM part. Here the build edges of the slices are assumed to be rectangular. This methodology involves a primitive volume approach, which assumes a complex part to be constructed from a combination of basic primitive volumes. More recently Massod [5] presented a generic approach in which tessellated CAD models are used in place of basic primitives which forms a part. Best part deposition orientation for tessellated CAD model is obtained by minimizing the volumetric error [3] [4]. Figure 2.1 Massod a Volumetric Error Based Approach 4 of 11

5 2.2 Model representation in SketchUp In order to get our hands on the model created in SketchUp, we first have to understand the relationship between SketchUp and Ruby API, and how a model is represented in SketchUp using the Ruby language. SketchUp commands are composed of SketchUp-specific terms, but the underlying language is Ruby. Ruby is a programming language whose primary use has been in the development of web applications (Ruby on Rails). Nearly every SketchUp script starts by accessing three basic data structure: Sketchup, Model and Entities [6]. The methods in the Sketchup module access properties related to the Sketchup application as a whole. The most important method in the Sketchup module is active_model. This returns the Model object corresponding to the currently open SketchUp design. The Model object represents a single SketchUp file (*.skp) which is a file containing the design information. It is like a single object containing multiple compartments, and each compartment may contain multiple objects. Figure 2.2 shows six of the most important object drawers contained within a Model object. The Entity class is the superclass for all drawable shapes in SketchUp. Figure 2.3 shows some of the hierarchy of Entity subclasses. In SketchUp, everything is made of one of two kinds of things: edges and faces. They are the building blocks of every model you make. For example, a basic cube is composed of 12 edges 5 of 11

6 and 6 faces. Edges are always straight. Arcs and circles are made of small straight-line segments. Faces are always flat. Surfaces that look curved are made up of multiple flat faces. Figure 2.2 Content of a SketchUp Model Object (Abridged) Figure 2.3 The Entity Class Hierarchy (Abridged) 6 of 11

7 2.3 Slicing model in any orientation After the model is created, it should first be made as a Group which is a collection of Entities objects. When operating on a Group, it operate on all of its Entity objects at once. This will make it possible for user to Rotate the whole model to a specified orientation, because a Group is treated as a whole instead of a bunch of Entities. Then the slice thickness is chosen by the user and the model is going to be sliced in Z direction just like the real-time scenario in which the layer is deposited in the XY plane and build up in +Z direction. The plugin starts by selecting faces of the model from the bottom up. The distance of the gap between two adjacent faces is the value of slice thickness chosen by the user. After all the faces are selected, call PushPull method on each face to extrude all faces in the +Z direction with distance equal to slice thickness. 2.4 Quantify volumetric error The volumetric error based approach used to determine suitable part deposition orientation presented by Massod provides us with a feasible way to quantify the surface quality. In most cases, the volume of the original model and the volume of the sliced model will be different. The difference of the two volumes is used to identify volumetric error and treated as an indicator of the surface quality. To further specify this approach, let the volume of original model be V(o), the volume of the sliced model be V(s), then the difference is V(o) - V(s). In the report, the value of V(o), V(s), V(o) - V(s) and V(o) - V(s) / V(o) are presented. To compare the same model with the same slice thickness under different orientations, V(o) - V(s) / V(o) will be a good indicator of the 7 of 11

8 surface quality. The smaller the figure is, the better the surface quality will be. Additionally, the number of slices is reported as well to indicate the build time. The larger the number of slices is, the longer the build time will take. 3. Achievements in the First Semester 3.1 Selecting faces in the model The general steps to slice a model are done as shown below: Call the method local_bounds on the model which is a group. The return value is an object called BoundingBox which is a three-dimensional box, alighted with the global axes, only large enough to exactly bound the model Group. Based on the height of model and the slice thickness, the number of slices needed is derived. Loop from the bottom to the top of the BoundingBox to create all the faces needed for later cutting. These faces have the same shape as the bottom/top face of the BoundingBox. The distance between any two adjacent faces is the value of slice thickness. All the faces form an Entities object. Use the cutting faces created just now to intersect with a copy of the model (Entities.intersect_with). This will create faces which do not exist before inside the copied model. After this is done, although you can only see some edges adding to the surface of the copied model, with the function Sketchup.active_model.selection.length which counts the total number of edges and faces, you can actually tell there is a face in the middle. 8 of 11

9 Loop through all the entities of the copied model to find all the entities which are both Face object and parallel to the XY plane. Call PushPull on each face with slice thickness as the parameter to create real layers. 3.2 Webdialogs as UI We created a WebDialog which is a combination of HTML and JavaScript to function as the user interface. Inside the plugin script, Observer classes are created and associated with the entities in the model. Whenever the user change the entities in the SketchUp model, the Ruby script will invoke the JavaScript function inside the WebDialog through the Observer. Thus, the changes in SketchUp model is shown in the WebDialog. The reverse way of using WebDialog to change model in SketchUp can also be achieved using a callback function. 4. Plan for Second Semester 4.1 Calculating volumetric error and UI enhancement It is easy to calculate the volume of some primitive 3D objects like cube with the help of some useful API functions. However, since the model created by the user can be in any shape, many models can not simply be made up of various primitive 3D objects; rather, they are PolygonMeshes which are essentially an array of polygons, and each polygon is an array of points. A method has be to find to calculate the volume of objects like these. On the other hand, since the sliced model are essentially multiple polygon prisms, their total volume is easier to 9 of 11

10 calculate. All the functionalities will be delivered to user through UI, thus a well designed UI is important and its appearance will be improved along the whole way. 4.2 Testing, debugging and improvement Testing will be carried out on the plugin using different models with different orientations and slice thickness. Specifically, the orientation comparison results generated by the plugin based on volumetric error can be tested against some existing experiments results. However, a potential problem would be to simulate the model used in the existing experiments accurately. A number of random tests should also be carried out to ensure that the plugin is bug free. Additionally, during the testing stage, slight changes and extra small functions can be made to improve the quality of the plugin from every possible aspect. 10 of 11

11 References 1. Pandey, P.M., RAPID PROTOTYPING TECHNOLOGIES, APPLICATIONS AND PART DEPOSITION PLANNING. 2. Bablani, M. and A. Bagchi, Quantification of errors in rapid prototyping processes, and determination of preferred orientation of parts. TRANSACTIONS-NORTH AMERICAN MANUFACTURING RESEARCH INSTITUTION OF SME, 1995: p Rattanawong, W., S. Masood, and P. Iovenitti, A volumetric approach to part-build orientations in rapid prototyping. Journal of Materials Processing Technology, (1): p Masood, S., W. Rattanawong, and P. Iovenitti, Part build orientations based on volumetric error in fused deposition modelling. The International Journal of Advanced Manufacturing Technology, (3): p Masood, S., W. Rattanawong, and P. Iovenitti, A generic algorithm for a best part orientation system for complex parts in rapid prototyping. Journal of materials processing technology, (1): p Scarpino, M., Automatic SketchUp: Creating 3-D Models in Ruby of 11