First Angle and Third Angle

First Angle and Third Angle

Orthographic projection shows an object by looking at it squarely from several directions. The views themselves are identical in both conventions — what differs is where each view is placed.

Third angle (mechanical work, SANS 0111): each view is drawn on the side you looked from. Look down at the top, and the top view goes above the front view. Look from the right, and the right side view goes on the right. The view comes toward you.

First angle (civil work and solid geometry, SANS 0143): each view is pushed through to the far side. The top view drops below; the right side view goes on the left.

Worked example. A part with a step on its right-hand side: in third angle that step appears on the right of the front view and the side view sits right. In first angle the identical part has its side view on the left. Read the drawing with the wrong convention and you machine the step on the wrong end — a mirrored part that fits nothing.

Core checkpoint: The object never changes. Only the arrangement of its views does — so the projection symbol is not decoration, it is what makes the sheet readable.