odontograph

An odontograph is a mechanical drafting instrument used to mark the precise outline of gear teeth on a blank wheel before cutting or hobbing. It holds a stylus or scribe that traces the involute curve profile of a tooth, transferring that shape onto the metal or casting surface. The instrument was essential in the era before CNC machining, when gear teeth were laid out by hand before being cut with a milling machine or hobbing tool.

The tool works by rolling a template or cam along a gear blank, with the stylus following the involute profile. Because involute teeth must mesh smoothly with mating gears at a precise pressure angle (typically 14.5 or 20 degrees), the odontograph ensures consistent tooth geometry from wheel to wheel. Variants include fixed odontographs mounted to a dividing head or milling machine table, and handheld versions with adjustable parameters for different module sizes and tooth counts.

Accuracy and limitations

The odontograph could produce layouts accurate to within 0.005 to 0.01 inch under careful use, sufficient for industrial gears running at moderate speeds. Precision depended on the quality of the template, the stability of the machine table, and the operator's skill. Heavy or high-speed gears requiring tighter tolerances still demanded final finishing by shaving or grinding, and modern production relies entirely on computer-controlled cutting.

The term combines the Greek odous (tooth) with graph (to write or mark). Though odontographs are now largely museum pieces, replaced by digital gear design and CAM programming, they remain useful in job shops and repair facilities where a few replacement gears must be made without re-programming a CNC machine. Some old odontographs are still calibrated and in use in small-batch spur and helical gear work.