An instrument or template used by engineers and pattern makers to draw the approximate involute or cycloidal profiles of gear teeth. Before computer-aided design, these tools allowed draftsmen to establish the correct base circle, pitch circle and tooth curvature for manufacturing templates. They rely on standardized geometric approximations to ensure gears will mesh properly.
The old blueprints from the foundry still show the layout marks made with an odontograph.
Related terms
Involute
The curve traced by the end of a taut string unwound from a circle, the shape of almost every gear tooth flank.
Base circle
The circle from which the involute tooth profile is unwound, smaller than the pitch circle.
Pitch diameter
The diameter of the pitch circle, the imaginary rolling circle on which tooth size and gear ratio are based.