1. Dimensioning Guidelines
Virtual Academy
Dimensioning Guidelines
My name is CJ and I would like to welcome you to this lesson on Dimensioning Guidelines.
This presentation will introduce you to the most common dimensioning guidelines that are used to carry out Activities, Projects, and Problems in Project Lead The Way curricula.
The list of guidelines and conventions is by no means complete, but this presentation does address areas of dimensioning for which mistakes are most commonly found.
A more complete list of dimensioning guidelines and conventions can be found in the books that are identified in the reference section of this presentation.
Project Lead The Way, Inc.
Copyright 2007

2. Dimensioning Guidelines
Virtual Academy
Terminology
Dimension Line
A dimension line is a thin line drawn perpendicular to extension lines with arrowheads at each end.
There are a few key terms that you will need to know before you can understand the basics of dimensioning guidelines.
One of these key terms is dimension line. A dimension line is used to identify linear distance, and is therefore always accompanied by a measurement value.
Arrows are located on either end of a dimension line. These arrows always point to and touch their respective extension lines.
If there is not enough room between the extension lines for the dimension line and its respective measurement value, then the measurement is usually located between the extension lines, and the dimension lines are inverted and moved to the outside of the extension lines.
If there is not enough room between the extension lines for either the dimension line or its measurement value, then both are located on the outside of the extension lines. This only occurs when dimensioning very small linear distances. Increasing the scale of a drawing can eliminate the need to do this, if most of an object’s dimensions are very small.
indicates linear distance between object edges or features
Project Lead The Way, Inc.
Copyright 2007

3. Dimension Line Geometry
Dimensioning Guidelines
Virtual Academy
Dimension Line Geometry
The first dimension lines should be placed 3/8” from the object, with each successive dimension line spaced ¼” apart.
A dimension line’s arrowheads are approximately 1/16” wide by 3/16” long.
The smallest dimension is placed closest to the object view, and is spaced approximately 3/8” away.
Successively larger dimensions are located outward from the object, and are spaced approximately ¼” apart.
Project Lead The Way, Inc.
Copyright 2007

4. Unidirectional Dimensions
Dimensioning Guidelines
Virtual Academy
Unidirectional Dimensions
In Mechanical Drafting, we use Unidirectional Dimensioning.
All dimension values should be oriented parallel to the bottom edge of the paper, and should read left-to-right (unidirectional).
Incorrect
Much as you would not turn a book 90° to read the text, a drawing should not be turned in order to read its dimensions.
Unidirectional dimensioning is a technique that requires all dimensions to be oriented in the same direction, and aligned parallel to the bottom edge of the drawing sheet.
The example given shows an incorrect way to dimension drawings of industrial objects.
Project Lead The Way, Inc.
Copyright 2007

5. Unidirectional Dimensions
Dimensioning Guidelines
Virtual Academy
Unidirectional Dimensions
All dimension values should be oriented parallel to the bottom edge of the paper, and should read left-to-right (unidirectional).
Correct
This example shows the correct way to orient dimension values.
Notice that all of the dimensions are now parallel to what would be the bottom edge of the drawing sheet, and be read from left-to-right.
Project Lead The Way, Inc.
Copyright 2007

6. Dimensioning Guidelines
Virtual Academy
Terminology
Extension Line
An extension line is a thin solid line that occurs perpendicular to a dimension line.
An extension line is used to extend an object edge so that it can be dimensioned to.
Note that a gap exists between the extension lines and their respective object lines. This is done intentionally to avoid overlap between the two line types.
extends an object line or centerline
shows the extents of a dimension
Project Lead The Way, Inc.
Copyright 2007

7. Extension Line Geometry
Dimensioning Guidelines
Virtual Academy
Extension Line Geometry
Extension lines start about 1/16” from the object, and extend about 1/8” past the last dimension. 1 8
Object
To keep an extension line from overlapping its respective object edge, a 1/16” gap is placed between the object line and the extension line. If this gap did not exist it would be difficult to tell where the object ends and the dimension starts.
An extension line continues out past its respective dimension line by a distance of approximately 1/8”. 1 16
Project Lead The Way, Inc.
Copyright 2007

8. Object Lines as Extension Lines
Dimensioning Guidelines
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Object Lines as Extension Lines
Do not use an object line as an extension line. Dimensions lines must terminate on extension lines.
Incorrect
A common mistake that people make when they are first learning how to dimension is to use an object line as an extension line. This is poor practice, and makes it harder to interpret the object’s geometry.
The example given shows an incorrect use of object lines being used as extension line.
An object should have a boundary of space surrounding it in which no dimension lines exist.
Extension lines are used to extend the edges of an object outward so that its dimension can be clearly communicated and interpreted.
Project Lead The Way, Inc.
Copyright 2007

9. Dimensioning Guidelines
Virtual Academy
Dimension Placement
If properly placed, dimensions will appear to taper from largest to smallest. The largest dimensions should occur furthest away from the object.
A well dimensioned drawing will have the largest dimensions located furthest away from its object views.
Successively smaller dimensions are placed in between the largest dimension and the object view, with the smallest dimensions located closest to the object.
If this practice is followed, the dimensions will appear to taper inward toward the object. This also helps to avoid the problem of crossing dimension and extension lines.
Project Lead The Way, Inc.
Copyright 2007

10. Contour (Shape) Dimensioning
Dimensioning Guidelines
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Contour (Shape) Dimensioning
Dimensions should be placed on the view that best illustrates the shape or contour of the feature.
Incorrect
A view that does the best job of illustrating an object’s contour or shape is the view to which associated dimensions should placed.
In the example, the object’s stepped geometry is clearly seen as such in the front view. Despite the fact that the edges of these steps are clearly visible on the right side view, these edges do not give the viewer the best illustration of the shape of these features.
Project Lead The Way, Inc.
Copyright 2007

11. Contour (Shape) Dimensioning
Dimensioning Guidelines
Virtual Academy
Contour (Shape) Dimensioning
Dimensions should be placed on the view that best illustrates the shape or contour of the feature.
Correct
Therefore, a more appropriate location for two of the height dimensions would be the front view, as seen in the corrected drawing.
Project Lead The Way, Inc.
Copyright 2007

12. Centering Dimension Values
Dimensioning Guidelines
Virtual Academy
Centering Dimension Values
In most cases, dimension values should be centered on their dimension lines or between their extension lines.
Incorrect
The most ideal location for a dimension value is the center location between the extension lines.
Observing this convention makes the drawing look neater.
There are situations where dimension values should be staggered, and thus located off-center between their extension lines. One such situation is when the diameters of multiple concentric cylindrical features are being dimensioned. An example would be an axle shaft that has multiple diameters.
Project Lead The Way, Inc.
Copyright 2007

13. Dimensioning Between Views
Dimensioning Guidelines
Virtual Academy
Dimensioning Between Views
Whenever possible, dimensions should be placed in the space between the front, right side, and top views.
Incorrect
To keep a dimensioned drawing looking neat, dimensions should be located in the space between the object views whenever possible.
Project Lead The Way, Inc.
Copyright 2007

14. Dimensioning Within an Object
Dimensioning Guidelines
Virtual Academy
Dimensioning Within an Object
Do NOT place dimensions within the object. Always locate dimensions outside of the object.
Incorrect
Dimensions belong outside of their respective object views.
When dimensions are placed within an object view, it makes it harder to interpret that object’s geometry.
Project Lead The Way, Inc.
Copyright 2007

15. Dimensioning Through an Object
Dimensioning Guidelines
Virtual Academy
Dimensioning Through an Object
Whenever possible, avoid sending an extension line through an object view.
Incorrect
If it can be avoided, place dimensions to the side of an object view that will not require extension lines to run through that view.
However, it should be realized that this is not always avoidable; especially with objects that have relatively complex geometries.
Whenever two guidelines appear to conflict, the dimensioning method that most clearly identifies the size information should be used.
Project Lead The Way, Inc.
Copyright 2007

16. Crossing Extension Lines
Dimensioning Guidelines
Virtual Academy
Crossing Extension Lines
Whenever possible, avoid crossing extension lines.
Incorrect
Though it is not always avoidable, crossing extension lines can make a drawing confusing to interpret.
Whenever possible, place the dimensions on multiple views to eliminate this problem.
However, do not adopt a solution that violates the contour rule, or has the dimension placed in such a way that the extension lines are made to run through the object.
Project Lead The Way, Inc.
Copyright 2007

17. Crossing Dimension Lines
Dimensioning Guidelines
Virtual Academy
Crossing Dimension Lines
Never cross a dimension line with another dimension line or an extension line.
Incorrect
There is no situation for which crossing dimension lines constitutes an appropriate dimensioning technique.
Recognize that the smallest dimensions occur closest to the object, with each successively larger dimension occurring outward. This helps to avoid the problem of crossing dimension lines.
Project Lead The Way, Inc.
Copyright 2007

18. Dimensioning Guidelines
Virtual Academy
Leader Line Conventions
Leader Line Definition
Leader Line Angles
Diameter versus Radius
Hole and Cylinder Dimensions
The next section of this lesson will explore what leader line dimensions are, where they are used, how they are applied to drawings, and their most common uses.
Project Lead The Way, Inc.
Copyright 2007

19. Dimensioning Guidelines
Virtual Academy
Terminology
Leader
A leader is a thin, solid line used to indicate features with which a dimension, note, or symbol is associated.
Points toward the center of the feature
Arrow on one end touches the part or detail
Text is extended from a short horizontal line or “shoulder” on the other end
A leader line is a special kind of dimension that is used to identify the size of geometric features, such as hole or cylinder diameters, chamfer sizes, thread types, and radius values.
Leader lines are accompanied by dimension values or notes.
They are similar to dimension lines, in that they have an arrow. But unlike a dimension line, a leader line arrow actually touches the feature to which it is referring.
Project Lead The Way, Inc.
Copyright 2007

20. Dimensioning Guidelines
Virtual Academy
Leader Line Angles
Whenever possible, a leader line dimension should occur at approximately 30°, 45°, or 60° from the horizontal or vertical.
Leaders should never be drawn horizontally or vertically
To keep a dimensioned drawing looking neat, leader lines should be placed at either 30°, 45°, or 60° angles to the horizontal or vertical.
Project Lead The Way, Inc.
Copyright 2007

21. Diameter versus Radius
Dimensioning Guidelines
Virtual Academy
Diameter versus Radius
A complete circular object is called out by its diameter.
A fillet or arc is identified by its arc radius.
Circles and cylindrical objects are always dimensioned by diameters.
A diameter is the distance across a circle, and is measured through the center of a circle.
Arcs, or partial circles, are identified by their radius values.
A radius is the distance from the center of an arc or circle to any point on its edge.
Project Lead The Way, Inc.
Copyright 2007

22. Hole and Cylinder Dimensions
Dimensioning Guidelines
Virtual Academy
Hole and Cylinder Dimensions
A hole is dimensioned on a view showing its true circular shape. A leader should be used for this purpose.
A cylinder, or solid cylindrical feature, is dimensioned on a side view using a linear dimension.
As previously stated, circles are always dimensioned by their diameter values. This is true whether the circle represents a cylindrical shaft of space, such as a hole, or a solid cylindrical object, such as an axle.
A hole diameter is called out using a leader line, whereas a solid cylindrical feature is dimensioned on its side profile using a linear dimension. In both instances, a diameter symbol is used to tell the viewer that the associated dimension is in fact a diameter value.
Project Lead The Way, Inc.
Copyright 2007

23. Dimensioning Guidelines
Virtual Academy
Common
Dimensioning
Errors
Unnecessary Dimensions
Duplicate Dimensions
Dimensioning to Hidden Lines
True Scale
The last section of this lesson will explore common dimensioning errors that are encountered, such as adding unnecessary dimensions, duplicating dimensions between views, dimensioning to hidden lines and features, and errors that sometimes occur when a scaled object is dimensioned.
Project Lead The Way, Inc.
Copyright 2007

24. Unnecessary Dimensions
Dimensioning Guidelines
Virtual Academy
Unnecessary Dimensions
A drawing must contain only those dimensions that are necessary to define the object’s geometry.
Incorrect
Over-dimensioning and omissions of critical information are common mistakes.
It takes a certain level of manufacturing knowledge to recognize how an object’s geometry is completely and appropriately defined through dimensioning. Manufacturing knowledge also helps the designer to understand that some dimensions have no bearing when it comes to actually creating a part.
The aligned dimensions that are shown in the example image do serve to define the object’s geometry, but they do not represent the simplest or most effective way. Replacing them with standard height and width dimensions would help the manufacturer avoid unnecessary calculations or assumptions.
The bottom line is, dimensions that appear on a drawing should provide a complete and appropriate geometric description of the object.
Project Lead The Way, Inc.
Copyright 2007

25. Dimensioning Guidelines
Virtual Academy
Duplicate Dimensions
Do not call out the same dimension on more than one view.
Incorrect
Another common error is dimensioning a feature more than once, by dimensioning identical features on two or more views.
Duplicate dimensions are redundant, and only add clutter to a drawing.
In the example, two of the height dimensions have been repeated on the right side view.
Project Lead The Way, Inc.
Copyright 2007

26. Dimensioning to Hidden Lines
Dimensioning Guidelines
Virtual Academy
Dimensioning to Hidden Lines
Never dimension to hidden lines. If necessary, generate an alternate view, or section view, where the feature appears as an object line.
Hidden lines are less visible than object lines, and should not be dimensioned to.
There are very few instances that you will ever encounter where it would be necessary to dimension to a hidden line.
In most cases, an alternate view should be used or created that shows the hidden feature as an object line.
If an edge or feature does not appear as an object line in any of the basic orthographic projections, then a sectional view should be created.
Project Lead The Way, Inc.
Copyright 2007

27. Dimensioning Guidelines
Virtual Academy
True Scale
Dimensions should reflect an object’s actual size; not its scaled size.
Lastly, the dimension values should always reflect the true size of the object, regardless of the scale of the drawing.
If a scaled drawing is being dimensioned, then a note indicating the drawing scale should be placed somewhere on the drawing.
Project Lead The Way, Inc.
Copyright 2007

28. Dimensioning Guidelines
Virtual Academy
References
Bertoline, G. R., & Wiebe, E. N. (2003). Technical graphics communication (3rd ed.). NY: McGraw-Hill Companies, Inc.
Lockhart, S., & Johnson, C. (2000). Engineering design communication. Upper Saddle River, NJ: Prentice Hall Inc.
Madsen D. A., Folkestad, J., Schertz, K. A., Shumaker, T. M., Stark, C., & Turpin, J. L. (2004). Engineering drawing and design (3rd ed.). Albany, NY: Delmar-Thompson Learning.
Spence, W. P. (1991). Drafting technology and practice (3rd ed.). NY: Glencoe-McGraw Hill Inc.
Wallach, P. (2003). Fundamental of modern drafting. Clifton Park, NY: Thomson Delmar Learning.
Now that you know proper dimensioning guidelines you can create drawings that are easily read and understood.
Project Lead The Way, Inc.
Copyright 2007

29. Dimensioning Guidelines
Virtual Academy
Credits:
Writer: Terry C. Nagy Jr.
Lesson Editor: Ed Hughes
Narration: CJ Amarosa
Production: CJ Amarosa
We would like to thank you for joining us for this lesson on dimensioning guidelines. We hope it was helpful and your Virtual Academy learning experience enjoyable.
Project Lead The Way, Inc.
Copyright 2007