Building a bigger 3018
Looking at the structure of the 3018, you can see that carriages are dependent on 8 mm rods (and, in some models, 10 mm rods). For short distances, this is fine. I have a Core XY 3D printer that uses 8 mm rods for the X and Y carriages that are some 24” long. When I upgraded the printer, I had to make sure that the X-carriage and Y-axis parts kept the assembly light enough to allow the rods to bend. Obviously, the longer the rods, the more there is a propensity to bend – and even a little bending is not good. The 3018 adds rigidity by having two rods, but even then, there is a chance, under load, of the rods flexing just enough to make your cut inaccurate enough to matter. That more likely applies to really long lengths of rod, so for the 3018, if you want to use the same method for motion, I suggest you do not make the work area more than twice the base size.
There is another factor to apply here. Even if you can use rods that are stiffened...