Sectional and Auxiliary Views
Standard multi-view orthographic drawings often struggle to clearly convey the complex internal geometry of machine parts or the true shape of angled surfaces. When an object has numerous internal holes, slots, or cavities, representing them all with hidden (dashed) lines creates a confusing, unreadable web. Sectional views solve this by virtually "cutting open" the object. Auxiliary views solve the problem of foreshortened, distorted inclined faces.
The Principle of Sectioning
A sectional view is created by passing an imaginary cutting plane through an object, removing the portion of the object between the viewer and the cutting plane, and drawing the newly exposed interior surfaces as visible lines.
Cutting Plane Line
A thick, heavy line (often drawn as a long dash followed by two short dashes, or a series of equal dashes) that indicates the path of the imaginary cut. Large arrowheads at the ends of the line point in the direction the observer is looking, effectively pointing towards the portion of the object that is kept.
Interactive Sectional View
Adjust the position of the cutting plane below to see how a section view is generated. Notice how the hatching appears only where the material is cut.
Interactive Sectional View
Slide to move the cutting plane.
0%: Full View (Exterior)
50%+: Full Section (Interior Revealed)
Types of Sectional Views
Common Section Types
- Full Section: The cutting plane passes straight through the entire object, usually along an axis of symmetry, splitting it into two halves. The front half is conceptually removed, exposing the entire interior.
- Half Section: The cutting plane passes only halfway through the object (typically a cylinder or symmetrical part), removing exactly one quarter of the object. This allows a single view to show both the exterior (on one half of the centerline) and the interior (on the other half). Hidden lines are generally omitted in the unsectioned half for clarity.
- Offset Section: When internal features (like holes in a flange) are not arranged in a straight line, a stepped cutting plane is used. The plane bends at 90° angles to pass through all significant features. The resulting view is drawn as if the features were all in one straight plane; the "steps" in the cutting plane are not shown as lines in the section view.
- Aligned Section: Used primarily for cylindrical objects with features (like spokes or holes) arranged in a circular pattern. The cutting plane passes through the feature, and then the feature is mathematically rotated into the plane of projection before being drawn. This prevents the severe distortion that a true orthographic projection would cause.
Localized Sections
- Revolved Section: Used to show the cross-sectional shape of an elongated object (like a spoke, bar, or structural I-beam). A thin slice is conceptually cut, revolved 90° in place, and drawn directly over the longitudinal view of the object.
- Removed Section: Similar to a revolved section, but the resulting cross-section is drawn outside the main view, often at a larger scale for clarity. It is usually labeled (e.g., "SECTION A-A").
- Broken-Out Section: Only a small, localized portion of the object's interior needs to be shown. A freehand break line is used to tear away a piece of the exterior surface, revealing the interior without needing a full cutting plane line.
Hatching (Section Lining) Conventions
The solid surfaces that are physically "cut" by the imaginary plane must be distinguished from the empty space (holes) behind them. This is done using section lines, commonly called hatching.
Hatching Rules
- Standard Pattern: The default symbol for general materials (like cast iron) is a series of uniformly spaced, thin, continuous lines drawn at a 45° angle.
- Material Specificity: Different hatch patterns represent different materials (e.g., ANSI 31 for Iron, ANSI 32 for Steel, ANSI 33 for Brass/Bronze, ANSI 38 for Magnesium/Aluminum). In civil engineering, standard hatches represent concrete, earth, or wood.
- Adjacent Parts: When an assembly of multiple parts is sectioned, the section lines for adjacent pieces must run in opposite directions (e.g., one part at 45° upper-right, the adjacent part at 45° upper-left) to clearly separate them.
- The Thin Web Rule: Long, thin features such as ribs, webs, gear spokes, shafts, bolts, nuts, rivets, and pins are typically NOT hatched when the cutting plane passes longitudinally (lengthwise) through them. Hatching a large web makes the object look like a solid mass rather than a thin stiffening feature, causing misinterpretation. They are only hatched if cut transversely (cross-wise).
Auxiliary Views
An inclined surface (a face that is not parallel to any of the principal glass box planes) will appear foreshortened and distorted in standard orthographic views. To accurately dimension this surface or show its true shape, an auxiliary view must be projected.
The Principle of True Shape
To see the true shape and size of any surface, the observer's line of sight must be exactly perpendicular (90°) to that surface. An auxiliary view is created by establishing a new projection plane that is perfectly parallel to the inclined face.
Types of Auxiliary Views
- Primary Auxiliary View: Projected directly from one of the principal views (Front, Top, or Side) where the inclined surface appears as an edge (a single line). The projectors are drawn perpendicular to this edge view.
- Secondary Auxiliary View: Used for oblique surfaces (faces that are inclined to all three principal planes, appearing foreshortened in all views). It is projected from a primary auxiliary view. The primary view establishes an edge view of the oblique plane, and the secondary view provides the true shape.
- Partial Auxiliary View: Because auxiliary views are often drawn solely to dimension the inclined face, drawing the entire foreshortened object behind it is unnecessary and confusing. A partial auxiliary view uses break lines to show only the true shape of the inclined feature, ignoring the rest of the object.
Key Takeaways
- Clarity Through Sectioning: Replaces confusing hidden lines with visible solid cuts and open spaces.
- The Cutting Plane: Arrows point to the portion of the object you are keeping to look at.
- The Rib Rule: Do not crosshatch ribs, webs, or spokes when cutting them lengthwise to prevent the illusion of massive solid material.
- Aligned Sections: Mathematically rotate circular features (like bolt holes in a flange) into the projection plane to avoid distortion.
- Auxiliary True Shape: Only a view projected perpendicular to an inclined surface will reveal its true shape, size, and angle for accurate dimensioning.