Thanks everybody for your advices
The basic conclusion is that I should replace the I beam with square profile. That was what I have in mind from the beginning. The reason that I choose I beam for the drivers support, was because it was easier to connect the two I beams ( upper and lower ) with bolt and nuts in order to have a fully adjustable table.
The plan that you suggest me to make Syliavski looks very strong and stiff. My question is how am I going to adjust that table if I have imperfections as cause of arc welding. The basic advantage of arc welding is the stiffness, on the other hand if I only weld and have no ability to adjust with bolts, I think is more than sure that my X axis rails wont be on the same plane. So I think that I will keep the solution of the bolt adjustment an replace the I beams with 80X80X3 or 4mmsquare beam. The Chinese says 1 picture 1000 words, I will design it and I will come back. Meanwhile I will be glad to hear any suggestion.

Thank for your time

Vagelis