My dad is an avid model airplane builder and he is currently building a new electrically-propelled glider of his own design. This particular airplane will have styrofoam-core wings and two weekends ago he was trying out a new way of making them.
Such wing profiles are traditionally cut out from blocks of styrofoam with a hot-wire cutter that is pulled by hand over plywood templates. However the width of the cut depends on the speed of the wire as it travels through the material and since you can't manually pull the wire at a perfectly constant speed the wings end up full of little pits and grooves. Of course, nowadays you can get computer controlled cutters that control the hot wire with servos and can cut out any shape with perfect steadiness and without the need for templates.
My dad however went for another approach: he made a purely mechanical device that pulls the wire over the templates with constant speed. The interesting bit here is that the two ends of the wire usually have to travel different lengths through the styrofoam in the same amount of time, depending on the wing taper ratio. He achieved this with an adjustable system of levers, pulleys and ropes that would make for a nice high-school mechanics class demonstration.
The whole thing is powered by gravity and an occasional nudge by hand. In fact, for perfect straight cuts just the weight of the hot-wire cutter is sufficient without additional mechanisms.
As you can see from the (long) video below, it takes one to two minutes to make one cut (and you need several per wing, depending on the number of segments), so it's not the fastest thing around. But it makes up for it with perfectly smooth cuts and if you only make a few wings per year it's perfectly sufficient.