[Johnno]

Why not just create the main thread 3D Printing and everything 3D related goes under that such as 3D Printers, 3D Printer chassis, 3D Printed Parts and the list goes on like that, be all of them under the one heading or implement sub-headings such as
3D Printing\
                        3D Printers (buying, recommendations, retailers, reviews)
                        3D Printing Chassis
                        3D Printing Parts
                        3D Printer 360 degree Scanners
                        3D etc

I have and still do use many Forums that have sub-forums, the indexing is rather easy.

Other than something like above guess we just throw the possible thread anywhere and see how it gets going.