How can i convert an autocad dwg or dxf file to a more 'common' format like OBJ, FBX, 3DS, etc. ?
I have tried a free software called abviewer 10 but the exported file is always empty.
You can use the AutoCAD I/O web service to batch convert files. See documentation at http://developer.autodesk.com
In summary you can upload a .dwg along with script contains a -FBXEXPORT command, which will make the export. After that you can download the exported file.