I had the same issue, and things were not staying "square" or linked when scaling.

Someone on here informed me that you click on : INVENTOR ICON LOGO>PRINT>SEND TO 3D PRINT SERVICE.

I am also using 2013. I have not has a scaling issue since I started using this method, as opposed to using export to cad format.

Good luck. If there are any other issues that I can help with, please let me know.