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Inside AutoCAD 14

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- 19 -

Advanced Dimensioning

by Michael Todd Peterson

One of AutoCAD's best features is its capability to control dimensions in a drawing. In other words, AutoCAD Release 14 provides you with several tools to edit dimensions, as well as control how a dimension appears in the drawing. This chapter focuses on the following topics:

Defining Dimension Styles

Dimension styles are your primary method for controlling how a dimension appears. By creating a dimension style, you define exactly how that dimension is going to appear in the drawing. This includes the dimension scale, the types of arrowheads, whether or not the dimension lines appear, and if so, what color the dimension lines are.

AutoCAD controls dimension styles through the use of Dimension Variables (DIMVARS). You can control these variables in two different ways. You can use the DDIM Dimension dialog box to access the variables using a graphical interface or you can type the variable at the Command: prompt and assign it a new value. There are approximately 58 dimension variables in AutoCAD Release 14. Most of the time, adjusting the dimension variables through the Dimension Styles dialog box is sufficient (see fig. 19.1). This dialog box is accessible through the Dimension toolbar or styles option of the Dimension menu.

Figure 19.1 The Dimension Styles dialog box enables you to control how a dimension is drawn.

Creating Dimension Style Families

Dimension style families enable you to create a single parent style with smaller variations in the different families. For example, you can create a parent family with a particular font for linear dimensions. Then, you can create a child style of the parent with a different font for radial and diameter dimensions, without having to create a whole new style.

When the parent style is the current style, the child style will be used when that particular type of dimension is used. In the preceding example, if you create a radius or diameter dimension when the parent style is current, the child style will automatically be used instead. This only applies, however, when creating radius and diameter dimensions. If a linear dimension is created, the parent style is automatically used.

There are six different families of dimension style properties that you can use to create child styles from. They are as follows:

Creating the Child in a Style Family

To make use of style families, you must pay attention to how you name your styles. The following steps should be followed to create the child style.

1. Create the parent style first.

2. Create the child style from the parent. Do this by saving the parent style name with a "$#" suffix.

3. Select a family within the child style to adjust. When you select a family for the child, only the dimension style options related to that particular family will be available in other areas of the Dimension Styles dialog box.

4. Adjust the child style properties as you like.

5. Save the child style under a name related to the parent style.

Child styles need to be named after their parents. For example, a parent style can be named ARCH. The child styles of ARCH should be named something like ARCH$1, ARCH$2--using numbers as the suffix. This enables AutoCAD to associate the child styles with the appropriate parent styles.

The following exercise shows you how to make use of style families.

A QUICK USE OF DIMENSION STYLE FAMILIES

1. Load the drawing 19TUT01.DWG from the accompanying CD-ROM.

2. Choose Dimension, Style. The Dimension Styles dialog box appears.

3. In the Name text box, replace STANDARD with the name CH19.

4. Click on Save. This renames the current style to CH19.

5. In the Family section of the Dimension Styles dialog box, select the Radial option.

6. Click on the Geometry button to access the Geometry dialog box.

7. Set the Arrowhead to Open 30 for the 1st option. The 2nd option will automatically be set identical to the 1st option.

8. Modify the Dimension Line Color to green.

9. Click on OK to close the Geometry dialog box.

10. In the Name text box, enter CH19$1.

11. Choose Save.

12. Under the Current drop-down list, set the current style to CH19.

13. Click on the Save button to save changes to the radial child.

14. Click on OK to close the Dimension Styles dialog box.

15. Select DIMLINEAR and dimension one side of the box in the drawing.

16. Select DIMRADIUS and dimension the circle. Figure 19.2 shows you the resulting drawing. Notice how the differences between the parent and child styles are automatically used, depending on the dimension command used.

Figure 19.2 The box and circle dimension with parent and child styles.

Dimension Style Options

Dimension styles provide you with a method for saving different sets of dimension variables for the various types of drawings you might create. There are many different options for defining how a dimension looks. To help you understand some of these options, figure 19.3 shows you a standard linear dimension with all the parts of the dimension labeled.

Figure 19.3 A dimension and all its parts.

AutoCAD enables you to define the style options in three different categories, as shown in the following list. When you click on one of these buttons in the Dimension Styles dialog box, a separate dialog box appears.

Each of these categories is explained in the following sections.

Geometry Options

The Geometry dialog box enables you to control all the dimension system variables related to the geometry of the dimension, except for the text (see fig. 19.4).

The Geometry dialog box is broken down into five distinct areas: Dimension Line, Arrowheads, Extension Line, Center, and Scale.

Figure 19.4 The Geometry dialog box enables you to define the geometry of the dimension.


Dimension Line Options

The Dimension Line section controls the appearance of the dimension line. In a linear dimension, this is the line beside or below the dimension text. In certain circumstances, you may want to create a dimension without the dimension line. For example, you may have a short dimension with large text. In this situation, you can suppress the first or second dimension line, or both. This option only takes effect when the dimension text is centered inside of the dimension line. When text is above the dimension line, the suppression options have no effect. The location of the dimension text is controlled under the format options, which are covered in the next section.

The Extension option, which is grayed out by default, is used in conjunction with certain arrowhead types. In particular, the oblique and architectural tick arrowheads make use of this option. When one of these two arrowheads is active, you may adjust the dimension line extension variable, which defines how far beyond the extension lines of the dimension it goes.

Probably the most used dimension line option is that of color. The default color of the dimension line is BYBLOCK, which means the line will take on the color of the dimension as a whole. The only reason to change this color is if you want to have a different line width for the dimension line. For example, you could have a thinner line for the dimension line versus the extension lines.

Extension Line Options

In the Extension Line area of the dialog box, the options perform the same function as those under dimension line. The notable exception is the Origin Offset option. When you create a dimension, such as a linear dimension, you select two points as the start and end points. These points are considered the origin points and are drawn in as part of the dimension. The origin offset defines the distance above these points that the extension line is started.

Arrowheads Options

The Arrowheads section of the Geometry dialog box provides you with complete control over the arrows. AutoCAD Release 14 provides you with additional standard arrowheads, including architectural tick, open 30, dot blanked, box filled, and a few others. Even with all these new arrowheads, you may want to create your own. To create your own arrowheads, under the 1st and 2nd drop-down lists, select the User Arrow option. This option enables you to select any block as any arrow, as long as that block is already defined in the current AutoCAD drawing. The arrowhead block should be created with an overall size of one unit in AutoCAD so it will be correctly scaled when used in the dimension. It also should be created using the right end of the dimension. The block will be rotated for the left end.

TIP: When you create a custom arrowhead, you should save the arrowhead as a block in a template file so it is available to all drawings based on that template. If you prefer, you can also use a block from an xref as well.

Figure 19.5 shows you the dialog box in which you can enter the block name for the arrowhead.

Figure 19.5 The User Arrow dialog box enables you to select custom arrowheads.

The following exercise shows you how to create your own arrowheads.

CREATING YOUR OWN ARROWHEADS

1. Start a new drawing from scratch.

2. Create an octagon using the polygon command. Make the radius of the polygon one unit.

3. Make a block of the polygon with an insertion point at the center of the polygon. Name the block P1.

4. From the Dimension pull-down menu, choose Style.

5. In the Dimension Styles dialog box, click on the Geometry button.

6. In the Arrowheads area of the Geometry dialog box, select user arrow from the 1st drop-down list.

7. In the User Arrow dialog box, enter p1 as the arrow name.

8. Click on OK to close the User Arrow dialog box.

9. Click on OK to close the Geometry dialog box and return to the Dimension Styles dialog box.

10. Choose Save to save the dimension style changes to Standard.

11. Click on OK.

12. Create a linear dimension. Figure 19.6 shows you a linear dimension created with a custom arrowhead.

Figure 19.6 A linear dimension with a custom arrowhead.

Center and Scale Options

The last two sections of the Geometry dialog box control center marks and scaling. The Center options define how center marks appear when used with radius and diameter dimensions. Probably the most important options in the Geometry dialog box, however, are the Scale options.

There are two scale options: Overall Scale and Scale to Paper Space. The Overall Scale controls how large all the features of the dimension, such as arrowheads, will appear in the drawing. This scale is directly related to the final plot scale for the drawing. For example, if you are plotting at an architectural scale of 1/4 inch=1'-0", your scale factor should be 48. This is obtained by taking the denominator of 1/4 inch and multiplying it by 12. For a scale such as 1:50, you can equate this to 1/50 =1, so the overall scale factor is 600.

When you set the scale factor, you define a scale multiplier by which all dimension size variables are multiplied. For example, arrowheads default to 0.18 units in size. If you have a scale factor of 48, the 0.18 is multiplied by 48 to arrive at the current size, correctly scaled for plotting.

If you are going to work in paper space, you can leave the Overall Scale factor set to 1 or you can turn on the Scale to Paper Space option. The Scale to Paper Space option sets the dimension variable DIMSCALE to 0. When working in paper space, a default value of 1.0 will be used. In this situation, when you are working in a model space viewport in paper space, you can create a dimension in either space and it will be scaled correctly. This assumes that you have used the Zoom command to correctly scale the geometry in the model space viewport. See Chapter 15, "Paper Space," for more information on paper space and model space viewports.

Format Options

The format options enable you to control the location of the dimension text. When you click on the Format button in the Dimension Styles dialog box, the Format dialog box appears (see fig. 19.7). This dialog box is broken down into four different areas: general, Text, Horizontal Justification, and Vertical Justification. All areas except general control the location of the text in the dimension and are self-explanatory.

Figure 19.7 The Format dialog box showing you the text location controls.

General Controls

The upper-left portion of the Format dialog box provides you with three general controls: User Defined, Force Line Inside, and Fit.

The User Defined option enables you to define how a dimension looks when you actually create the dimension. For example, if you create a linear dimension with the User Defined option enabled, you will be given options such as horizontal or vertical to determine the direction of the dimension. You can create a separate style with User Defined enabled to give you the most flexibility when creating a wide variety of dimensions.

The Force Line Inside option controls the placement of the dimension line when the dimension text is longer than the distance between the extension lines, as shown in figure 19.8. Force Line Inside always assures that a dimension line will appear between the two extension lines.

Figure 19.8 A dimension showing where force line inside is helpful.

Last, there is the Fit option. This is similar to the Force Line Inside option and is applied in the same conditions. In the Fit drop-down list, you will find six different options. Each option is briefly described in the following list:

The Format options are generally configured the same for all dimension styles you might use in a drawing. You should try to be consistent with your Format options simply to maintain consistency in your drawings.

TIP: In the architectural field, typically dimension text should be above the dimension line. If you adjust an existing dimension with this property by using the Grip feature and drag the dimension to the side, however, a leader is drawn underneath the text. This is typically undesirable and annoying. A quick fix to this is to use the DimOverRide command. Upon requesting this command, the system will ask you for a dimension variable to override. Type DIMTAD (DIMension Text Above Dimension), set it to 1, and then select the newly moved dimension with the incorrect leader format. After this is complete, the settings will be returned to normal.

Annotation Options

The Annotation option of the Dimension Styles dialog box enables you to control how the text looks in your dimensions. Figure 19.9 shows the Annotation dialog box.

Figure 19.9 The Annotation dialog box enables you to control how text appears in your dimensions.

The Annotation dialog box is broken down into four separate sections: Primary Units, Alternate Units, Tolerance, and Text. Each of these areas is explained in the sections that follow.

Primary and Alternate Units

The Primary Units and Alternate Units sections are used to define the units that dimensions will use in their text. Unfortunately, AutoCAD does not automatically set the dimension units to match your Units setting in AutoCAD. So, you must correctly define the units for your dimensions separately.

NOTE: The reason AutoCAD does not automatically use the Units setting from the drawing is because there are additional unit types available to you in the dimensions.

When you click on the Units button, the Primary Units dialog box shown in figure 19.10 appears.

Figure 19.10 The Primary Units dialog box enables you to define units for your dimensions.

There are two new unit types: Architectural Stacked and Fractional Stacked. The stacked unit types vary from the normal unit types in one way. When you have a fraction in the dimension, the fraction will use AutoCAD's stacking capabilities to create a correctly stacked fraction instead of an inline fraction such as 1/2. Figure 19.11 shows you a dimension with a stacked fraction.

The dialog box shown in figure 19.10 should look very similar to the Units dialog box discussed in Chapter 3, "Setting Up the AutoCAD 14 Drawing Environment." You can select the type of units as well as the type of angle measurements to use. In both cases, you can specify the accuracy. Of particular interest here are two options: Zero Suppression and Linear Scale.

Figure 19.11 A dimension showing you the use of the architectural stacked unit type.

Zero Suppression is used to control when a 0 appears in a dimension. For example, 6' is a valid dimension when in architectural units. But, 6' is easy to confuse with 6", especially if the blueprint of the drawing is not very good. In both cases, the leading or trailing zeros have been suppressed. These dimensions read much easier as 6'-0" and 0'-6". You can set this up by disabling zero suppression for feet and inches. You can also control zero suppression for leading and trailing zeros such as 0.6 and 6.000.

The Linear scale option is used to adjust how the distance between the start and end points of the dimension is measured. Most of the time, this option is reserved for paper space. In paper space, when you place a dimension, it measures the dimension in paper space units, not model space units.

In a viewport that is scaled to 1/4"=1'-0", for example, a 4' line will measure 1". This is because of the underlying scale factor difference between paper space and model space. Just as you have to adjust the overall scale factor of a dimension style for model space, you must adjust the linear scale to match paper space. It is calculated in the same way as the overall scale factor. In the above example, a linear scale of 48 is correct.

After you have set your primary units, click on OK to return to the Annotation dialog box. Alternate units are configured in the same way. After secondary units are enabled, they are used to provide a different unit measure for the same dimension. Figure 19.12 shows you a dimension with alternate units enabled.

Figure 19.12 A dimension showing alternate units.

After you have established the units for your dimension, you can further modify the text by adding a prefix, suffix, or both to the dimension. For example, you can further clarify a dimension by using a suffix of CM to indicate centimeters.

Tolerance Types

The Tolerance section of the Annotation dialog box enables you to add tolerances to the end of the dimension text. These tolerances are different from the tolerance command discussed in Chapter 18, "Productive Dimensioning." There are five different types of tolerances. Each tolerance is briefly described in the following list:

After you select the tolerance method, you may apply an upper and lower value, as well as justification of the text in the dimension line. Figure 19.13 shows you a dimension with a symmetrical tolerance applied.

Figure 19.13 A dimension showing the use of symmetrical tolerance.

Text Properties

The last section of the Annotation dialog box is the Text section, where you can specify how the text in the dimension will appear. In the Style drop-down list, you can select any previously defined text style. (See Chapter 16, "Text Annotation," for more on how to define a text style.) After you select a style, you can apply other properties to the style, such as height and color. The Height option, in particular, depends on how the text style is defined. If the text style is defined with a fixed height, that height will be used. If the text style's height is 0, the height specified in the Annotation dialog box will be used instead.

Round Off

The last option in the Annotation dialog box is Round Off, which is used to determine how a measured dimension is rounded. For example, if you set the round off to 0.5, all dimensions will be measured to the nearest half unit. A setting of 1.0 will round off to the nearest integer value.

The following exercise ties together all the information you have learned in this chapter so far. It shows you how to quickly and easily set up a complete dimension style for use in AutoCAD.

CREATING A DIMENSION STYLE FOR A MECHANICAL DRAWING

1. Start a new drawing.

2. From the Dimension menu, choose Style.

3. In the Dimension Styles dialog box, rename the style MECH1.

4. Click on Save.

5. Click on the Geometry button.

6. In the Center area of the Geometry dialog box, select Line.

7. In the Arrowheads area, select Dot.

8. In the Extension Line area, select Red as the Color.

9. Click on OK to close the Geometry dialog box.

10. Click on the Format button.

11. In the Vertical Justification area of the Format dialog box, select Above.

12. In the Text area, turn off Inside Horizontal and Outside Horizontal.

13. Click on OK to close the Format dialog box.

14. Click on the Annotation button.

15. In the Primary Units area of the Annotation dialog box, click on the Units button.

16. In the Primary Units dialog box, set the Dimension precision to 0.00.

17. In the Angles area, set the Tolerance precision to 0.0.

18. Click on OK to close the Primary Units dialog box.

19. In the Tolerance area, set the tolerance Method to Symmetrical.

20. Set the Upper Value to 0.2.

21. Set the Text dimension Color to green.

22. Click on OK to close the Annotation dialog box.

23. Click on Save to save the dimension style.

24. Click on OK to close the Dimension Styles dialog box and return to AutoCAD.

25. Create a couple of dimensions in this style. Figure 19.14 shows you a few possibilities.

Figure 19.14 A few dimensions created in the MECH1 style.

At this point, you would normally create other dimension styles and save them to a template file. For a little extra practice, see if you can create a series of child styles using MECH1 as a parent.

Now that the processes and steps for working with style families have been covered, the following section introduces some important tips that will help you optimize your dimension styles.

Tips for Creating Effective Dimension Styles

The following are a few tips and techniques concerning dimension styles:

Modifying Dimensions

After you have created your dimension styles and created a variety of dimensions in your drawing, you will need to eventually be able to modify the dimensions. Some reasons why you might need to modify your existing dimensions include:

The sections that follow discuss the various techniques for modifying existing dimensions.

NOTE: The rest of this chapter assumes that Associative Dimensioning is turned on. This is controlled through the DIMASO dimension variable, which should be set to On. Without associative dimensioning, you cannot update or modify your dimensions because they are broken down into individual entities and are not considered a dimension entity after they are created.
Leaders, of course, do not make use of associativity and therefore are slightly different when it comes to dimension editing. In most cases, you will simply edit leaders as normal AutoCAD entities.

Grip Editing

One of the most powerful methods of editing in AutoCAD is grip editing. Just like you can grip edit most objects in AutoCAD, you can grip edit dimension as well. Of course, you can only use grip editing if PICKFIRST and GRIPS are enabled and set to a value of 1.

To grip edit a dimension, simply click on the dimension to highlight it. You will immediately see the grip boxes appear on a linear dimension, as shown in figure 19.15.

Figure 19.15 Grip boxes used to edit a linear dimension.

Of course, the exact location and effect of each grip will differ from dimension type to dimension type. Figure 19.16 shows you the grip layout for a radius dimension.

Figure 19.16 The dimension grips for a radius dimension.

To edit a grip, simply click on one of the blue grip boxes. The box will turn red to indicate it is selected. Then, right-click on the box to view the grip editing pop-up menu shown in figure 19.17.

Most of the time, you will use the Move option to reposition the dimension text, the dimension line, or the start or end points of the dimension. After you select the option you want, simply grip edit the dimension just like you would any other object. See Chapter 10, "Basic Object Editing," for more information on grip editing.

Figure 19.17 Select the type of grip edit you want to use from the grip editing pop-up menu.

A couple of problems you might encounter are:

The following exercise shows you how to make use of grip editing with dimensions.

GRIP EDITING A DIMENSION

1. Load the file 19TUT02.DWG from the accompanying CD-ROM.

2. Turn on a running object snap mode of Endpoint.

3. Click on the one existing dimension to highlight it and show the grips.

NOTE: Pickfirst and grips must both be enabled. They are enabled by default in AutoCAD. If you disabled them, re-enable them for this exercise.

4. Click on the lower-right grip to highlight it.

5. Right-click on the same grip to display the pop-up menu.

6. Choose Copy.

7. Select each corner going to the right to create three more dimensions.

8. Press Esc twice.

9. Click on the dimension farthest to the right.

10. Select the grip at the intersection of the dimension and extension lines and move the dimension up into position, as shown in figure 19.18.

Figure 19.18 The drawing with the first dimension in position.

11. Repeat steps 2-10 for the other two dimensions. Figure 19.19 shows you the final dimensioned drawing.

Figure 19.19 The drawing with all dimensions correctly positioned.

Editing Dimension Text

One of the most common editing tasks for a dimension is the ability to change the dimension text after the dimension has been created. The easiest way to edit the text is to simply choose Object, Text from the Modify pull-down menu. This executes the DDEDIT command. If you select the dimension object, the Multiline Text Editor will appear as shown in figure 19.20.

Figure 19.20 The Multiline Text Editor dialog box when used with a dimension.

The only thing that appears in the dialog box is a <>. This symbol indicates the measured AutoCAD value. To replace the measured value, delete the <> and replace it with the value you want. Otherwise, add the text before and/or after the symbol as you see fit. Just make sure to pay attention to how large the text will be inside the dimension. You don't want to put more text than there is room for.

TIP: You can have stacked text in a dimension because of the MTEXT object contained in it. Place a return after the <> text to place the next line below the first. If the text is set to Middle justification, the first line of text will move up and new lines will go below it. If the justification is Above, it will all move up to be above the dimension line. To keep the first dimension line in place and generate any new lines below the first line, place a \X at the end of the first line of text, followed immediately by the second line. Then step any following lines with a return as usual.

When it comes to leaders, the DDEDIT command works just fine for editing the text.

In addition to changing the value of the text in a dimension, you can also rotate and reposition the text. The fastest and easiest way to reposition text is to simply grip edit the dimension. Alternatively, you can also use the DIMTEDIT command, which is accessed by clicking on the Dimension Text Edit button on the Dimension toolbar or by choosing Align Text from the Dimension pull-down menu. For the pull-down menu version, each DIMTEDIT option is listed individually on the Align Text flyout.

DIMTEDIT enables you to reposition the text, as well as align it to the left or right side of the dimension. If you make a mistake, DIMTEDIT also has a Home option that you can use to move the text back to the original position it was in when the dimension was created. The last DIMTEDIT option is Rotate, which enables you to rotate the text of a dimension without rotating the dimension itself.

The following exercise shows you how to edit the text of a dimension.

EDITING THE DIMENSION TEXT

1. Load the file 19TUT03.DWG from the accompanying CD.

2. Choose Object, Text from the Modify pull-down menu and click on the 4.000 dimension. This will bring up the Mtext dialog box.

3. After the <> symbol, add the text Overall Length.

4. Click on OK to close the Mtext dialog box. Figure 19.21 shows you the resulting dimension.

Figure 19.21 The dimension with the modified text.

Updating Dimensions

Another popular dimension editing task is to update an existing dimension to the current dimension style. This occurs frequently when users create drawings with many different dimension styles. It is very easy to create dimensions in the wrong style by accident, under this condition.

The three different ways to update the style of a dimension are as follows:

If you modify a dimension style that is currently in use in the drawing, when you save the style and exit the Dimension Styles dialog box, all dimensions using that style will automatically be updated to the new version. In some instances, you may have to regen the screen to see the changes.

If you want to change a dimension to a different style, you must first set the new style as the current dimension style. You can do this in the Dimension Styles dialog box or use the DIMSTYLE system variable. After the style is set to current, you can choose Dimension, Update or use the Dimension Update tool on the Dimension toolbar. Then, select the dimension and it will be updated to match the new style.

NOTE: There is an inconsistency in AutoCAD with the way updates are handled. The pull-down makes uses of the DIMSTYLE command, whereas the toolbar makes use of the DIM: UPDATE command. Both end up with the same result. Use whichever method you are most comfortable with.

The following exercise shows you how to update AutoCAD dimensions.

UPDATING DIMENSIONS IN AUTOCAD

1. Open the file 19TUT04.DWG from the accompanying CD-ROM.

2. Choose Dimension, Style to open the Dimension Styles dialog box.

3. Set the style 19TUT04 as the current style.

4. Click on OK to close the Dimension Styles dialog box.

5. Choose Dimension, Update.

6. Select all the dimensions in the drawing and press Enter. The dimensions are updated, as shown in figure 19.22.

7. Save the files as 19TUT05.DWG someplace on your hard drive for use in the next exercise.

Figure 19.22 The updated dimensions.

Using DIMEDIT

DIMEDIT is another AutoCAD dimension editing tool. This command is available by typing DIMEDIT at the Command: prompt, or by selecting the Dimension Edit tool from the Dimension toolbar. It is not available from the Dimension pull-down menu.

DIMEDIT enables you to reposition the dimension text back to the home position, rotate the text, and replace the dimension text, just like DIMTEDIT does. What is new is the capability to add an obliquing angle to a dimension. An obliquing angle forces the vertical extensions lines off from vertical by the angle specified. This is more of a cosmetic adjustment to make dimension look more interesting. Obliquing a dimension does not affect the text, dimension line, arrowheads, or origin points. It only affects the extension lines.

The following exercise shows you how to use DIMEDIT.

USING DIMEDIT ON A DIMENSION

1. Continue from the last exercise, or load the file 19TUT05.DWG from the accompanying CD-ROM.

2. Open the Dimension toolbar if it is not open already and select the Dimension Edit tool.

3. At the Command: prompt, type O for Oblique.

4. Select all the dimensions in the drawing and press Enter.

5. Enter 85 as the oblique angle and press Enter. Figure 19.23 shows the resulting drawing.

Figure 19.23 The drawing with oblique dimensions.

Overriding Dimension Variables

A lesser-known method of modifying a dimension is to override a dimension variable. When you are creating a dimension in a specific style, it is possible to override one or more dimension variables in the style. This enables you to change the color of the dimension text for a couple of dimensions and then revert back to the original if you so desire.

There are several ways to implement dimension variable overriding. The easiest way is to override the dimension variable when you are creating the dimension. Unfortunately, to do this, you must know the name of the dimension variable you want to override. When you select the dimension command, such as DIMLINEAR, simply enter the name of the dimension variable you want to override. Give it the new value and that value will be used until you clear the override. For example, DIMASZ controls the size of the arrowheads. You can override this variable with a larger or smaller value than that found in the dimension style.

To clear a dimension override, you must use the DIMOVERRIDE command, which is available on the Dimension pull-down menu as Override. At the Command: prompt, you will be asked for the dimension variable to override. If you type Clear at this prompt, you will clear all overrides and revert back to the original style definition. Alternatively, you can enter any dimension variable, override it, and apply it to existing dimensions.

Overrides stay valid until the CLEAR command is executed, a new style is chosen, or you change the override to another value.

To help you make use of the override command, table 19.1 quickly lists all the dimension variables and what each does.

Table 19.1 Dimension Variables and Meanings

Variable Function
DIMALT Enables the use of alternate dimensions units.
DIMALTD Controls the decimal places used in alternate units.
DIMALTF Controls the alternate unit scale factor.
DIMALTTD Number of decimals in a tolerance in an alternate unit.
DIMATTZ Toggles suppression of zeros for tolerances.
DIMALTU Unit format for alternate units except for angular dims.
DIMALTZ Controls suppression of zeros for alternate units.
DIMAPOST The text prefix or suffix for alternate dimensions except angular.
DIMASO Enables associative dimensions.
DIMASZ Controls the arrowhead sizes.
DIMAUNIT Angle format for angular dimensions.
DIMBLK Name of block to be drawn instead of regular arrowhead.
DIMBLK1 User defined arrowhead 1.
DIMBLK2 User defined arrowhead 2.
DIMCEN Enables use of center marks.
DIMCLRD Color of the dimension line.
DIMCLRE Color of the extension line.
DIMCLRT Color of the dimension text.
DIMDEC Number of decimal places for primary tolerances.
DIMDLE Controls extension of dimension line when oblique or architectural
tick arrowheads are used.
DIMDLI Dimension line spacing for baseline dimensions.
DIMEXE Distance extension lines extend beyond the dimension line.
DIMEXO Extension line offset.
DIMFIT Placement of arrows and dimension lines inside of extension lines.
DIMGAP Gap around dimension text.
DIMJUST Horizontal dimension text position.
DIMLFAC Global scale factor for linear measurements.
DIMLIM Generates dimension limits as default text.
DIMPOST Prefix or suffix for text.
DIMRND Dimension rounding value.
DIMSAH Enables use of user defined arrowheads.
DIMSCALE Overall scale factor.
DIMSD1 First dimension line suppression.
DIMSD2 Second dimension line suppression.
DIMSE1 First extension line suppression.
DIMSE2 Second extension line suppression.
DIMSHQ Controls redefinition of dimension when dragged.
DIMSOXD Suppresses drawing of dimension lines outside extension lines.
DIMSTYlE Current dimension style.
DIMTAD Vertical position of text in relation to the dimension line.
DIMTDEC Number of decimals in a tolerance.
DIMTFAC Scale factor for text height in tolerances.
DIMTIH Position of text inside extension lines.
DIMTIX Draws text between extension lines
DIMTM Lower tolerance limit.
DIMTOFL Forces drawing of dimension line.
DIMTOH Position of text outside of extension lines.
DIMTOL Appends tolerances to text.
DIMTOJ Vertical justification of tolerances.
DIMTP Upper tolerance limit.
DIMTSZ Size of oblique dimension arrowheads.
DIMTVP Vertical position of text.
DIMTXTSTY Text style for the dimension.
DIMTZIN Zero suppression of tolerance values.
DIMUNIT Unit format for dimensions except angular.
DIMUPT Cursor functionality for user positioned text.
DIMZIN Suppression of primary unit value.

For most of the dimension variables, you may need to look up exactly what values you can use. Many are simply 1 or 0, and others accept text strings like DIMSTYLE does. If you are going to use overrides, though, you need to know which variables you want to override and how you want to override them.

The following exercise shows you how to make use of dimension overrides.

OVERRIDING DIMENSION VARIABLES

1. Load the file 19TUT02.DWG from the accompanying CD-ROM.

2. Erase the first dimension.

3. Choose Linear from the Dimension pull-down menu and create an overall dimension.

4. Select DIMLINEAR again by pressing Enter.

5. At the First Extension line prompt, type in DIMCLRT and press Enter.

6. Enter a value of 1 for color RED.

7. Dimension the smaller horizontal lengths of the block, as shown in figure 19.24.

Figure 19.24 The drawing with correctly placed dimensions.

8. Select DIMLINEAR.

9. Enter DIMCLRT again and set it back to BYBLOCK.

10. Dimension the right vertical edge of the block. Figure 19.25 shows you the final drawing.

Figure 19.25 The drawing with four overridden dimensions.

Summary

AutoCAD provides you with a fair amount of control over dimensions through dimension styles. By making good use of templates, you can save your dimension styles and not have to re-create them.

After you create dimensions, of course, you need to be able to edit those dimen-sions. The primary methods for editing dimensions are grip editing, DDEDIT, and DIMTEDIT. Each gives you various ways of editing the dimension, from text editing to text positioning.

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