It's worthy to note that the recommended dimensions for the extrusions for this design are 420mm, 340mm, and 300mm. For my machine I scaled the design by 200mm in all axes. (so 620mm, 540, and 500 respectively) I will from here on out be referring to the longest extrusions as A, the middle as B, and the shortest as C. I purchased my extrusions from Misumi, which I have been highly impressed by so far.
If you are going to be building a MendelMax I would highly recommend either starting with a stock size printer or a more mild scaling. The rather large scaling I'm pursuing is because for the fiber winder and next printer I will be making I need a rather large build envelope. This is almost certainly going to cause problems, especially in the Y axis and build table. Unless you have a specific reason to go big, do yourself a favor and stick with the stock dimensions.
The first step of the build is to tap the center hole of the extrusions with an M5 tap. Tap all 4 of the B and C extrusions. It would be a good idea to start by taping the A extrusions since they don't actually need to be threaded and if you haven't manually tapped something before this will be good practice where it doesn't matter.
The next step is to drill four tool access holes in two of the A extrusions that will form the top cross bars. If you have the printed jig that is included in the design on Thingiverse use that. If not the center holes are 70mm from the end of the extrusion. My printed parts didn't include the jig so I had to do my best with calipers. I then used a friends drill press to drill the holes using a 1/4" drill bit.
At this point you should have to completed sub-frames that look like this.
full BOM for specific hardware, it's late and I can't recall what I used at the moment) Once the frame vertices have been attached via the threaded bottom secure the trapped T slot nuts to the vertices through the holes in the printed parts to completely secure the vertices.
At this point the only thing left to do is install the bottom C extrusions across the bottom vertices. Depending on the specific Y axis you will be using you will need to trap T slot nuts on the inside of C extrusions. The remaining four A extrusions will be installed along the long axis of the machine, each will need to have 4 T slot nuts trapped in order to mount them and to accommodate the installation of the motor mounts.
Unfortunately I will be leaving for the annual EAA fly-in at Oshkosh on Sunday, and then leaving from there to New Space 2012 in San Jose on Wednesday so it will be a while before my next assembly post. That being said I can't wait for both Airventure and New Space, I'll be posting from both.