Standards

Metric Dimensioning Guide

Written by Allan Wise

Created 2-July-2004

Lines

Projection lines indicate extremities of a dimension. A gap of 1mm is generally drawn from the object to the start of the projection line. An extension of 2mm is generally drawn past the dimension line.

Dimension lines run between the projection lines. They generally terminate with and arrowhead or tick. Arrowheads are usually 3x1mm and ticks, 2mm long.

Thin linework should be used of dimension and projection lines.

For dimensions that cannot be drawn to there true termination point, the free end is terminated in a double arrowhead.

15 x scale is the formula for working out the gap between dimension lines and the object.

Notation

Dimensions are usually in millimetres (millimeters). Place a note in the title block stating "ALL DIMENSIONS IN mm" to avoid the need to specify "mm" after every number.

Dimensions lest than 1 should lead with a "0" before the decimal point. Example: 0.5

Angular dimesions are specified in decimal degrees, degrees and minutes or degrees minutes and seconds. Example: 10.5° - 10°30' - 10°30'30"

When specifying an overall dimension, one non-critical component dimension should be omitted. Where all component dimesions must be specified, and overall lenght should be specified as an auxiliary dimension.

Auxiliary dimensions should be shown in brackets or noted "REFERENCE".

Dimensions that are not to scale are underlined or noted "NTS".

Where the "plus minus" symbol appears after a number, the number has been rounded of to the nearest whole number. Generally this is not required, unless in clears up why a dimension string does not close off.

Alignment

There are two common methods of aligning notations, "Aligned" is the most common.

Aligned: The dimensions are parallel to the dimension line. Dimensions can always be readable either from the bottom or the right of the drawing.

Unidirectional: All dimensions are written horizontally.

Dimensioning rules of thumb

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