Associative Dimensioning
Ever since the development of Paper Space, users have been torn with which “space” to create dimensions. Another common problem is automatic updating of dimensions as the objects they define change. With AutoCAD 2002, great strides at answering these problems have been made with the new Associative Dimensioning system.
The following sections will explore possible uses for this new system and how they can benefit you in your everyday work.
Controlling Associativity
With all previous versions of AutoCAD you had two types of dimensions: Associative and Exploded. This basically meant you could make a smart dimension object, or just “dimension” entities made up of lines and such. Most users worth their weight would always use associative dimensions enabling them access for editing the dimension afterwards. This option was controlled via the DIMASO dimension variable, which was stored within the DIMSTYLE currently defined.
As AutoCAD is refined with each new version, methods to handle legacy drawings are implemented, as the case with AutoCAD 2002 Associative Dimensioning. When activated the DIMASO dimension variable is defunct and the DIMASSOC system variable takes over. So with AutoCAD 2002 a new definition is used to describe an Associative Dimension. Now that term really refers to a dimension that is linked to a object.
This new system variable is controlled via a toggle in the Options dialog box in the User Preferences tab panel (see Figure 18.25). The small DWG logo next to the control indicates that it only affects the current drawing.
Figure 18.25. The Options dialog provides the control for Associative Dimensioning.
If this toggle is checked, any new dimensions you create will “link” to the object that is used to define its node points.
Tip
The new Associative Dimensioning system relies on the use of Osnaps in order to “select” the point to link to. As you use an Osnap, the system is also able to record the object that the Osnap actually snapped to, thereby provide the object link. Obviously, if you do not use Osnaps to define the node points then you will not create a truly associative dimension. Another great reason to use Osnaps.
Using Associative Dimensioning
Until now you have been able to modify dimensions using Grips, and you can of course, use the STRETCH command. Both of these methods provide a good level of productivity, but neither can compare to the enhanced productivity with new Associative Dimensioning. In most circumstances your first task of modification is to change the dimensioned object. Then you have to modify the dimensions that refer to that object. With AutoCAD 2002 and Associative Dimensioning, your task would end after the first step because the dimension would “fix” itself after you modified the object. The following exercise examines this process.
Exercise 18.9 Applying Associative Dimensioning
1. |
Load the 18tut06.DWG from the accompanying CD. |
2. |
Be sure that Osnaps are turned on and Endpoint or Intersection is one of your options. You can right-click the Osnaps button on the status bar and choose Settings to check. |
3. |
From the Dimension pull-down menu, choose Linear, and then select the object on the second horizontal top segment from the left. |
4. |
Drag the cursor toward the top of the display to create a horizontal dimension. |
5. |
From the Tools pull-down menu, choose Options. Click the User Preferences tab and then check the Associative Dimensioning option (refer to Figure 18.25). Click Apply, and then close the dialog. |
6. |
Repeat the Linear dimension process in step 2, but drag toward the bottom of the display (see Figure 18.26). Now you have two dimensions for one segment of the object. Figure 18.26. The drawing with two seemingly identical dimensions.
|
7. |
From the Modify pull-down menu, choose Move and pick on the object anywhere. At the Specify base point or displacement: prompt, pick anywhere on the drawing. |
8. |
At the Specify second point of displacement or <use first point as displacement>: prompt, type @1<0
to move the object 1 unit to the right. Notice that the object moved as well as the last dimension you made, but not the first dimension—it got left behind. The first dimension was made with the Associative Dimensioning toggle (DIMASSOC) turned off. But there is something you can do about that to fix it. |
9. |
From the Dimension pull-down menu, scroll all the way to the bottom and select Reassociate Dimensions. |
10. |
At the Select dimensions to reassociate…: prompt select the top dimension object. Press Enter to stop selecting dimensions. |
11. |
At the Specify first extension line origin or [Select object] <next>: prompt, type S
and then press Enter. Select the same object segment used originally. The dimension then reassociates itself with the object geometry, keeping the same length extensions as well. |
12. |
Keep this drawing open for the following exercise. |
Associative Dimensioning and Layouts
As stated earlier in this chapter, placing dimensions in Paper Space Layouts does offer advantages in some cases. Up until now the biggest drawback was the values that the dimension text value had to be changed so that it reflected the true model space distance. Another big drawback was the inability to co-exist with objects in model space. This then required multiple editing sequences to get the dimension to line up with the new locations of objects in model space.
Now with AutoCAD 2002 you can create dimensions in a Layout and due to the new Associative Dimensioning feature, actually link them to their model space objects.
The following exercise shows the power of this new feature.
Exercise 18.10 Associative Dimensioning in a Layout
1. |
Continue with the drawing from the previous exercise, or if needed load the 18tut07.DWG from the accompanying CD. |
2. |
Click on the Layout1 tab to open Paper Space. |
3. |
Remembering you have the Associative Dimensioning option set to On, from the Dimension pull-down menu choose Linear and place a dimension across the entire width of the object (see Figure 18.27). Figure 18.27. The drawing with one overall associative dimension.
|
4. |
Now using DIMASSOC, turn off Associative Dimensioning, type DIMASSOC
, and set it to 1
. |
5. |
Create another dimension using the entire width of the object and place this one at the bottom of the display (see Figure 18.28). Figure 18.28. The drawing with two overall dimensions showing different values.
Note that one dimension shows 4.00 and one shows 6.00. The one with the 4.00 value is actually “linked” to the object in model space and extracting its distance value and modifying its DIMLFAC value to factor it to read 4.00. |
6. |
Double-click in the viewport to move to Model Space. |
7. |
Use the MOVE command, select the object, and press Enter. At the Specify base point or displacement: prompt pick any point. |
8. |
At the Specify second point of displacement or <use first point as displacement>: prompt type @1<180
to move the object 1 unit to the left. Note that the object moved along with its two model space associative dimensions and the one paper space associative dimension that read 4.00. |
9. |
You can now close this drawing without saving the changes. |
The final issue when using this feature is getting the dimension styles to match when it comes to size. This example had a zoom factor for the viewport of 1.5xp. This was done to show that that scale of a dimension object respects the current space it is in. One would need to adjust DIMSCALE values to match dimensions between spaces. Generally, paper space dimensions would have a DIMSCALE value of 1 and then model space dimensions would use a scale factor such as 48.0 for 1/4" scale DIMSCALE setting with a corresponding 1/48xp viewport scale factor.
With Associative Dimensioning, your decision on where to place dimensions becomes that much more complicated. Through proper planning before you start placing dimensions, you can determine the best method to fully recognize the potential with this new feature.