Extension lines are fundamental components of dimensions in AutoCAD, extending from the measured feature to the dimension line to indicate what is being dimensioned. While they are automatically generated when you create dimensions, you have full control over their appearance and behavior through dimension style settings.
What are Extension Lines?
In technical drawings and within AutoCAD, extension lines (sometimes called witness lines) are lines that originate from points on the object you are measuring and extend outward. They stop at the dimension line, which contains the dimension text. They clearly show the precise points between which the measurement is taken, preventing ambiguity.
How Extension Lines are Used (Creating Dimensions)
Extension lines appear automatically when you use standard dimensioning commands in AutoCAD, such as:
DIMLINEAR(Linear dimension)DIMALIGNED(Aligned dimension)DIMANGULAR(Angular dimension)DIMRADIUS(Radius dimension)DIMDIAMETER(Diameter dimension)
When you select the points on your object to dimension, AutoCAD draws the extension lines from those points to the location where you place the dimension line and text, based on the current dimension style settings.
Modifying Extension Line Settings (Based on Reference)
To customize how extension lines look and behave, you need to modify the settings within your Dimension Style. This allows you to control aspects like their color, linetype, lineweight, how far they extend past the dimension line, their offset from the object, and whether they are suppressed.
Here are the steps to modify extension line settings, drawing directly from the provided reference:
- Click Home tab > Annotation panel > Dimension Style.
- In the Dimension Style Manager, select the style you want to change. If you want to create a new style based on an existing one, click New.
- In the Modify Dimension Style dialog box (or Create New Dimension Style dialog box), go to the Lines tab.
- Under the Extension Lines section, change the settings as needed.
- Click OK to apply the changes to the style.
If you modified the current style, any new dimensions you create will use these settings. Existing dimensions using that style may also update depending on your DIMASSOC setting.
Understanding Extension Line Options
Within the Extension Lines section of the Lines tab, you'll find several crucial settings:
- Suppress Ext Line 1 / Suppress Ext Line 2: You can choose to hide one or both extension lines. This is sometimes used for clarity in specific drafting situations but should be used carefully as it can make dimensions harder to read.
- Extend beyond dim lines: Controls how far the extension lines extend past the dimension line. A value greater than zero creates a small overshoot, which is a common drafting standard.
- Offset from origin: Determines the distance between the point you selected on the object and the beginning of the extension line. A small offset is typically used so the line doesn't touch the object geometry directly.
- Fixed length extension lines: If checked, you can set a specific total length for the extension lines instead of them automatically extending to the dimension line. The extension line starts at the offset distance from the origin and ends at the specified fixed length.
These settings allow fine-tuning of the visual appearance and adherence to drafting standards.
By understanding that extension lines are integral to dimensions and knowing how to adjust their settings via the Dimension Style Manager, you can effectively "use" and control them in your AutoCAD drawings.
