1. Engineering Graphics II Projection Theory and Multiview Drawings
Prof. Jon Southen
October 10, 2008

2. Text Reference
Bertoline-Wiebe, Fundamentals of Graphics Communication, Chapter 5

3. Projection Theory
Scheme for representing 3-dimensional objects in a 2-dimensional medium (paper or computer screen)
Can represent all 3 dimensions in a single image or use multiple views
Working drawings are the practical result

5. Projection Theory Based on two concepts: Line of Sight
An imaginary ray of light between the observer’s eye and the object
Start at single point for perspective projections and are parallel in parallel projections (surprising!)
Plane of Projection
An imaginary flat plane (paper, computer screen) upon which the image created by the lines of sight is projected

6. Perspective Projection

7. Parallel Projection

8. Projection Theory
Perspective projections mimic what the eye sees, but are more difficult to construct and do not show the true dimensions of the object
Parallel projections are most commonly used in engineering graphics

9. Orthographic Projection
A parallel projection where the plane of projection is perpendicular to the lines of sight
Can produce either pictorial drawings that show all three dimensions at once, or multiview drawings that show only two dimensions at a time

10. Multiview Drawings
Engineering drawings are used to manufacture or construct whatever the object is
Pictorial drawings (perspective or orthographic) distort dimensions and/or angles, making fabrication difficult
Multiview drawings show the true size and shape of the two dimensions shown

11. Pictorial Distortion

12. A Multiview Drawing by Albrecht Dürer (1471-1528)

13. Multiview Projection True size and shape Angles are correct
Circles are round
Lengths are to scale

14. Multiview Projection Set of 2-D views representing 3-D object.
Views are projections on sides of ‘box’ containing object.

15. Drawing formed by unfolding box

16. Drawing formed by unfolding box

17. Most Common Views Top (or Plan) Front View (or Elevation)
Right Side View (or Profile)
Cross sections show interior of object – later!

18. Projection Angle In Europe, First Angle Projection is used
In North America, and in ES050, Third Angle Projection is used
Be sure to select Third Angle Projection for your drawings

19. First-Angle Projection (Europe)

20. Third-Angle Projection (North America)

21. Standard Practices Choose front view as most descriptive
Determine views to best represent object
Use minimum number of views to completely describe object
Align views correctly

23. Line Conventions Hidden Lines
Some features may not be visible in a given view, but are still important.
E.g. holes, surfaces, change of planes
Hidden features are represented with a dashed line

24. Hidden Lines

25. Line Conventions Center Lines
Used to indicate the axes of symmetrical parts and features (holes, cylinders, etc.)
Consist of alternating long and short thin dashes

26. Center Lines

27. Line Precedence Visible lines have top priority
Hidden lines do not cross or have priority over visible lines
Centre lines do not cross or have priority over visible or hidden lines

28. How to Create 3-View Drawings
Block in front, top and right side views using the overall width, height and depth.
Sketch the front.
Using construction lines project to top and right.
Block in major features seen in each view

29. How to Create 3-View Drawings

30. How to Create 3-View Drawings

31. How to Create 3-View Drawings

32. Draw Multiview
E-drawing solution

33. Draw Multiview

34. Draw Multiview

35. Draw Multiview