Chapter 4: Dimensioning in AutoCAD – Machine Drawing with AutoCAD

Chapter 4

Dimensioning in AutoCAD

Chapter Outline

GENERAL CONCEPTS AND STANDARD PRACTICES

Dimensions enable us to indicate the true shape and size of different objects in a drawing. It also helps us to understand the positions of these objects in realtion to one another or with some reference points or objects. When you draw an object on the computer screen, you represent the physical shape of the object. It will not be meaningful until and unless you associate these shapes with their respective measurements (sizes). When the drawing will go to workshop, workers must understand the basic sizes of the objects, diameter and location of holes, any other special features, tolerances and so on. All these bits of information constitute dimension and which have to be incorporated in the drawing to make it useful and complete. Fig. 4.1 illustrates the typical drawing of a baseplate indicating all the necessary dimensions, properly placed. Before going into the details of dimensioning, you should know the different components of the Dimension label required to represent it in the drawing.

 

FIG. 4.1   A dimensioned drawing

COMPONENTS OF DIMENSIONS LABEL

The different components of a dimension label includes projection line (or extension line), dimension line, leader line, dimension line termination (arrowhead or tick), extension line origin (definition point), and dimension text. All these components are illustrated in Fig. 4.1. Dimension lines, leader lines and extension lines are drawn as thin continuous lines. Extension lines should be drawn perpendicular to the object line being dimensioned and they should extend slightly beyond the respective dimension lines. Wherever necessary, they may be drawn at an angle but parallel to each other as shown in Fig. 4.2.

 

FIG. 4.2   Extension lines at an angle

 

An arrowhead, or to put it more correctly, a pair of arrowheads is the most important constituent of a dimension label. Its pointed head touches an outline or an extension line or the center of the outline. It is convention to keep the length of the arrowhead about three times its maximum width. It may be filled in or blank. Sometimes, in architectural drawings, arrowheads are replaced by slant cut marks (tick marks) or dots (Fig. 4.3). AutoCAD has a library of different types of arrowheads which we shall explore later.

 

FIG. 4.3   Types of arrowheads

TYPES OF DIMENSIONS

Dimensions should be indicated on a drawing following any one of the styles mentioned below.

  1. Aligned system All dimensions in this system shall be placed parallel to their dimension lines and preferably near the middle and above the dimension line—Fig. 4.4a. The dimension text should be indicated so that it can be read from the bottom or from the right hand side of the drawing.
  2. Unidirectional system All dimensions in this system are placed in one direction and in such a manner that they can be read from the bottom edge of the drawing sheet—Fig. 4.4b. The dimension lines are broken in the middle to accommodate the dimension text. Generally this system is followed in large drawings where reading the dimensions from both sides is inconvenient.

 

FIG. 4.4a   Aligned dimensioning

 

FIG. 4.4b   Unidirectional dimensioning

STANDARD PRACTICE IN DIMENSIONING

There are some general standard practices which must be adhered to while deciding where and how to place the dimensions in a drawing. These decisions may be guided by the following general principles.

 

FIG. 4.5   Placement of dimension

 

FIG. 4.6   Dimension placed within a view

 

FIG. 4.7   The 30°Prohibitive zone

 

  1. All dimensions should be placed outside the view of the object as far as possible as shown in Fig. 4.5. However, where it is unavoidable, dimensions may be placed, within a view (Fig. 4.6).
  2. As far as possible, all dimensions for one particular operation should be specified in one view. For example, the diameter and depth of a hole, size and depth of a threaded hole and should appear in the same view.
  3. Dimensions should not be very close to each other or to the object to be dimensioned.
  4. Intersection of dimension lines with object lines, hidden lines or center lines should be avoided. Under unavoidable circumstances, none of the lines should be broken. Extension lines and dimension lines (of other dimensions) should not cross each other as far as possible.
  5. The overall dimension line should be placed outside the intermediate dimension lines.
  6. Dimensions should be complete. They should not need further calculation or measurements. Repetition of dimension must be avoided.
  7. As far as possible, all dimensions in one drawing should be expressed in one system of units. It is always preferable to use millimetres (mm) as the unit. It is not neccesary to write mm after the dimension.
  8. It is advisable to use the Aligned system of dimensioning. Dimension lines should not be placed in the 30° zone as shown by the hatched region in Fig. 4.7.
TYPES OF DIMENSIONING

Now that you have gained thorough knowledge of the drawing conventions being followed in a dimensioning practice, let us see how AutoCAD executes them.

Let us examine Fig. 4.1 once again. You will notice that different types of dimensioning have been used in the drawing. The lengths and the diameters are not shown by the same type of dimension. The radius has been measured in some other way. In fact, for general use, we need the following five basic types of dimension to delineate all physical shapes.

 

Linear To show all linear measurements
Angular To show all angular measurements
Ordinate To show measurements referring to a datum coordinate
Diametric To show measurements of diameters
Radial To show measurements of radii

 

Illustrations of each type of dimension are shown in Fig. 4.8.

 

FIG. 4.8   The five basic types of dimensions

DIMENSIONS IN AutoCAD

Dimensioning an engineering drawing accurately and beautifully is a daunting task. It requires a great deal of experience and exposure to master it completely. However, in AutoCAD, the system of dimensioning has incorporated in it, almost all conceivable standard conventions and practices of dimensioning, making the draftsman's life easier. In the following discussions we shall explore in detail the methods employed in AutoCAD to give dimension to a drawing.

Using the dimensioning commands of AutoCAD, all the five types of dimensioning can be implemented. We shall see now how to give the Dimension command in AutoCAD for dimensioning objects in a drawing. When you work in any computerised environment, you must understand the properties of the entities you are going to handle. In AutoCAD, dimensions are associative, that is, the dimensions and all its components such as dimension lines, extension lines, arrowheads, and text are created as a single drawing object and are linked to the objects that they dimension. If you change the size of the object, the dimensions automatically change. However, you can create non-associative dimensions by turning the Dimaso system variable off.

TOOLBAR AND COMMANDS

The Dimension commands in AutoCAD need some more explanation. In earlier releases of AutoCAD, the Dim command had to be invoked to put the command line in the Dimensioning mode (Dim: Mode) whereby the prompt would change to Dim: and then the actual dimensioning subcommands such as Linear, Angular and so on had to be issued. You had to type Exit or press Esc to return to the usual command prompt. To maintain compatibility, AutoCAD still continues this system. In addition, you can invoke the commands directly from the command line with the Dim prefix taken as a whole such as Dimlinear, Dimangular, and so on. But some commands continue to remain as Dim: Sub command.

You can invoke the command as usual by any one of the three modes. The dimension toolbar, shown in Fig. 4.9, provides buttons for employing the frequently used dimension commands. You can dock the toolbar at the edge of the screen or leave it floating inside the drawing area.

 

FIG. 4.9   Dimension toolbar

Linear Dimensions—Dimlinear

You can invoke this command for lines, a straight segment of a polyline, or a straight segment in a block. If you choose a circle or an arc to put linear dimensions, this command will select the end points of the diameter of the circle and the linear length of the arc (not the whole perimeter). The prompt sequence is as follows.

 

 

AutoCAD Commands and Prompts Steps/Feedback/Options
Command: Dimlinear ↵ Activates the Dimlinear command.
Specify first extension line origin or < select object >: You can pick the first extension line origin as shown in Fig. 4.10. If you opt for select object option, just press Enter.
Select object to dimension: Pick the object. In the above example (Fig. 4.10), if you try dimensioning the circle using this option, you will find that AutoCAD selects the two ends of its diameter to calculate the length of the diameter.
Specify second extension line origin: Pick the second extension line origin. Once these two points are defined, AutoCAD acquires the dimension of the object from its database. The user need not supply it.
Specify dimension line location or [Mtext/Text/Angle/Horizontal/ Vertical/Rotation]: Fix a point for the location of the dimension line. When you move the cursor you can see the result dynamically. You may also enter an exact location using relative coordinates.
  Once the choice of location is finalised, AutoCAD places the dimension at that point. While putting linear dimensions, keep in mind the standard practices discussed earlier.

 

FIG. 4.10   Linear dimensions

 

Other options   You can also select any one of the options offered at the command prompt to further control the dimension text. In order to invoke a particular option, enter the first letter of the same.

Mtext   This option allows you to add prefix, suffix, notes or any other text of your choice by replacing the dimension text that has been extracted by AutoCAD from its database. When selected, it opens the Mtext Editor dialog box with the original text (or numbers) inside < >. If you want to add a prefix, place the cursor before the bracket and type your text. Similarly, you can add a suffix by placing the cursor after the bracket. You can also delete the bracket and type your own text. In that case the Dimension will loose associativity (discussed earlier).

Text   The end result of this option is almost same as Mtext but here the Mtext editor dialog box is not displayed. You have to type the new text at the command line.

Angle   The angle orientation (45°) of the Dimension text can be changed by entering a new angle (0 = horizontal) at the “Specify angle of dimension text:” prompt displayed as a consequence of this option.

Horizontal/Vertical   With this option, you can give the dimension between two points running horizontally or vertically (Fig. 4.10). It is very useful when you want to dimension only the X or Y length of an angled object.

NOTE

Make it a standard habit to use Object Snap to indicate extension line origins when putting dimensions to any object.

 

Rotated   This option allows you to dimension a length which is not parallel to the extension line origins. Instead, it measures an imaginary line that is parallel to the dimension line. When selected, this option will prompt “Specify angle of dimension line < 0 >:” and you will have to type the angle or pick two points. AutoCAD will draw the rotated dimension as shown in Fig. 4.11.

 

FIG. 4.11   Example of Dimlinear dimensions

Aligned Dimension—Dimaligned

By this command you can align your dimension with two points or a linear object which is not orthogonal. Unlike rotated dimension, the dimension lines here are always parallel to the object.

 

 

FIG. 4.12   Dimaligned dimension

 

An application of the Dimaligned command is shown in Fig. 4.12. Since the command prompts are similar to that of the Dimlinear command they are not being repeated here.

By the Dimangular command, it is possible to put dimension labels to the angles described by an arc, circle, or two lines, or by a set of three points. You may need to define the vertex of the angle sometimes. An arc with dimension arrows at each end is drawn and the value of the angle is placed following the current text style (Fig. 4.13). The command works in the sequence given below.

 

FIG. 4.13   Angular dimension

 

AutoCAD Commands and Prompts Steps/ Feedback/Options
Command: Dimangular ↵ Initiates the Angular Dimension command.
Select an arc, circle, line or <specify vertex>: ↵ There are four options here: You can press Enter to choose the specify vertex option where you may define the angle by the 3 points method (Fig. 4.13).
Specify angle vertex: Specify the angle vertex by picking a point.
Specify first angle end point: Specify the first angle end point.
Specify second angle end point: Specify the second angle end point.
Specify dimension arc line location or [Mtext/Text/Angle]: Drag your cursor and pick a point to show the location of the dimension line and AutoCAD creates the dimension.
Arc option: Select an arc. AutoCAD will use the center of the radius of the arc as the vertex and continue to prompt for the dimension line location (Fig. 4.13).
Circle option: Select the circle. AutoCAD will use the circle's center as the vertex and then wait for the user to supply the two end points to define the angle at the appropriate prompts.
Line option: Select two lines within which to place the angle dimension.

 

In all the three options, once you complete the definition of the angle, the following usual AutoCAD prompt appears.

 

AutoCAD Commands and Prompts Steps/Feedback/Options
Specify Dimension arc line location or [Mtext/Text/Angle]: Specify the location of the dimension and AutoCAD will create the angular dimension.
NOTE

We know that when two lines meet to produce an angle, they actually produce two angles—major angle and minor angle. There is also a supplementary angle as shown in Fig. 4.14. To put the dimensions for these angles appropriately, select the dimension arc line location accordingly, as shown in Fig. 4.14.

 

FIG. 4.14   Dimensioning major and minor angles

Radius Dimension—Dimradius

 

With this command you can annotate diameters and radii of circles and arcs. According to your need and choice you can also put a center mark or center lines with your dimensions. For creating diameter and radius dimensions, you should invoke Dimdiameter and Dimradius commands, respectively.

When you create these dimensions, the position and appearance of the dimension depend on the size of the circle or arc, dimension line location you specify and the settings of the different system variables. Fig. 4.15 clarifies the previous statement. The flow of command prompts for both these commands are same. We discuss only the Diameter command here.

 

FIG. 4.15   Dimensioning circles and radii

 

AutoCAD Commands and Prompts Steps/Feedback/Options
Command: Dimdiameter ↵ Initiates the command.
Select arc or circle: Pick the arc or circle and press Enter.
Specify dimension line location or [Mtext/Text/Angle]: The leader line may be in a dragging mode. Pick a point to locate its position. You can choose other options which work as discussed before.

To Mark the Center of an Arc or Circle—Dimcenter

With the help of this command, you can put a cross at the center point of a selected arc or circle. The only prompt here is: Select arc or circle: Pick an arc or a circle, and the mark will appear.

 

NOTE
  1. If you do not modify the text, the dimension value will be preceded by a diameter symbol (Ø) and the radius value will be preceded by an R. You can place the dimension horizontally or vertically by using the quadrant option of Osnap over-ride.

  2. If the system variable Dimtext is On, the dimension is placed inside the circle.

NOTE

AutoCAD places a center mark by default at the center of the arc or circle when you use the Dimdia or Dimradius commands.

 

Dimcen   Using this system variable, you can change the size of the center mark. You can change the center mark to a center line by entering a negative value for the size.

Baseline and Continued Dimensions—Dimbaseline and Dimcontinue

Sometimes it is necessary to create a series of dimensions which are attached or connected. For example, showing the distance of the center lines of bolt holes from a datum line (base line) such as the edge outline of an object (Fig. 4.16). You have two options provided by AutoCAD for doing that.

 

1. Base line dimensions   They show a series of dimensions which start from one point. The first extension line for all these dimensions is the same and is known as the base line. The second extension line covers the distance from the base line and an additional one.

 

FIG. 4.16   Baseline dimension

 

 

The prompts of this command are somewhat different from the other commands as the prompts will ask you only to specify the second extension line origin. It will assume the first extension line of the previous dimension as the baseline for the current one.

AutoCAD Commands and Prompts Steps/Feedback/Options
Command: Dimbaseline ↵ Initiates the Dimbaseline command.
Specify a second extension line origin or [Undo/Select] < select >: If the first dimension was a linear, angular or ordinate dimension, AutoCAD uses its first extension line origin as the base for the new dimension. Specify the origin of the second extension line and AutoCAD will create the Baseline dimension (Fig. 4.16).
If you do not want to use the previous dimension, press Enter and AutoCAD will prompt:
Select base dimension:
The starting point of your base dimension (baseline). Then continue to pick the second extension line origin. AutoCAD will continue to mark the dimensions accordingly.

2. Continued dimension   The continued dimensions work exactly like the Baseline dimension. Only the command and access are given here with a figure of continued dimension (Fig. 4.17). Note that you can use the Dimfit system variable to control the positioning of the dimension lines and text between the selected points.

 

You can do a hands-on exercise to put the Baseline and Continued Dimension with the drawing of a Gusset plate with holes given below. Use the end point Object Snap to select the extension line origins. Use a separate layer for Dimension. The command sequence is given below.

 

FIG. 4.17   Continued dimension

 

AutoCAD Commands and Prompts Steps/Feedback/Options
Command: Dimlinear ↵ Initiates the Linear dimensioning command to give the base dimension first.
AutoCAD will prompt as follows:  
Specify first extension line origin or < select object >: Select point A ↵
Specify second extension line origin:
Specify dimension line location or [Mtext/Text/
Angle/Horizontal
/Vertical/Rotated]
:
Select point B ↵
Dimension text = Select point C ↵
Command: Dimcontinue ↵ The Continued Dimension command is given.
Specify a second extension line origin or [Undo/Select] < select >: ↵ Select point D ↵
Specify second extension line origin or [Undo/select] < select >: Select point E ↵
Specify second extension line origin or [Undo/select] < select >: Select point F ↵
Specify second extension line origin or [Undo/select] < select >: Enter
Select Continued Dimension: Enter
Command: Baseline ↵ Initiates the Baseline Dimension command.
Specify second extension line origin or [Undo/select] < select >: Press Enter to opt for the Select Object option.
Select base dimension: Select the first dimension near point C.
Specify a second extension line origin or [Undo/Select] < Select >: Select point F ↵
Specify a second extension line origin or [Undo/Select] < Select >: Press Enter
Select base dimension: Press Enter

Your drawing will look like the figure below with all the dimensions put in their places:

 

FIG. 4.18   Exercise on Continued and Baseline dimension

Ordinate Dimension—Dimordinate

In mechanical design drawings it is often more convenient to express all dimensions with reference to a datum value (or origin). This is called Ordinate dimensioning. Ordinate Dimensions constitute of an X or Y coordinate and a leader line. The X coordinate measures distance along the X axis and the Y coordinate measures distance along the Y axis. This type of dimensioning helps prevent errors by dimensioning from the same reference point or datum. Since this method has limited use, it is not mentioned here. Interested readers are advised to go through Appendix IV.

Adding Dimensions/Text with Leaders

Leaders lines are widely used in mechanical drawings to connect some annotations to a feature in the drawing. They are mostly used to incorporate Shop Notes in the drawings. Remember that leader lines are not associative in AutoCAD. This means no dimension text is calculated by AutoCAD while drawing leaders. You can also edit the leaders and their annotations separately. But any modification in the position of the annotation text changes the orientation of the Hook line.

AutoCAD provides two different commands for creating leaders. The older Leader is still available but most users prefer the Qleader command because it is more powerful and easier to use.

 

 

AutoCAD Commands and Prompts Steps/Feedback/Options
Command: Qleader ↵ Initiates the Qleader command.
Specify first leader point or [Settings] < Settings >: Pick the arrowhead end of the leader or choose to change the settings by pressing Enter.
Specify next point: Specify another point.
Specify text width < 0.0000 >: Specify the width of your annotation block so that AutoCAD may automatically wrap the text if it gets too long. Enter a value or pick points to do that.
After you fix the length of the text AutoCAD will prompt:
Enter first line of annotation text < M text >:
Press Enter to open the Mtext Editor box to write your annotation. You can also enter text by typing when the earlier prompt for annotation will be repeated until you press Enter twice.

Creating Multiple Leaders

You can easily change the geometry of the leader line or copy it to create multiple leaders from the same annotation by using the Grip editing.

  1. Click the first leader so that Grips are visible.
  2. Click the Grip at the arrowhead to make it hot grip. AutoCAD prompts: ** STRETCH ** Specify stretch point or [Base point/Copy/Undo/Exit]:
  3. Type C and press Enter to select Copy option.
  4. Click to the position of the new leader.
  5. Press Enter to exit from GRIP editing and then press Esc twice to clear the grips.
  6. Fig. 4.19 shows the application of multiple leaders (copy option) where the first dimension is created by Qleader command.

 

FIG. 4.19   Creating multiple leader lines

EDITING DIMENSIONS IN AutoCAD

The default dimension style of AutoCAD is created keeping in mind the needs and practices of mechanical engineering. For Architecture, Civil, or even Electrical engineering, this style may not be suitable and needs changes or modifications to suit their conventional practices. Sometimes, even in mechanical drawings, editing of one or more components of a dimension is required for better understanding of the drawing. For example, a text may need to be positioned in some other location, or its height increased to make it legible, or the arrow sizes, or extension line positions may require modification. To achieve these changes, we use the editing operations on the Dimensions entries of the existing style. This is called Editing Dimensions.

There are various methods provided by AutoCAD to edit dimension labels either individually or as a whole. In addition, it is possible to override the default dimension style of AutoCAD. Interested readers are requested to go through Appendix V where a detailed description with suitable illustrations is given for easy understanding of their operations.

ILLUSTRATIVE EXAMPLE OF DIMENSIONING

In the following exercise, we shall discuss how dimensioning is actually done in an AutoCAD drawing. We have taken here the drawing of the Plummer block cap that we drew in the previous chapter. For your understanding, we have given below two figures—Fig. 4.20a and b—which exhibit the outline drawing (to be dimensioned) and the dimensioned drawing respectively.

NOTE

The space between the Hook line segment and the dimension text is set with the Dimgap system variable.

 

FIG. 4.20   The Plummer block cap

 

Before putting any dimension, examine the drawing thoroughly and check the following points:

  1. The system of dimensioning to be followed—aligned or unidirectional system of dimensioning. Once the system of dimensioning is selected, tune the dimension style of AutoCAD to suit the chosen system.
  2. Check how many linear dimensions are to be given and their relative position so that the final arrangement complies with the convention of dimensioning.
  3. Similarly, check the number of circles, arcs that are to be dimensioned and how we have to place them so that there is minimum or no interference with other dimensions.
  4. If any sectional views are to be dimensioned, check how the cutting plane line and the associative text entries may be placed.

Once the planning part is complete, begin the process of dimensioning for the given drawing stepwise.

 

Step I

Putting the linear dimensions Start with the linear dimension. Put on the Osnap and check the Endpoint box. The method of putting a linear dimension has been already discussed. Fig. 4.21 shows the object with the linear dimensions added in the drawing.

 

FIG. 4.21   Putting the linear dimensions

 

Step II

Dimensioning the arcs and circles In this step, the circles and arcs are dimensioned with the relevant commands. Care is taken to see that the dimension and extension lines do not cross each other. Place dimension text horizontally except in the case of R42. Fig. 4.22 shows the position of the circle/arc dimensions in the drawing.

 

FIG. 4.22   The drawing after putting the circular dimensions

 

Step III

Putting text to cutting lines Though strictly speaking this step does not come under dimensioning, we execute this step alongwith the dimensioning exercise. Use the DT command and enter the appropriate input, for example, text size, to achieve proper visibility. After the completion of this step, the final view of the drawing is as shown in Fig. 4.20b.

 

EXERCISE
  1. Draw the orthographic views of the objects shown in Figs. 2.29 and 2.30. Add appropriate dimensions necessary in each view.
  2. Draw the three views shown in Figs. 3.68 and 3.69. Add dimensions to the respective views as shown in the figures.
  3. Pictorial views of various objects are given in Figs. 5.30 to 5.61. In each case, the viewing direction for the front view is specified by means of arrows. Draw the front view, one of the side views, and the top view of these objects. Add dimensions to the respective views. Any dimensions missing may be assumed proportionately.