Dimensioning Nonorthogonal Objects


Most of your dimensions will be horizontal or vertical, which are handled easily with the linear dimension. But eventually you'll need to apply many other types of nonorthogonal dimensions. For example, you might need to dimension a hexagonal shape whose sides are at 30° angles. In the following sections, you'll find out how to add dimensions to objects that are turned at an angle. You'll also learn about dimensioning arcs and circles.

Adding Nonorthogonal Linear Dimensions

You can add a linear dimension to an object that is not in a horizontal or vertical orientation using the aligned dimension. This type of dimension aligns the dimension line with the two points you select for dimensioning, as described in the following steps:

  1. Choose Dimension è Aligned or enter dal to start the aligned dimension.

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    USING OSNAPS WHILE DIMENSIONING

    Dimensions can adjust themselves to changes in the objects they are measuring. To take advantage of this feature, make sure you use osnaps while adding dimensions. The osnaps ensure that you're selecting exact points on objects so that AutoCAD can keep track of what it is you're dimensioning. If you don't know how to use osnaps, see Chapter 2.

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    In addition, you can dimension an object by selecting it instead of pointing to locations on the object. This also enables the dimension to "follow" the changes made to the dimensioned object.

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  2. At the Specify first extension line origin or <select object>: prompt, use an osnap to select the first end of the object being dimensioned.

  3. At the Specify second extension line origin: prompt, use an osnap again to select the other end of the object being dimensioned.

  4. At the Specify dimension line location or [Mtext/Text/Angle]: prompt, you'll see a dimension follow the cursor. Click a point to place the dimension. The dimension appears in the drawing, as shown in Figure 10.5.

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Figure 10.5: The aligned dimension of a nonorthogonal line

Just as with linear dimensions, you can enter t in step 4 to enter alternate text for the dimension.

You might also need to place a dimension at an angle that is not necessarily aligned with the drawing feature being dimensioned, as shown in Figure 10.6.

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Figure 10.6: A nonorthogonal linear dimension that is not aligned with the feature being dimensioned

Here's how to draw this type of dimension:

  1. Click the Linear tool on the Dimension toolbar.

  2. At the Specify first extension line origin or <select> object: prompt, use an osnap to select an appropriate location on the object being dimensioned.

  3. At the Specify second extension line origin: prompt, use an osnap again to select the other end of the object being dimensioned.

  4. At the Specify dimension line location or [Mtext/Text/Angle/Horizontal/Vertical/Rotated]: prompt, enter t to use the Rotated option.

  5. At the Specify angle of dimension line <0>: prompt, specify the angle at which the dimension line is to be placed. The angle value you enter is in relation to the x-axis of the drawing, so if you enter 0, for example, the dimension line will be horizontal.

  6. At the Specify dimension line location or [Mtext/Text/Angle/Horizontal/Vertical/Rotated]: prompt, you'll see the dimension line follow your cursor. Select a point to place the dimension line.

Dimensioning Arcs and Circles

Dimensioning diameters of arcs or circles is fairly simple:

  1. Choose Dimension è Diameter, or enter ddi.

  2. At the Select arc or circle: prompt, select the circle or arc you want to dimension.

  3. At the Specify dimension line location or [Mtext/Text/Angle]: prompt, you'll see the Diameter dimension drag along the circle as you move the cursor. If you move the cursor outside the circle, the dimension changes to display the dimension on the outside. (See the right image in Figure 10.7.)

  4. Place the dimension where you want it, and then click.

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Figure 10.7: Dimension showing the diameter of a circle

AutoCAD gives you the option to place the dimension text inside or outside the circle as you drag the temporary dimension over the circle. Be aware that, although AutoCAD gives you great deal of flexibility in the placement of dimensions, certain dimensioning standards define specific rules for dimensioning circles.

The Radius tool on the Dimension toolbar gives you a radius dimension just as the diameter dimension provides a circle's diameter. Figure 10.8 shows a radius dimension on the outside of the circle, but you can place it inside in a manner similar to the diameter dimension. The Center Mark tool on the Dimension toolbar just places a cross in the center of the selected arc or circle.

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Figure 10.8: A radius dimension shown on the outside of the circle

If you're dimensioning a large arc radius and you can't show the center of the arc within the area of your drawing, you can use a jogged radius dimension (choose Dimension è Jogged) instead of the usual Dimension è Radius option. The Jogged option places a jog in the dimension line to indicate that the center is not shown in the drawing.

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Dimensioning Angles and Arc Lengths

Dimensions aren't always used to show distances. You can use the angular dimension to indicate the angle between features in your drawing. Here's how:

  1. Choose Dimension è Angular or enter dan to start the angular dimension.

  2. At the Select arc, circle, line, or <specify vertex>: prompt, select the first line whose angle you want to dimension.

  3. At the Select second line: prompt, select the other line whose angle you want to dimension.

  4. At the Specify dimension arc line location or [Mtext/Text/Angle]: prompt, the dimension moves to different locations as you move the cursor. This allows you to select the angle to dimension, as shown in Figure 10.9.

  5. When the dimension appears in the location you want, click that point to fix the dimension in place.

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Figure 10.9: Two examples of an angular dimension

In addition to the angle of an arc, you can also dimension the length of an arc. Choose Dimension è Arc Length, and then select the arc whose length you want to dimension. The dimension appears and drags with the cursor. You can then click to place the Arc Length dimension.

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WHY DO MY ANGULAR DIMENSIONS SHOW ONLY WHOLE DEGREES?

By default, AutoCAD's dimensioning feature rounds off angular dimensions to the nearest degree. Follow these steps to show angles less than a whole degree:

  1. Select the angular dimension, then right-click, and finally select Precision.

  2. Click the Angle Precision option, and select the value you want for the dimension.

You can also change the Angle Precision setting for the current dimension style using the Modify Dimension Style dialog box, which is the same as the New Dimension Style dialog box. See the "Setting Up the Dimension's Appearance" section later in this chapter.

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Introducing AutoCAD 2008
Introducing AutoCAD 2008
ISBN: 0470121505
EAN: 2147483647
Year: 2007
Pages: 147
Authors: George Omura

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