I have existing AutoCad files .dwg extensions that I need to open on my Mac. I saved all these files from my Windows based system, but now need to open them and alter. Looking for a Cad application for Mac that will open these files and let me make changes and save. I have searched the web and have tried a couple of "try" before you buy applications with no luck. I can't afford to purchase an expensive program like Vector. I am open to all suggestions. Thx.