Becoming Proficient at Productive Dimensioning

To become proficient at dimensioning a drawing, you need a little practice and a little understanding about some of the various options made available to you by AutoCAD. The most commonly used dimensioning type is linear dimensioning.

Linear Dimensioning

Linear dimensions, of course, define a specific length, whether it is horizontal, vertical, or aligned to the object you are dimensioning. AutoCAD provides you with five different linear dimensioning commands, including DIMLINEAR, DIMCONTINUE, DIMBASELINE, DIMALIGNED, and DIMROTATED. Each of these commands can be accessed through either the pull-down menu (see Figure 17.1), the Dimensioning toolbar (see Figure 17.2), or entered at the Command: prompt. You should access this command using the method you are most comfortable with. For example, DIMLINEAR is the Command: prompt command for a linear dimension, whereas it appears as Linear on the pull-down and toolbar. All are the same command.

The base linear command, DIMLINEAR, is fairly straightforward and easy to use. But, you may not be aware of one or two options of the command, which are covered in the next section.

Linear Options

The DIMLINEAR command is based on selecting three points to create the dimension. These points are the starting and ending points of the dimension, and the location of the dimension line. When choosing the first two points for the linear dimension, you are prompted to select the first and second extension line origins on the object(s) being dimensioned.

An alternative method is to press Enter when prompted to select the first extension line origin. At that point, you can select the line, polyline, circle, or arc you want to dimension. Then, all you have to do is place the dimension. When selecting a line or using this method, the endpoints of the dimension are automatically determined. This alternate method works well when you are dimensioning a single line, arc, circle, or polyline segment that is precisely the length you need it to be. When this method is applied to a multi-segmented polyline, only the segment you select will be dimensioned. If you use this method with a circle, you can dimension the diameter of the circle with a linear dimension. DIMLINEAR will recognize objects that it cannot dimension and will issue the following informational message: Object selected is not a line, arc, or circle. Figure 17.3 shows you some example dimensions created with two clicks.

The following exercise gives you a taste of how to use the DIMLINEAR command.

Exercise 17.1 Dimensioning with DIMLINEAR

1.	Load the file 17TUT01.DWG from the accompanying CD. This file has several objects—five circles and a rectangle—that you will dimension.
2.	From the Dimension pull-down menu, select Linear.
3.	At the Command: prompt, you will be prompted to select the first extension line or select by object. Press Enter to choose Select by Object.
4.	Click on the bottom line of the rectangle. You will immediately see a linear dimension of that edge of the rectangle.
5.	Move the mouse down and pick a position for the dimension line a comfortable distance away from the rectangle. Now, let’s see how to manually create a linear dimension.
6.	Press Enter to bring the command back up again.
7.	When prompted for the first extension line, type CEN for center point snap, press Enter, then click on the left circle.
8.	You will then be prompted for the second extension line. Again, type CEN , press Enter, and then select the right circle.
9.	When you do, you will see the dimension line appear. If you move the cursor up and down and left to right, you will see that you can create either a horizontal or vertical dimension for the two circles, simply based on where you place the dimension line. Place the line to the right of the rectangle and you are finished.
10.	You may close this drawing without saving.

The DIMLINEAR by selection option, however, does not solve every situation for linear dimensions. You may still need to resort to using construction lines in conjunction with object snap modes. In these cases, AutoCAD 2002’s tracking feature is very helpful. See Chapter 6, “Accuracy in Creating Drawings with AutoCAD 2002,” for more information on how to use tracking.

Ultimately, to get more productivity when creating linear dimensions, you need to explore a few more commands—in particular, DIMBASELINE and DIMCONTINUE. Both commands are used after creating an initial linear dimension to quickly create additional dimensions. These commands are discussed in the following sections.

Baseline Dimensions

Baseline dimensions are used to quickly and easily create a series of dimensions from a single basepoint. If you want to dimension various objects along a wall, but want all the dimensions to measure from one end of the wall, for example, baseline is the method to use.

To make use of the baseline command, you must create a linear, aligned, or rotated dimension before using the baseline command. After you have the initial dimension, choose Baseline from the Dimension pull-down menu or the toolbar. When inside the command, select the endpoint of the next dimension. Each dimension is then automatically placed next to or above the previous dimension with a user-specified spacing. When using DIMBASELINE and DIMCONTINUE, AutoCAD remembers the position of the last dimension placed.

Figure 17.4 shows you the click points necessary to create a series of dimensions using the baseline command.

The following exercise shows you how to dimension a steel plate quickly and efficiently using baseline dimensions.

Exercise 17.2 Creating Linear Dimensions by Using Baselines

1.	Load the drawing 17TUT01.DWG from the accompanying CD.
2.	Create a linear dimension by using the DIMLINEAR command from the left end of the block to the center point of the first circle, as shown in Figure 17.5. Figure 17.5. The block with the first linear dimension applied.
3.	Choose Dimension, Baseline from the pull-down menu or the Dimension toolbar.
4.	When prompted for the second extension line, select a center object snap mode and select the center of the second circle.
5.	Continue using center snap modes and select the center of the rest of the circles, moving from left to right.
6.	When you are finished with the circles, select the endpoint of the upper-right corner of the block. Figure 17.6 shows you the block with all the dimensions applied. Figure 17.6. The block dimensioned using baseline dimensions.
7.	Close this drawing without saving, or undo these dimensions and leave open for a following exercise.

As you can see from this exercise, after you have created the first linear dimension, regardless of whether the dimension was created using DIMLINEAR, DIMALIGNED, or other linear commands, the block is dimensioned quickly with a minimal amount of mouse operations.

To further test the baseline command, try rotating the plate 45 degrees before you create the first dimension. Then, create a DIMALIGNED dimension for the first dimension. When you use the baseline command again, you will see that it works perfectly.

Quick Dimensions

Before looking at other dimensioning commands available in AutoCAD, let’s take a quick look at quick dimensions. Quick dimensions, or QDIM, is an automated system for quickly dimensioning a series of objects originally introduced in AutoCAD 2000. It works by selecting the objects you want to dimension, then placing the dimension line. To illustrate this, let’s run through the previous exercise again, but this time use QDIM to create the dimensions.

Exercise 17.3 Dimensioning a Block with QDIM

1.	Continue from the previous exercise or reload the file 17TUT01.DWG from the accompanying CD.
2.	Choose Dimensions, QDIM from the pull-down menu.
3.	Select all of the objects in the scene and press Enter.
4.	Click above the block to place the dimensions, and you’re done. Figure 17.7 shows you the block. Figure 17.7. The block after using QDIM to create the dimensions.
5.	Close the drawing—there is no need to save.

As you can see from the previous exercise, QDIM is very quick and easy to use to create a set of linear dimensions. As you work with AutoCAD 2002, you will find that this command is a great time saver.

Continue Dimensions

Continue dimensions are very similar to baseline dimensions with one exception: Rather than basing all the dimensions off a single point, they are based off the endpoint of the last dimension drawn. Continue dimensions automatically line up the dimension lines to create crisp, clean dimensions. For example, a wall is generally dimensioned from centerline to centerline of the components of the wall, such as doors and windows. Using the continue dimension makes this very easy.

If you have to create a series of dimensions, one after the other on a single dimension line, use the continue command, because it automates the placement of additional dimensions, much like the baseline command did. Figure 17.8 shows you an example of a continue dimension.

Like baseline dimensions, continue dimensions rely on having one linear dimension type already created, followed by the continue dimension command.

For an exercise on how to use this command, repeat the baseline exercise, but use the continue command instead. Both commands work the same way, but produce different results. As you may have guessed, in a lot of ways, QDIM can replace continue dimensions, but there will still be instances in which it will be easier to use continue.

Aligned and Rotated Dimensions

The last two linear dimension types are aligned and rotated. Both of these types are similar to each other in the fact that they are not horizontal or vertical dimensions. Aligned and rotated dimensions are the only linear dimensions in which the dimension line is not horizontal or vertical.

Aligned dimensions arrange the dimension line to match the angle produced between the start and endpoints of the dimension. Rotated dimensions have the dimension line rotated a specific angle amount before the start and endpoints are selected. Figure 17.9 shows you examples of both types of dimensions.

As you can see in Figure 17.9, you can use the rotated command to create linear dimensions with any orientation. The aligned command, however, is forced to align itself along the start and endpoints of the dimension. Also note that a different dimension is measured because of the dimension angle, even though both dimensions use the same endpoints.

The aligned dimension command may be accessed through the pull-down menu, toolbar, or entered at the Command: prompt. The rotated command, however, is only available at the Command: prompt. Simply type DIMROTATED to access the command.

The following exercise shows you how to use the rotated command to create a few dimensions. The exercise also shows you why you need to be careful when using this dimension type, because a rotated linear dimension may end up with a different measured length than the original.

Exercise 17.4 Creating a Rotated Dimension

1.	Load the file 17TUT02.DWG from the accompanying CD. This drawing shows three circles that you are going to dimension from center point to center point.
2.	At the Command: prompt, type the command DIMROTATED .
3.	When prompted for an angle, type 38 , which sets the angle of the dimension line.
4.	Select the center of the left circle as the start point.
5.	Select the center of the middle circle as the end point. Pick a point above and to the left to position the rotated dimension. The distance between the left and middle circles should measure out to 4.000.
6.	Press Enter to bring up the rotate command again. This time, enter an angle of 315 (45 degrees down and to the right).
7.	Select the center point of the middle circle as the start point.
8.	Select the center point of the circle on the right. Figure 17.10 shows you the three circles dimensioned. Figure 17.10. A 38-degree and a 45-degree rotated dimension showing what happens when you use the rotate command on a linear dimension.
9.	When you are finished with the rotate command, type exit to return to the standard AutoCAD Command: prompt. Close the drawing without saving.