Orthographic Projection and Multi-View Drawing
Projection systems
First-angle projection places the top view below the front view and the right-side view on the left; third-angle places the top above and the right side to the right. The projection symbol in the title block fixes the system, so never mix arrangements. Six principal views are possible; use only the views that remove ambiguity.
View rules
- Align views: front and top share width; front and right share height; top and right share depth through a 45° miter line if you construct one.
- Line precedence: visible outline, then hidden line, then centreline. Do not draw a hidden line where a visible line covers it.
- Show hidden lines for internal features unless a section is used; omit them when they would clutter a feature already defined in another view.
- Centre holes, cylinders and bolt circles with long-dash-dot centrelines extending slightly past the outline.
- Tangent edges between a fillet and a flat are not drawn as visible lines in orthographic views under the simplified convention.
Worked example
An L-bracket has a vertical leg 60 mm high, a horizontal leg 80 mm long, both 40 mm deep, with a Ø12 hole centred 20 mm from the vertical leg's top and through the depth. The front view shows the L outline and the hole as a circle if the hole axis is normal to the front plane; the top view shows the 80 × 40 rectangle plus the hole's hidden lines; the right view shows height 60 and depth 40 with the hole hidden. Each hole appears once as a circle and twice as hidden lines, which is the check for correct view selection.
Traps
Placing the top view above the front view in first angle, omitting hidden lines for the hole, drawing duplicate edges where faces are coplanar, and using a centreline as an outline.