Since publishing the Filament Server System in 2013, I've sought an improved system requiring less printing, fewer loose parts, less assembly, and easier spool changeovers. When I created the pipe-based, wall-mounted Spool Storage Rack last year, I became intrigued by the idea of feeding directly from spools sitting on pipes, but the friction of the spool edges on iron pipes is too great.
As it turns out, the black drainage pipe sold in Home Depot and Lowe's, from "Charlotte Pipe" is made from a very "slippery" material that allows most (but not all) filament spools to turn freely. Normal PVC plumbing pipe will not work, it has too much friction. For spools that don't turn freely on the Charlotte pipe (cardboard, for one), or for situations requiring absolute minimal pulling force, I designed a snap-on axle support, two of which can support a piece of 2"/50mm OD plastic pipe acting as an axle. The scheme can be seen in action here: https://youtu.be/O7FAWeHpkeU
The plastic pipe slides over 3/4" (23.4mm OD) electrical conduit, which is cheaper, lighter, cleaner, and easier to cut than iron pipe, and has no threaded ends needing to be concealed. I'm not sure what the nominal size of the Charlotte pipe is, but its OD is about 1.25" or 30mm.
This system also addresses a need I had for a free-standing rack not requiring wall-mounting. The vertical members are 1-1/4" perforated square "Boltmaster" tubing from SteelWorks, available from Amazon and elsewhere. In my setup these uprights are bolted to a butcher block workbench with the help of some bracketry. The "mid" and "top" rail guides are a tight friction fit into the square tubing and might require a whack or two with a mallet. The conduit snaps smartly into the rail guides, as do the filament guides and axle supports into the plastic pipe.
The system can feed from spools at a significant lateral distance from the printer, but if not prevented from doing so, the spool will creep sideways and run into other spools, increasing friction. To prevent this, and to provide a smooth, low-friction path for the filament regardless of feed angle, snap-on multifunction guides are provided. Spools can simply sit between them, or the spool edge can be placed in the "groove" provided. There's also a notch-y thing at the front of each guide where a loose filament end can be secured.
IMPORTANT: When feeding from a spool resting on the pipe rails, it is imperative that the filament feed from "under" the spool, between the rails. If you feed from "over the top" of the spool, the spool will get pulled off the rails and topple over (don't ask me how I know). Spools using the axle holder can feed either way.