Technical drawing

A technical drawing is a drawing of a single part that contains everything needed to manufacture and inspect it: the shape in orthographic views, dimensions, tolerances, surface requirements, material and delivery condition. It is also called a detail drawing, part drawing or manufacturing drawing. Unlike an assembly drawing, which shows how parts fit together, it always describes one part, unambiguously enough for any manufacturer to make and inspect it.

It consists of views, sections and details in orthographic projection (usually first-angle in Europe, third-angle in the US), dimensions with tolerances, geometric tolerances referenced to datums, and surface roughness symbols, for example Ra 1.6 µm on mating faces. Requirements such as heat treatment, surface finish or edge condition are added as notes. The title block carries the drawing number, part name, material (for example EN AW-6082-T6, or 1.4301 for 304 stainless steel), scale, a reference to general tolerances and the revision.

Drafting rules are standardized by ISO: ISO 128 for presentation, ISO 129-1 for dimensioning, ISO 5457 for sheet sizes and ISO 7200 for the title block. Geometric tolerances follow ISO 1101. Dimensions without a stated tolerance are covered by general tolerances, most often to ISO 2768, with class m (medium) typical for machined parts. Many US companies use ASME Y14.5 instead, so the drawing should state which system applies.

Today the drawing is usually derived from the 3D CAD model, so the geometry of both stays linked. Some companies are moving to model-based definition (MBD), where the 3D data itself carries tolerances and requirements as PMI. In everyday supplier work, however, the drawing, usually sent as a PDF, remains the main document for manufacturing, inspection and quality claims.

You need a technical drawing for every part made by an outside supplier where accuracy matters: machined components, sheet metal parts with bends and threads, weldments, or castings with machined surfaces. It is the basis for the quote (it shows the manufacturer the critical dimensions and how demanding inspection will be), for manufacturing, and for incoming inspection and the measurement report.

For quick 3D printed prototypes or simple laser-cut blanks, a 3D model or a DXF file with a short note on material and thickness is often enough. But once the part has to fit other parts, be made repeatedly with the same result, or be produced by different suppliers, a drawing always pays off.

The most common mistake is an incomplete drawing: no general tolerances, a vague material ("aluminum", "stainless"), no surface finish or edge condition, and a manufacturer left guessing. Redundant dimensions (one size defined by two routes) and chain dimensioning, where tolerances add up, also cause trouble. Dimension functional features from the datums the part actually locates on in the assembly.

Do not dimension features that cannot be measured, and specify tight tolerances only where the function needs them. If you send a drawing with a 3D model, agree with the manufacturer which document governs in a conflict, and keep both at the same revision. Never omit the projection symbol from the title block: suppliers used to the other method may misread the views.