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7-2 Terminology and Conventions—ANSI

Dimensions are added to drawings to define the part and guide manufacturing. General rules and conventions are used to dimension a drawing in a complete, orderly, and succinct manner.

Common Terms

Figure 7-2 shows both ANSI- and ISO-style dimensions. The terms apply to both styles.

  • Dimension lines: In mechanical drawings, lines between extension lines that end with an arrowhead and include a numerical dimensional value located within the line.

  • Extension lines: Lines that extend away from an object and allow dimensions to be located off the surface of an object.

  • Leader lines: Lines drawn at an angle, not horizontal or vertical, that are used to dimension specific shapes such as holes. The start point of a leader line includes an arrowhead. Numerical values are drawn at the end opposite the arrowhead.

  • Linear dimensions: Dimensions that define the straight-line distance between two points.

  • Angular dimensions: Dimensions that define the angular value, measured in degrees, between two straight lines.

Dimensioning Conventions

There are general guidelines you should follow when dimensioning drawings. Figure 7-3 shows some of the following guidelines applied to a dimensioned part.

  • Dimension lines should be drawn evenly spaced; that is, the distance between dimension lines should be uniform. A general rule of thumb is to locate dimension lines about 1/2in. or 15mm apart.

  • There should be a noticeable gap between the edge of a part and the beginning of an extension line. This serves as a visual break between the object and the extension line. The visual difference between the line types can be enhanced by using different colors for the two types of lines.

  • Leader lines are used to define the size of holes and should be positioned so that the arrowhead points toward the center of the hole.

  • Centerlines may be used as extension lines. No gap is used when a centerline is extended beyond the edge lines of an object.

  • Align dimension lines whenever possible to give the drawing a neat, organized appearance.

Common Errors to Avoid

See Figure 7-4.

  • Avoid crossing extension lines. Place longer dimensions farther away from the object than shorter dimensions.

  • Do not locate dimensions within cutouts; always use extension lines.

  • Do not locate any dimension close to the object. Dimension lines should be at least 1/2in. or 15mm from the edge of the object.

  • Avoid long extension lines. Locate dimensions in the same general area as the feature being defined.

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