Sometimes parts need to be attached to each other for various reasons:
- Printing area not large enough.
- Visual effect alternative to printing in multiple colors, desired 'inlay effect'.
Of course, there is no reason that just one of these is used in one case. If two parts are designed for each other, they are likely already interlocking to some extent.
Stuff that clicks together, often with little pegs that bend a bit. Note(again) that really small stuff tends to be much weaker along the directions of print.
Disadvantage is that you need to put a clamp in the design.(But maybe you can choose an existing one) Plastic-based springs are okay, but not so good under constant tension, as they creep.
Large enough threads are printable, so things can be screwed together. You have to get both the positive&negative threads correct to have it hold on properly.
Screws and nuts are a straightforward and easy-to-get-decent way of connecting things. Disadvantage is that the Vitamin 'screw' has different lengths and sizes, making it less convenient.
Advantage is that it is easy to control the tightness. (Many uses, for instance locked nut to set something at some particular distance)
Using (rigid)metal wire(See Roping for flexible wire), can sometimes be used when something was designed for screws.(But obviously much scrappier) Usually it is more convenient to lay the wires closely in parallel before twisting them together. Note that bends often dont pull out easily so it is better to have all the wire layed out nicely before putting it under tension.
Wire is usually a present vitamin, and you can easily cut off different lengths, and it is easier to fit into designs than screws. Disadvantage is that the wire may rust, and the methods are more tricky. Also, note that if there is any give and it wobbles, it will fail eventually!
Tightly interlocking parts
These can take quite a bit of force to compress together, in some cases needing a pipe wrench.
Advantage is that it doesnt take any vitamins, disadvantage is that it needs to be designed in.
See Glue.(It is rather too terse at the moment)