centerlines

Centerlines are a type of line used in technical drawings and computer-aided design to indicate the central axis or the center of symmetry of a feature. They do not represent physical edges but rather the center around which a feature is formed or about which an object rotates. Centerlines are commonly used to locate the centers of circular or cylindrical features such as holes, bores, and shafts, as well as the axes of symmetry in parts and assemblies. They also mark the midline of a part or the centerline of a path or corridor in civil engineering projects.

Notation and appearance: In most drafting standards, centerlines are drawn as a line consisting of alternating long and short dashes; the spacing and length conventions vary by standard. When extending across multiple features, centerlines are typically broken at boundaries and extended as needed to show alignment with other features. Centerlines must be differentiated from dimension lines, extension lines, construction lines, and leader lines.

Applications and guidance: On a hole pattern, a centerline runs through the centers of all holes to define their alignment and pitch. In an assembly, centerlines indicate the axis of rotation or the symmetry of mating parts, guiding interpretation for manufacturing, inspection, and assembly.

Standards: Centerline conventions are specified in drafting standards such as ANSI/ASME Y14.5 and ISO 128. Adherence ensures consistent communication among engineers, machinists, and inspectors.