GlossaryKind of term: Standard and conventionProjection method (first and third angle)
What is a projection method?
The projection method, first or third angle and shown by a symbol, sets where the views on a drawing sit relative to the front view.
A technical drawing shows a part in orthographic views: front, top, bottom, left, right and rear. The projection method sets where each view sits relative to the front view. In first angle projection, the usual convention in Europe, the top view sits below the front view and the left view to the right of it. In third angle projection, the North American convention, the top view sits above the front view and the left view on the left. A symbol showing a truncated cone in two views, in or near the title block, states which method applies.
What does the standard say about the projection method?
The methods and their symbols are set out in ISO 5456-2. That standard recognises both methods, and also the reference arrow method, in which each view is tied to an arrow and a letter on the front view and may sit anywhere on the sheet. So the standard does not require first angle in Europe; that is custom. In North America third angle is the default under ASME Y14.3. Without the symbol, the arrangement of the views is ambiguous unless each view is identified with an arrow and a letter.
How does the projection method work in practice?
A misread projection swaps the sides features sit on and can produce a mirrored part: a pocket on the wrong side, an angled hole pointing the other way. A STEP model fixes the shape, but tolerances, roughness and datums are often only on the drawing, and attaching a requirement to the wrong face is the same error. So read the symbol first. If it is missing, or the model and drawing disagree, ask before you settle the route and the setups. What a mistake costs depends on when it is caught: at programming, at first article inspection or only at the customer.
What is the projection method often confused with?
The projection method is not isometric projection: an isometric view is a pictorial image of the part, while the projection method concerns where the orthographic views are placed. Nor is a view a section: a section shows the part as if cut along a marked line, and its viewing direction follows from the arrows on that line.
Frequently asked questions
How do I tell first angle from third angle on a drawing?
By the truncated-cone symbol in or near the title block. If it is missing, you can sometimes work the method out from features that appear in two views, but that remains an assumption. In that case, ask the customer.
What if the STEP model and the drawing disagree?
Ask the customer which source leads before you price. Sometimes a drawing note or the purchasing terms already settle it, for example that the model leads for shape and the drawing for requirements.