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Having difficulty finding rectangular steel tubing for X-Axis. Could I substitute a slightly undersized Imperial sized steel tubing (size 3"x1.5"x 1/8")? And then fill the void in the width with something that doesn't compress like washers or metal shims. I don't think the undersized vertical dimension is important to be exact as you are looking for strength.
I think the creator is possibly on vacation but as an engineer I feek I could step in meanwhile to suggest some things to consider.
The steel bar is for bending and twisting rigidity. Since it is not attached at the mounting plates on the side, it adds more rigidity against bending down the x axis than twisting the x bar.
To have the exact same effect with a somewhat thinner steel tube, it's important to realize that to transfer all the forces from the alu profile to the steel tube, the profile must be screwed so tughtly against the steel that the surface friction is higher than any forces trying to move the alu profile.
If the friction is not high enough, the bending force would land on the bolts and then transfer to the steel tube. But the bolts would experience cutting and twisting forces. So if you just put shims or washers on the bolts, you won't get the support you want.
To make this work, you would be better off cutting a steel sheet of a sufficiant gauge and fill the space entirely.
The tube itself is definitely strong enough but make sure the alu profile rests in the entire length of ty steel tube on its surface
This is generally speaking, but unless you are going to push the machine very hard like cutting metal etc you could probably get away with washers. Personally I wouldn't do that though.
I am at the moment building this machine too (buggest size, rack and pinion), but as I live in Sweden, I fortunately got the correct measure steel bar from a supply just half a mile from home. Easier for me than you, but no worries, you will figure this out.
Good luck!
Thank you Steve and you are correct, the steel tube is more for bending than twisting, but for regular use shims will work, you can also use a 60x80 aluminiumextrusion bar, but steel is better. And yes im on vacation and my replies will be a bit late,