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14 November 2003

Mechanical Drawing the Old-Fashioned Way: Dimensions

by Brian Mansfield

A mechanical drawing is only a pretty picture and doesn’t give the viewer the full picture until it is dimensioned and annotated. This week’s article focuses on dimensioning your mechanical drawings.

Dimensions come in many flavors of which, the linear dimension is used most. Linear dimensions show length measurements. The stroke of the linear dimension, like all the other dimensions, is thinner than the stroke for visible lines similar to the width of centerlines. They consist of end lines and a length line with arrows at the end. See Figure 1 for an example of linear dimensions. Notice the measurement is placed in the middle of the length line. This number usually does not have units (the units are usually placed in the title block which will be explained in a future article). Linear dimensions come in variety of types such as horizontal, vertical and general. See Figure 2 for examples of each.

Figure 1: Linear Dimensions

Figure 2: Horizontal, Vertical and General Linear Dimensions

Another type of dimension is the radial and diametral. See Figure 3 for the variety of these types of dimensions.

Figure 3: Radial and Diametral Dimensions

The last type of dimension we want to present is the angular dimension. These dimensions are similar to the linear dimensions, but an arc is used the length line. See Figure 4 for an example of the angular dimension.

Figure 4: Angular Dimensions

As you can see the arrowheads are very important to the dimensions. There are many rules to follow when drawing arrowheads. The length of the arrow should be as long as your letters are high. The width of the arrow should be 1/3^rd its length. The arrowhead can be open or colored in but should be consistent throughout your drawing.

Also, there are good ways to dimension a drawing and bad ways to dimension a drawing. Some rules that you will want to follow are illustrated in Figures 5-8.

Figure 5: Try not to cross dimensions with leader lines. Share leader lines when possible

Figure 6: Try to dimension visible lines on an orthographic projection drawing

Figure 7: Use centerlines as leaders when dimensioning circles of arcs and circles

Figure 8: Leave out dimensions that can be inferred

Homework: Dimension some the drawings you have made in previous excercises.