Learn/Shape Generator/Built-in profiles
Involute Gear
At a glance
- External (spur), Internal (ring), and Rack types
- Module (mm) or Diametral Pitch (1/in) — automatic conversion displayed
- Pressure angle: freely settable to any value
- Profile shift coefficient (external gears)
- Bore option for external; wall thickness for internal; rack height for rack
- Warns on undercut potential
The Involute Gear shape (Power Transmission category) cuts true involute tooth profiles for spur gears, ring gears, and racks — the profiles that actually mesh, not decorative gear outlines. Pick it from the Shape Generator dialog in Drafting Sketch, Design, Laser, or Plasma.
Types and sizing
Type selects External (spur), Internal (ring), or Rack. Sizing System switches between Module (mm) — default module 1.0 — and Diametral Pitch (teeth per inch of pitch diameter, default 32). Any two gears cut with the same module (or DP) and pressure angle will mesh, so size a mating pair from the same settings and only change the tooth counts.
Shared parameters: Num Teeth (default 20; external and internal gears need at least 4, a rack needs at least 1) and Pressure Angle (default 20°, anything above 5° is accepted — use 14.5° for older imperial gearing). External and internal gears add a Profile Shift (x) coefficient; positive shift thickens the tooth at the root, which is how you avoid undercut on low tooth counts. Each type has one extra dimension: Bore Dia for external, Wall Thickness outside the root circle for internal, and Rack Height (body depth below the teeth) for rack.
How the teeth are built
Each flank is a genuine involute unwound from the base circle, sampled and then simplified so the DXF stays light without losing the curve. Tip and root circles come out as true arcs. Tooth proportions are standard full-depth: addendum of one module, dedendum of 1.25 modules, both adjusted by profile shift. When the root circle falls inside the base circle (small tooth counts), the generator connects the flanks to the root arc with radial drops — the standard fabrication compromise, since no involute exists below the base circle. Internal gears invert the tooth (wide at the outer root, narrow at the inner tip) and wrap it with an outer wall circle; racks get straight-flanked trapezoidal teeth at the pressure angle on a solid body.
Tips
- Module is metric-native: it is entered in millimeters even though the shape is inch-authored — the conversion is handled for you.
- For a working pair, give a low-tooth-count pinion positive profile shift rather than accepting
a thin, undercut root — watch the preview as you raise
x. - Plasma-cut gears mesh better after a cleanup pass on the flanks; laser and router output is usually usable as-cut. See live preview to sanity-check the tooth form before cutting.