Module and diametral pitch are the two most common ways to describe tooth size on a gear drawing, and they are the parameters that must match between mating gears. Getting them wrong is one of the most frequent causes of gear orders that do not fit. This article explains what they mean, how to convert between them, and what to check before you send a drawing.
Module, written as M, is the metric system of tooth sizing. It is the ratio of the pitch circle diameter in millimetres to the number of teeth, which is the same for two gears that mesh, and it describes how large the teeth are: a module 2 gear has teeth twice as big as a module 1 gear at the same tooth count. Standard modules follow a preferred series such as 0.5, 0.8, 1, 1.25, 1.5, 2, 2.5, 3, 4, 5, and 6, which keeps tooling standard and costs down.
Diametral pitch, written as DP, is the imperial system equivalent. It is the number of teeth per inch of pitch circle diameter, so a higher DP number means smaller teeth. Because the two systems are built on different units, they are not interchangeable: a module 2 gear will never mesh with a 12 DP gear. The conversion is M equals 25.4 divided by DP, so 12 DP is approximately module 2.12, and 8 DP is module 3.175. Using a converter is easy, but mixing systems on a drawing is a red flag for the supplier.
The pressure angle is the other number that must match. Most modern gears use 20 degrees, while older designs and some American replacements use 14.5 degrees. Two gears with the same module but different pressure angles will not mesh correctly even though they look similar. The drawing should also show the addendum height, whole depth, backlash class and any profile modification, because all of them affect how the pair runs.
When an old or worn gear must be reproduced, measuring the module is straightforward if the pitch circle diameter and tooth count are known, but those values can be hard to obtain from a worn part. In that case the fastest route is measurement across pins or over teeth, or simply sending a sample or good photos of the gear with its bore size to the manufacturer. A competent supplier can reverse-engineer the module, pressure angle and profile from the sample.
For buyers, the takeaway is simple: write the module or DP, pressure angle and tooth count clearly on the drawing, and state whether the standard is metric or imperial. If any value is uncertain, say so, and send a sample or photos; guessing a module wastes a week of lead time. At our factory, gears from module 0.5 upwards are cut on hobbing and shaping machines, and we routinely reverse-engineer samples for replacement orders.