3DPrinting
3DPrinting is a place where makers of all skill levels and walks of life can learn about and discuss 3D printing and development of 3D printed parts and devices.
The r/functionalprint community is now located at: !functionalprint@kbin.social or !functionalprint@fedia.io
There are CAD communities available at: !cad@lemmy.world or !freecad@lemmy.ml
Rules
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No bigotry - including racism, sexism, ableism, homophobia, transphobia, or xenophobia. Code of Conduct.
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Be respectful, especially when disagreeing. Everyone should feel welcome here.
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No porn (NSFW prints are acceptable but must be marked NSFW)
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No Ads / Spamming / Guerrilla Marketing
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Do not create links to reddit
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If you see an issue please flag it
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No guns
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No injury gore posts
If you need an easy way to host pictures, https://catbox.moe may be an option. Be ethical about what you post and donate if you are able or use this a lot. It is just an individual hosting content, not a company. The image embedding syntax for Lemmy is 
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First off I'll say that what programs you'd want to use depend on what kind of things you're wanting to design. If you're wanting to design something artistic (a sculpture or miniature or model of a space ship or whatever), Blender's a great choice. If you're wanting to design something that's more for mechanical use (a custom wall mount for something, a replacement gear for machinery, a case for a Raspberry Pi, whatever), you'll probably be better served using a CAD (Computer-Aided Design) program like TinkerCAD, FreeCAD, AutoCAD, or OpenSCAD.
I do a lot of CAD sort of model designs and I personally love OpenSCAD, but it's made very particularly for programmers. TinkerCAD has a much lower learning curve and is free to use.
I don't do much artistic model designing for 3d printing. I dabble in Blender too, but mostly just to make minor tweaks to models I found online.
About your cylinder question, you're right that any slicer would be pretty confused if you gave it a shape like that. To make it work, you'd need a shape that had volume. Not something that's an infinitely thin surface.
To get a "tube," you'd have to decide how thick you want the walls and deal with that in whatever program you're using to make the 3d model. In a CAD program, the way you'd think about this is that you'd add a cylinder and then "subtract" another (concentric) cylinder with a smaller radius. Like, if you wanted a cylinder with an outer diameter of 3cm and a thickness of 2mm, you'd make a 3cm cylinder and "subtract" from that cylinder another cylinder with a diameter of 3 centimeters minus twice 2 millimeters or 2.6 cm. (The second cylinder would also need to be of equal or greater height than the first.)
The result would be an object with volume and with a very specific defined thickness that your slicer would be able to handle nicely.
Where CAD programs let you think in terms of "volumes" of various shapes and combining them in interesting ways, programs like Blender work more in terms of "polygons." I'm not terribly familiar with Blender, but just off the top of my head, if I were trying to do the same thing in Blender, I might make a cylinder 3cm in diameter, another one 2.6cm in diameter (at the same location with the same height), remove the top and bottom faces of both cylinders, reverse the normals on all faces of the second cylinder, and then manually, one by one, create faces to fill in the "ring" around the top and the "ring" around the bottom.
Does that all make sense?