436 Part II Drawing in Two Dimensions

Dimension System Variables

All of the settings that you make in the Dimension Styles dialog box are stored in a large number of system variables devoted to dimensions. (See Chapter 5 for more information about system variables).

To read about the dimension variables, choose Help Help and click the Contents tab. Double-click Command Reference and then double-click System Variables. Double-click D System Variables. All of the variables starting with DIM are dimension system variables.

Once upon a time, the only way to manage dimensions was by knowing all of the dimension system variables and individually setting each one. Today, the Dimension Style Manager makes it much easier to manage dimensions. However, if you want to create scripts or AutoLISP routines to manage your dimensions, you need to understand how the dimension system variables work, because scripts and AutoLISP routines cannot access dialog boxes.

You can use the DIMSTYLE command on the command line to list all of the system variable settings for a dimension style. Type -dimstyle at the command line. Use the STatus option to display the current style’s system variable values. You can get a great education in dimension system variables by printing out and perusing this list.

The other DIMSTYLE options are:

Save: Saves a dimension style.

Restore: Makes a dimension style current.

Variables: Lists dimension variables, such as the STatus option, but lets you first choose the dimension style for which you want to list the variables.

Apply: Updates dimensions to the current style, including overrides. This is equivalent to choosing Update from the Dimension toolbar.

?: Lists all dimension styles in the drawing.

You can also create dimension style overrides by using the DIMOVERRIDE command (Dimension Override). You need to know the name of the system variable and the setting code that you want. The command displays the Enter dimension variable name to override (or Clear overrides): prompt. To create an override, type the system variable and its setting. Then select the dimensions for which you want to override dimension style settings. To clear all dimension style overrides, type c . Then select the dimensions for which you want to remove overrides.

Creating Geometric Tolerances

You can use the TOLERANCE command to create geometric tolerances. (For another way to specify tolerances, see the “Formatting tolerances” section earlier in this chapter.) This command creates feature control frames, which define tolerances. This method of denoting tolerances conforms to international standards such as ISO (International Standards Organization), ANSI (American National Standards Institute), or JIS (Japanese Industrial Standards). Figure 15-36 shows a drawing using tolerance feature-control frames.

Tolerance symbol for concentricity

Tolerance value

Datum

Figure 15-36: An example of tolerance feature-control frames.

Thanks to Jerry Butterfield of Clow Value Company, Oskaloosa, Iowa, for this drawing.

Starting the tolerance frame

Creating a tolerance frame is a step-by-step process that depends on what information you want to include. To start the frame, choose Tolerance from the Dimension toolbar, which opens the Geometric Tolerance dialog box, shown in Figure 15-37.

Figure 15-37: The Geometric Tolerance dialog box.

Use this dialog box to build the frame. The frame enables you to create two rows of two tolerances and three datum references (for up to three dimensions), as well as a projected tolerance zone value, and a symbol and datum identifier. You’ll rarely, if ever, use all of the features in the frame.

Follow these steps to build the frame:

1. Click the first Sym box to open the Symbol dialog box, shown in Figure 15-38.

Figure 15-38: Use the Symbol dialog box

to choose the symbol for the type of geometry for which you want to specify tolerance.

438 Part II Drawing in Two Dimensions

2.Choose the symbol for the geometric characteristic that you are tolerancing. (Table 15-1 explains these symbols.) If you don’t need a symbol, click the blank box. The Symbol dialog box disappears.

3.To insert a diameter symbol before the first tolerance, click the black Dia box to the left of the text box in the Tolerance 1 section.

4.Type the tolerance value in the Value box.

5.If you want to specify a material condition, click the black MC box to the right of the text box. The Material Condition dialog box opens, shown in Figure 15-39. Choose the symbol that you want. The dialog box disappears.

Figure 15-39: The Material Condition dialog box.

6.If desired, complete a second tolerance.

7.If desired, type a datum in the Datum box of the Datum 1 section, usually A.

8.If desired, add a material condition, using the same method described in Step 5.

9.If desired, type in datum references in the Datum 2 and Datum 3 sections, usually B and C with material conditions.

10.If you need to specify a projected tolerance zone for a perpendicular part, type a value in the Height box. Then click the Projected Tolerance Zone box to insert the Projected Tolerance Zone symbol.

11.Finally, if you want to specify a datum identifier, type the identifier letter in the Datum Identifier box.

12.Click OK to return to your drawing.

If you choose a material condition symbol and then change your mind, click the material condition box again and choose the blank square to delete your symbol.

Table 15-1: Tolerance Symbols

Editing a tolerance frame

To edit a geometric tolerance, double-click the tolerance frame. The Geometric Tolerance dialog box opens, and you can make any changes that you need. Click OK to return to your drawing.

STEPS: Creating Geometric Tolerances

1.Open ab15-c.dwg from your CD-ROM.

2.Save the file as ab15-06.dwg in your AutoCAD Bible folder. This drawing of a gear operator is shown in Figure 15-40. The Dim layer is current. If the Dimension toolbar is not visible, right-click any toolbar and choose Dimension.

1

Figure 15-40: A mechanical drawing using geometric tolerances.