Well here goes... Aluminium frame router design

[CharlieRam]

Hi all, I am currently still designing the machine, It will be made mainly using 20mm alu plate and 100x50 5mm box section, 20mm profiled rails sourced from chai (Thanks Jazz) and 1610 ballscrews hopefully belt driven.
Rail lengths are 1050 on X, 850 on Y and 350 on Z. Bearings are spaced 250mm apart on all axis (from outer dimension) which should give me an efective machining area of 800x600x100 excluding tool width.
Now I have used the gantry stiffness5 excel sheet and I think using the box section on the gantry I will end up with 13micron bending deflection and 30micron tool deflection. Would this be considered acceptable or the norm? If not Can you give me some Ideas to stiffen up the gantry design, I have attached a sketchup model in the zip file so feel free to modify and or tear it apart if you can help :-) Its obviously not complete yet but I dont want to spend hours on it if I am going in the wrong direction.
Will 5mm of aluminium be ok for the rails to attach to or should I glue some plate behind the box section for better securing.
One last question, I am hoping to use timing belts and pulleys, probably reduce the ratio to 75% so I should be able to get the speed for woods/ plastics but keep accuracy for aluminium, does that sound ok?
I look forward to some feedback, good or bad

Cheers, Charlie