Now that I've teamed up with Blueprinted to run some free CAD and 3D Printing courses, we thought it would be a good idea to publish a brief write up of the difference between what most people think of when we say "3D Modelling" and the more engineering focused CAD 3D Modelling which is typically used more often in conjunction with 3D Printing.
When most people use the term "3D Modelling" they are referring to what, in the industry, is called Mesh Modelling, that is, creating a 3D figure in a computer program through the manipulation of polygons. The result of this 3D Modelling process is a detailed mesh that forms a shape. To understand this, let's imagine a fishing net, magically held into place to form whatever shape you like, where the holes in the net are considered the polygons. Seen below are two images taken from a widely used, free, open source, 3D Modelling program, Blender. In the image on the left you can see the mesh I am referring to, a wire frame that forms the 3D shape. The image on the right is with the wire frame turned off, so you can see the final 3D shape. The object in this screen shot is dutchmogul's "Hold Guardian" found on Thingiverse.
In many cases, the creator would start this modelling process by pressing a button to insert a cube, sphere, cylinder, or another basic shape into the 3D space within the program and using variables to control how many polygons this shape is made up of. We can then use various tools in the software to move points, lines and faces in order to manipulate this shape. There are also other tools to join, smooth, flatten, divide (you name it) these shapes to make them look nicer.
Programs such as ZBrush, that let you push/pull/carve/mould/etc a shape in a more tactile way, are just giving you a new way to control this same process, moving polygons around to create a mesh.
To be clear, CAD stands for either Computer Aided Drawing or Computer Aided Design. CAM stands for Computer Aided Manufacturing. These two terms come hand-in-hand.
In most cases, you wouldn't bother with CAD if you didn't have CAM in mind.
CAD gives the user a way to create practical, engineering focused 3D Models which they plan to manufacture to use for a purpose in real life. The key difference between Mesh Modelling and CAD is measurement.
A CAD program needs you to input real life measurements at every turn. Even if you are lazily rushing through making your object, and you don't specify how big something is, the program gives it a real life measurement, which imports into CAM software at a 1:1 scale. If you created something at 3.75mm in your CAD program, assuming your machinery is calibrated correctly, your end product in real life, will also be 3.75mm.
Below are two images taken from my CAD program of choice, Onshape (also free, web-based, cloud-focused, etc), demonstrating the use of real life measurements, to show how they control an object. In the image on the left, I have drawn 2D "Sketch" on a flat plane, which is controlled by dimensions, and in the image on the right, I have used an "Extrude" tool to tell the program that this flat, 2D sketch should stack on top of itself until it exists to a thickness of 25mm. To understand how this works, imagine you drew my sketch by hand on paper, and cut out. Then you cut out another one and stuck them together. Then you cut out the third one and stuck it ontop of that, and a fourth one, and so on... until the result was 25mm thick.
Well, both of these have their uses. Don't be the guy who see's these images and recognises that that awesome Dwarf Hold Guardian looks cooler than my random Onshape sketch (it does) and therefore write off CAD forever. As makers who make things, not just Artists or Designers who want pretty stuff, I would argue that CAD is more useful when combined with CAM.
Typically, Mesh Modelling exists and is used to create models that can be animated over time, in order to create movies, video games, etc. Where they need to look realistic in an on-screen environment to simulate a virtual reality.
CAD exists to give engineers a method to accurately create parts which can be used for manufacturing in the real world, not just in an on-screen environment.
When you use programs like Blender, 3DSMax or Maya to create 3D meshes, the programs let you control and change numbers in order to specify how big parts of your mesh are. These numbers are not a real life measurement system. They are their own, dare I say arbitrary unit system which does not correlate to real life measurement at all.... not even the imperial system! A mesh you create in one of these programs might tell you it is 300 "blender-units" big, for example, and when you import that into a CAM program ready to make it, it turns out it might be 4.76mm or 98733621mm big.
You can somewhat control this in CAM programs by using the Scale function to increase/decrease the size of an object in order to make them the size you want them. It's just a bit of an afterthought, and will never be perfect. Often when you download files off the internet, like the Dwarf up there, the creator has done this for you, so that the STL file you end up with already scales 1:1 with a certain size. But, imagine you were making an engine, and the bore of the piston chamber needed to be manufactured specifically to, say, 62.776mm. If you do this in a Mesh modelling program, and then scale it up/down in CAM, there is no way you'll get all the measurements right, and no way you'll end up with a working engine.
In CAD programs, as seen above, these measurements are input at every point, so the file you end up with is very, very accurate, and can be easily imported into CAM programs and manufactured using real life measurements.
All that said, please understand that, at this stage, Blueprinted and I are offering free CAD 3D Modelling courses, specifically using Onshape (all CAD programs are very similar, you will easily be able to adapt what you learn here to other CAD programs). Mesh Modelling is something we may consider at some point in the distant future.