Cycloidal Scribbling-Engine

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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 place
Push-pins work fine, but map pins are thinner, and will make smaller holes in your paper. Also, they have shorter points, so they don't need as thick a piece of foamcore. The No. 100 Series is very similar to the original Spirograph pins, and they come in lots of nice colors.
NOTE! 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.
foamcore on which to pin your gears and paper
Found at art supply stores, craft stores, and probably some office supplies. Make sure the foamcore is thick enough that the pins don't go all the way through it and poke holes in your table. (If you can't find the thicker kind of foamcore, stack up two pieces, or put a sheet of corrugated cardboard underneath.) Eventually your foamcore will become so thoroughly perforated with pin-holes that the surface will be all bumpy and drawing will be difficult, and you will need a new piece. If it's getting close to that stage, but you don't have a replacement handy, it helps to put a sheet of waste-paper under your drawing paper, and pin through both of them.
paper
Or indeed any surface that seems like it might be fun to draw on.
pens
The most important consideration for pens is that they need to have long and narrow enough tips to fit easily through the holes in the gears.
A selection of pens

Pictured above are several kinds of pens.
  1. Bic Cristal
  2. Zebra Sarasa Clip
  3. Gelly Roll gel pen
  4. Ballpoint from a dollar-store multi-pack
  5. Sakura Micron
  6. Sharpie Ultra Fine Point
Notice the wide tapered cone of the Bic. This pen will not work! The acrylic material of the gears is too thick for the ballpoint to reach the paper, without making the holes prohibitively large. All the other examples work fine, even the gel pen which has a relatively fat tip.

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.

Basic Forces

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.

Helpful Forces

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.

Unhelpful Forces

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.



© 2010 Morgen Bell.