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Engineering Graphics and CAD · Lesson 8 of 35

Section views: cutting planes and hatching

Reveal internal geometry by cutting the object, the primary tool for parts with internal features.

Readiness check

Learning objectives

By the end of this lesson you can:

Explain why and when a section view is used instead of hidden lines.
Place and label a cutting-plane line with its viewing direction.
Draw the resulting section with correct hatching on cut material only.
Show only what lies on and behind the cutting plane.
Convert hidden internal detail into visible section detail.

Check your starting point

Five to ten minutes.

On a part with a deep internal bore, would the bore appear as visible or hidden lines in a normal front view?
If you sliced an apple in half and looked at the cut face, what would you see that you could not see from outside?
What do you think hatching (evenly spaced diagonal lines) inside a view represents?

Interpretation.

Q1: Hidden lines (dashed), because the bore is inside the material. Many hidden lines make a view hard to read, which is the problem sections solve.
Q2: The internal structure (core, seeds) that the skin hid. A section view does the same for a part.
Q3: The solid material that the cut passed through. This lesson makes that precise.

You need L5-L7 (views and line types). A cutting-plane line is a new member of the line alphabet.

0 or 1 weak itemsContinue with this lesson.

2 weak itemsReview Lesson 5, Lesson 7, then return.

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

The core idea

What it is. A section view is a view of an object as if it were cut open along a stated plane, with the near part removed, so that internal features appear as visible edges on the exposed face. The exposed cut material is marked with hatching.

Why an engineer needs it. Parts with internal geometry (bores, counterbores, internal steps, cavities) produce a tangle of hidden lines in ordinary views. A section replaces that tangle with clear visible edges, making the inside readable and dimensionable.

What problem it solves. It removes hidden-line clutter by showing the interior directly, so internal features can be seen, understood, and dimensioned without ambiguity.

What goes wrong when it is ignored. Complex interiors drawn only with hidden lines are hard to read and easy to misinterpret. Dimensioning to hidden lines (discouraged in L11) compounds the problem. A missing or misplaced section leaves the interior unclear.

A simple mechanical example. A cylindrical sleeve with a central bore and an internal shoulder shows, from the outside, only its outer circle and dashed lines for the bore. Cut it lengthwise through the axis and the bore, shoulder, and wall thickness appear as clean visible edges on the exposed face, with the solid wall hatched.

How a section is made (the mental procedure):

Pass an imaginary cutting plane through the feature you want to reveal.
Remove the part of the object between you and the plane.
Look at the exposed face in the stated direction.
Hatch the material the plane actually cut through.
Show features on and behind the plane as visible edges; internal edges that were hidden now become visible section edges.

The cutting-plane line (a chain line, wide at its ends, with arrows) shows where the cut is and which way you look. Hatching (also called section lining) is drawn as evenly spaced narrow lines, conventionally at 45 degrees, consistent across one part.

Part 2: Internal and inclined geometry.

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

The concept. A cutting plane removes the near material. Hatch only the material actually cut, and show the bore as visible internal geometry.

The skills, taught in order

Skill 8.1 - Decide when to section

Concept. Use a section when internal features would otherwise need confusing hidden lines. Terminology. A section view replaces or accompanies a normal view to show the interior. Procedure. If a view has more than a couple of hidden lines from internal features, consider a section that reveals them as visible edges. Reasoning. Sections trade hidden clutter for visible clarity; without internal features, they add nothing. Failure mode. Sectioning a part with no internal features (needless), or leaving a complex interior in hidden lines (unclear). Check. Given a part, state whether a section helps and why.

Skill 8.2 - Place and label the cutting plane

Concept. The cutting-plane line shows the cut location and viewing direction, and the section is labelled to match. Terminology. The cutting-plane line marks the cut; arrows show the viewing direction; letters (for example A-A) label the section. Procedure. Draw the cutting-plane line through the feature on a normal view, add arrows in the viewing direction, label both ends (A-A), and title the section "SECTION A-A." Reasoning. The label and arrows tell the reader exactly which cut, seen from where, the section shows. Failure mode. Omitting arrows or labels, so the reader cannot tell where or how the part was cut. Check. On a view, place a labelled cutting plane with a clear viewing direction.

Skill 8.3 - Hatch the cut material only

Concept. Only material the plane actually passed through is hatched. Terminology. Hatching (section lining) marks cut solid material; voids (holes, cavities) are not hatched. Procedure. Identify the solid the plane cut, and fill only that with evenly spaced 45-degree lines, consistent in angle and spacing across the part. Leave holes and open spaces unhatched. Reasoning. Hatching communicates "this was solid here"; hatching a void would falsely imply material. Failure mode. Hatching the whole section outline, including the bore, implying a solid where there is a hole. Check. In a bored part, confirm the bore is left unhatched.

Skill 8.4 - Turn hidden edges into visible section edges

Concept. Internal edges that were hidden in the normal view become visible edges on the exposed cut face. Terminology. A section edge is an internal edge now shown as a visible (wide) line on the cut face. Procedure. On the exposed face, draw every internal edge the cut reveals as a visible line. Generally omit hidden lines behind the plane unless one is essential for clarity. Reasoning. The purpose of sectioning is to make the interior visible; keeping it dashed defeats the purpose. Failure mode. Leaving revealed internal edges dashed, or cluttering the section with unnecessary hidden lines behind the cut. Check. Confirm the bore and shoulder edges are drawn as visible lines in the section.

Worked example 1: full section of a bored sleeve

Problem. A cylindrical sleeve is 40 in outer diameter and 60 long, with a central through-bore of diameter 20 and an internal shoulder that reduces the bore to diameter 28 for the last 20 of length. Produce a full section through the axis and hatch it correctly.

Planning. Pass the cutting plane through the axis lengthwise, remove the near half, draw the exposed face, and hatch only the wall material.

Solution.

Cutting plane. Place the cutting-plane line along the axis on the circular (end) view, arrows pointing to the viewing direction, labelled A-A.
Exposed face. Looking at the lengthwise cut, the outer profile is a 40 by 60 rectangle. The bore appears as an opening 20 wide for part of the length, stepping to 28 wide for the last 20, with the internal shoulder shown as a visible edge where the bore changes.
Wall material. The solid wall between the outer surface and the bore is the only cut material. Hatch it at 45 degrees, both walls at the same angle and spacing. The bore itself is a void, left unhatched.
Section edges. The bore wall, the shoulder, and the step are drawn as visible (wide) lines, not dashed.
Label. Title it SECTION A-A.

Result. A full section showing the wall hatched, the bore and its shoulder as visible edges, and the void unhatched, all clearly readable where a normal view would have shown only dashed lines.

Why the method works. Cutting through the axis exposes the full internal profile in one plane, and hatching only the wall communicates exactly where material is.

How to verify independently. The hatched wall thickness on each side should equal (outer radius minus bore radius): (20 minus 10) equals 10 in the 20-bore region, and (20 minus 14) equals 6 in the 28-bore region. If the drawn wall matches these, the section is consistent.

Worked example 2: a feature off the cutting plane

Problem. A round flange, 80 in diameter and 12 thick, has a central bore of diameter 30 and four bolt holes of diameter 8 on a bolt circle of diameter 60. A straight cutting plane is placed horizontally through the centre (through the bore). Decide what appears in the section, and explain why an offset section (previewed for L9) would be used to catch a bolt hole. The complication is that the bolt holes do not lie on a single straight cut through the centre.

Planning. See which features the straight central cut passes through, then consider how to also reveal a bolt hole.

Solution.

What the straight cut passes through. A horizontal plane through the centre cuts the flange body and the central bore. If the four bolt holes are at the 45-degree positions (not on the horizontal axis), the straight horizontal plane misses them entirely.
The section from the straight cut. It shows the flange body hatched, the central bore as a void with visible edges, and no bolt holes, because the plane did not pass through them. A reader would not see the bolt holes in this section.
The problem. The bolt holes are functionally important, but a single straight cut cannot pass through the central bore and the off-axis bolt holes at once.
The fix (offset section, L9). An offset section uses a cutting plane that jogs (steps) so it passes through the central bore and then offsets to pass through one bolt hole, revealing both in one section. The jog is not drawn as a material edge in the section; it is a device to capture features on different planes.
Comparison. The straight section is simpler but misses the bolt hole; the offset section is slightly more complex but captures the functional features. For this flange, the offset section communicates more of what matters.

Result. The straight central section shows the body and bore but misses the off-axis bolt holes; an offset section is the correct tool to reveal a bolt hole together with the bore.

Independent check. Ask whether every functionally important internal feature appears in the section. If a bolt hole is absent, the cut plane needs to offset to catch it, confirming the reasoning.

Misconceptions and diagnostics

Misconception	Why it seems reasonable	Why it is wrong	Evidence that reveals it	Correction	Diagnostic question
"Hatch the whole section outline."	The section looks solid.	Only material the plane cut is hatched; voids are not.	A hatched bore implies solid where there is a hole.	Hatch cut material only; leave voids clear.	"Did the plane cut solid here, or is this a void?"
"Internal edges stay hidden (dashed) in a section."	They were hidden before.	Sectioning exists to make them visible; they become wide visible edges.	Dashed internal edges in a section defeat its purpose.	Draw revealed internal edges as visible lines.	"Is this revealed edge drawn visible, as a section should?"
"One straight cut can catch any features."	A plane goes anywhere.	A straight plane misses features not on it; an offset section is needed.	Bolt holes off the axis do not appear in a straight central section.	Use an offset section to capture off-plane features.	"Do the important features actually lie on this plane?"

Practice ladder

Level A - Recognition

Task. On six given sections, mark what is correctly hatched and what should not be hatched (voids), and identify the cutting-plane line. Deliverable. Six annotated judgements. Success criteria. At least five correct; voids correctly excluded from hatching. Answer guidance. Trace where the plane cut solid; only that is hatched. Common errors. Treating a bore as hatched material. Difficulty. Low.

Level B - Guided application

Task. Complete a hatched full section of a bored part from a scaffolded cut (outline and cutting plane given). Deliverable. The finished, hatched section. Success criteria. Wall hatched consistently; bore unhatched; internal edges visible. Answer guidance. Hatch both walls at the same angle and spacing. Common errors. Inconsistent hatch angle between the two walls. Difficulty. Medium.

Level C - Independent application

Task. Produce a full section of the flanged shaft (Project P3) through its axis, showing internal and stepped features. Deliverable. A labelled full section (SECTION A-A). Success criteria. Correct cutting-plane line and label; wall hatched; steps and any bore as visible edges; voids clear. Answer guidance. Cut through the axis to expose the full internal profile. Common errors. Missing the section label or arrows. Difficulty. Medium.

Level D - Transfer and design

Task. For a part with internal features, choose the cutting plane that best reveals its function, produce the section, and justify why that plane (versus an alternative) communicates the interior best. Deliverable. A section plus a short justification. Success criteria. The chosen plane reveals the functional interior with least clutter; justification compares at least one alternative. Answer guidance. Cut where the important internal features live. Common errors. Choosing a plane that misses a key feature. Difficulty. Medium to high.

Working with AI, and proving it yourself

Use AI as a tutor

Useful AI support:

Ask it to explain when a section beats hidden lines using your part.
Ask for practice sections to hatch, with answers.
Ask it to list what should not be hatched and confirm against this lesson.

Limits:

A text assistant cannot see whether your hatching or cutting plane is correct.
It may hatch voids or keep internal edges dashed in its descriptions.

Verify AI output against: the hatch-only-cut-material rule, the visible-section-edge rule, and the cutting-plane-line convention.

Prove it yourself

A plausible but incorrect AI answer, and how to catch it. You ask, "In a section view, should I hatch across the holes too so the face looks complete?" and the assistant replies: "Yes, hatch the entire cut face including the holes for a uniform look."

This is wrong and would mislead a maker. Detect it with the core rule: hatching means "solid material was here," so hatching a hole falsely claims material where there is a void. The evidence is functional: a machinist reading a hatched bore might not recognize the hole. Correct conclusion: hatch only the material the plane actually cut; leave bores and cavities unhatched.

Retrieval and spaced review

What is the purpose of a section view?
What material is hatched, and what is not?
What does the cutting-plane line show?
What happens to internal hidden edges in a section?
Why does a straight cut sometimes miss important features?
What does hatching communicate about a region?
Cumulative (L7): Which line-alphabet member is the cutting-plane line, and how is it drawn?
Reconstruction task: From memory, sketch the bored-sleeve section from Worked Example 1, hatching only the wall.

Answers. 1: to reveal internal geometry clearly by cutting the object open, replacing hidden-line clutter. 2: only the material the plane cut is hatched; voids are not. 3: the cut location and the viewing direction (with arrows and a label). 4: they become visible (wide) section edges. 5: because features not on the straight plane are not cut; an offset section is needed. 6: that solid material was present there. 7: a chain line wide at its ends with direction arrows.

Suggested review intervals. 1 day, 3 days, 7 days. Re-hatch a fresh bored 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: decide when to section; place and label a cutting plane; hatch cut material only; and turn hidden internal edges into visible section edges.

Self-assessment checklist.

I section only when internal features need it.
I label the cutting plane with arrows and letters.
I hatch cut material and leave voids clear.
I draw revealed internal edges as visible lines.
My hatching is consistent in angle and spacing.

Next lesson: L9 - Section types and conventions. Why it follows: you can now make a full section; next you learn the family of section types (half, offset, broken-out, revolved, removed) and the conventions (ribs, fasteners, aligned features) that keep sections from misleading, including the offset section previewed here.

Required files or submissions: submit your Level C full section of Project P3. Optional extension: take a part with a bore and a counterbore and section it, checking that each internal step reads as a visible edge.