Dimensioning

When engineers design parts that must fit together, they cannot make everything perfectly. Tolerances are the acceptable differences in size allowed. Think of manufacturing car engine parts in Lagos—a piston must slide smoothly inside a cylinder, but not so loose it wastes fuel. The upper and lower size limits define how much variation is acceptable.

An allowance is the intentional gap between two mating parts. Fits describe how parts work together: a tight fit means little space between parts, while a loose fit allows movement. When dimensioning technical drawings, you write the basic dimension plus the tolerance range. For example, a shaft might be dimensioned as 20mm +0.05mm/-0.03mm, meaning it can be between 20.05mm and 19.97mm.