3 differences between first angle projection and third angle projection
First angle projection and third angle projection are two different methods used in technical drawing to represent three-dimensional objects on a two-dimensional surface. Here are three differences between the two:
1. Placement of views: In first angle projection, the object is imagined to be in the first quadrant and the views (front, top, and side) are placed around it. The front view is placed above the top view, and the side view is placed on the right side of the top view. On the other hand, in third angle projection, the object is imagined to be in the third quadrant and the views are placed around it. The front view is placed below the top view, and the side view is placed on the left side of the top view.
2. Symbol conventions: First angle projection uses a specific symbol convention, where the cutting plane lines are represented by thick, continuous lines, and the visible outlines are shown with thin lines. In contrast, third angle projection uses the opposite convention, where the visible outlines are represented with thick lines, and the cutting plane lines are shown with thin, dashed lines.
3. Orthographic projection: The way the object is projected onto the two-dimensional surface also differs in the two methods. In first angle projection, the object is imagined to project onto the planes of projection from the perspective of the object, meaning the object itself is located between the observer and the planes. In third angle projection, the object is imagined to project onto the planes of projection from the perspective of the observer, meaning the object is located behind the observer.
To determine which projection method is being used, look for the symbol or note provided in the technical drawing. The symbol for first angle projection is a square with arrows pointing towards it, and the symbol for third angle projection is a square with arrows pointing away from it.