Parallel joining of Misumi extrusions -- some thoughts.
I bought some 8020 and 6020 extrusions to make a replacement for the stock vinyl table. To join them, I plan on using a combination of end cap 2020 extrusions, superglue, buttonhead screw and t-slot nut. If you drill a 6mm hole through the center of an extrusion at the right place along the length of the extrusion, you can use an allen or torx wrench to tighten an M5 button head bolt that attaches one extrusion to its neighbor. The neighbor may have a t-slot nut or its center hole tapped M5.
I'm not sure whether to keep the other vinyl parts of the Y carriage, or to replace them with aluminum also. Clearly aluminum would be more rigid, but it's more work and cost. I just finished building a MendelMax 3D printer out of Misumi extrusions, and am very impressed with the combination of cost, rigidity and construction time/tooling needed.
As I did with the MendelMax, I will prep the extrusions by drilling, tapping, and inserting screws and nuts in the right places. Then lay the 3 inside and 2 endcap extrusions out carefully on a flat table (I use a 3/4" piece of marble slab that I've found to be flat to better than .01" across its 24" length). Then, after some clamping and verification of flatness, hit the joints with super glue. Flip it and repeat on the other side. After the superglue cures, tighten the screws. Repeat this process, adding extrusions to the sides until the table is complete.
The table can be lapped or ground flat before and/or after installation.
I'll try to post pictures as I build.