Sectional Views and Conventional Representations

Introduction

Engineering drawings communicate the shape, size, and functional relationships of machine components. External views are often insufficient when important internal features—holes, keyways, cavities, bearings, or intersecting passages—must be shown. A sectional view solves this problem by imagining that a portion of the object has been cut away and removed, exposing the internal surfaces to the observer.

Sectional drawing is not merely a pictorial technique. It is a standardized graphical language. The cutting plane, direction of sight, hatch pattern, treatment of ribs and shafts, and representation of repeated features must all be selected so that the drawing is unambiguous and economical. Incorrect sectioning can make a solid appear hollow, conceal a fastening method, or imply a false material boundary.

This chapter develops the principles and practical methods used to produce full, half, offset, broken-out, revolved, and removed sections. It also explains hatching conventions and the special treatment of ribs, webs, shafts, fasteners, repeated features, and standard machine elements.

Learning Objectives

On completing this chapter, a student should be able to:

• explain why sectional views are used and identify the cutting plane and viewing direction;
• construct full, half, offset, broken-out, revolved, and removed sections;
• apply appropriate section lining, spacing, direction, and termination rules;
• distinguish parts that are normally sectioned from parts conventionally shown unsectioned;
• represent ribs, webs, shafts, keys, bolts, nuts, bearings, gears, springs, and repeated features correctly;
• select a suitable section type for a manufacturing or inspection purpose; and
• interpret sectional drawings while recognizing common drafting errors.

1. Fundamental Principles

A sectional view is obtained by passing an imaginary cutting plane through an object, removing the material between the observer and the plane, and projecting the remaining portion onto the drawing plane. The surfaces actually cut by the plane are identified by section lining, commonly called hatching.

The cutting plane is shown in another view by a thick chain line or a similar standardized line pattern. Arrows indicate the direction from which the section is viewed. Arrowheads are placed at the ends of the cutting-plane indicator and are labelled with matching letters, such as A–A. The sectioned view is then identified as “SECTION A–A.”

Only surfaces intersected by the cutting plane are hatched. Surfaces merely visible behind the cutting plane are drawn with normal visible outlines; they are not hatched unless the cutting plane actually passes through them. Hidden lines are generally omitted in a sectional view when the section makes the internal form clear. They may be retained if they are necessary to define an important feature that is not otherwise apparent.

A section is a projection, not a perspective sketch. Therefore, the normal rules of orthographic projection still apply: corresponding points must align, dimensions must be taken from the correct view, and line weights must distinguish object outlines from construction and indication lines.

Cutting-plane selection

The cutting plane should reveal the maximum useful information with minimum complexity. It is normally passed through:

1. the axes of cylindrical holes and shafts;
2. the centre plane of symmetrical components;
3. important internal steps, seats, shoulders, and recesses; and
4. features whose relationship cannot be understood from external views.

A cutting plane should not be chosen merely to produce a visually attractive drawing. It must represent a physically meaningful section and should avoid unnecessary bends unless an offset section is required.

Figure: A cutting plane through a component and the corresponding hatched sectional view.

2. Types of Sectional Views

2.1 Full section

A full section uses one cutting plane passing completely through the object. One half of the object in front of the plane is conceptually removed. The resulting view is appropriate when the component is internally complex and symmetry does not provide a sufficient alternative.

The cutting plane may be parallel to a principal projection plane or perpendicular to it. Its location is selected to pass through the relevant internal features. A full longitudinal section through a shaft or housing commonly reveals bores, shoulders, keyways, and bearing seats.

2.2 Half section

A half section is used mainly for symmetrical objects. Two perpendicular cutting planes, or a conventional equivalent, remove one quarter of the object. One half of the resulting view is sectional and the other half remains an external view.

The centre line separates the two halves. It is not normally drawn as a thick visible edge. Hidden lines in the external half should be used sparingly. Half sections are particularly effective for pulleys, flanged hubs, valves, and bearings because they show both exterior form and internal construction in one view.

2.3 Offset section

An offset section uses a cutting plane that bends to pass through several non-aligned features. In the sectional view, the separate portions of the cutting plane are treated as if they lay in one plane. The bends are not shown in the resulting section; only the features exposed by the complete cutting path appear.

The cutting-plane line must clearly show each change in direction, and the arrows must indicate the viewing direction. The plane should normally change direction at a centre line or another well-defined location. An offset section must not be interpreted as a true oblique geometric projection: it is a conventional representation designed to expose aligned features.

2.4 Broken-out section

A broken-out, or local, section removes only a small portion of the object. The boundary of the removed material is drawn with a freehand irregular break line or an appropriate thin zigzag line. No complete cutting-plane indicator is required when the local nature of the section is obvious.

Broken-out sections are useful for showing a keyway, wall thickness, internal groove, or small bore without sacrificing the rest of the external view. The break line must not coincide with an important visible edge, and it should clearly enclose the region considered removed.

2.5 Revolved section

A revolved section is made by rotating a cross-section through 90° about the feature’s longitudinal axis and drawing that section directly on the view. It is useful for showing the cross-sectional shape of spokes, arms, ribs, channels, and uniform members.

A thin continuous line usually encloses the revolved section when it is superimposed on the parent view. The section is hatched according to the same rules as an ordinary section. Where the revolved section would obscure essential outlines, it may be drawn adjacent to the feature or omitted in favour of a removed section.

2.6 Removed section

A removed section is taken away from the main view and placed elsewhere on the drawing, often on the extension of the cutting-plane line. It is labelled with the cutting-plane letters, for example, “SECTION B–B.”

Removed sections are preferable when a revolved section would be too small or would interfere with the main outline. They are frequently used for long members with changing cross-sections, rails, shafts, and structural profiles. The removed section may be drawn at a different scale only if that scale is clearly identified.

3. Section Lining and Hatching Conventions

Section lines are thin, evenly spaced lines—commonly at approximately 45° to the principal outlines. The angle is not a rigid visual rule when it would be parallel or nearly parallel to a prominent edge; a different angle, such as 30° or 60°, may improve clarity. Spacing should be uniform within a particular area and sufficiently wide to avoid a dark, unreadable patch.

The hatch pattern indicates that material has been cut, not necessarily a particular material type. Material identification should be given by a legend or a recognized standard when required. Different adjacent parts in an assembly are distinguished by changing the hatch angle, spacing, or both. The same component should retain the same hatch treatment in all views.

Hatching should stop at a visible outline. It should not cross a gap, a hole, or an unsectioned component. Small areas may be filled with solid black only when the drawing standard and scale permit it; thin walls should not be made visually heavier than their outlines.

Dimensions are placed on the sectioned view when the section provides the clearest measurement location. Section lines should not be mistaken for dimension lines, centre lines, or hidden lines. Hatching is generally interrupted where dimension text or arrowheads would otherwise become illegible.

4. Parts Normally Shown Unsectioned

Several machine elements are conventionally not hatched when the cutting plane passes longitudinally through them. This convention prevents a familiar component from being mistaken for a solid block and reduces visual clutter.

Shafts, axles, pins, and rods

A shaft or rod shown in longitudinal section is normally drawn without hatching. Its outline, centre line, shoulders, and keyway remain visible. If the cutting plane passes transversely through the shaft, the resulting circular area may be sectioned because the material is genuinely cut across its cross-section.

Ribs, webs, and spokes

A rib or web is not hatched when the cutting plane passes lengthwise through its thin dimension. Instead, it is shown by its outlines. If the plane cuts across the thickness of the rib, the cut area is hatched. This convention prevents a thin reinforcing feature from appearing to be a massive solid member.

Fasteners

Bolts, screws, studs, nuts, washers, rivets, and similar standard fasteners are generally drawn unsectioned in longitudinal assembly sections. Their external outlines and threads are represented conventionally. A transverse section through a fastener may be hatched when necessary to communicate the actual cut.

Keys and small standard parts

Keys, cotters, bearings, and some other standard parts are commonly left unsectioned in longitudinal assembly sections, subject to the applicable drafting standard. The purpose is recognition and clarity, not a claim that the cutting plane has physically bypassed the part.

5. Conventional Representation of Common Features

Holes and cylindrical features

A through hole is shown by its visible circular or cylindrical boundaries and centre lines. In a section passing through the hole axis, the empty space remains unhatched. Counterbores and countersinks must be shown by their stepped or conical profiles, with dimensions or standard callouts as appropriate.

Threads

Internal and external threads are represented by simplified major- and minor-diameter lines rather than by drawing every helical turn. A longitudinal section may show the thread profile schematically at the engagement region. Thread notation, such as a standard designation and pitch, supplies the manufacturing information.

Bearings and gears

Rolling-element bearings are normally represented with simplified standard symbols or conventional outlines. Individual balls or rollers need not be fully sectioned in an assembly unless their arrangement is the subject of the drawing. Gears are usually shown by pitch circles, addendum and root outlines, and a limited number of teeth; drawing every tooth in a small assembly view is poor practice.

Springs

Compression springs may be represented by a conventional zigzag or helical line, while their end conditions and free length are specified separately. In a longitudinal assembly section, a spring is not necessarily hatched as a solid body.

Repeated features

Repeated holes, teeth, spokes, slots, and fasteners may be shown fully only when their exact number and distribution are important. Otherwise, one or more representative features may be drawn and the remainder indicated by centre lines, pitch circles, or a note such as “6 HOLES Ø10 EQ SP ON Ø80 PCD.” The abbreviation “EQ SP” means equally spaced, and PCD means pitch-circle diameter.

A circular pattern should not be represented by copying a feature in a way that suggests an incorrect angular position. The drawing must communicate count, spacing, orientation, and reference diameter.

6. Assemblies and Adjacent Components

In an assembly section, adjacent components should be visually distinguishable. Hatch angles are alternated between neighbouring parts, and spacing may be varied. A component should not change its hatch direction arbitrarily between views unless required for legibility.

Contact surfaces must be represented accurately. A clearance should be shown as a genuine gap at the drawing scale, while a press fit or mating surface should not be drawn with an accidental gap. Centre lines identify axes of rotation and symmetry but do not represent physical edges.

The section should support functional interpretation. For example, a bearing section should reveal which surfaces locate the bearing, how axial movement is controlled, and whether a seal or retaining ring is present. A section that exposes geometry but omits the relationship between components is technically incomplete for assembly communication.

7. A Systematic Method for Producing a Section

A reliable construction sequence is as follows:

1. Define the purpose. Decide which internal features must be communicated.
2. Select the section type. Use a full section for general internal complexity, a half section for symmetry, an offset section for non-aligned features, and a local section for limited detail.
3. Locate the cutting plane. Pass it through axes, centre planes, seats, holes, or other important details.
4. Draw the cutting-plane indicator. Use the correct line type, arrows, and identifying letters.
5. Project the section. Transfer all intersected edges and changes in profile using the selected orthographic method.
6. Remove or suppress hidden lines where appropriate. Do not delete lines that are needed to interpret an unexposed feature.
7. Apply conventional treatment. Leave shafts, fasteners, ribs, and webs unhatched where the convention applies.
8. Add hatch lines. Maintain consistent angle, spacing, and termination at boundaries.
9. Complete centre lines and dimensions. Add only those dimensions supported clearly by the section.
10. Inspect the result. Check alignment, hatch boundaries, feature count, section labels, line hierarchy, and consistency with other views.

8. Worked Application: Sectioning a Flanged Bearing Housing

Consider an illustrative bearing housing consisting of a rectangular body, a central cylindrical bore, two mounting holes, a counterbore for the bearing, and a transverse reinforcing rib. The design team needs a drawing for machining and assembly inspection.

A full longitudinal section through the bore would reveal the counterbore and bearing seat, but it would not clearly show both mounting holes if they are offset from the bore axis. A half section could show the bore and external body, but symmetry is incomplete because the rib is present on only one side. The most effective solution is an offset section.

The cutting plane begins at the centre of one mounting hole, passes through the bore axis, and then bends to pass through the second mounting hole. In the cutting-plane view, each bend is shown clearly with arrows. In the resulting section, the mounting holes appear aligned with the bore even though they are not collinear in the physical object; this is the accepted convention of an offset section.

The housing material is hatched at 45°. The bearing, bolts, and shaft are drawn using assembly conventions and are not hatched longitudinally. The rib is intersected lengthwise, so it is shown by its outlines without hatch lines. The bore is left open, and the counterbore step is drawn as a visible shoulder. Hidden lines are omitted because the section directly reveals the internal form.

The final drawing should include the bore diameter, counterbore diameter and depth, mounting-hole diameter, relevant outside dimensions, and a note identifying the bearing standard. The result serves both manufacturing and inspection: the machinist sees the required seats and holes, while the assembler can understand how the bearing and shaft relate to the housing.

This example illustrates an important principle: the best section is not necessarily the simplest geometric cut. It is the view that communicates functional information with the fewest additional views and the least possibility of misinterpretation.

9. Errors, Limitations, and Professional Practice

Common errors include hatching surfaces that were not cut, hatching a longitudinal shaft, drawing hidden lines unnecessarily, failing to show a cutting-plane direction, using inconsistent hatch angles for the same part, and placing a section without a corresponding label. Another serious error is treating an offset section as though it were a true physical plane and then using it to infer an incorrect dimension.

Sectional views also have limitations. They may conceal external features on the removed side, simplify complex three-dimensional intersections, and create a false impression of alignment in offset sections. For this reason, a section should normally be interpreted together with at least one external view, detail view, or auxiliary view where necessary.

Modern computer-aided design systems can generate section views automatically, but automatic output is not automatically correct. The drafter must verify the section depth, excluded components, hatch associations, line hierarchy, and display of threads and standard parts. Model-based definition and three-dimensional visualization improve coordination, yet manufacturing drawings remain essential where tolerances, datum structures, surface requirements, and inspection information must be formally controlled.

Professional practice requires adherence to the applicable national or international drafting standard, revision control, clear units, traceable design changes, and disciplined use of standard symbols. AI-assisted drafting may suggest views or detect inconsistent hatching, but engineering responsibility remains with the qualified designer. The final representation must be checked against design intent, manufacturing feasibility, safety requirements, and contractual documentation standards.

Chapter Summary

Sectional views expose internal geometry by conventionally removing material in front of a cutting plane. Full sections reveal extensive internal detail; half sections combine an internal and external view of symmetrical parts; offset sections expose non-aligned features; broken-out sections show local detail; revolved sections display a cross-section in place; and removed sections place that cross-section separately for clarity.

Only surfaces cut by the plane are hatched. Hatch angle, spacing, and termination must be consistent and must distinguish adjacent components. Shafts, fasteners, keys, ribs, webs, and similar elements are often left unsectioned in longitudinal assembly sections to preserve recognition and avoid implying incorrect solidity. Repeated features may be simplified when their number and distribution are stated clearly.

A good sectional drawing is selected for communication purpose, constructed by systematic projection, checked against orthographic relationships, and interpreted with the applicable drafting standard. CAD tools assist production, but professional judgement is required to ensure that the result represents functional, manufacturing, and inspection requirements accurately.

Key Terms

Sectional view: An orthographic view showing internal features exposed by an imaginary cut.

Cutting plane: The imaginary plane through which an object is considered cut.

Section lining: Thin lines used to identify surfaces intersected by a cutting plane.

Hatching: The patterned application of section lines.

Full section: A view produced by a cutting plane passing completely through an object.

Half section: A view combining a sectioned half with an external half, usually for a symmetrical object.

Offset section: A section using a cutting plane with one or more changes in direction.

Broken-out section: A local section bounded by an irregular break line.

Revolved section: A cross-section rotated into the plane of the drawing at its original location.

Removed section: A cross-section placed away from the parent view.

Rib: A thin reinforcing member.

Web: A thin plate-like structural member joining or stiffening parts.

Centre line: A thin line indicating an axis, centre, or line of symmetry.

Visible line: A heavier line showing an observable edge or boundary.

Hidden line: A line showing an edge not directly visible in a normal view.

Counterbore: A cylindrical enlargement at the mouth of a hole.

Countersink: A conical enlargement at the mouth of a hole.

PCD: Pitch-circle diameter used to locate equally spaced circular features.

Conventional representation: A standardized simplified depiction of a feature or component.

Review Questions

Short Answer Questions

1. What is the purpose of a sectional view?
2. How does a half section differ from a full section?
3. Why are shafts commonly left unhatched in a longitudinal assembly section?
4. State two rules governing section lining.
5. What information should accompany a repeated circular pattern?

Long Answer Questions

1. Explain the construction and applications of full, half, offset, broken-out, revolved, and removed sections.
2. Discuss the principles governing hatch angle, spacing, boundaries, and adjacent components in an assembly section.
3. Explain the conventional treatment of ribs, webs, shafts, fasteners, keys, bearings, and gears.
4. Describe a systematic procedure for creating and checking a sectional engineering drawing.
5. Evaluate the advantages and limitations of using sectional views for manufacturing and assembly communication.

Analytical / Application Questions

1. A symmetrical pulley contains a hub bore, keyway, and internal recess. Select the most suitable section type and justify your choice.
2. A housing has two mounting holes that do not align with its central bore. Explain how an offset section could reveal all three features and identify the possible interpretive limitation.
3. In an assembly section, a cutting plane passes lengthwise through a shaft, rib, bolt, and cast housing. State which elements should normally be hatched and explain the reasoning for each decision.

Multiple Choice Questions with Answers

Question 1

Which sectional view is most appropriate for a symmetrical component when both internal and external forms must be shown in one view?

A. Half section

B. Broken-out section

C. Removed section

D. Offset section

Correct Answer: A. Half section

Question 2

What do arrows on a cutting-plane indicator identify?

A. The material specification

B. The direction of viewing

C. The hatch-line spacing

D. The drawing scale

Correct Answer: B. The direction of viewing

Question 3

Which surfaces are hatched in a sectional view?

A. All visible surfaces

B. Only hidden surfaces

C. Only surfaces intersected by the cutting plane

D. Only cylindrical surfaces

Correct Answer: C. Only surfaces intersected by the cutting plane

Question 4

A cutting plane is bent to pass through several non-aligned holes. Which section is required?

A. Full section

B. Half section

C. Revolved section

D. Offset section

Correct Answer: D. Offset section

Question 5

How is a rib normally represented when a cutting plane passes lengthwise through its thin dimension?

A. It is filled solid black

B. It is hatched heavily

C. It is shown by outlines without hatching

D. It is omitted completely

Correct Answer: C. It is shown by outlines without hatching

Question 6

What is the principal purpose of a broken-out section?

A. To show a limited internal area without cutting the whole object

B. To show only repeated external features

C. To replace all orthographic views

D. To indicate material hardness

Correct Answer: A. To show a limited internal area without cutting the whole object

Question 7

In an assembly section, adjacent components are commonly distinguished by changing their:

A. Overall scale

B. Hatch angle or spacing

C. Centre-line thickness only

D. Dimension units

Correct Answer: B. Hatch angle or spacing

Question 8

Which feature is commonly represented by the abbreviation PCD?

A. Parallel centre distance

B. Primary clearance depth

C. Pitch-circle diameter

D. Projected cutting direction

Correct Answer: C. Pitch-circle diameter

Question 9

A revolved section is produced by:

A. Removing the entire front half of an object

B. Rotating a cross-section into the drawing plane

C. Bending the cutting plane through several holes

D. Projecting a section at a different scale only

Correct Answer: B. Rotating a cross-section into the drawing plane

Question 10

Why are hidden lines often omitted from a sectional view?

A. They are prohibited in every engineering drawing

B. The section normally makes the internal form clear

C. They indicate material type incorrectly

D. They would change the object dimensions

Correct Answer: B. The section normally makes the internal form clear