Orthographic projection
Orthographic projection shows a 3D object as a set of flat views. Learn the principal views, the two projection systems used worldwide, and how to choose and align views.
In this lesson you will
- Describe principal orthographic views
- Tell first-angle from third-angle projection
- Choose the best front view
- Project features between aligned views
Views
Imagine looking at an object straight on from each side. Each view shows true lengths of edges parallel to the viewing plane. The usual set is front, top and side — enough for most parts; complex parts may need more, simple ones fewer.
First and third angle
Third-angle projection (common in North America, and in Australia under AS 1100): the top view is placed above the front view, the right view to the right.
First-angle projection (common in Europe and much of Asia): the top view goes below the front view, the right-side view to the left.
Every drawing shows the projection symbol (a truncated cone in two views) in the title block. Getting it wrong mirrors features — a hole on the wrong side.
Choosing the front view
Pick the view that shows the most characteristic shape, with the fewest hidden lines, in the part's natural or functional orientation. Then add only the views needed to define every feature.
Projecting between views
Views are aligned: widths line up between front and top, heights between front and side. Features are projected across with construction lines. In CAD, object snap tracking does this; the principle is the same.
Common mistakes
Placing views in the wrong positions for the projection system.
Check the projection symbol and place top and side views accordingly.
Adding views that show nothing new.
Use the minimum number of views that fully define the part.
Exercise
Three views of a block
- Sketch an L-shaped block with a hole through its short leg.
- Draw front, top and right views in third-angle projection.
- Redraw the arrangement in first-angle projection.
- Add the correct projection symbol to each.
You should end up with: The same part in both systems, correctly labelled.
Checklist
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Lesson quiz
In third-angle projection, where is the top view?
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