Dimensioning and details for manufacture

Manufacturing drawings must communicate enough information for a component to be made accurately, consistently and without ambiguity.

Important information can include:

overall and component dimensions
hole diameters and positions
radii and angles
material and thickness
tolerances where variation matters
surface finish or treatment
assembly and fixing details.

Dimensioning and datums

Dimensions should be stated clearly rather than forcing a manufacturer to calculate important values from several other measurements. A datum is a common reference edge, point or surface from which related dimensions are taken, reducing the build-up of marking-out errors.

Tolerances

A tolerance states the permitted variation from a nominal dimension. Tight tolerances are important where parts must fit, align or move precisely, but unnecessary precision increases machining and inspection cost. The tolerance should therefore match the function of the feature.

Geometric dimensioning and tolerancing (GD&T)

GD&T is a standardised system of engineering drawing symbols used to communicate allowable variation in the geometry and position of features. It can specify requirements such as the permitted position, orientation or form of a feature in relation to a datum.

GD&T is useful when simple linear dimensions do not fully describe how accurately parts must relate to one another, particularly for interchangeable components and precision assemblies.

Other manufacturing information

Sectional views, exploded views, material specifications, quantities, process notes and finishes may also be needed. Clear manufacturing information reduces interpretation errors and makes consistent production and assembly more likely.