fair point on the accuracy, I'd imagine the steel fabrication would potentially be a little out of square! - check out this link for large CNC milling machines - http://www.davymarkham.com/machining.htm

It's a photo of some of Davy Markhams equipment, one of the bed plates for the machine is the whole factory floor! with 5 huge horizontal borers around the edges!!...........I saw a editorial on a job that Davy's won a while ago and one of the guys made comment on how they had to employ someone full time to sweep up the swarf created on the one job......unreal!