This project is started to convert complex mechanical CAD designs to X3D for convenience of online browsing. The x3d_step_assy.py runs in FreeCAD and processes STEP models created by other mechanical CAD programs.
- Update: Here is the result of such conversion of 800+ parts and a few assemblies: http://wiki.elphel.com/index.php?title=Elphel_camera_assemblies
This macro converts assembly CAD model to X3D. It tries to recognize individual parts (provided as STEP files) in the assembly model, converts each part to X3D and then generates assembly X3D file that includes inline references to the recognized part files, applying appropriate transformations (rotations and translations).
First thing the program does is it scans all the STEP models under the specified directory and collects general properties of each file, including volume, surface area, center of mass, gyration radii and axes, as well as per-color centers. Normally each part should contain just one solid, but if there are more than one only the largest (by volume) will be used for identification in the assembly (in that case assembly may show multiple not matched solids that will still be correctly rendered in the final model with each part).
This information will be saved in 'info' directory under the specified working directory, same file name as the original STEP model but with extension '.pickle' (and yes, they are just Python pickle files). These files are saved in one directory, so each original part file have to have unique name, even when stored in different directories. This file basename (last segment of the OS path without the extension) will be used as a part name and used in 'id' and 'class' properties of the result x3d files. The program only processes the part files if the corresponding info file does not exist or has the modification timestamp earlier than the STEP model.
During the next step the assembly object is analyzed and the same properties are extracted for each solid, then the each is compared to the library part and the parts with the same values (to the specified precision) are selected as potential candidates. Parts material is not used, so distinguish between similar screws that have the same geometry the color may be used. This allows to find the position of the center of volume of the part in the assembly, but getting the correct orientation is trickier. For the asymmetrical (having all 3 different radii of gyration) it is rather easy (only 4 variants to check as the gyration axes can have opposite direction), it also works for the parts with full cylindrical or spherical symmetry where the axes match is not required, but it is more difficult to deal with the discrete rotational symmetry. When resolving such cases the program relies on colored faces of the parts. Coloring just a single hole (not on the axis of the symmetry) in the part (and then using it in the assembly) breaks ambiguity. Parts that do not have faces that can be easily colored can be modified with boolean operations that preserves the shape but add color asymmetry
When the solids are matched, the program generates missing/old (by timestamp) x3d files of the individual parts and assembly in the 'x3d' subdirectory of the working directory. It also generates and shows the parts that are not recognized (they might be 'other' solids of the part files and so will be available in the generated model).
This method can work with most modern CAD systems, and does not require special export - the colored STEP files are still good for production. In some systems the assembly model should be flattened (removed assembly status) before STEP export, it is also advised to import individual parts that are provided to you as STEP models to the CAD that is used for the assembly and re-exporting to STEP so both part and assembly STEP files will be generated by the same software. )