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Engineering Graphics and CAD · Lesson 9 of 35

Section types and conventions

Choose and draw the right kind of section, and apply the conventions that prevent misleading sections.

Readiness check

Learning objectives

By the end of this lesson you can:

Distinguish full, half, offset, broken-out, revolved, and removed sections.
Select the appropriate section type for a given feature.
Apply the rib, web, and spoke non-sectioning conventions.
Represent aligned features (odd-count ribs, holes, spokes) correctly.
Diagnose an incorrectly sectioned rib or standard fastener.

Check your starting point

Five to ten minutes.

If a part is symmetric, could you show its outside and its inside in one view? How?
If a thin rib is cut lengthwise and hatched like a solid block, what false impression might it give?
Why might a standard bolt not be sectioned even when a cutting plane passes through it?

Interpretation.

Q1: Yes, a half section shows one half outside and the other half cut open. If unsure, Skill 9.1 covers it.
Q2: It could look like a solid disc or heavy mass rather than a thin rib, exaggerating strength and material. This drives the rib convention.
Q3: Sectioning a solid bolt lengthwise adds nothing and clutters; convention leaves standard fasteners unsectioned. Skill 9.3 explains.

You need L8 (basic sectioning and hatching).

0 or 1 weak itemsContinue with this lesson.

2 weak itemsReview Lesson 8, then return.

3 or more weak itemsWork through the prerequisite examples before continuing.

The core idea

What it is. Section types are the standard ways to cut a part for different needs, and section conventions are the rules that stop sections from misleading the reader.

Why an engineer needs it. One kind of cut does not suit every part. A symmetric part is best shown half-sectioned; features on two planes need an offset; a thin arm is best shown by a revolved section. And some features (ribs, fasteners) must follow special conventions or the section will misrepresent the part.

What problem it solves. It matches the cut to the part and prevents the classic errors (a hatched rib that looks like a solid mass, a sectioned bolt that clutters and misleads).

What goes wrong when it is ignored. A rib hatched like solid material implies a heavy disc; sectioning standard fasteners adds noise; failing to align odd-count features distorts their spacing. Each misleads the maker.

A simple mechanical example. A pulley (a symmetric wheel with a hub, a thin web, and a rim) is ideally shown as a half section: one half gives the outside profile, the other half reveals the hub bore and web thickness. But the web, cut lengthwise, is not hatched, or it would look like a solid disc.

The section types:

Full section: the plane passes fully through; the whole view is sectioned. (L8.)
Half section: for symmetric parts, one half is cut and one half is left as an external view, showing inside and outside together.
Offset section: the plane jogs to pass through features on different planes; the jog is not shown as a material edge.
Broken-out section: a small local area is broken away to reveal a feature, bounded by a freehand break line.
Revolved section: the cross-section of an arm, rib, or spoke is rotated in place and drawn on the view.
Removed section: the same cross-section is drawn off to the side, often to a larger scale.

Key conventions:

Ribs, webs, and spokes cut lengthwise are not hatched, so they do not look like solid masses.
Standard parts (bolts, nuts, screws, washers, keys, pins, rivets, shafts) cut lengthwise are not sectioned; they are drawn as external views.
Aligned sections: odd-count features (three ribs, five spokes, holes on a bolt circle) are imagined rotated into the cutting plane so their true spacing and count read correctly.

Part 2: Internal and inclined geometry.

Check: explain the decision in your own words before using a CAD command.

The concept. Full, half, and offset sections answer different questions. Conventions keep ribs, webs, spokes, shafts, and fasteners from looking falsely solid.

The skills, taught in order

Skill 9.1 - Choose the section type

Concept. Match the section type to what the part needs shown. Terminology. Each type (full, half, offset, broken-out, revolved, removed) suits a different situation. Procedure. Symmetric part needing inside and outside: half. Features on two planes: offset. Local peek: broken-out. Cross-section of an arm: revolved or removed. Whole interior: full. Reasoning. The right type shows the needed information with the least clutter. Failure mode. Forcing a full section where a half or revolved section would communicate better. Check. Given a part need, name the best section type.

Skill 9.2 - Do not hatch ribs, webs, and spokes cut lengthwise

Concept. Thin connecting features cut along their length are left unhatched. Terminology. A rib or web is a thin reinforcing wall; a spoke connects hub to rim. Procedure. When the plane runs lengthwise along a rib, web, or spoke, draw its edges but do not hatch it; hatch the surrounding solid normally. Reasoning. Hatching a thin rib makes it read as a solid mass, exaggerating material and strength; leaving it clear signals it is a thin feature. Failure mode. Hatching a rib like the surrounding solid, implying a heavy disc. Check. In a ribbed section, confirm the lengthwise rib is not hatched.

Skill 9.3 - Do not section standard parts cut lengthwise

Concept. Fasteners, shafts, keys, and pins are drawn as external views when the plane passes along them. Terminology. Standard parts are catalog items (bolts, nuts, washers, keys, pins, shafts). Procedure. Where the plane runs lengthwise through a standard part, draw it externally (unsectioned) even though the surrounding material is sectioned. Reasoning. Sectioning a solid fastener adds no information and clutters; the convention keeps it recognizable. Failure mode. Hatching a bolt shank lengthwise, cluttering the section and obscuring the fastener. Check. In an assembly section, confirm the bolt is drawn external, not hatched.

Skill 9.4 - Align odd-count features

Concept. Odd-count ribs, spokes, and holes are rotated into the plane so their count and spacing read correctly. Terminology. An aligned section imagines symmetric features rotated into the cutting plane. Procedure. For features spaced around an axis, rotate one into the cutting plane before projecting, so the section shows true radial distance and the correct number. Reasoning. A literal straight cut would foreshorten or omit off-plane features, misrepresenting spacing; alignment shows them truly. Failure mode. Drawing off-plane spokes foreshortened, implying wrong spacing. Check. In a three-spoke wheel section, confirm a spoke is aligned into the plane.

Worked example 1: half section of a symmetric pulley

Problem. A pulley has an outer rim diameter 100, a hub of diameter 40 with a bore of diameter 20, joined by a thin web 8 thick. Produce a half section that shows the outside profile and the internal bore and web.

Planning. Because the pulley is symmetric, cut one half through the axis and leave the other half as an external view. Do not hatch the web.

Solution.

Choose half section. The pulley is symmetric about its axis, so a half section shows inside and outside in one view.
Cut one half. On the cut half, the plane passes through the axis, revealing the hub bore (20), the hub wall, and the web joining hub to rim.
Leave the other half external. The uncut half shows the outer profile (rim, web, hub) as a normal external view, with a centre line dividing the two halves (no visible line between them on a half section).
Hatch selectively. Hatch the cut hub and rim material. Do not hatch the web, because it is a thin connecting feature cut lengthwise; hatching it would make the pulley look like a solid disc.
Section edges. The bore and internal steps on the cut half are visible edges.

Result. A half section: one half external (profile), one half cut (bore, hub, web visible), hub and rim hatched, web unhatched, divided by a centre line.

Why the method works. The half section exploits symmetry to show outside and inside together, and the web convention keeps the thin feature from reading as solid.

How to verify independently. The web should appear as its true 8 thickness, bounded by visible edges but unhatched. If it is hatched, the pulley wrongly looks like a solid disc, signalling the convention was missed.

Worked example 2: a ribbed bracket, right and wrong, plus an offset

Problem. A bracket has a base, an upright, and a single triangular rib joining them, plus a mounting hole in the base that is off the rib's plane. Show the section done correctly (rib not hatched) and incorrectly (rib hatched), explain the false impression of the wrong version, and use an offset to also capture the mounting hole. The complication combines the rib convention with an off-plane feature.

Planning. Cut lengthwise through the rib and upright, apply the rib convention, then offset the plane to pass through the mounting hole.

Solution.

Correct section. The plane runs lengthwise through the rib. Draw the rib's outline but leave it unhatched; hatch the base and upright solid that the plane cut. The section reads as a base and upright joined by a thin rib.
Incorrect section (for contrast). If the rib is hatched like the surrounding solid, the section looks like a solid triangular gusset filling the corner, implying far more material and strength than a thin rib provides. A maker might infer a solid web.
Why the convention exists. Not hatching the lengthwise rib signals it is thin; this is a deliberate readability convention, not an oversight.
Add the offset for the hole. The mounting hole in the base is off the rib's plane. Jog the cutting plane so it passes through the rib/upright and then offsets to pass through the mounting hole, revealing the hole in the same section. The jog itself is not drawn as a material edge.
Result of the offset. One section now shows the rib (unhatched), the cut base and upright (hatched), and the mounting hole (as a void with visible edges).

Comparison of approaches. A single straight section through the rib misses the mounting hole; hatching the rib misrepresents it as solid. The correct approach combines the rib convention with an offset, communicating both the thin rib and the functional hole.

Result. The correct section leaves the lengthwise rib unhatched and offsets to capture the mounting hole; the incorrect hatched-rib version falsely implies a solid gusset.

Independent check. In your section, the rib should be bounded by visible edges but carry no hatching, and the mounting hole should be present. If the rib is hatched or the hole is missing, revise.

Misconceptions and diagnostics

Misconception	Why it seems reasonable	Why it is wrong	Evidence that reveals it	Correction	Diagnostic question
"Ribs are hatched like any cut material."	The plane did pass through the rib.	Hatching a lengthwise rib makes it read as a solid mass; convention leaves it unhatched.	The section looks like a solid disc or gusset.	Leave lengthwise ribs, webs, and spokes unhatched.	"Is this a thin rib cut lengthwise? Then do not hatch it."
"Section every part the plane touches, including bolts."	The plane cuts them too.	Standard parts cut lengthwise are drawn external; sectioning them clutters and misleads.	A hatched bolt shank obscures the fastener.	Draw standard parts external when cut lengthwise.	"Is this a standard fastener cut lengthwise?"
"Draw spokes exactly where they are, even off the plane."	It is literally true.	Off-plane spokes foreshorten, misrepresenting spacing; align them into the plane.	A three-spoke wheel shows uneven, foreshortened spokes.	Use an aligned section to rotate a spoke into the plane.	"Would a literal cut foreshorten these radial features?"

Practice ladder

Level A - Recognition

Task. Match eight sections to their type (full, half, offset, broken-out, revolved, removed) and flag any incorrectly hatched ribs. Deliverable. Eight labelled judgements with rib flags. Success criteria. At least six types correct; hatched ribs flagged. Answer guidance. Identify the cut extent and any thin features. Common errors. Confusing offset with full. Difficulty. Low.

Level B - Guided application

Task. Convert a given full section of a symmetric part into a half section, with prompts, applying the web convention. Deliverable. The half section. Success criteria. One half external, one half cut; correct centre-line divide; web unhatched. Answer guidance. Use symmetry; hatch hub and rim, not the web. Common errors. Hatching the web. Difficulty. Medium.

Level C - Independent application

Task. Produce an offset section of the bearing block (Project P4) that captures features on two different planes. Deliverable. A labelled offset section. Success criteria. The jog captures both feature sets; jog not drawn as a material edge; correct hatching and conventions. Answer guidance. Route the plane through the first feature, then offset to the second. Common errors. Drawing the jog as a visible edge. Difficulty. Medium to high.

Level D - Transfer and design

Task. For a ribbed, holed, multi-feature part, choose the section type(s), apply all relevant conventions, and justify your choices against alternatives. Deliverable. A section (or sections) plus a justification. Success criteria. Correct type chosen; rib/fastener/alignment conventions applied; justification compares an alternative. Answer guidance. Combine a section type with the conventions the features demand. Common errors. Ignoring the rib or alignment convention. Difficulty. High.

Working with AI, and proving it yourself

Use AI as a tutor

Useful AI support:

Ask it to explain when to use a half versus offset section for your part.
Ask it to list the non-sectioning conventions and confirm against this lesson.
Ask for practice parts needing different section types.

Limits:

A text assistant cannot see whether you hatched a rib or aligned a spoke.
It may over-apply hatching to thin features.

Verify AI output against: the rib/web/spoke non-hatching rule, the standard-parts rule, and the aligned-section rule.

Prove it yourself

A plausible but incorrect AI answer, and how to catch it. You ask, "When I section a part with a thin rib, should I hatch the rib?" and the assistant replies: "Yes, hatch every region the cutting plane passes through, including ribs, for consistency."

This violates a standard convention. Detect it with this lesson's rule: a rib cut lengthwise is left unhatched so it does not read as a solid mass. The evidence is the false impression: a hatched rib looks like a solid gusset, implying more material and strength than exists. Correct conclusion: do not hatch ribs, webs, or spokes cut lengthwise, even though the plane passes through them.

Retrieval and spaced review

Name the six section types.
When is a half section appropriate?
Which features are not hatched when cut lengthwise?
Which parts are not sectioned when cut lengthwise?
What does an aligned section fix?
What is an offset section for, and how is the jog treated?
Cumulative (L8): In any section, what is hatched and what is not?
Reconstruction task: From memory, sketch the ribbed-bracket section with the rib unhatched and the offset hole present.

Answers. 1: full, half, offset, broken-out, revolved, removed. 2: for a symmetric part, to show inside and outside together. 3: ribs, webs, and spokes. 4: standard parts (bolts, nuts, screws, keys, pins, shafts). 5: it shows odd-count radial features at true spacing by rotating one into the plane. 6: to capture features on different planes; the jog is not drawn as a material edge. 7: only the material the plane cut is hatched; voids are not.

Suggested review intervals. 1 day, 3 days, 7 days. Re-section a ribbed part at day 7.

Reference mapping and next step

Read further

Giesecke ch.7
ISO 128-3:2022.

Standards details must be checked against the current official edition used by your institution or employer.

Finish the lesson

You can now: choose among the six section types; apply the rib, web, spoke, and standard-part conventions; align odd-count features; and produce a correct offset section.

Self-assessment checklist.

I match section type to the part's need.
I never hatch a lengthwise rib, web, or spoke.
I draw standard parts external when cut lengthwise.
I align odd-count radial features.
My offset jog is not drawn as a material edge.

Next lesson: L10 - Auxiliary views and true shape. Why it follows: sections reveal internal geometry on principal planes; next you learn to reveal the true shape of surfaces that are inclined to all principal planes, which no principal or section view shows without distortion.

Required files or submissions: submit your Level C offset section of Project P4. Optional extension: find a cast part with ribs and decide how you would section it without misrepresenting the ribs.