Whilst looking a parts for a Kossel, a large Kossel (see Kossel 3D Printer), I came across these aluminium vertices for 2040 aluminium from RobotDigg, see 2040 or 3030 Alu Vertex for Kossel XXL or XXXL. On that product page they recommend using Nema 23, en lieu of Nema 17, stepper motors and, indeed, offer a vertex machined especially to take a Nema 23 stepper motor.
Now, Nema 17 stepper motors are pretty well covered in the RepRap forums, and there are three common favourites. However, the choice is not so clear for Nema 23 stepper motors. I decided to do some research on which Nema 23 stepper motors would be appropriate.
Seeing on ReprapWiki – Kossel, that the Kossel uses OpenBeam, which is very similar to 2020 extruded aluminium, except that it is 1515, and as I had already sourced some inexpensive 2020 for my Wilson II 3D printer, in Bangkok, it would not be much of a problem to go and get some more;
Having watched the following video on the Micromake 3D printer, that uses 2020 aluminium and is similar to the Kossel (inasmuch as it is a Delta printer). See this explanatory video:
However, as I have stated above, the main reason was that I had the idea of sourcing the aluminium for the Kossel, whilst I was in Bangkok, as aluminium is relatively cheap in Bangkok, when compared to Europe.