Engineering Graphics and CAD · Lesson 5 of 35
Orthographic projection and projection angle
The working definition of a part is built from aligned, square-on views. This lesson covers the glass-box model, the six principal views, choosing the front view, and the first-angle and third-angle conventions that tell a reader how the views are arranged.
Readiness check
Tick only what you can do closed-notes.
- Say which two views share a part's width.
- Recall where the top view usually sits relative to the front.
- Explain why a slanted view distorts a face.
- Rotate a part mentally to face a chosen direction.
- Accept that two standard conventions arrange views differently.
The core idea
Orthographic projection represents a 3D object by flat views seen square-on, so faces parallel to the viewing plane show true shape. Two conventions, first-angle and third-angle, arrange the same views differently; a projection symbol states which is used.
square-on views preserve true shapethird-angle: top above, right-side rightfirst-angle: top below, sides swappedImagine the object inside a glass box and project each face square-on onto the nearest pane, then unfold: the six panes become the front, top, bottom, left, right, and rear views, though most parts need only two or three. The front view is chosen to show the most characteristic shape with the fewest hidden lines. Views are locked in alignment: front and top share width, front and side share height, and depth is transferred between top and side with a 45-degree miter line. In third-angle projection (common in North America) the top view sits above the front and the right-side view to the right. In first-angle (the ISO default this course teaches) the top view sits below and the sides are swapped. The view content is identical; only the arrangement differs, so a missing projection symbol can cause a reader to mirror the part.
The skills, taught in order
Four skills produce a correct multiview.
5.1 Build views with the glass box
Project each face square-on onto its pane and unfold. Square-on projection preserves true shape for faces parallel to the plane, which is what makes the views measurable.
5.2 Choose the front view
Orient the object so its most descriptive profile faces front, long axis usually horizontal, minimizing hidden detail. A good front view reduces the number of views needed.
5.3 Align views and read the convention
Keep the top directly above or below the front and the side directly left or right. Read the projection symbol to know whether the arrangement is first- or third-angle, so you do not mirror the part.
5.4 Transfer depth between top and side
Project heights across from the front to the side; take depths from the top and swing them through a 45-degree miter line so the top and side agree on depth.
| Shared dimension | Between |
|---|---|
| Width | front and top |
| Height | front and side |
| Depth | top and side (via miter) |
Alignment is what lets you transfer dimensions without re-measuring.
Worked example 1: three views of the stepped block (third-angle)
Draw the front, top, and right-side views of the stepped block (base 60 wide, 40 deep, 20 tall; raised block 30 wide, 40 deep, 20 tall, centred) in third-angle.
- ProblemProduce an aligned third-angle three-view.
- FrontThe 60 by 40 silhouette: a 60 by 20 base with a 30 by 20 block centred, leaving 15 shoulders.
- Top (above)The 60 wide by 40 deep footprint, with the raised block as a 30-wide central band; place it above the front, sharing width.
- Right side (right)Depth 40, height 40, with a mid-height line where the base meets the block; place it right of the front, sharing height.
- Depth transferCarry the 40 depth from the top into the side via a 45-degree miter line.
- CheckWidths match front-top, heights match front-side, depth matches top-side.
Worked example 2: the same block in first-angle
Render the same block in first-angle, place the projection symbol, and show how identical view content lands in different positions.
- ProblemRearrange the same views the first-angle way.
- FrontIdentical to before: the stepped silhouette.
- Top (below)The same top outline, now placed below the front.
- Side (swapped)The view seen from the left is placed to the right; the shape is the same rectangle, its position moved.
- SymbolPlace the first-angle truncated-cone symbol in the title-block area; it is the mirror of the third-angle symbol.
- ConsequenceWithout the symbol, a reader assuming third-angle would mirror an asymmetric feature and build a left-handed part.
Misconceptions and diagnostics
| Mistake | Symptom | Diagnostic question | Correction |
|---|---|---|---|
| "The front view is the physically front face" | Many hidden lines, unclear shape | "Which orientation shows most shape with fewest hidden lines?" | Choose the front for clarity, not by label. |
| "First- and third-angle differ in content" | Redrawing views for each convention | "Did the view's shape change, or just its position?" | Only the arrangement differs; read the symbol. |
| "Views can be nudged for a tidy layout" | Projected lines no longer line up | "Is the top exactly above the front?" | Keep strict alignment. |
Practice ladder
Identify given three-view drawings as first-angle or third-angle from the arrangement and symbol.
Show answer
Third-angle: top above, right-side right. First-angle: top below, sides swapped. Check the symbol too.
Complete the missing third view of a part in third-angle, transferring depth with a miter line.
Show answer
Project heights from the front; swing depths from the top via the miter line so the side agrees with the top.
Produce a full three-view of the stepped block in both conventions, each with its symbol.
Show answer
Draw the three views once, then arrange twice; identical view shapes, correct arrangement and symbol for each.
For a given part, select the best front view and justify it, then produce the minimum set of views.
What good work looks like
The front minimizes hidden lines and shows characteristic shape; the view set is minimal yet complete, justified against the least-hidden-detail rule.
Working with AI, and proving it yourself
Use AI as a tutor, not a black box
Prove it yourself
An assistant may say the first-angle top view goes above the front, like third-angle. Catch it with the glass-box logic: in first-angle the object sits between observer and plane, so looking down projects the top onto a plane below, placing the top view below. Verify against the arrangement rules.
Retrieval and spaced review
Closed notes. Answer out loud, then reveal.
1. What does "orthographic" ensure?
Square-on projection, so faces parallel to the plane show true shape.
2. How is the front view chosen?
Most characteristic shape with the fewest hidden lines.
3. Third-angle: where do top and right-side go?
Top above, right-side to the right. First-angle: top below, sides swapped.
4. What does the projection symbol tell you?
Which convention is used; it sits in the title block.
5. What is the miter line for?
Transferring depth consistently between top and side.
Reference mapping
This lesson follows Giesecke and ISO drawing standards. Use these to read further.
| Topic in this lesson | Where to read more |
|---|---|
| Orthographic projection and views | Giesecke, Orthographic Projection |
| Views, sections, and cuts | ISO 128-3 (views, sections and cuts) |
| The projection-angle symbol | ISO 5456-2 (projection methods) |
Standards are named for study; copy the exact projection symbol from the official text. Introductory teaching rules are distinguished from full requirements.