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First and Third Angle Projections - Coggle Diagram
First and Third Angle Projections
First and third angle projections are techniques used in engineering drawings to represent 3D objects in 2D. In first angle projection, views are projected onto the opposite side of the plane, while in third angle projection, views are projected onto the same side. Understanding these methods is key to interpreting technical drawings.
First Angle is commonly used in European countries.
The orthographic projections are on the opposite side.
Third Angle Projection is commonly used in U.S.
The orthographic projections are on the same side.
Visible Lines
Visible lines in engineering drawings are solid, thick lines that represent features seen in a view, helping clearly convey the object's shape and details.
Hidden Lines
Hidden lines, shown as dashed lines, represent features not visible in a view. They reveal internal details essential for understanding the full structure of an object.
Center Lines
Center lines, drawn as alternating long and short dashes, mark the centers of symmetrical features and help accurately locate and align them in engineering drawings.
Example of Visible, Hidden and Centre Lines in an Object
For a roll of toilet paper, its orthographic projection in third angle is shown below.
(Recall that for third angle projection, the right-side view is placed on the right of the front view, the top view is placed on top of the front view, and so on)
First Angle
Third Angle
