1. Dimensioning Guidelines
Virtual Academy
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.
Project Lead The Way, Inc.
Copyright 2007

2. Dimensioning Guidelines
Virtual Academy
Terminology
Dimensioning
Dimension Amount
The process of describing the size of an object, and locating important features of the object.
Extension Line
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.
Dimension Line
Project Lead The Way, Inc.
Copyright 2007

3. Dimensioning Guidelines
Virtual Academy
Terminology
Dimension Line
Green Lines
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 and locates features such as holes.
Project Lead The Way, Inc.
Copyright 2007

4. 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

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. Unidirectional Dimensions
Dimensioning Guidelines
Virtual Academy
Unidirectional Dimensions
The example below is incorrect because the numbers are oriented towards the right side of the page.
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

7. Dimensioning Guidelines
Virtual Academy
Terminology
Extension Line
An extension line is a thin solid line that is drawn perpendicular to a dimension line.
Green Lines
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

8. 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

9. Object Lines as Extension Lines
Dimensioning Guidelines
Virtual Academy
Object Lines as Extension Lines
Object lines should never be used 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

10. 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

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.
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

12. 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

13. 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

14. 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

15. 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