Dimensions should be applied to features and arranged in such a way as to represent the function of the feature.

All types of dimensions have:

• nominal value - ideal size of feature
• tolerance - amount the value may vary above and below nominal value

The simplest dimensioning just specifies distances between points e.g. length, accompanied by a plus or minus tolerance.

The development of geometric dimensioning and tolerancing allows tolerancing of both geometry and dimensions (shapes, locations and sizes).

• linear - two parallel lines called extensions lines spaces at the distance between the two features are shown at each of the features. A line perpendicular to the extension lines with arrows at its endpoints is the dimension line with number midpoint.
• Ordinate - one horizontal and one vertical extension line establish an origin for the entire view.

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Good practice online modules

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• Dimensional
• Overview of basic design and interpretation of an engineering drawing
• An introduction to datum
• An introduction to geometric tolerancing
• An introduction to the designer's role when designing a component
• An introduction to the manufacturer's role when designing a component
• An introduction to the inspector's role when designing a component