Modifying Dimensions

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

• The drawing plot scale changes.

• You make a change to dimensioned geometry.

• You want to override the AutoCAD measurement for the dimension.

• You want to re-position the dimension text for the purpose of clarity.

• You want to change the settings for some elements of a dimension without having to re-create the dimension.

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

Grip Editing

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

To grip edit a dimension, click on the dimension to highlight it. If the dimension is associative, the five grip boxes will appear on a linear dimension, as shown in Figure 18.14.

Of course, the exact location and the effect of each grip differs from dimension type to dimension type. Figure 18.15 shows the grip layout for a radius dimension.

To edit a grip, click on one of the blue grip boxes. The box turns red to indicate that it is selected. Then right-click on the box to view the grip editing pop-up menu shown in Figure 18.16.

With the enhanced right-click capabilities in AutoCAD 2000, you will find that not only can you choose options for working with the selected grip, you can also find options for editing the dimension itself. This enables you to control options such as the placement of the dimension text, the precision of the measurement, and even the dimension style itself.

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 as you would any other object. See Chapter 11, “Advanced Geometry Editing,” for more information on grip editing.

You might encounter a couple of problems when editing dimensions with grips:

• If you select the grip that is nearest to the dimension text and select Rotate, the dimension will rotate around the text. The text itself does not rotate. You must use a special dimension editing command (DIMEDIT) if you want to rotate the text and not the dimension line.

• If you are working with a radius or diameter dimension, you can grip edit the center point of the dimension. If you reposition the center point, the dimension text will change. AutoCAD does not maintain a link between the dimensioned object’s center and the dimension itself. Always make sure you move the point back to the center of the dimensioned object.

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

Exercise 18.4 Grip Editing a Dimension

1.	Load the file 18TUT01.DWG from the accompanying CD-ROM, and then turn on a running object snap mode of Endpoint.
2.	Click on the dimension to highlight it and show the grips.
3.	Click on the lower-right grip to highlight it.
4.	Right-click on the same grip and choose Copy from the pop-up menu.
5.	Select each corner going to the right to create three more dimensions.
6.	Press Enter once.
7.	Click on the dimension farthest to the right.
8.	Select the grip at the intersection of the dimension and extension lines and move the dimension up into position, as shown in Figure 18.17. Figure 18.17. The first dimension is in position, but the copies overlay it.
9.	Repeat steps 2–10 for the other two dimensions. Figure 18.18 shows you the final dimensioned drawing. Figure 18.18. All dimensions are now correctly positioned in the drawing.
10.	Close the drawing without saving.

Editing Dimension Text

One of the most common editing tasks for a dimension is changing the dimension text after the dimension has been created. The easiest way to edit the text is to choose Text from the Modify pull-down menu. This executes the DDEDIT command. If you select the dimension object, the Multiline Text Editor appears, as shown in Figure 18.19.

The only thing that appears in the dialog box is < >. 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. Pay attention to the amount and size of the text for the dimension. You don’t want to put in more information than there is room for.

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 use the DIMTEDIT command, which you access 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 cascade menu.

DIMTEDIT enables you to reposition the text and to align it to the left or right side of the dimension. If you make a mistake, DIMTEDIT also has a Home option you can use to move the text back to its original position. 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.

Exercise 18.5 Editing the Dimension Text

1.	Load the file 18TUT02.DWG from the accompanying CD.
2.	Choose Text from the Modify pull-down menu and click on the 4.000 dimension. AutoCAD displays the Mtext dialog box.
3.	After the < > symbol, type the text Overall Length .
4.	Click OK to close the Mtext dialog box. Figure 18.20 shows the resulting dimension. Figure 18.20. The dimension with the modified text.
5.	Now suppose you want to rotate the text of the dimension 45 degrees. You can accomplish this with the DIMTEDIT command.
6.	From the Dimension pull-down menu, select Align Text then select the Angle option.
7.	Select the dimension you just modified.
8.	Enter a value of 45 to rotate the text, and then press Enter.
9.	If you make a mistake when editing the position and rotation of the dimension text with DIMTEDIT, or if you simply want to return the dimension to its original condition, you can use the Home option to restore it. Choose Dimension, Align, Home.
10.	Select the dimension and press Enter. The text is returned to its original position.
11.	You can close the drawing without saving it.

Updating Dimensions

Another popular dimension editing task is updating an existing dimension to the current dimension style. This is often necessary when a user creates drawings with many different dimension styles. When you use many dimension styles in a drawing, it is very easy to create a dimension in the wrong style by accident. AutoCAD offers three ways to update the style of a dimension:

• Modify the style

• Update the dimension with DIMSTYLE

• Revise the dimension with Update

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

If you want to change a dimension to a differently named style, you must first set the active current style to the new style desired. You can do this in the Dimension Style dialog box, or you can use the DIMSTYLE system variable. After resetting the current style, 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.

The following exercise shows you how to update AutoCAD dimensions.

Exercise 18.6 Updating Dimensions in AutoCAD

1.	Open the file 18TUT03.DWG from the accompanying CD-ROM.
2.	Choose Dimension, Style to open the Dimension Style dialog box.
3.	Set the style CH18TUT3 as the current style.
4.	Click Close to close the Dimension Style 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 18.21. Figure 18.21. The updated dimensions.
7.	Close this drawing and do not save; your changes are not needed in the next exercise.

Using DIMEDIT

DIMEDIT is another AutoCAD dimension editing tool. To use this command, you type DIMEDIT at the Command: prompt, or select 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 as DIMEDIT does. What is unique about DIMEDIT is its capability to add an obliquing angle setting to a dimension. An obliquing angle forces the vertical extension lines off from vertical by the angle specified. This is more of a cosmetic adjustment you might use to make a dimension look more interesting. Obliquing a dimension does not affect the text, dimension line, arrowheads, or origin points. It affects only the extension lines.

The following exercise shows you how to use DIMEDIT.

Exercise 18.7 Using DIMEDIT on a Dimension

1.	Continue from the last exercise, or load the file 18TUT04.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 18.22 shows the resulting drawing. Figure 18.22. The drawing with oblique dimensions.
6.	Close this drawing without saving.

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 current style. For example, you may want 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 a dimension variable override. 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 start the dimension command, such as DIMLINEAR, 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 can 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. You can type Clear at this prompt to clear all overrides and revert to the original style definition. Alternatively, you can enter any dimension variable, override it, and apply it to existing dimensions.

Alternately, and probably more conveniently, you can override dimension style variables from the Dimension Style dialog box. To do so, select the style you want to override and click on the Override button. The Modify dialog box appears. Override any styles you like. Then click OK to return to the Dimension Style dialog box, and you will see the Dimension style listed in an outline format with a Style Overrides listing below it. To remove the overrides, right-click on the listing and choose Delete. You can also choose to rename those overrides or to incorporate them into the style permanently.

Overrides stay valid until you execute the CLEAR command, choose a new style, or change the override to another value.

To help you make effective use of the OVERRIDE command, Table 18.1 lists all the dimension variables and what each does.

Table 18.1. Dimension Variables and Meanings

Variable	Function
DIMADEC	Lets you control angle precision.
DIMALT	Enables the use of alternate dimensions units.
DIMALTD	Controls the decimal places used in alternate units.
DIMALTF	Controls the alternate unit scale factor.
DIMALTRND	Controls the alternate unit roundoff.
DIMALTTD	Indicates the number of decimals in a tolerance in an alternate unit.
DIMATTZ	Toggles suppression of zeros for tolerances.
DIMALTU	Controls unit format for alternate units except for angular dims.
DIMALTZ	Controls suppression of zeros for alternate units.
DIMAPOST	Allows you to specify the text prefix or suffix for alternate dimensions except angular.
DIMASO	Enables associative dimensions. This is now obsolete with use of the DIMASSOC system variable.
DIMASZ	Controls the arrowhead sizes.
DIMATFIT	Controls Fit: Arrows and Text.
DIMAUNIT	Specifies angle format for angular dimensions.
DIMAZIN	Indicates Angle Zero Suppression.
DIMBLK	Lets you specify the name of the block to be drawn instead of a regular arrowhead.
DIMBLK1	Allows the use of a user-defined arrowhead 1.
DIMBLK2	Allows the use of a user-defined arrowhead 2.
DIMCEN	Enables use of center marks.
DIMCLRD	Sets the color of the dimension line.
DIMCLRE	Sets the color of the extension line.
DIMCLRT	Sets the color of the dimension text.
DIMDEC	Controls the number of decimal places for primary tolerances.
DIMDLE	Controls extension of dimension line when oblique or architectural tick arrowheads are used.
DIMDLI	Controls dimension line spacing for baseline dimensions.
DIMDSEP	Indicates decimal separator.
DIMEXE	Allows you to specify the distance extension lines extend beyond the dimension line.
DIMEXO	Controls the extension line offset.
DIMFIT	Indicates placement of arrows and dimension lines inside of extension lines.
DIMFRAC	Specifies Dimension Fraction Format.
DIMGAP	Controls the gap around dimension text.
DIMJUST	Allows you to specify horizontal dimension text position.
DIMLDRBLK	Controls the Leader Arrow Block.
DIMLFAC	Sets the global scale factor for linear measurements.
DIMLIM	Generates dimension limits as default text.
DIMLUNIT	Sets units for all dimension types except Angular.
DIMLWD	Controls Dimension Line Lineweight.
DIMLWE	Controls Dimension Line Extension Lineweight.
DIMPOST	Allows you to specify a prefix or suffix for text.
DIMRND	Sets dimension rounding value.
DIMSAH	Enables use of user-defined arrowheads.
DIMSCALE	Indicates overall scale factor.
DIMSD1	Controls first dimension line suppression.
DIMSD2	Controls second dimension line suppression.
DIMSE1	Controls first extension line suppression.
DIMSE2	Controls second extension line suppression.
DIMSHO	Controls redefinition of dimension when dragged.
DIMSOXD	Suppresses drawing of dimension lines outside extension lines.
DIMSTYLE	Indicates current dimension style.
DIMTAD	Allows you to control vertical position of text in relation to the dimension line.
DIMTDEC	Sets number of decimals in a tolerance.
DIMTFAC	Indicates scale factor for text height in tolerances.
DIMTIH	Controls position of text inside extension lines.
DIMTIX	Draws text between extension lines.
DIMTM	Lower tolerance limit.
DIMTMOVE	Controls Fit: Text movement.
DIMTOFL	Forces drawing of dimension line.
DIMTOH	Indicates position of text outside of extension lines.
DIMTOL	Appends tolerances to text.
DIMTOLJ	Allows you to specify vertical justification of tolerances.
DIMTP	Sets upper tolerance limit.
DIMTSZ	Indicates size of oblique dimension arrowheads.
DIMTVP	Lets you set vertical position of text.
DIMTXSTY	Indicates text style for the dimension.
DIMTXT	Controls text height.
DIMTZIN	Sets zero suppression of tolerance values.
DIMUNIT	Controls unit format for dimensions except angular.
DIMUPT	Specifies cursor functionality for user-positioned text.
DIMZIN	Controls suppression of primary unit value.

For most dimension variables, you may need to look up exactly what values can be used. Many are simply 1 or 0 (on and off); others, such as DIMSTYLE, accept text strings. 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.

Exercise 18.8 Overriding Dimension Variables

1.	Load the file 18TUT05.DWG from the accompanying CD-ROM.
2.	Turn on the ENDPOINT running OSNAP if it’s not already active.
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 DIMBLK and press Enter.
6.	Enter the string value of ARCHTEXT to set a new arrowhead format.
7.	Dimension the smaller horizontal lengths of the block, as shown in Figure 18.23. Figure 18.23. The drawing with correctly placed dimensions.
8.	Select DIMLINEAR.
9.	Enter DIMBLK again and set it back to Closed Filled by entering just a period (.). That is a quick way to revert to a standard filled arrowhead.
10.	Dimension the right vertical edge of the block. Figure 18.24 shows you the final drawing. Figure 18.24. The drawing with four overridden dimensions.
11.	Close this drawing without saving.