AutoCAD & AutoCAD LT All-In-One Desk Reference For Dummies (2006)

212 Book III: Annotating Drawings

Chapter 2: Dimensioning

In This Chapter

Understanding what a dimension is made of

Types of dimensions

Using and creating dimension styles

Creating dimensions

Editing dimensions

Leaders

Geometric tolerances

Dimensions are a form of annotation that provides information about how a product should be created or assembled. Typically, dimensions

provide information on how long a part is, the diameter of a hole, or a callout in the form of a leader. In this chapter, you find out how to create and work with dimensions, leaders, and geometric tolerances.

AutoCAD and AutoCAD LT offer a number of dimension-related commands to help convey your design ideas to the people who are doing the construction (in the case of a building) or manufacturing (in the case of a machine part). Just as text styles manage the appearance of text in a drawing, dimension styles do the same thing for dimensions. Dimension styles allow you to define visually how dimensions appear in a drawing, but are much more complex than text styles.

Understanding What a Dimension Is Made Of

Dimensions are one of the most complex objects in AutoCAD and AutoCAD LT because of the many options that are available to control how they appear in a drawing. Dimensions are made up of several sub-objects or components that define what the dimension looks like. Some of the sub-objects that define a dimension are extension lines, dimension lines, arrowheads, dimension text, and origin points.

214 Understanding What a Dimension Is Made Of

Before you begin creating dimensions, you should understand some of the common terminology for the different parts of a dimension (see Figure 2-1). Although each of the different dimension types has its own purpose, they do share some things in common.

	Arrowhead		Dimension text		Dimension line
Figure 2-1:									Extension line
The parts of
									Origin point
a dimension.

Here are the five main parts of a dimension:

Origin points: Origin points are used to define the measurement value for the dimension text and are small non-printing points that are attached to the dimension. Origin points are typically placed on an object, which allows you to move or reposition a dimension that might need to be updated.

Extension lines: Extension lines extend up and away from the object that is being dimensioned to the dimension line and arrowheads. Extension lines typically have a gap between them and the origin points of the dimension object and extend past the dimension line. A gap helps show that the dimension is not part of the actual object.

Arrowheads: Arrowheads appear at the end of a dimension line and are used to indicate where a dimension starts and ends. Arrowheads can be displayed as open or closed arrows, architectural ticks, dots, or userdefined arrows, among other arrowhead styles.

Dimension line: Dimension lines are displayed typically between the extension lines and go outward from the center of the dimension object towards the extension line. Based on where the dimension text is placed, the dimension line may appear broken or might appear on the outside of the extension lines.

Dimension text: Dimension text provides the actual value of measurement that the dimension was created with and can be formatted in a variety of different ways. Some of the formatting options are the units the measurement is expressed in, the type of decimal separator that should be used, and how the value should be rounded.

Each of the parts of a dimension can be formatted globally using dimension styles, or individually using what are known as dimension overrides. We talk more about dimension styles and dimension overrides later in this chapter.

Types of Dimensions 215

Types of Dimensions

Dimensions are a special type of block that AutoCAD treats differently from the blocks that you can create and insert into drawings; we cover blocks later in Chapter 1 of Book VI. Because dimensions are a special type of object, you interact with them differently. Dimensions can be created so that if the object that was dimensioned is changed, the dimension is updated automatically. Dimensions can also be created so that they are exploded after they have been created (which is not recommended because you won’t be able to quickly update the dimension when the geometry in the drawing changes). They can also be created so that if the original object that is dimensioned is modified, the dimension doesn’t update. No matter what type of dimension is created, they all look the same, but they are edited in much different ways.

Associative dimensions

Associative dimensions are created as a single object — so when you select any part of the dimension, the entire dimension is selected. An associative dimension is linked to the original object that was dimensioned, so when the original object is modified, the dimension is updated accordingly. Modifying the original dimensioned object by moving, rotating, stretching, or scaling it causes the dimension to update automatically. Fully associated dimensions were first introduced in AutoCAD 2002; prior to AutoCAD 2002, associated dimensions meant that the dimension was all one object and had no association with the dimensioned object.

Non-associative dimensions

Non-associative dimensions are created as a single object, just like associative dimensions are. Unlike associative dimensions however, non-associative dimensions are not linked to the original dimensioned object and therefore are not updated when the original dimensioned object is modified. If you select the dimension when modifying the original dimensioned object, it is still updated based on the type of modify command that is being used on both the object and dimension. Prior to AutoCAD 2002, non-associated dimensions were referred to as associative dimensions because they were

a single object instead of individual objects.

Exploded dimensions

Exploded dimensions are created as individual 2D objects and are very hard to update because the original objects have to be deleted each time a change is made. We recommend that you don’t create exploded dimensions because it can lead to inaccuracies to the dimensions if the text is not

Book III

Chapter 2

Dimensioning

216 Specifying the Type of Dimension to Create

updated. If you really need to create an exploded dimension, it is best to create associative dimensions and then use the EXPLODE command on a dimension.

Specifying the Type of Dimension to Create

To specify whether new dimensions are created as associative or nonassociative dimensions, you can check or uncheck the Make New Dimensions Associative check box under the Associative Dimensioning section on the User Preferences tab (see Figure 2-2).

Figure 2-2:

Specifying whether new dimensions should be created as associative.

Follow these steps to determine whether new dimensions should be created as associative or non-associative.

1.From the Tools menu, choose Options.

The Options dialog box is displayed.

2.In the Options dialog box, click the User Preferences tab.

3.On the User Preferences tab under the Associative Dimensioning section, check or uncheck the Make New Dimensions Associative check box.

If the option Make New Dimensions Associative is checked, all new dimensions are created as associative.

The setting to create new dimensions associative is saved in the drawing. When you get a drawing from a vendor or client, be sure that the setting is set to create new dimensions as associative.

4.Click OK.

The changes are saved and the Options dialog box is closed.

Using and Creating Dimension Styles 217

Using and Creating Dimension Styles

Dimensions contain a large number of display options. Both AutoCAD and AutoCAD LT use dimension styles to contain groups of settings that control the appearance of dimensions. Dimension styles tell AutoCAD or AutoCAD LT how the dimension lines, extensions lines, dimension text, and arrowheads should be displayed for any dimension that is created in a drawing. Dimensions that reference a dimension style are updated when a change is made to the style, unless an override has been applied at the dimension object level.

Working with the Dimension Style Manager

Dimension styles are stored in a drawing file and are referenced by the dimensions that are created in the drawing. Each dimension that is created can reference a different dimension style. Dimension styles are created, modified, and managed through the Dimensions Style Manager dialog box (see Figure 2-3), which is displayed with the DDIM command.

Figure 2-3:

The Dimension Style Manager dialog box.

To start the DDIM command and display the Dimensions Style Manager dialog box, follow one of the methods outlined below:

Format or Dimension menu: Choose Format or Dimension Dimension Style.

Styles or Dimension toolbar: Click the Dimension Style button on the Styles or Dimension toolbar.

Keyboard input: Type DDIM and press Enter.

Book III

Chapter 2

Dimensioning

218 Using and Creating Dimension Styles

Creating a dimension style

Dimension styles can only be created from an existing style that already exists in the drawing; by default, each drawing contains either a dimensions style called Standard (imperial units) or ISO-25 (metric units). The Dimension Style Manager is used to create the new dimension style and then allows you to make any necessary property changes to the dimension style that was just created. The following procedure creates a new dimension style:

1.Use one of the previously mentioned methods to open the Dimension Style Manager dialog box.

2.In the Dimension Style Manager dialog box, click New.

The Create New Dimension Style dialog box (see Figure 2-4) is displayed.

Figure 2-4:

Creating a new dimension style.

3.In the Create New Dimension Style dialog box, enter a name in the New Style Name text box for the dimension style that you are creating.

The name entered is used to set the dimension style current when you want to create new dimensions using that dimension style.

4.Select the dimension style that you want to base the new dimension style on from the Start With drop-down list.

The dimension style that you base the new dimension style on will be identical to the one you are creating until you make changes to the new dimension style.

5.Select All Dimensions from the Use For drop-down list.

The Use For drop-down list allows you to create substyles for a dimension style. Substyles allow you to define specific characteristics for certain types of dimensions that can be created using a single dimension style. Substyles are used when the parent dimension style is used.

6.Click Continue.

The New Dimension Style dialog box is displayed and allows you to change the settings of the new dimension style.

7.In the New Dimension Style dialog box, adjust the properties of the dimension style or the substyle as necessary.

Using and Creating Dimension Styles 219

8.Click OK.

The New Dimension Style dialog box closes and the dimension style is saved in the drawing.

9.In the Dimension Style Manager dialog box, click Close.

The Dimension Style Manager dialog box closes.

The New Dimension Style dialog box

The New Dimension Style dialog box allows you to define how dimensions in a drawing look when they are created. The New Dimension Style dialog box is divided up into seven tabs, which help to organize the properties of a dimension style. As you change the properties for the dimension style, the preview of the dimension style in the upper-right corner of the dialog box is updated.

The Lines tab

The Lines tab (see Figure 2-5) allows you to define the display of dimension and extension lines. On the Lines tab, you can specify the color, linetype, lineweight, which extension and dimension lines are suppressed, and some spacing/offset values that help to define what a dimension should look like when assigned the dimension style.

Figure 2-5:

The Lines tab of the New

Dimension Style dialog box.

Book III

Chapter 2

Dimensioning

The Symbols and Arrows tab

The Symbols and Arrows tab (see Figure 2-6) allows you to define the characteristics of the terminators and symbols for dimensions. On the Symbols and Arrows tab, you can specify the type of arrowheads that are used, how center marks are drawn, the symbol that should be used for arc length dimensions, and the jog angle for jogged radial dimensions.

220 Using and Creating Dimension Styles

Figure 2-6:

The Symbols and Arrows tab of

the New

Dimension Style dialog box.

The Text tab

The Text tab (see Figure 2-7) allows you to define how dimension text is placed along the dimension line and how it appears. On the Text tab, you can specify the color, text style, background fill color, height, and some other properties that affect how text is created, placed, and aligned with the dimension line.

If you assign a text style that has a specified text height to a dimension style, the text height that is specified on the Text tab is overridden by the value specified in the text style. For the most control, use a height of 0 (zero) when defining a text style.

Figure 2-7:

The Text tab of the New Dimension Style dialog box.

Using and Creating Dimension Styles 221

The Fit tab

The Fit tab (see Figure 2-8) allows you to define how dimension text and arrowheads are placed between extension lines. You can specify how dimension text and arrowheads are placed in relation to the amount of space between the extension lines; the scale of dimension text in model space and on a layout tab; whether dimension text is placed with a leader; and whether a dimension line should be drawn between the extension lines.

Figure 2-8:

The Fit tab of the New Dimension Style dialog box.

The Primary Units tab

The Primary Units tab (see Figure 2-9) allows you to define how dimension text should appear. On the Primary Units tab, you can specify how dimension text is formatted including decimal separators, rounding factor, measurement units, prefix and suffix for the dimension text, measurement scale factor, and how leading and trailing zeros are displayed. The settings affect both linear and angular measurement values for dimensions.

The Alternate Units tab

The Alternate Units tab (see Figure 2-10) allows you to define how alternate measurements appear as part of the dimension text. The Alternate Units tab allows you to control whether an alternate measurement value is placed with the primary measurement value. Typically, alternate units are used when you want to display dimensions with both imperial and metric values. The Alternate Units tab contains options that are used for the formatting of the alternate measurement text and its placement.

Book III

Chapter 2

Dimensioning