Machine Design #78: Sections and Cutaway Views — Cut the Right Place to Show the Right Construction
The design question is not only whether section view works once, but whether the same result can be made, measured, operated, recovered, and audited when the boundary moves.
Start with the function
Write the input condition, operating condition, expected result, acceptance limit, failure symptom, and measurement method before choosing a component or note. Do not hide unknowns behind a large safety factor. Assign an owner and a test.
Core checks
- Question the section must answer: write down the one question the section exists to answer before choosing where to cut; a section that answers nothing is decoration.
- Cutting plane and direction chosen from function: put the cutting plane through the feature that carries the load or the seal, and mark the viewing direction so the reader cannot reverse it.
- Hatching, material, and adjacent-part clarity: hatch adjacent parts in different directions, and keep the hatch angle away from the outline so edges stay readable.
- Fasteners, seals, clearances, and hidden interfaces: do not section fasteners, shafts, keys or ribs along their axis; show them whole so the reader still recognises them.
- Detail callout and scale for manufacture: call out a detail at a larger scale where the geometry decides the fit, and state that scale next to the detail.
- Consistency with 3D model, BOM, and inspection: check the section against the 3D model, the parts list and the inspection plan so all four describe the same construction.
Failure modes
| Failure mode | Symptom | Verification |
|---|
| Optimistic boundary | Works in a demo, fails in real duty | Test minimum, maximum, and fault case |
| Ambiguous interface | Wrong state, fit, view, or permission | Review datums, ownership, and access |
| Omitted process step | Drift, missing record, or difficult recovery | Walk the real route and measure each step |
| Maintenance not designed | Long replacement or unsafe restart | Run a first-time maintenance trial |
Five kinds of section and when to use each
Sectioning is not about "seeing more clearly" but about replacing hidden lines with visible lines. Choose the wrong kind and you either lose information or add a view without adding understanding.
| Kind | Japanese | Use when |
|---|
| Full section | 全断面図 | The internal structure is complex along the whole length and all of it has to be seen |
| Half section | 半断面図 | Symmetric parts — one half shows the outside, the other the inside, saving a view |
| Offset section | 階段断面図 | The features that have to be seen do not lie in one plane |
| Local section | 部分断面図 | Only a small area needs to be seen inside, not worth sectioning the whole part |
| Revolved or removed section | 回転図示断面図 | The cross-section of a rib, spoke or handle, rotated into the plane of projection |
On a symmetric part, a half section is usually the most economical choice: one view carries both the external shape and the internal structure.
Items that by convention are NOT sectioned lengthwise
This is where drawings go wrong most often, and the CAD software does not prevent it. When the cutting plane runs lengthwise through the items below, the convention is to draw them without hatching:
| Not sectioned lengthwise | Why |
|---|
| Stiffening ribs | Hatching a rib lengthwise makes it look like solid material and the reader misjudges the real thickness |
| Spokes and handles | Same reason as ribs |
| Solid shafts, pins, keys | Sectioning lengthwise adds no information and only clutters the view |
| Bolts, nuts, washers | Standard items whose internal form is already known |
| Balls and rollers in bearings | Same reason |
That is exactly why the decision table above says an offset section avoids cutting lengthwise through a rib — it is not a trick to make the drawing look neat but a convention that prevents communicating the wrong thickness.
Cutting plane lines and hatching
- The cutting plane trace is drawn with a heavy chain line at both ends and at each change of
direction, with arrows showing the direction of sight and a letter at each end (A-A, B-B).
- Hatching runs at an angle, usually 45 degrees to the main outline. If the outline itself runs at
45 degrees, change to another angle so the two do not coincide.
- Two adjacent parts in an assembly drawing must be hatched in **different directions or at
different spacings**, otherwise the reader takes them as one solid piece.
- The same part is hatched identically in every section throughout the drawing.
- Very thin items such as shims and gaskets may be filled solid instead of hatched.
Omitting the direction-of-sight arrows is the quietest fault in this group: the section view is drawn correctly, but the reader does not know which side it is seen from and reads left and right reversed.
Choose the cutting plane by what has to be proved
The question before sectioning is not "where does a cut look good" but "what does this section have to prove". Answer that and the position of the cutting plane follows.
| What has to be proved | Where to cut |
|---|
| Enough wall remains after pocketing | Through the thinnest place, not the convenient one |
| The tapped hole has enough thread engagement | Along the axis of the tapped hole, showing full thread and run-out |
| Whether two parts touch | Through the suspected contact area, in the assembled state |
| The oil passage is continuous | Along the passage, using an offset section if it changes direction |
| The seal groove has the right section | Perpendicular to the groove, with an enlarged detail |
The consequence: a section through a place where nothing happens proves nothing. It is still geometrically correct, but it answers no question and takes the space that the section you actually need should have had.
Three mistakes that make a section useless
Cutting through a region of constant section. The section comes out as a plain flat area adding no information. This happens when the cutting plane is placed on the part centre line out of habit rather than through the feature that has to be seen.
Cutting so the relationship between two features is lost. Two holes whose relationship matters, but the cutting plane passes through only one of them. The reader sees each hole but not the distance between them. The fix is an offset section through both, or stating that relationship on the view rather than on the section.
Leaving hidden lines on the section. The purpose of sectioning is to turn hidden lines into visible lines. Keeping hidden lines on the section itself cancels that purpose and produces a view where two kinds of line describe the same internal structure.
Detail views: when a section still is not enough
Some features cannot be read even on a section at the general scale: seal grooves, multi-step chamfers, thread roots, the clearance between two parts. That is what an enlarged detail view is for.
Three things a usable detail view must have:
- A marked area on the parent view with a name, so the reader knows where it comes from.
- Its own scale stated next to it, because it differs from the general scale of the drawing.
- All the dimensions of that area moved onto the detail, none left on the parent view.
Splitting dimensions between the two views is a sure way to get something dimensioned twice or missed entirely.
MINATA release checklist
- [ ] Function, boundary, duty cycle, and failure symptom are written.
- [ ] Interfaces, ownership, and mistake-proofing are clear.
- [ ] Six topic checks have evidence and pass/fail limits.
- [ ] Manufacturing, assembly, operation, and maintenance were tried.
- [ ] Fault, recovery, and safe stop paths were tested.
- [ ] Revision, supplier, inspection, and traceability records agree.
Good engineering is a chain of explicit assumptions that survives manufacture, operation, maintenance, and change. For section view, the standard is evidence that the result continues to work.
Frequently asked questions
When is a half section better than a full section?
When the part is symmetric and both the external shape and the internal structure have to be seen. One half shows the outside, the other the section — saving a view without losing information.
Why must a stiffening rib not be sectioned lengthwise?
Because a hatched rib looks like solid material and the reader misjudges the real thickness of the part. The convention is to draw the rib unhatched when the cutting plane runs along it. Solid shafts, pins, keys, bolts and bearing balls follow the same convention.
How should hatching be drawn?
At an angle, usually 45 degrees to the main outline; if the outline already runs at 45 degrees, change the angle. In an assembly drawing, adjacent parts must be hatched in different directions or spacings so they are not read as one piece, while the same part is hatched identically in every section.
What happens if the direction-of-sight arrows are omitted?
The section view is still correct, but the reader does not know from which side it is viewed, so left and right can be reversed. It is a hard fault to spot because the drawing still looks complete — it surfaces only when the machined part comes out mirrored.
How are very thin items hatched?
Filled solid rather than hatched. On shims, gaskets and thin sheet, two hatch lines that close together become unreadable, so a solid fill is clearer.
Frequently asked questions, continued
Where should the cutting plane go?
Where it proves what you need to prove. To show remaining wall thickness, cut through the thinnest place; to show thread engagement, cut along the axis of the tapped hole. Cutting along the centre line out of habit usually gives a correct view that answers no question.
Why does my section seem to add nothing?
Most likely it passes through a region of constant section. A section only earns its place when the cutting plane meets a feature: a hole, a groove, a step, a change of wall thickness.
May hidden lines be kept on a section?
Better not. The point of sectioning is to convert hidden lines into visible ones; keeping them cancels that and leaves a view where two kinds of line describe the same structure.
When is a detail view better than enlarging the whole drawing?
When only a small area needs close reading. Enlarging the whole drawing to make one seal groove readable pushes the drawing onto a larger sheet that the rest does not need. A detail view solves exactly the area that needs it.
Where are the dimensions of a detailed area placed?
All on the detail view. Splitting them between the parent view and the detail is a sure way to have something dimensioned twice, and at the next revision the two places disagree.
Conclusion
Sections and Cutaway Views is not paperwork done to make a file look tidy. It is how intent becomes a result that can be manufactured, assembled and measured repeatedly. A good drawing does not need the designer standing beside it to explain it; the structure of the information has to do that work.
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