AutoCAD  Training

Dimension

 

Dimensioning Principles

There are two basic principles to follow when you add dimensions to a drawing:

  1. Assignment of more than one dimension to a feature should be avoided.
  2. The reader of the drawing should not be required to do calculation in order to obtain the dimension of a feature.

 

Components of a Dimension

Refer to Figure 1. A dimension consists of four components:

  1. A dimension value.
  2. A dimension line that is parallel to the direction of the described feature.
  3. A pair of arrowheads.
  4. A pair of extension lines projecting from the feature to which the dimension refers. There should be a small gap between the end of the extension line and the feature. The extension line should project a short distance away from the intersection with the dimension line.

 

FIG1.gif (5346 bytes)

Figure 1 Components of a dimension

Definition Points

In addition to the four general components of a dimension, AutoCAD includes definition points in a dimension block for reference. These points are stored in a special layer called DEFPOINTS. You may leave this layer ON all the time because the entities on this layer will not be plotted. When you edit an associative dimension, AutoCAD refers to these points. See Figure 2.

FIG2.gif (2322 bytes)
Figure 2 Definition points

Associative Dimension

The components of a dimension that are listed above can be grouped together as a block or treated as separate entities. If they are grouped as a block, the dimension is called an associative dimension. You can edit or modify the appearance of an associative dimension as a single unit. The dimensions will update automatically after you edit a related feature. On the other hand, if they are created as separate entities, you can edit them independently using general editing commands.

 

Dimension Style

The way a dimension appears depends on the current dimension style. To set up a dimension style, you may use the DDIM command or the DIMSTYLE command. The DDIM command works graphically by providing interactive dialog boxes. The DIMSTYLE command works at the command line interface.

From the Dimension pull-down menu, select the Style... item to run the DDIM command. See Figure 3.

<Dimension> <Style...>

Command: DDIM

FIG3.gif (6307 bytes)

Figure 3  Dimension Styles dialog box

Family

For each dimension style, there is a family of seven members. These consist of a parent and six other family members linear, diameter, radial, ordinate, angular, and leader. The parent is a set of variables that applies to all types of dimensions. The other family members are subsets of variable settings that are specific to the types of dimensions that they refer to.

Parent enables you to set the overall dimension style.
Linear enables you to set the linear dimension style.
Diameter enables you to set the diameter dimension style.
Radial enables you to set the radial dimension style.
Ordinate enables you to set the ordinate dimension style.
Angular enables you to set the angular dimension style.
Leader enables you to set the leader style.

 

Geometry

The geometry of a dimension includes the dimension line, the arrowheads, the extension lines, the center mark, and the overall geometry scale. See Figure 4..

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Figure 4  Geometry dialog box

There are five areas in the Geometry dialog box.

1. Dimension Line

Suppress: 1st enables you to suppress the display of the part of the dimension line that starts from the first selected end point of a dimension.
Suppress: 2nd enables you to suppress the display of the part of the dimension line that starts from the second selected end point of a dimension.
Extension: enables you to set the length of extension of the dimension line beyond the extension line. It works only with the architectural tick arrowhead.
Spacing: enables you to set the incremental distance between consecutive baseline dimensions.
Color... enables you to set dimension line color.

2. Arrowheads

1st: enables you to choose an arrowhead at the first selected end point of a dimension.
2nd: enables you to choose an arrowhead at the second selected end point of a dimension.
Size: sets the size of an arrowhead, the size of an architectural tick, or the overall scale of a user-defined arrowhead block.

3. Extension Line

Suppress: 1st enables you to suppress the display of the extension line at the first selected end point of a dimension.
Suppress: 2nd enables you to suppress the display of the extension line at the second selected end point of a dimension
Extension: enables you to set the length of the extension line that extends beyond the dimension line.
Origin Offset: enables you to set the distance of the starting point of the extension line from the selected end points of an object.
Color... enables you to set extension line color.

4. Center

Mark enables you to cause the DIMCENTER command to produce a pair of center marks.
Line enables you to cause the DIMCENTER command to produce a pair of center lines
None disables the DIMCENTER command.
Size: enables you to set the size of the center marks or center lines.

5. Scale

Overall Scale: enables you to scale the dimension geometry.
Scale to Paper Space enables you to scale the dimension geometry relative to paper space environment. (Paper space environment will be discussed in Chapter 9.)

 

Format

Dimension format includes the placement of dimension text, arrowheads, and leader lines. See Figure 5.

 FIG5.gif (8011 bytes)

Figure 5  Format dialog box

There are four areas in the Format dialog box.

1. Unnamed

User Defined enables you to select a dimension text location.
Force Line Inside enables you to place the dimension line within the pair of extension lines in case the dimension text is placed outside the extension lines.
Fit: enables you to decide how the dimension text and arrowheads are placed within the pair of extension lines in case there is not enough space for both.

2. Text

Inside Horizontal enables you to force the dimension text placed within the extension lines to be horizontal.
Outside Horizontal enables you to force the dimension text placed outside the extension line to be horizontal.

3. Horizontal

Justification enables you to set the dimension text horizontal location.

4. Vertical

Justification enables you to set the dimension text vertical location.

Annotation

Annotation concerns the way the dimension text is displayed. See Figure 6.

 FIG6.gif (10055 bytes)

Figure 6  Annotation dialog box

There are five areas in the Annotation dialog box.

1. Primary Units

Units... enables you to bring out the Primary Units dialog box. See Figure 7.
Prefix: enables you to set the primary annotation prefix.
Suffix: enables you to set the primary annotation suffix.
Display box displays the primary annotation format.

FIG7.gif (7932 bytes)

Figure 7  Primary Units dialog box

In the Primary Units dialog box, there are five areas.

a. Units enables you to set linear unit format.
b. Angles enables you to set angular unit format.
c. Dimension  
Precision: enables you to set the number of decimal places of the primary dimension text.
Zero Suppression enables you to decide whether zero inch or zero feet is displayed if English units are used.

d. Tolerance

 
Precision: enables you to set the number of decimal places of the primary tolerance text.
Zero Suppression enables you to decide whether zero inch or zero feet is displayed in the tolerance text if English units are used

e. Scale

 
Linear: enables you to scale the primary dimension value
Paper Space Only enables you to scale the primary dimension value only in paper space environment.

 

2. Alternate Units

Enables Units enables you to control whether alternate units are enabled
Units... enables you to bring out the Alternate Units dialog box. See Figure 8.
Prefix: enables you to set the alternate annotation prefix.
Suffix enables you to set the alternate annotation suffix.
Display box displays the alternate annotation format.

FIG8.gif (8555 bytes)

Figure 8  Alternate Units dialog box

There are four useable areas in the Alternate Units dialog box.

a. Units enables you to set linear unit format.
b. Dimension  
Precision: enables you to set the number of decimal places of the alternate dimension text.
Zero Suppression enables you to decide whether zero inch or zero feet is displayed if English units are used.
c. Tolerance  
Precision: enables you to set the number of decimal places of the alternate tolerance text.
Zero Suppression enables you to decide whether zero inch or zero feet is displayed in the tolerance text if English units are used.
d. Scale  
Linear: enables you to scale the alternate dimension value.
Paper Space Only enables you to scale the alternate dimension value only in paper space environment.

3. Tolerance

Method: enables you to select the tolerance method: None, Symmetrical, Deviation, Limits, or Basic.
Upper Value: enables you to set the positive deviation of size.
Lower Value: enables you to set the negative deviation of size.
Justification: enables you to decide how the tolerance text is justified with the dimension text.
Height: enables you to set the tolerance text height relative to the dimension text.

 

4. Text

Style: enables you to select the annotation text style.
Height: enables you to set the annotation text height.
Gap: enables you to set the distance between the annotation text and other adjacent geometry.
Color... enables you to set the annotation text color.

 

5.

Round Off sets the annotation text rounding-off value.

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