That was based upon a load of 3kg, will have to wait until the spindle gets here so I can get an accurate weight. But am confident I will have no real issues with a build size of 800mm X 1500mm. Going with the table mounted rails will limit flexing on the Y axis greatly, as the rail will be fully supported along its entire length.
check this out. Its for 2 2080s, 1m long on my X. Thats pretty damn flat for me!
https://8020.net/deflection-calculator is what I used
What profile did you choose?
I used the 40-4080 as that’s the same dimension as 2 2080 side by side
Take a look at the Charging Ox. I am duplicating this build and making a 1000 x 1000. It uses Angle to support the sides and there is no need for the front/back v-slot rail.
I bought some 6"x6" aluminum angle and had it cut into 4" wide pieces for rail support.
http://www.openbuilds.com/builds/charging-ox-cnc-machine.1607/
Thanks for the info Dean. I have settled on an 800mm X 1500mm build. The rails are mounted to the side of a torsion box table.
Have a pic of the design?
For the torsion box? If so look at http://www.openbuilds.com/builds/lfox-large-format-ox-deriviative.3496/ There is a pdf of the torsion box table.
Thanks!
A lot to think about when taking V-slot to lengths in a build.
The deflection calculations for V-slot is a good one, actually started at SMW3D… Thinking about it don’t recall seeing any atta boy…Anyway it was a pain when engineers started arguing basic assumptions. Glad Mark took that one and resources made it amazing.
Back to the point, if your building a standard OX a point load is overly simplifying the situation. The load is distributed across multiple points, great add by Mark, especially if you use 8 wheels across the top. Unfortunately to calculate this, the math is not as simple… Well, still simple if you use integrals often… 
The issue with such a long X is deflection in the X/Y plane. Let’s say your getting after it in the Y direction on a good chip load. The end mill will translate a moment arm to the X and cause a “roll”, that causes a deflection in the Y, cut suffers.
I do completely agree with Peter’s mount to a table, that is awesome!! That makes for a much more stable base and removes Z deflection from the Y beams. Not to mention you could dual purpose the table by putting it on a hinge and allowing it to flip over like one of the R7 guys did, pretty sick!