This is a customizer for gears and plates used in my Spiro Machine recently published (http://www.thingiverse.com/thing:2039878)
I've used the Parametric Involute Bevel and Spur Gears by GregFrost
http://www.thingiverse.com/thing:3575 code for this one.
Why a customizer?
Number of teeths on gears and plates used in a spirograph determines the number of loops (also called points) of drawn shapes.
In the Spiro Machine files i've included a bunch of gears and plates but you may want to ry others configurations. Customizer creates gears and plates with any number of teeths.
The Excel File
There's a relationship between plate and gears teeths so i also included a Excel file as a calculator for your custom gears.
How it works?
When you create a gear/plate set the number of teeths in plate determines the variations of shapes depending of gears teeths.
With this file you can find a plate's teeths number with many variations and discard the plates 'bored' or 'weirds'
Once you have some 'interesting' plates, look for gears that can draw nice shapes.
In the file uploaded i've highlighted the plates and gears that i've used for the Spiro Machine published.
Mmmm....I don't know if i explained well. English it's not my native language and have big grammar lacks. Use comments for asking some doubts or things related to this.
Hope you find it useful, explore your own variations and good luck!!!