newton millimeter

The newton millimetre is the native torque of the millimetre-built engineering model. Every mainstream finite element package lets you work in newtons and millimetres, and the choice has a famous pair of consequences. The pleasant one: stress arrives directly in $\mathrm{N/mm^2}$, which is exactly the megapascal. The unpleasant one: mass must then be carried in tonnes, so the density of steel becomes $7.85 \times 10^{-9}$ tonnes per cubic millimetre, and typing the familiar 7850 into an N-mm model makes the structure a trillion times too dense, a mistake most analysts make exactly once. Reaction moments, section forces and every torque such a model outputs arrive in N·mm.

Outside analysis, the unit belongs to small hardware. Tightening torques for the smallest machine screws in electronics are specified in newton millimetres because the same figures in N·m would begin with a string of zeros, and a small hobby servo rated 10 kgf·cm works out to about 980 N·mm, just under one newton metre. The rule from the caution on this page bears repeating in the working direction: divide N·mm by 1000 to reach N·m, always.

On this site the torque converters move N·mm across to N·m, kgf·cm and ounce-inches, and the motor torque-power-speed solver in the power transmission catalog accepts it directly, which suits the small-motor datasheets the unit lives on.