The most convenient way to handle .ai graphics would be to use the original Adobe Illustrator which is the program that produces these files in the first place.
If you are looking for ai to dwg conversion, both versions of Adobe Illustrator for Mac and for Windows are able to export graphics in AutoCAD drawing file format, through the File ► Export... ► Save as type: AutoCAD drawing menu.
Some alternative would be to use a 3rd party graphics editors for this job, but you may find some compatibility issues with latest versions of .ai files.