If the undercuts are ignored the resulting gears will jam if machined and assembled precisely. For an elegant description of the issue including a recipe for how to graphically create correct spur gears see Michal Zalewski’s corresponding section in part six of his excellent Guerrilla guide to CNC machining, mold making, and resin casting series.
However, I have not found a freely available tool that correctly caters for undercuts that occur for smaller tooth counts (around 10 or less): An involute curve is fairly easy to calculate and quite a number of freely available tools and scripts exist that use this fact to create involute gear profiles (see e.g., Gear template generator, Parametric Involute Bevel and Spur Gears, OpenJsCad’s gear demo, and many more). Still here and interested in some background? Perfect! Involute gears are by far the most commonly used gears today. For information about version 2 see this post. UPDATE: This post refers to an old version.
For the impatient: Just head over to the Online Involute Spur Gear Builder page.
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