There was a mod called "UbioZur Welding Ltd." http://forum.kerbalspaceprogram.com/threads/38577-0-22-UbioZur-Welding-Ltd-2-0-Dev-Slowdown where you can build your craft and then make one part from that design, there was some limitations that some of parts cannot be welded but even that problem, the craft / station that had 300-400 parts after using welding tool had only 20-30 parts. All of you know what does it mean for fps and whole pleasure you take from game. There are a lot of great suggestions here but in my humble opinion in game welding tool is jus essential. Please share your opinions. .. and please support this idea. THANKS !
