I'm wondering why go through the trouble to precicely cope the tubes if you're assembling with gussets anyway?
That has been discussed by the coffee group at our EAA: One could argue that the copes become a secondary load path under compression, or they straddle the tube preventing movement side to side, or if you needed to you could add epoxy to the joint, or it is a starting point for a welded structure, or at the very least if you have the something in the wrong place it will definitely not go together.
Final answer: it just looks cooler.I have used this system to create assemblies that were welded (larger aluminum tube assemblies, engine mounts, non-gusseted assemblies, etc.) so the copes were necessary. However, back to the original topic...
It is kind of inefficient to use a bunch of time to accurately cope small tubing. In my humble opinion, 1/2 inch is the tipping point. Anything smaller and you can just straight cut at the correct angle and fill when welding. At 1/2 inch you need a little bit of cope, but close is good enough. At 5/8 inch tube your TIG welds start to look like ugly MIG welds if you have sloppy copes. At 3/4 and up, coping accurately is necessary and really shows in your final product. Almost the whole fuselage of the Legal Eagle is in the 1/2 to 5/8 inch neighborhood. So whether you CNC cut or hand cut you end up with about the same aircraft. I personally would rather CNC cut tubes because it makes welding a bit easier: I am great at cutting and grinding, I am not so great at TIG welding tiny objects (thank you old eyes and shaky hands).
I hope this helps,
Robert
