Chapter 14 Drawing Dimensions 377

Dimensioning Arcs and Circles

When you dimension an arc or a circle, you measure its radius or diameter. It’s also common to mark arc and circle centers to clarify what you’re measuring. Arc and circle dimensions are most commonly used in mechanical drawings.

Marking arc and circle centers

Circle and arc centers are often marked in mechanical drawings because the center is an important aspect of a circle or arc but is not obvious without a mark. You set the size and type of mark when you create a dimension style, as explained in the next chapter. You can use a center mark (a small cross) or centerlines, as shown in Figure 14-14.

Choose Center Mark from the Dimension toolbar. At the Select arc or circle: prompt, pick the arc or circle you want to mark. The command draws the mark or lines.

Figure 14-14: Circles — one with a center mark and the other with centerlines.

Center mark

Centerlines

Dimensioning arc lengths

To dimension the length of an arc, choose Arc Length from the Dimension toolbar. You can dimension the length of an arc or an arc segment in a polyline. At the Select arc

or polyline arc segment: prompt, select an arc. At the Specify arc length dimension location, or [Mtext/Text/Angle/Partial/Leader]: prompt, pick where you want the dimension line (which is an arc) to appear. The command automatically adds an arc symbol before the measurement. You can specify the arc symbol above the measurement or choose to display no symbol — you set this in your dimension style. Figure 14-15 shows an example of an arc length dimension.

378 Part II Drawing in Two Dimensions

You can also choose the MText, Text, or Angle option, as described in the section “Drawing Linear Dimensions.” The Partial option lets you dimension part of an arc. At the prompts, you specify where you want to start and end the dimension. The Leader option inserts an arrow pointing to the arc.

Creating radial dimensions

To dimension the radius of a circle or arc, choose Radius Dimension from the Dimension toolbar. The command responds with the Select arc or circle: prompt. Select an arc or circle. At the Specify dimension line location or [Mtext/Text/Angle]: prompt, pick

where you want the dimension line to appear. The command automatically adds an R before the measurement to indicate the radius, as shown in Figure 14-15.

You can also choose the MText, Text, or Angle option, as described in the “Drawing Linear Dimensions” section earlier in this chapter.

A radius dimension usually passes through the center of the arc or circle. If you are dimensioning a large arc or circle, you may find that you can’t see the center at the same time

as you see the circumference without zooming out too far. In this instance, you can use the new jogged radial dimension, which lets you specify an arbitrary center for the arc or circle. Choose Dimension Jogged.

Figure 14-15: The circle’s radius dimension uses a leader (a line and arrow pointing to the object) because the circle is too small to place the dimension inside it. The arc displays an arc length dimension.

Chapter 14 Drawing Dimensions 379

Creating diameter dimensions

To dimension the diameter of an arc or circle, choose Diameter Dimension from the Dimension toolbar. The command responds with the Select arc or circle: prompt.

Select an arc or circle. At the Specify dimension line location or [Mtext/Text/Angle]: prompt, pick where you want the dimension line to appear. The command automatically adds the diameter symbol before the measurement to indicate the dimension, as shown in Figure 14-16.

Figure 14-16: A diameter dimension.

You can also choose the MText, Text, or Angle option, as described in the section “Drawing Linear Dimensions.”

Dimensioning Angles

You have several options for dimensioning angles. You may want to dimension the angular relationship between two lines, but the lines may intersect at their midpoints or may not intersect at all. Therefore, you need to be able to specify the vertex of the angle you want to dimension. Figure 14-17 shows an angular dimension with the points used to define it.

To create an angular dimension, choose Angular Dimension from the Dimension toolbar. The command displays the Select arc, circle, line, or <specify vertex>:

prompt, and responds differently, depending on what you select:

If you press Enter, the command asks for the angle vertex, the first angle endpoint, and the second angle endpoint. These three points define the angle.

If you select an arc, the command dimensions the entire arc, using the arc’s center as the angle vertex.

If you select a circle, the command uses the pick point as the first angle endpoint and the circle’s center as the angle vertex. You then see the Specify second angle endpoint: prompt. Pick a point on the circle.

380 Part II Drawing in Two Dimensions

If you select a line, the command asks for a second line. The command measures the angle between the two lines. If the lines don’t intersect, the command uses their implied intersection as the angle vertex.

Angle vertex

Figure 14-17: An angular dimension.

Thanks to Mary Redfern of the Bethlehem Corporation, Easton, Pennsylvania, for this drawing.

After you define the angle, the command responds with the Specify dimension arc line location or [Mtext/Text/Angle]: prompt. Pick a point for the dimension arc line — which is the same thing as a dimension line, except that the command uses an arc for angular dimensions.

You can also choose the MText, Text, or Angle option, as covered in the section “Drawing Linear Dimensions.”

In the following exercise, you get to practice drawing radial, diameter, and angular dimensions.

STEPS: Drawing Radial, Diameter, and Angular Dimensions

1.Open ab14-c.dwg from your CD-ROM.

2.Save the file as ab14-04.dwg in your AutoCAD Bible folder. This is a view of a bearing housing for an industrial washing machine, as shown in Figure 14-18. OSNAP should be on. Set running object snaps to endpoint, intersection, and center. If the Dimension toolbar isn’t visible, right-click any toolbar and check Dimension.

3.Choose Center Mark from the Dimension toolbar. At the Select arc or circle: prompt, pick one of the four small circles at the corners of the model. Repeat the

command for the other three circles.

Chapter 14 Drawing Dimensions 381

Dimensioning Minor, Major, and Supplemental Angles

When two lines meet at an angle, they create two angles, the minor angle and the major angle. The angle that is less than 180 degrees is the minor angle. The major angle is always more than 180 degrees. You can also measure the supplemental angle, which is the difference between 180 degrees and the minor angle. These angles are shown here.

Here’s how you create each type of dimension. Start the DIMANGULAR command. The command responds with the Select arc, circle, line, or <specify vertex>: prompt.

To dimension the minor angle, select both lines. Then at the Specify dimension arc line location or [Mtext/Text/Angle]: prompt, place the dimension arc line inside the angle, as shown in the figure. (You can also press Enter, specify the angle vertex and the two lines, and place the dimension arc line inside the angle.)

To dimension the major angle, press Enter. Do not select the lines. At the prompts, specify the angle vertex and the two lines. At the Specify dimension arc line location or [Mtext/ Text/Angle]: prompt, place the dimension arc line outside the angle, as shown in the figure.

To dimension the supplemental angle, select both lines. At the Specify dimension arc line location or [Mtext/Text/Angle]: prompt, place the dimension arc line outside the angle, as shown in the figure.

As you can see, how you specify the angle and where you place the dimension arc line determine which angle you measure.

4.Choose Diameter Dimension from the Dimension toolbar. At the Select arc or circle: prompt, choose the outer of the two circles at 1 in Figure 14-18. At the

Specify dimension line location or [Mtext/Text/Angle]: prompt, pick a location for the dimension line.

5.Choose Radius Dimension from the Dimension toolbar. At the Select arc or circle: prompt, choose 2 from Figure 14-18. At the Specify dimension line

location or [Mtext/Text/Angle]: prompt, pick a location for the dimension line. The line may appear to the left of the angle; try moving the cursor until the dimension appears to its right.

382 Part II Drawing in Two Dimensions

7

Figure 14-18: A bearing housing for an industrial washing machine.

Thanks to Robert Mack of the Dexter Company, Fairfield, Iowa, for this drawing.

6.Choose Angular Dimension from the Dimension toolbar. Follow the prompts:

Select arc, circle, line, or <specify vertex>: Specify angle vertex: Pick 3 in Figure 14-18.

Specify first angle endpoint: Pick the endpoint at 4. (Press Tab if necessary until you see the endpoint tooltip.)

Specify second angle endpoint: Pick the endpoint at 5.

Specify dimension arc line location or [Mtext/Text/Angle]: Choose a location for the dimension line.

7.Repeat the DIMANGULAR command. At the Select arc, circle, line, or <specify vertex>: prompt, pick the arc at 6. At the Specify dimension arc line location or [Mtext/Text/Angle]: prompt, pick a location for the dimension line.

8.Choose Arc Length from the Dimension toolbar. At the Select arc or polyline arc segment: prompt, again pick the arc at 6. At the Specify arc length

dimension location, or [Mtext/Text/Angle/Partial]: prompt, pick a location for the dimension line further out than the previous dimension that you drew.

9.Start the DIMANGULAR command. At the Select arc, circle, line, or press ENTER: prompt, pick 7 in Figure 14-18. At the Second line: prompt, pick 8. At the

Dimension arc line location (Mtext/Text/Angle): prompt, pick a location for the dimension line to the left of the model.

10.Save your drawing. It should look like Figure 14-19.