A frequent question in 3D modeling is what file format to use when exporting a solid model to use with another program or to give to someone else. Typically all 3D modeling programs (AutoCAD, Inventor, CATIA, Solidworks, Solid Edge, etc.) have their own file format(s), proprietary or not, and so the question comes up as to which format is going to yield the most interoperability between programs. Also, programs for creating meshes for numerical simulation typically do not accept any of these program-specific file formats. Though the method of storing feature information is basically the same (BREP, Boundary REPresentation), each format has a slightly different way of storing information, and thus importing/meshing using different formats will yield different results.
The big contenders in interoperability are:
- STEP (ISO 10303), an ISO standard initially published in 1994, file extension: .stp or .step
- IGES (NBSIR 80-1978), a U.S. National Bureau of Standards standard initially published in 1980, file extension: .iges
There are several reasons why STEP is better:
- ISO standardized, i.e. international acceptance as opposed to standardization by the US; there are a lot of CAD/CAE/PLM applications from European countries (almost everything except AutoCAD and Inventor)
- Newer and still maintained, the last revision of IGES (5.3) was in 1996
- From personal experience: more applications accept STEP files
- From personal experience: better results with STEP files; meshing models with complex geometries are more likely to have a clean mesh, while IGES files will likely result in mesh defects in these complex geometry areas (lofts, for example)
- More understandable/human-readable file format, see ISO 10303-21; easier to implement
All that being said, I recommend using STEP files for data exchange with IGES as a fall-back. Other formats, such as ACIS, have their place: AutoCAD can't import/export STEP or IGES, then again AutoCAD isn't really meant for serious 3D modeling.
Some discussions on the topic: