Voron has open source printer plans ~~i think~~. The og is reprap
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The entirety of voron is an open source project, they do not make or sell printers or parts. None of their printers are designed for this size, though. People have definitely made custom ones larger than the schematics but there are drawbacks for going even 50-100mm over the normal max of 350mm, doubling it would certainly require more engineering than "print some parts longer"
IIRC there's someplace on their website or manual where they explain why 350mm is the max they chose, and a big reason was the wobble/flex that you'd get from going over--you'd have to compensate for that by designing a much more rigid frame and not just scaling the Voron design
I might go hardcore and build a welded frame for it.
How good a welder are you? introducing a 1-2mm. difference is really easy when welding, not to mention the squareness of the frame. Extrusions allow for corrections.
I've helped build a couple 350mm Voron v2 for the local library makerspace.
Those are some big linear rails!! Can't wait to see what you make with that. Maybe a belt printer would be good, lot of the huge ones are belt based for some reason. Like how about this one?
https://3ddistributed.com/mrrf-2019/white-knight-3d-printer/
I built one, but it was a (top-down) resin printer, so I don't think I have any useful advice to offer. Also, I kinda burnt out on the project after I came in one morning and found the resin had melted the vat and leaked onto the floor--somehow I just totally failed to consider that the solvents in the resin could obviously dissolve a lot of plastics. Huge mess, huge pain to remediate. At least it was a concrete floor. I got a replacement vat in glass, but I never worked up the will to work on it again. It did basically print before that, but I had a lot of off-layer curing and didn't get to do much work on tuning that in before the accident.
So of your dont want to build a very long/tall 3d printer i recommend looking into building a flat long CNC frame for a laser/Plotter. They want to be long and flat, fitting your parts. I build a voron, several cartesians from scratch aswell as a giant laser frame. A Laser frame is much more simple compared to sth as overengineered as a voron(Perfection), far better suited to waste some spare parts with out getting lost in a one year project. Id have loved those rails for my laser.
I built a Prusa Mendel from scratch eons ago. With enough of effort, and a more modern nozzle and extruder, you should be good. You probably want to have some 3d printed parts as part of the assembly, so see if you can find someone nearby.
What size rails? Double carriages makes me think DIY desktop cnc.