MetaMesh import from SolidWorks and Alibre
During the slow time over the holidays, I had an opportunity to investigate the transfer of geometric information from two solid-modeling programs: SolidWorks® and Alibre®. The medium for data transfer is the neutral STL (stereo-lithography) format. In principle, it should be possible to work with all 3D CAD programs. My tests showed that the quality of STL export can vary considerably.
On the positive side, there is a close correspondence of concepts between MetaMesh, SolidWorks, Alibre and the STL format. In the three programs, the PART is the basic building block. A PART is a contiguous solid body with a single surface. The topological definition of single surface is that you can reach any point on the surface by a path that lies on the surface. Within this definition, the surface of a PART may be quite complex, including through holes, convolutions, etc. The STL format defines a PART through a set of triangular facets on its closed surface. A valid STL file may contain only a single PART (i.e., one contiguous surface).
There are two relevant working spaces in SolidWorks and Alibre:
- PART. Fabricate and modify a single part in a local coordinate system.
- ASSEMBLY. Shift and orient multiple PARTs to create a system with complex objects.
These environments mirror the WorkBench and Mesh spaces of MetaMesh (Chap. 16 of the manual).
The utility of a solid modeler for MetaMesh input depends on whether STL files may be created from information in the PART and/or ASSEMBLY spaces.
- When an STL object is exported from the PART space, it contains information on the potentially complex surface of the object, but does not contain information about absolute position and orientation in an assembly. In this case, it is necessary to set the object in place using the SHIFT and ROTATE commands of MetaMesh. This task may involve extended calculations and/or trial-and-error.
- In principle, export from the ASSEMBLY space gives absolute facet coordinates that specify the position and orientation of the object. In this case, the objects may be directly included in the MetaMesh script with no calculations required. To create valid STL files, it is essential that the program makes individual files for all selected parts.
SolidWorks meets all criteria for generating good STL files. In export from the PART space, the program has several options to control the quality of the surface representation and generates a preview of the facets. It is possible to suppress unneeded details (e.g., bolt holes, fillets,…) so that they will not be included in the STL model. In export from the ASSEMBLY space, you can suppress entire parts so that they are not recorded. SolidWorks makes an individual STL file for each active part with the correct absolute coordinates to place the part in the assembly. There is one precaution. During my tests I found that the 64-bit version of SolidWorks 2009 had a bug that prevented setting any STL options. This should be corrected with the release of Service Pack 2 this month. The present 32-bit version of the program works correctly 90% of the time.
Alibre is more limited. It does support STL export from the PART space. The tesselation is good. You can set some parameters to control the triangle shapes, although there is no preview option. Alibre cannot export from the ASSEMBLY space. There are two problems:
- For a simple export, Alibre records all PARTs in a single STL file as a single solid. The coordinates are correct, but the invalid STL file is useless.
- If you suppress all other parts and export the single remaining PART, the program records the PART coordinates rather than the ASSEMBLY coordinates.
I expect that it would be easy for Alibre Inc. to fix these problems. I reported the issue to their tech help department. The response was that the program worked just fine. They gave me a couple operation sequences to follow, neither of which worked. The bottom line is that if you are considering a solid modeling program for MetaMesh input, be sure to test the STL export capabilities.