Thanks everybody for your answers
Reading all these useful comments I want to say that I this machine won’t be used to achieve magic tolerances. As Dean said
“…router designed for mostly for wood……… is to be used for cutting aluminum then again the tolerances needed are still low because it will be mostly limited to and probably used mostly for profile cutting”

To be honest I don’t know yet what exactly I am going to use the machine for. I just build a “weapon for hunting” I have never been in the “forest” before with that kind of automated “weapons”. Till now I hunt with catapult (traditional handy tools).
I get involved with many kinds of materials ( wood, steel, alloy) and like to mix them in my construction. That is why the machine is a middle solution (all around machine). That of course means that when you earn in one level you will miss in another level. I think an all purpose machine is impossible to be build as there are parameters opposite each other ( accuracy vs size, stiffness vs light weight and all these vs cost ). The main principal that I try to achieve ( not only in cnc construction but also in my way of life ) is to have the best value for money. So I am trying to achieve the better result with the means and the efforts that I have.
The added value of course at this attempt is your help, even your well disposed disagreements that make me understand things better. In my country we use to say that “sciolism is worst than ignorance”. Many times here in mycncuk I realized that I had sciolism for many aspects that had to do with constructions. Many myths in my mind have been basted especially with Deans help who is something more than an “auto-answer machine” . I could not not mention Jonathans help especially through his Servo sizing software and his inspired constructions, and also I cant forget that the main idea for my machine frame, come out from Syliavskis thread. And all the others that these last months participate to encourage me, correct me, and generally help go one step beyond. I know that I am not too fast in making steps, but I also know that ( for me ) the most important is to make small steps than trivially jumps. This attempt for me is like a trip. I prefer to make the “trip” with a slow train looking “out of the window” , more than ride a fast car and reach the end some days earlier. A Greek poem describes it better. Whoever want to read it (in English ) can find it here
http://www.cavafy.com/poems/content.asp?cat=1&id=74
Thanks for your time