What the projection method decides
An orthographic drawing shows the part as separate flat views: front, top, side and others, each looking straight at one face. The views themselves look the same in both methods. What differs is where each one is placed on the sheet relative to the front view. A person who reads a first-angle drawing as third-angle swaps the left and right views and the top and bottom views, and can model a mirror image of the part, or put a feature on the wrong face.
ISO 5456-2 defines both methods and their symbols, and ISO 128 permits either. ASME Y14.3 uses third-angle projection as its normal method. In practice, drawings from North America are mostly third-angle, and many drawings from Europe and other ISO users are first-angle. Neither habit is a rule for a given drawing: the sheet's own symbol is.
Third-angle projection
Imagine the part inside a glass box and look at it through each face. In third-angle projection, the image on each face is unfolded so that it lies on the same side as the face you looked through. The view seen from above is drawn above the front view, and the view seen from the right is drawn to the right of it.
First-angle projection
In first-angle projection, the part sits in front of the projection planes and each view is projected through the part onto the plane behind it. Each view therefore lands on the side opposite the one it is seen from. The view seen from above is drawn below the front view, and the view seen from the left is drawn to the right of it.
| View seen from | Third-angle | First-angle |
|---|---|---|
| Above (top view) | Above | Below |
| Below (bottom view) | Below | Above |
| The right | To the right | To the left |
| The left | To the left | To the right |
| The back | Beside a side view, at the far end | Beside a side view, at the far end |
Reading the projection symbol
The symbol is a truncated cone drawn in two views: a side view, a trapezoid, and an end view, two concentric circles. The end view shows both circles as solid lines, so it is the view from the cone's small end. That makes the symbol self-explaining. In the third-angle symbol, the small end of the trapezoid points toward the circles, because the view sits on the side it is seen from. In the first-angle symbol, the small end points away from the circles.
The symbol normally sits in or beside the title block. Some drawings write the method out in words instead, as THIRD ANGLE PROJECTION. A drawing with neither is incomplete: the views may settle the question, but raise it with the drawing's owner rather than infer it.
Checking the views against each other
01 Read the symbol
Find the projection symbol or note in the title block, and note which method it states.
02 Find the front view
The front view is usually the most descriptive view, and the others are aligned with it horizontally or vertically.
03 Trace a feature between views
Pick a feature that appears on only one side of the part, such as a hole near one end or a step. Follow it along the projection lines into the neighboring view and check that it lands where the stated method says it should.
04 Check hidden lines
A feature on the far side of a view shows as hidden (dashed) lines there. If the hidden lines only make sense under the other method, the symbol or a view is wrong: raise it.
05 Treat auxiliary and section views separately
Auxiliary views, sections and detail views are placed by their own viewing arrows and labels, not by the projection method.
When the symbol and the views disagree
A symbol that contradicts the views is a drawing defect. Do not choose the method that makes the part look reasonable. Ask the drawing's owner, and record the answer with the model.
Errors that look like projection errors
Some mistakes produce the same mirrored or misplaced result: a view labeled and placed out of its normal position (allowed when it carries a viewing arrow and label), a left-hand part drawn as its right-hand twin with a note to make it opposite, and a section taken looking the other way. The guide to drawing interpretation errors covers these with the other common misreadings.
Questions
Which projection does the US use?
ASME Y14.3 uses third-angle projection as its normal method, and most North American drawings are third-angle. The drawing's own symbol still decides for any given sheet.
How do I remember the symbol?
In the third-angle symbol the small end of the cone points toward the circles; in the first-angle symbol it points away from them.
What happens if I read a first-angle drawing as third-angle?
The views swap sides, so the part you model can be a mirror image, or have features on the wrong faces. Checking one asymmetric feature across the views catches it.
Do section views follow the projection method?
No. A section view is placed and read by its cutting-plane line, arrows and label, whichever projection the rest of the sheet uses.
Sources
- ISO 5456-2, Technical drawings, Projection methods, Part 2: Orthographic representations · ISOFirst-angle and third-angle projection, the placement of views and the graphical symbols.
- ISO 128 (series), Technical product documentation: General principles of representation · ISOViews, sections and the general rules of representation.
- ASME Y14.3, Orthographic and Pictorial Views · ASMEThird-angle projection as the normal method on ASME drawings; view placement and labeling.