AutoCAD  Training

There are two basic principles to follow when you add dimensions to a drawing:

1. Assignment of more than one dimension to a feature should be avoided.
2. The reader of the drawing should not be required to do calculation in order to obtain the dimension of a feature.

Components of a Dimension

Refer to Figure 1. A dimension consists of four components:

1. A dimension value.
2. A dimension line that is parallel to the direction of the described feature.
3. A pair of arrowheads.
4. A pair of extension lines projecting from the feature to which the dimension refers. There should be a small gap between the end of the extension line and the feature. The extension line should project a short distance away from the intersection with the dimension line.

Figure 1 Components of a dimension

In addition to the four general components of a dimension, AutoCAD includes definition points in a dimension block for reference. These points are stored in a special layer called DEFPOINTS. You may leave this layer ON all the time because the entities on this layer will not be plotted. When you edit an associative dimension, AutoCAD refers to these points. See Figure 2.

Figure 2 Definition points

The components of a dimension that are listed above can be grouped together as a block or treated as separate entities. If they are grouped as a block, the dimension is called an associative dimension. You can edit or modify the appearance of an associative dimension as a single unit. The dimensions will update automatically after you edit a related feature. On the other hand, if they are created as separate entities, you can edit them independently using general editing commands.

The way a dimension appears depends on the current dimension style. To set up a dimension style, you may use the DDIM command or the DIMSTYLE command. The DDIM command works graphically by providing interactive dialog boxes. The DIMSTYLE command works at the command line interface.

From the Dimension pull-down menu, select the Style... item to run the DDIM command. See Figure 3.

<Dimension> <Style...>

Command: DDIM

Figure 3  Dimension Styles dialog box

For each dimension style, there is a family of seven members. These consist of a parent and six other family members ¾ linear, diameter, radial, ordinate, angular, and leader. The parent is a set of variables that applies to all types of dimensions. The other family members are subsets of variable settings that are specific to the types of dimensions that they refer to.

Geometry

The geometry of a dimension includes the dimension line, the arrowheads, the extension lines, the center mark, and the overall geometry scale. See Figure 4..

Figure 4  Geometry dialog box

There are five areas in the Geometry dialog box.

1. Dimension Line

2. Arrowheads

3. Extension Line

4. Center

5. Scale

Dimension format includes the placement of dimension text, arrowheads, and leader lines. See Figure 5.

Figure 5  Format dialog box

There are four areas in the Format dialog box.

1. Unnamed

2. Text

3. Horizontal

4. Vertical

Annotation concerns the way the dimension text is displayed. See Figure 6.

Figure 6  Annotation dialog box

There are five areas in the Annotation dialog box.

Figure 7  Primary Units dialog box

In the Primary Units dialog box, there are five areas.

Figure 8  Alternate Units dialog box

There are four useable areas in the Alternate Units dialog box.

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