Netfabb before Autodesk fucked it will fix a lot of hanging vectors, Sketchup is just way easier to use than any CAD software I've every used but may or may not be able to open files included with Thingiverse downloads, depending on what was used to make them.

DM me if you need a copy of either. I've installed both on Linux in the past as well and have a link to some documentation on that.

In my view CAD is always worth learning, especially if you understand what prints best on your printer.

FreeCAD is basically the worse possible beginners tools. Don't get me wrong when you learn it, it's good and comparable to professional CAD software. But the learning curve is dumb.

Learn the basics first. TinkerCAD is free (from Autodesk) and will get you started. But if you want something which would take you further Fusion360 is fantastic for beginners.

The workflow of CAD is as follows. Sketch -> Action -> Sketch -> Action. Lets say you want to make a box with a hole in it. Sketch the outer box -> Extrude it -> Sketch the inner box -> Extrude/Cut it.

I was in a simiar boat, but once I started learning it really took off and I couldn't find enough things to model/fix! I started with meshmixer, it's pretty simplified but still somewhat powerful and I think Windows only. Once I found stuff I couldn't do with it I switched to blender and now I'm much faster with it. I found there are some things each implements better so if having an issue I'll switch and try something else. I highly recommend picking one and watching youtube videos specific to what you want to change in a model

Edit: you may be able to get by on some of the connectors by lowering flow too, depends on how dialed in your printer is, everyone needs different tolerances

Tinkercad (free, closed source, in browser) is what I use. It's very basic, but easy to learn.

I find that being able to sketch up things I need in CAD and then print them is both cool and really useful. It's the main reason I bought a printer in the first place. Thus far I've tried out FreeCAD, Solid Edge, and Blender. With any modeling package, you will have to dedicate time on a regular basis to really get used to them. FreeCAD is certainly nice for the fact it's free, just as it implies. I used it to design a few parts that were functional. It works, has some useful workbenches and add-ons. My problems were the software having bugs that caused models to break when trying to make changes, and available training info was often outdated. Siemens offers a free version of Solid Edge to makers, which is really nice, even with some of the advanced features turned off. It's a much more polished program with great training resources. You can only export designs as stl files, but that's fine for 3d printing. Solid Edge will slice and print, but I always import files into Orca and go from there. Blender looks really amazing for modeling, but I admit I haven't spent enough time learning it yet. You can use it to manipulate meshes, which is useful for customizing and fixing models. I've used it to Frankenstein together different models for custom prints I wanted. But yeah, while you don't have to learn to use modeling software to do prints, it opens up so many options for you to be creative. I think it's worth while.

Google's sketchup (last free version is from 2017, but can still be found) is easy tool and still has bunch of mantained add-on's.

Even though I have FreeCAD installed (and I thinkered in it) I use SketchUp for all my prints.
Plus in comparison to FreeCAD SketchUp has no learning curve (plus I learned some tricks from free tutorial videos Trimble uploads)

The time and effort you put in to learning 3D Cad will more than pay off for you in your ability to make things you need that someone else hasn't yet made and posted online. Sure, it will be a bit of learning curve, but what are we here for but to learn and grow?

I use FreeCAD which is FOSS. There's a great series of Youtube Videos by Mango Jelly on how to use it.

I've been reading the other comments, and while people are encouraging, their comments seem a bit too "you HAVE to learn CAD." You definitely don't NEED to know CAD. I made basic parts and modifications in the slicer for nearly a year after I started printing, and it worked really well. However, if you are considering learning a full CAD program, I have two pieces of advice.

First pertains to if you are working with functional parts. Then you are talking about a parametric CAD program (fusion, onshape, FreeCAD, etc.). In this case, I think it's worth learning for you, and it's not as hard as it seems. You say you have SketchUp experience, so I'm assuming you have decent spatial reasoning. I know someone with no tech literacy nor programming experience who learned a CAD program very well in less than a month of following tutorials in her free time. Just give it a try, and it's a skill you'll be happy to have.

If you are working with cosmetic parts like miniatures and helmets, then you might need to use something like Blender. Admittedly, that can be even more challenging than the other CAD programs I mentioned. However, if you spend a few hours learning some basics from YouTube, you should be able to do fundemental things like fixing holes.

Hope this helps. Good luck with your printing endeavors!

Most of the CAD software mentioned here will not be super helpful in modifying meshes (like STLs), but they're great if you want to redesign the part from scratch :)

Tinkercad is the exception, as it works with meshes and booleans.

If you happen to have any background with coding, I recommend OpenSCAD. There are a (relatively) limited set of instructions, easy to pick up if you do any other coding.

IMO it's worth the time and effort to learn the basics. You don't need to be an elite level modeler because you're not modeling for animation or games. I've had good luck with simple fixes and mods in Blender.

My experience is entirely with SLA, however, FDM might be a bit trickier.

If all you have are STLs then you're pretty limited. I use Meshmixer to do basic modifications or tweaks to STLs but it's not super user friendly.

This post turned out to be a bit of a rant about what drives me to model my own designs most of the time. In short, it isn't required, but I highly recommend it.

I'd say that most people who own 3D printers have little to no skill in modelling and are happy printing whatever they can download online. Maybe they hit a point where they want more, but until then learning modelling isn't a useful skill for them.

Personally, I'm a designer at the end of the day. 3D modelling is a crucial tool in taking my ideas and bringing them to life in a way that can be passed to a manufacturing process and made into a physical object. 3D printing just happens to be the manufacturing tool I use most often for personal projects because it is what I have the easiest access to. If I had a machine shop, I'd use that too. When working on high volume products I'll design for injection molding, die casting, sheet metal, compression molding, etc.

I'm not against utilizing models people have already put online that solve the problem I want, that is just efficient use of resources. But I agree, most models out there are very poor quality so I pretty rarely use downloaded models. Heck, I just re-modelled Gridfinity bins because I couldn't find a parametrically adjustable model for SolidWorks that I was happy with (on that note, the dimensional documentation for Gridfinity is straight garbage and I'm still not sure I have it right) and those are some of the most widely available models out there.

I also absolutely despise STL and other non-parametric file formats for sharing designs. They are terrible, inefficient formats that make files very hard to edit. Most people don't export them in high enough resolution resulting in horrible looking faceted models. The community needs to fully accept STEP as the file format of choice now that any slicer worth using can import them properly.

I was recently roughly in the same place. I have played around in the past with a few things, but there were reasons I didn't really get into any of them: Fusion360 and their increasingly limited free options, OnShape and their online only thing... so on...

FreeCad has a new release candidate for what they're calling version 1.0, meaning they consider it a mature and functional piece of software. I've been working with it, and yeah... it's not perfect, but it's definitely usable now. I figured if I was going to put all that effort into learning something, I might as well learn something that would always be free.

Get SketchUp Make 2017 from the wayback machine. Is super intuitive and you can import stl files directly.

I had a booth about this at the Bay Area Maker Faire lately.

If we're all printing the same object on our 3D printers, it's proooobably a lot less trouble to just have someone injection mold it and save us all the trouble. 3D printers are really great for one-offs and mass-customization and things like that. Aaaaaand, I feel like it's kind of an under-appreciated problem in 3D printing. Because, yeah, CAD is hard and we're never going to reach a world where every 3D printer owner is very very comfortable with CAD, and so it should be more of a concrete goal for the 3D printing community to make sure that we're focusing on this problem. It's important that every 3D printer owner can do at least some amount of tweaking and customizing, otherwise we're failing as a community.

Now, I don't Tinkshame. I spent a lot of time learning Blender, FreeCAD, and OpenSCAD to prove Naomi Wu's assertion that we should all just get over ourselves and use TinkerCAD. The only real problem with it is that it's not really free, it's "free at the pleasure of AutoDesk" where they could raise the "Mission Accomplished" banner at some point and turn it off. And there's not really an open source version of it for roughly the same reason that random thingiverse models are always kinda halfassed and bad. Doing a good TinkerCAD-but-actually-free-by-some-definition is actual work to get everything right and polished and documented and bug-free and nobody really wants to pay for it.

Also, maybe I am pedantic and obsessive, but I don't really like screwing around too heavily with models in a slicer, so I'd rather they take some of the magical code in the OrcaSlicer/PrusaSlicer/SuperSlicer tree and actually organize it into something that could be TinkerCAD-esque?

Anyway, the core of the talk of my booth was systems and libraries of 3D printable objects. So, for example, there's the Honeycomb Storage Wall system and some of us have been writing some neat lil OpenSCAD libraries and models for it (and another group of people have been doing similar things in Fusion) where you can make a parametric model so you can measure your flashlight and print a cute 40mm holder for it based on the measurement without having to model things from scratch and it'll click into the HSW wall and it's fine unless you are married to someone who has ommetaphobia and then you need to make sure that the honeycomb is the same color as the wall. And the same is true for Gridfinity, just you can put that in the drawer.

And there's also a lot of parametric models. I'm not sure what you are looking to print, but there's a decent selection of people who have done stuff in Fusion or FreeCAD or OpenSCAD where you can download the model and change the parameters to get it a lot closer to what you want without going through all of the drama of making it all over again.

I love using OpenSCAD. I've got a buncha years of experience using various 3D modelling tools at various times and so I can use Blender or FreeCAD quite well actually, but in the end, I do a lot of functional bits and it's so darn easy to just write some code because, actually, I've been working as a professional software engineer for quite some time.

So... dono, it depends on your aspirations? There were a good number of Gridfinity-like systems that were around before Gridfinity came out and they were ... ok, but not great, but then Gridfinity came along and did a boxy-box system just like was already there but with some interesting tweaks and making it more amenable to real customization and suddenly everybody went gonzo over Gridfinity in particular. So you might not be just making a thing that exists in a dozen forms better if you borrow an idea and make your version of it.

Also, I learned 3D modelling tools mumble mumble years ago in a failed attempt and/or dodged-bullet because I'd wanted to do games or special effects as a kid. The software I learned on is long gone, but it turns out that once you are thinking about things, it tends to stick? Which means that I learned pottery while visualizing the objects I was making on the wheel as if they were in the CAD window of my mind, got good at photographic lighting based on what I'd observed in the 3D program, and then transitioned back to CAD because I wanted to make things, so it's kinda one of those things where you probably won't waste the time spent.

tl;dr: I learned OpenSCAD, FreeCAD, and Blender to prove that Naomi Wu is right and we should all get over ourselves and use TinkerCAD and ... she's still probably right, LOL.