Helpful Hints for the Cycloidal Scribbling-Engine
Velocipede's Cycloidal Scribbling-Engine is my homage and companion to the Spirograph. It's a set of laser-cut gears and rings, available on Ponoko. This page is meant to be a sort of help file to go with it.
Additional Things You Will Need:
pins to hold stationary pieces in placeNOTE! This drawing tool uses pins with sharp points. It is not recommended that you give it to a small child (or foolish grown-up person) who might eat the pins. Eating pins is pretty much always a bad idea.
Pictured above are several kinds of pens.
- Bic Cristal
- Zebra Sarasa Clip
- Gelly Roll gel pen
- Ballpoint from a dollar-store multi-pack
- Sakura Micron
- Sharpie Ultra Fine Point
Addendum: Since I first wrote this page, I've discovered that a website called Sound Feelings has all sorts of Spirograph-related supplies: pins, baseboards, pens both plain and fancy, even replacement ballpoint springs. One stop shopping!
Making Drawings
The laser-cutting process leaves a slight residue on the cut surfaces. This means that the first time you mesh one gear with another, there is likely to be a slight stickiness which makes it difficult to draw smoothly. Don't panic! After a couple of times around, the residue will wear off, and things will smooth out.
Everyone who has used a classic Spirograph will remember the discouraging frustration of getting two-thirds finished with a pattern, only to have the gear skip its track and send the pen skidding across the paper, ruining the drawing entirely. Very annoying! However, by thinking about what happens when you're making the drawing, it is possible to reduce this problem.
First, consider what's happening when a gear travels around the inside of a ring. You push the gear around in a circular path, while at the same time also pushing slightly outward, so the teeth remain in contact.
If the gear had a hole in the exact center, and the pushing-force—say, a pen—was in this central hole, the gear would go around in a tidy circle, and would be unlikely to skip off the track. But when the pen-hole is offset from the center, its path stops being circular and starts to make those lovely cycloidal shapes that are the whole point of the spirograph. Thus, the pushing of the pen also goes in different directions, following the path it's traveling along. At some points in the pattern, this works to your advantage.
In the picture above, the pen is tending to push the gear toward the outer ring. But as the pen approaches the point closest to the outer edge, its path of direction changes sharply. It does a rapid hairpin turn, and starts pushing the gear away from the ring.
It is at this stage of things that the gear is most likely to disengage. So, to prevent that happening, you give the gear a gentle nudge with your other hand, to keep it on track. It's especially important to do this with the larger gears. For one thing, they're heavier, so they need a bigger push to get them moving. That bigger push is more likely to send them off in the wrong direction. For another thing, their diameters are larger, which means that the pen is farther away from the nice stable point in the center of the gear. More distance gives you more leverage, which magnifies the push.
The ideal situation is to make sure that the gear is kept moving smoothly and steadily in a circle, and let it pull the pen in the proper path, instead of using the pen to drive the gear. In actual practice it works out to be a sort of back-and-forth thing, where the pen is pushing some of the time and being pulled some of the time. Keep the speed moderate; don't try to whip the gear around as fast as it will go, but allow enough time for the line to be drawn. The more you practice it, the more your hands will get used to feeling what needs to happen, and when.