Read the top, front and side views of a part and relate them to an isometric picture.
Why it matters
A photo of a part shows what it looks like; a technical drawing shows exactly what to make. To do that on flat paper, drawings use standard ways of projecting a 3D object into 2D. If you can read them, you can build, install, inspect or repair from a print. If you misread them, you can drill a hole on the wrong face.
Orthographic projection
Orthographic projection shows an object as a set of flat views, each looking straight at one face, with lines of sight parallel and perpendicular to that face. Each view shows only two of the three dimensions:
| View | Shows | Hides |
|---|---|---|
| Front | width and height | depth |
| Top | width and depth | height |
| Right side | depth and height | width |
Most parts need three views; simple parts (a washer, a shaft) may need only one or two plus a note. The front view is chosen to show the most characteristic shape of the part, not necessarily the side that faces you in use.
The views line up
Views share dimensions, so they are aligned:
- The top view sits directly above the front view and has the same width.
- The right side view sits directly to the right of the front view and has the same height.
- Depth appears in both the top and side views.
You can project features from one view to another with light construction lines. If a hole appears in the front view, find it in the top view directly above.
Third-angle and first-angle projection
There are two conventions:
- Third-angle projection (used in the US and Canada, ASME standards): top view above the front, right view on the right.
- First-angle projection (used in much of Europe and Asia, ISO standards): the top view goes below the front, and the right side view goes on the left.
The title block shows which is used with a small symbol of a truncated cone. Always check it, especially on prints from overseas suppliers. Misreading the projection mirrors the part.
Hidden and centre lines
Features you cannot see in a view, such as a hole through the back, are drawn as hidden lines (short dashes). The axis of a hole or cylinder is shown with a centre line (long and short dashes). Lesson 2 covers all line types.
Isometric (pictorial) views
An isometric drawing shows the object in one picture, tilted so that three faces are visible. The three axes are drawn 120 degrees apart: vertical edges stay vertical, and width and depth edges run at 30 degrees from horizontal. In an isometric drawing, lengths along the three axes are drawn at true scale, so you can measure them.
Isometric views help people visualise a part. They are often shown in a corner of a print or in assembly instructions, but the orthographic views carry the dimensions.
Worked example: an L-shaped block
Picture a block 60 mm wide, 40 mm deep and 30 mm tall, with a 30 mm wide by 15 mm tall notch cut out of the top right along its full depth, leaving an L shape when seen from the front. A 10 mm hole goes straight down through the low (right) part.
- Front view: an L outline 60 wide and 30 tall; the step is 15 mm down, 30 mm from the right. The hole, which runs vertically, appears as two hidden lines with a centre line between them, in the right half.
- Top view (above the front): a 60 × 40 rectangle with a line across it 30 mm from the right edge (the step edge). The hole is a circle in the right half, with crossing centre lines.
- Right side view (right of the front): a 40 × 30 rectangle with a visible line 15 mm down where the step is seen edge-on. The hole shows as hidden lines.
Check: the top and front views both span 60 mm wide; the front and side views are both 30 mm tall; the top and side views are both 40 mm deep.
How to read views quickly
- Find the front view (usually the largest, bottom left in third-angle).
- Check the projection symbol.
- Take one feature at a time and find it in every view.
- Build the 3D shape in your head, or sketch a quick isometric.
- Confirm hidden lines match features you expect.
Common mistakes
- Assuming third-angle on a first-angle drawing.
- Reading one view alone. A circle in the top view could be a hole, a boss or a cylinder; the other views tell you which.
- Ignoring hidden lines.
- Measuring an isometric drawing for dimensions. Use the dimensioned views.
Recap
- Orthographic views: front (width × height), top (width × depth), side (depth × height), aligned with each other.
- Third-angle (US) puts the top above and the right side on the right; first-angle reverses them. Check the symbol.
- Isometric drawings show one 3D picture with axes at 30 degrees from horizontal.