Sectional and Auxiliary Views
Learning Objectives
- Construct a sectional view from the actual intersection between a cutting plane and the object.
- Distinguish material, holes, voids, geometry beyond the plane, and longitudinal section exceptions.
- Select among full, half, offset, aligned, revolved, removed, and broken-out sections.
- Explain why cutting-plane bends are normally omitted from an offset section result.
- Construct primary and secondary auxiliary views using folding lines and perpendicular projectors.
- Quantify foreshortening and verify that the target surface has reached true shape.
- Audit sectional and auxiliary drawings for geometry, convention, reference, and dimensioning errors.
Principal orthographic views do not always communicate interior or inclined geometry efficiently. A sectional view reveals selected internal features by imagining that part of the object has been removed. An auxiliary view changes the projection plane so an inclined or oblique surface can be seen in true shape. Both methods remain orthographic: every result must follow a defined line of sight, projector direction, reference line, and distance-transfer procedure.
Project Standards Govern Conventions
Section-line patterns, cutting-plane symbols, hidden-line treatment, longitudinal exceptions, view placement, and auxiliary reference notation vary by discipline and organization. This lesson teaches transferable geometry and review logic; use the project drawing standard for issued work.
1. What a Section View Represents
Cutting Plane
An imaginary plane that identifies where the object is conceptually cut. Its line, identifying letters, and arrows communicate the path and viewing direction.
Section Lining
Thin, evenly spaced lines or an approved material pattern applied only to surfaces physically intersected by the cutting plane.
Four Geometric Categories in a Section
- Cut material: receives section lining.
- Void or opening: remains clear even when enclosed by cut material.
- Geometry behind the plane: may remain visible as object outlines where needed.
- Geometry removed in front of the plane: is not shown in the resulting view.
Chord Created by an Offset Plane
For a plane at radial offset e cutting a circular boundary of radius R.
Variables
| Symbol | Description | Unit |
|---|---|---|
| Chord length intersected by the cutting plane | mm | |
| Circular boundary radius | mm | |
| Perpendicular offset from the circle centre | mm |
Offset Cut Through a Hollow Sleeve
For an outer radius of 40 mm and a plane offset 24 mm from the centre:
For an 18 mm bore, , so the plane does not intersect the bore. The resulting section is a solid 64 mm-wide chord through the sleeve height rather than an annular opening.
Interactive Section Geometry Laboratory
Move the cutting plane continuously through a hollow sleeve. The top-view intersections, outer chord, bore chord, approximate cut area, ligament, and resulting hatching update from one geometric model. Then compare longitudinal and transverse treatment of ribs, webs, shafts, fasteners, and adjacent components.
Cutting planes, actual intersections, hatching, and drawing conventions
Section Geometry and Convention Laboratory
Move a cutting plane through real geometry, distinguish material from void, apply longitudinal exceptions, and audit sectional-view decisions.
Technical drawing viewport
Continuous annular cutting-plane geometry
Interpretation limit
2. Building a Correct Section
Section Construction Workflow
- Select a cutting plane that passes through the features needed for the drawing decision.
- Draw and identify the cutting-plane line according to the selected standard.
- Confirm the viewing direction from the arrows.
- Imagine removing material between the observer and the plane.
- Project the remaining outlines orthographically.
- Identify every surface physically intersected by the plane.
- Hatch cut material while leaving holes and voids clear.
- Apply longitudinal exceptions and adjacent-part contrast deliberately.
- Omit unnecessary hidden lines unless they communicate essential information.
- Add dimensions and notes to visible, unambiguous geometry.
Hatching Quality
- Keep spacing and angle consistent within one component.
- Vary angle or spacing between adjacent parts.
- Do not hatch across a hole, slot, bore, or cavity.
- Break hatching around text or dimensions when required for legibility.
- Avoid making section lines graphically stronger than object outlines.
- Use material-specific symbols only when required and clearly defined.
3. Longitudinal Exceptions
Thin structural or fastening features can be misleading when sectioned lengthwise. Hatching a longitudinal rib or web may imply that its broad face represents actual thickness. Many drawing standards therefore show these features in outline without section lining when the cutting plane runs along them. When cut across their thickness, the same features are normally hatched.
Features Commonly Reviewed as Exceptions
- Ribs and webs
- Spokes and arms
- Shafts and pins
- Bolts, nuts, washers, and similar fasteners
- Keys and cotters
- Thin reinforcing plates cut along their length
An Exception Is Not Missing Material
Leaving a longitudinal feature unhatched does not mean the feature is hollow or absent. Preserve its visible outline and apply the convention consistently so the reader understands the geometry.
4. Choosing the Section Method
Seven Section Methods
- Full section: the entire projected view is sectioned by one plane.
- Half section: one half shows the interior and the other half shows the exterior of a symmetric object.
- Offset section: a stepped plane includes non-collinear features and presents them in one continuous section.
- Aligned section: a radial feature is rotated into the cutting plane by convention.
- Revolved section: a local cross-section is rotated into the parent longitudinal view.
- Removed section: a cross-section is placed away from the parent view and referenced clearly.
- Broken-out section: an irregular local break exposes only a nearby interior region.
Seven-Method Section Builder
Use one reference component to compare all seven section methods. Select the features the section must communicate, inspect the cutting path and result, and solve realistic method-selection cases.
Section selection, cutting paths, conventional rotation, and communication intent
Seven-Method Section View Builder
Compare full, half, offset, aligned, revolved, removed, and broken-out sections using one reference component, then choose the method that best solves each drawing problem.
Technical drawing viewport
Full section comparison drawing
Offset and Aligned Sections Are Conventional Results
An offset section is not drawn with visible steps in the result. The selected features are treated as if they lie in one cutting plane. An aligned section rotates a radial feature into the plane before projection. These conventions improve communication but must not imply a literal geometric relationship that does not exist.
Section View Audit
- Cutting-plane path and viewing direction are identifiable.
- The selected plane passes through the intended controlling features.
- Only intersected material is hatched.
- Openings remain clear.
- Longitudinal exceptions are treated according to the drawing standard.
- Adjacent parts remain distinguishable.
- Hidden lines are limited to essential information.
- Offset or aligned conventions do not create false dimensions.
- Section labels and parent-view references agree.
- Dimensions use visible geometry and do not duplicate controlling information.
5. Why Inclined Faces Need Auxiliary Views
Auxiliary View
An orthographic view projected onto a plane other than a principal projection plane, normally used to reveal the true shape or true size of an inclined or oblique feature.
A plane surface reaches true shape only when the projection plane is parallel to that surface. In a principal view, an inclined face is foreshortened. The construction begins from a view where the target face appears as an edge. A new folding line is drawn parallel to that edge, and projectors are drawn perpendicular to the folding line.
Foreshortened Area
Simplified area relationship when a planar face is inclined by angle θ to the projection plane.
Apparent Area of an Inclined Rectangular Face
A 120 mm × 70 mm face has true area . At an inclination of 37°:
The smaller principal-view area is not a change in the object; it is projection foreshortening.
6. Primary Auxiliary Construction
Primary Auxiliary Workflow
- Identify the principal view where the target surface appears as an edge.
- Draw folding line parallel to that edge.
- Draw projectors perpendicular to from every controlling vertex and feature centre.
- Transfer distances from the related principal view along those projectors.
- Connect corresponding points in sequence.
- Add holes, slots, centre marks, and dimensions only after the true-shape boundary is established.
- Verify that known right angles, parallel edges, and circles recover their true geometry.
Folding-Line Distance
Moving the folding line closer to or farther from the parent view changes drawing placement only. It must not change the size or shape of the auxiliary result. If geometry changes when the folding line moves, distances are being transferred incorrectly.
7. Secondary and Partial Auxiliary Views
A surface oblique to all principal planes may not appear as an edge in any principal view. A primary auxiliary is first constructed to create an edge view. A second folding line parallel to that edge then establishes a secondary auxiliary plane parallel to the target surface. When only one feature requires clarification, a partial auxiliary view can reduce clutter.
Primary and Secondary Auxiliary Studio
Adjust face inclination, folding-line placement, projector accuracy, secondary obliquity, and partial-view extent. The studio quantifies apparent area and deliberately introduces projector error so the visual consequences can be diagnosed.
Folding lines, perpendicular projectors, distance transfer, and true-shape recovery
Auxiliary View Construction Studio
Construct primary and secondary auxiliary views, quantify foreshortening, introduce controlled projector errors, and audit projection logic.
Technical drawing viewport
Primary auxiliary view construction
Do Not Stretch a Foreshortened Face
An auxiliary view is not a visually corrected or scaled copy of an inclined polygon. Each point must be projected normally and transferred from the correct source view.
Auxiliary View Audit
- The target face and required true-shape purpose are stated.
- The source view shows the target face as an edge, or a primary auxiliary creates that condition.
- Folding line is parallel to the edge view.
- Projectors are perpendicular to the folding line.
- Distances are transferred from the correct related view.
- Corresponding vertices are connected in the correct sequence.
- Circular features become circles only in the true-shape view.
- Partial boundaries do not omit controlling geometry.
- Dimensions are placed on the view that actually shows true size.
- A section is controlled by actual plane-object intersections, not by visual approximation.
- Hatch only cut material; keep holes and voids clear.
- Longitudinal ribs, webs, shafts, and fasteners require deliberate convention review.
- Full, half, offset, aligned, revolved, removed, and broken-out sections solve different communication problems.
- True shape appears only on a projection plane parallel to the target surface.
- Auxiliary projectors are perpendicular to the folding line, and distances transfer from a related view.
- A secondary auxiliary may be required before an oblique face reaches true shape.