Some design points
I lied, next is design. From baby rattles to skyscrapers, you have to have a plan. For a lot of stuff, a sketch on a piece of paper is fine, but other things, especially things that need to be documented, and things that utilize more complex manufacturing, like CNC machines, plans will be good. For things like 3D printers, you pretty much have to use a CAD program of some sort, because you need a 3D model file for the printer to actually turn into a physical object.
There are three basic kinds of programs to look at here, artistic 3D modeling, CAD 3D and 2D modeling. 2D is just drawings, which is find for a lot of stuff, and also puts out the “vector” files things like laser cutters use. They’re the most prolific of the free software, and reasonably powerful. 3D is harder to find, and software that approaches the Solidworks level of easy to use even harder. There’s a reason you pay for professional software. I’ll not spend much time on specific software, you’ll have to find one that works well for you, and/or you can find training for it. Artistic modeling is of limited use, since it’s for sculpting, not machine parts. Blender is a pretty good program, but except for a relatively recent mod that you have to install separate and learn to use, it’s hard to get things to precise dimensions, for say a bolt hole. Solidworks on the other hand will have a wizard where you just tell it where the hole is, what bold you’re using, and it’ll figure out the rest.
3D design is of three basic kinds, and we can define it with three programs—Autocad, Solidworks, and Rhino. Autocad, and a lot of 2D systems, use a command based system—each line or feature is defined by a command line type code, which tells the software where to draw lines. It gets even more complicated when you go to 3D, since a bit of behind-the-scenes work is used to defined a solid, and you don’t really have a solid until it’s saved in an appropriate format. Solidworks goes 3D from the start, and you run from sketches that are extruded into solids, and the solids modified. Solidworks and it’s little brother Geomagic design, which uses a similar GUI and interface, are also designed to work with assemblies of multiple parts, with Solidworks having native support for extensive simulation and testing before putting something into reality. Rhino is different, it’s default system having template solids, which are then sized, arranged, and “boolean” functions are used to add or subtract from the object. It’s a little awkward when learning Solidworks to begin with.
I’ll not mess much more with all that, a good google search will find most of the programs, and show you how they work. You’ll need something though, and some of the lower end but still paid for programs will prove their worth in slightly better interface and ability. Geomagic is a good example of this, more or less replicating the UI of the $4,000 Solidworks, but for less than $500 for the basic version. Sketchup, google’s free one, is similar, with the sketch-and-extrude system, but lacks many qualities of the other software, like a history system and it’s not “parametric” where dimensions are more thoroughly connected to features, allowing easy modification of a design. More advanced versions of some software will have special stuff, for example Solidworks and the higher levels of Geomagic have “weldment” and sheet metal tools that let you work directly with structural members and sheet metal, automatically calculating angles and fillets and bend angles and stuff like that.
The other important things are books. Drafting is well standardized, and this allows easy discussion of matters between disparate people. A lot of standards are in the Machinery’s Handbook, and others are published as drafting references. Good idea to get something you can use to tell what you’re saying to someone, and what someone else is telling you.