Animation Fundamentals: Threads and Rods
Threads
Threads—usually fine nylon monofilament—can do things that cannot be accomplished any other way. Let's start right off with an example: swings. Can you think of another means to make the swings move that's practically invisible?
In this case, the monofilament threads run up the legs of the swing set to a tiny bell crank at the top. The threads are held taut and alternately pulled to make the swing move. Threads are likewise used for the seesaws. Sometimes the threads are allowed to be visible for lack of a way to conceal them; in these situations, about all you can do is create a distracting background. For the frisbee game, I located it near the edge of an overgrown field full of brambles and bushes to distract the eye. On the other hand, the threads may represent something real; for instance, with the water tower, the water spout is suspended by a pair of cables. Special mention must be made for the fisherman, because in that case the thread is representing itself.
When planning on using thread, bear in mind that the objects they move almost always need to be under total control; that is to say, in order to achieve realistic action, very often two threads are required, and very often held taut. Relying on gravity to move a tiny object reliably is not recommended. For one thing, tiny objects mean tiny gravitational forces; that, combined with a comparatively huge amount of friction in a mechanical system, can spell failure. Also, motorized devices used to move the threads must almost always be very low RPM, because tiny objects move tiny distances, and realism is broken if they move too quickly. Keeping the threads taut has another benefit: it keeps them from slipping off of sheaves or pulleys. Also, when threads must negotiate complex paths, points where the thread changes direction must be smooth and at least somewhat curved (no sharp corners) to prevent excessive wear.
Most of the time, the very finest monofilament is required. What I use is sewing thread, sometimes called "invisible mending thread," available at craft or sewing stores. The thread is surprisingly strong, although it's still delicate, so care must be taken when handling it. You will likely find it an enormous challenge handling the thread, especially when tying knots, as it tends to have a mind of its own.
Finally, I must point out that the excavator is a project that makes extensive use of thread, so it serves as an excellent tutorial on the subject.
Rods
Rods may be thought of as rigid threads, and because of their rigidity, they can be used to push or rotate objects. A good subject to study for using rods is the dump truck, because the gate is moved by a rod. It also serves as a reminder about how gravity cannot be scaled down; the gate cannot be allowed to swing freely because it will jiggle quickly, rather than sway slowly. Hence, the rod offers complete control of its behavior.
One thing to bear in mind when using rods: whenever practical, provide a guide for as much of the rod as possible; it will otherwise want to bend out of shape and fail to move the object to which it's connected. It is rare that a super-fine rods and wires will remain straight when you want them to, especially when the object being moved and the motorized mechanism are not in precise alignment. An example of an exception is the enginehouse doors: they are small and light enough, and the distance short enough, that the wire remains straight.
Rods can be any diameter, from the very finest stainless steel 0.04" music wire to 1/16" rod. Rarely would one want to use anything other than stainless steel spring or music wire, or possibly phosphor bronze, because music wire retains its shape, as opposed to, say, brass or copper wire, which is ductile and easily deformed permanently.
As with threads, rods are usually hidden; a typical example of this is the oil column (above), where the rod runs up the column itself. However, occasionally rods are deliberately allowed to be seen; an example of this would be the flying model airplane. This example also highlights one of the drawbacks of visible rods: when the rod is very fine, the movement of the object is usually compromised by jittering. There isn't much to be done about this except to use the thickest rod possible that doesn't look absurd.
Sometimes you don't want the rod to be visible, but have no other option but to leave it exposed. Such was the case for one of my barber poles (above). In this instance, I used 0.04" music wire tinted black with a Sharpie; this rendered the rod very nearly invisible.
Finally, special mention goes out to the Bobcat, which uses both threads and rods to operate.