This is my first BUILD please help.

[EddyCurrent]

Please do not take my comments as critisism because your machine is excellent, it's just that for me there looks to be a mismatch between the gantry and bed frame.
Well I'm no expert on rails but from what I've read and found with my own machine I think you would have done better using the low profile rails such as Hiwin. Lots of people use the rails you have and they seem to work well but somehow they are not in keeping with that substantial base. Hiwin rails however would have been more expensive.
There 's one thing for sure though and that is the frame and bed will have enough stiffness.

As an experiment maybe you could add some test results to this thread ?
http://www.mycncuk.com/forums/gantry...cnc-mk3-2.html