Open linear bearings?

I just got a Prusa I3 and I love it. One of the things I’d like to do is cast an aluminum cylinder head for an odd engine, which would require a printer just the size of the Prusa, which is why I bought it, only with about 20cm more in the Y direction. On the Prusa, that could be accomplished by removing the rear aluminum extrusions and bolting in some new longer ones, using the same rear plate, and using a longer timing belt and longer linear rails. Other people who have done this have moved to 12mm rails, which makes sense.
What I’m considering, and I don’t understand why this isn’t ubiquitous, is using open bearings and railroad-track-shaped linear rails, like
Even just free-hanging, these should have 4x the stiffness of round rod. Bolted to a trued surface, they could have as much stiffness as you want.
Why doesn’t anyone use these? I’ve never seen them on a 3d printer. For that matter, I’ve never seen them on a cnc router, even though they’re not that expensive and come in 1-3 meter sections.

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I’ve definitely seen them on a CNC router. It was the first time I became aware of them. They were a much larger size than you would use on a 3D printer!

I think that by the time you are looking at those bearings, most of the time people end up with Hiwin or “chiwin” style linear bearing blocks instead.

For information on making some “chiwin” blocks run more smoothly, see this comment and following:


I have a bunch of THK linear rails with carriages, and like using them for very heavy projects like a taper cutting attachment for my lathe, but they are so totally overkill for a 3d printer. Although, they’re cheap on ebay… maybe I’ll just go with that.