Been making very slow progress on my design lately and I'm looking at redesigning my frame but can't decide what size tubing to use. Originally I was using 50x50x3 for most of the frame with 100x50x4 for the rail support, I'm toying with increasing this to 50x50x4, 50x50x5 or possibly even 60x60x5. Obviously there is a cost difference associated with each option so I'd like to do some sort of stress / deflection calculations to work out whether the cost is justified. Can anyone tell me what sort of loading I should be applying to my design to work this out please? I have seen example calculations with 50N as the load on the spindle which seems more than enough from my experience using a hand router - is this a good number to work with? If so I don't imagine I will get much flex from the frame using 50x50x3 let alone the heavier sections, I think most deflection will come from the bearings and aluminium extrusion gantry. What is an acceptable amount of deflection? I understand the argument for increasing mass to reduce vibration but if I can reduce the material costs for the frame and find another way of adding mass to it (sand / epoxy filling the frame?) then I can spend the money elsewhere.

Thanks for your help!