406 Part II Drawing in Two Dimensions

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 15-35 shows a drawing using tolerance feature control frames.

Tolerance symbol for concentricity

Tolerance value

Datum

Figure 15-35: 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 15-36.

Figure 15-36: The Geometric Tolerance dialog box.

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Use this dialog box to build the frame. The frame enables you to create two rows of two tolerances, three datum references (for up to three dimensions), as well as a projected tolerance zone value and symbol and a datum identifier. You’ll rarely, if ever, use all 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 15-37.

Figure 15-37: Use the Symbol dialog box to choose the symbol for the type of geometry for which you want to specify tolerance.

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

3.To insert a diameter symbol before the first tolerance, click the 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 MC box to the right of the text box. The Material Condition dialog box opens, shown in Figure 15-38. Choose the symbol you want and click OK.

Figure 15-38: 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 in the height. 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.

408 Part II Drawing in Two Dimensions

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

Symbol	Name	Symbol	Name
	Position		Flatness
	Concentricity		Circularity
	Symmetry		Straightness
	Parallelism		Surface profile
	Perpendicularity		Line profile
	Angularity		Circular runout
	Cylindricity		Total runout

	Inserting the tolerance frame
	After you complete the frame, you’re returned to your drawing with the Enter tolerance
	location: prompt on the command line. Specify any point to insert the frame.
Tip	You can create a matching Datum reference to place on your model by creating a tolerance
	frame with no symbol and only the Datum letter.

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 you need. Click OK to return to your drawing.

On the	The drawing used in the following Step-by-Step exercise on creating geometric tolerances,
CD-ROM	ab15-c.dwg, is in the Drawings folder on the CD-ROM.

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STEP-BY-STEP: 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 15-39. The Dim layer is current. If the Dimension toolbar is not visible, right-click any toolbar and choose Dimension.

1

Figure 15-39: A mechanical drawing using geometric tolerances.

3.Choose Tolerance from the Dimension toolbar. In the Geometric Tolerance dialog box, click the top-left box, labeled Sym.

4.The Symbol dialog box opens. Choose the top-left symbol (for position).

5.In the Tolerance 1 section of the Geometric Tolerance dialog box, click the next black box, the Dia box, to insert the diameter symbol. In the Value box, type .004. Click the next black box, the MC box. In the Material Condition dialog box, choose the first image tile (for Maximum material condition).

6.In the Datum 1 section, type B in the Datum box. Click OK.

7.At the Enter tolerance location: prompt, pick 1 in Figure 15-39 to place the geometric tolerance.

8.Save your drawing. It should look like Figure 15-40.

Figure 15-40: The drawing with the added geometric tolerance frame.