The Pendulum Pivot

The pivot is the most delicate component of the pendulum. It must have cylindrical symmetry. That is, it must look the same from every direction, as it hangs vertically. The method of creating a symmetric pivot here is to carve it from the existing nylon guitar string.

The diameter of the guitar string is about 1 mm. The string is thinned, over a length of about 1 cm, to a diameter of about 0.3 mm, or less. This can be done with sandpaper, stroking down, and turning the string about the vertical axis after each stroke. In practice the string tends to turn to a ribbon instead of a cylinder, but the procedure has yielded successful pivots for this pendulum.

The method used currently involves more equipment, but reduces preparation time from days to minutes. A sketch is shown at the right. The string hangs from the chuck of a drill press, tensioned with a weight of about 200 grams at the bottom. A Dremel tool with a grinding cutter in place is mounted on a translating stage so that the cutter moves sideways to touch the string, just below the chuck jaws.

The drill press chuck is rotated at about 1 revolution per minute while the cutter is pressed a little closer with each revolution. To make the drill press rotate that slowly, the motor from a barbecue rotisserie is fastened to the ordinary drive shaft, with a piece of car radiator hose. The rotisserie motor is clamped to the lid of the drill press pulley chamber.

The cutter makes a quarter-moon shaped groove around the string. When the thinnest part is about 0.3mm in diameter, the Dremel is pulled away. The drill press chuck is loosened and the string is lowered about 2 mm. The process is repeated, so that the string has the appearance of a string of beads, about 1cm long.

The finished guitar string is mounted in the pin vise. The pin vise collet is 1 to 2 mm above the top most groove of the pivot. This insures that the string flexes in the grooved section, and not at the pin vise collet. The pin vise collet is not cylindrically symmetric, and will create preferred directions, if the pivot point is at the collet.

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