THE PROBLEM SECTIONS SOLVE: Internal features are shown by HIDDEN (dashed) lines. For a simple part that works. For a part with several internal features at different depths, the view becomes a thicket of overlapping dashed lines in which nothing can be traced, and — critically — DIMENSIONING TO A HIDDEN LINE IS BAD PRACTICE because the reader cannot be sure which hidden edge the dimension refers to. THE SOLUTION: IMAGINE THE OBJECT CUT THROUGH BY A PLANE, THE NEAR PORTION REMOVED, AND WHAT REMAINS DRAWN AS AN ORDINARY VIEW. Interior features that were hidden edges become VISIBLE OUTLINES — continuous thick lines — and can be dimensioned with confidence. THE FOUR CONVENTIONS THAT MAKE IT READABLE: 1. THE CUTTING PLANE LINE — chain line, thin, with THICK ENDS and at changes of direction. It carries ARROWS at its ends showing THE DIRECTION OF VIEW, and LETTERS: A—A, B—B. The resulting view is titled "SECTION A—A". THE ARROWS POINT IN THE DIRECTION THE OBSERVER LOOKS, toward the retained material. Reversing them reverses the view, and this is a common exam error. 2. HATCHING (SECTION LINING) marks the CUT SURFACE — the material the plane actually passed through. · thin continuous lines at 45° · evenly spaced, 2–4 mm apart depending on the area · HATCHING GOES ONLY ON MATERIAL THAT WAS CUT. Anything visible BEYOND the plane is drawn normally, unhatched — a very frequently made mistake. · IN AN ASSEMBLY, ADJACENT PARTS ARE HATCHED DIFFERENTLY: opposite directions (45° and 135°), or different spacings, so that the boundary between two parts is unmistakable. THE SAME PART IS HATCHED IDENTICALLY IN EVERY VIEW AND EVERY SECTION. · Large areas may be hatched only around the boundary (outline hatching) to save effort. · Very thin sections (sheet metal, gaskets, structural webs under about 2 mm) are FILLED SOLID BLACK, because hatching would be illegible; adjacent thin parts are separated by a small white gap. 3. HIDDEN LINES ARE OMITTED IN THE SECTIONED PORTION. The whole point was to remove them; drawing them again defeats it. They are shown only where genuinely needed to avoid ambiguity. 4. THE OTHER VIEWS ARE UNAFFECTED. Cutting is an imaginary act performed for one view only — THE MATERIAL IS NOT ACTUALLY REMOVED, so the front view still shows the complete object even when the side view is sectioned. Students frequently and wrongly remove the material everywhere.

1. FULL SECTION The cutting plane passes ENTIRELY THROUGH the object along a single plane; half is removed. THE DEFAULT AND MOST COMMON. Used when the interior is the main subject. 2. HALF SECTION Used on SYMMETRICAL objects. The plane cuts through only ONE QUARTER, so ONE HALF OF THE VIEW IS SECTIONED AND THE OTHER HALF IS SHOWN EXTERNALLY. ONE DRAWING GIVES BOTH THE INSIDE AND THE OUTSIDE — its great economy, and the reason it is preferred for cylindrical parts such as pulleys, flanges and covers. · The two halves are divided by a CENTRE LINE, never a solid line, because no edge exists there. · Hidden lines are omitted from BOTH halves. · Dimensioning across the boundary needs care: a diameter is dimensioned with one arrowhead only, the line stopping just past the centre line. 3. OFFSET SECTION The cutting plane is STEPPED — bent at right angles — so that it passes through several features that do not lie on one straight line. CRITICAL CONVENTION: THE BENDS IN THE PLANE ARE NOT SHOWN IN THE SECTIONED VIEW. No line appears where the plane stepped, because no such edge exists in the object. The steps appear ONLY on the cutting plane line in the other view. This is the single most examined detail of offset sections. 4. REVOLVED SECTION The cross-sectional shape of a bar, spoke or rib is drawn DIRECTLY ON THE VIEW, rotated 90° into the plane of the paper, with THIN CONTINUOUS outlines. Used to show that a handle is round, hexagonal or I-shaped without a separate view. The original outline may be broken away on either side to leave room. 5. REMOVED SECTION The same idea, but the cross-section is drawn ELSEWHERE on the sheet — beside the view or on another sheet — and labelled. Drawn with THICK outlines, unlike revolved sections. Preferred when the section is complex, needs dimensioning, or must be drawn at a larger scale. 6. BROKEN-OUT (LOCAL) SECTION Only a SMALL REGION is cut away, bounded by an IRREGULAR FREEHAND BREAK LINE (the thin wavy line). No cutting plane line is needed. Used when just one feature — a single hole or boss — needs explaining and a full section would be excessive. 7. ALIGNED (REVOLVED) SECTION For features arranged radially — spokes, ribs, holes on a bolt circle — that do not lie on the cutting plane. THE FEATURE IS ROTATED INTO THE PLANE and drawn at its true radial distance. DELIBERATELY UNTRUE TO THE PROJECTION, and justified because the alternative — showing a spoke foreshortened — would misrepresent its length and confuse the reader more than the rotation does. 8. AUXILIARY SECTION A section taken on a plane inclined to the principal planes, to show an angled internal feature in true shape. THE CONVENTIONS THAT OVERRIDE THE RULES — always examined, because they look like errors until they are known: CERTAIN FEATURES ARE NEVER HATCHED EVEN WHEN THE CUTTING PLANE PASSES THROUGH THEM ALONG THEIR LENGTH: · RIBS AND WEBS · SPOKES AND ARMS of wheels and pulleys · SHAFTS, and solid round parts · BOLTS, NUTS, SCREWS, STUDS, RIVETS, PINS, KEYS · BALLS AND ROLLERS in bearings · GEAR TEETH THE REASON, and this is the answer that earns the mark: HATCHING THESE WOULD GIVE A FALSE IMPRESSION OF SOLIDITY OR MASS. A hatched rib looks like a thick solid flange rather than a thin stiffening plate, and a hatched web makes a cast wheel look like a solid disc. THE CONVENTION SACRIFICES LITERAL ACCURACY TO PRESERVE THE CORRECT IMPRESSION — exactly the same trade-off cabinet oblique makes. NOTE THE QUALIFICATION: a rib is not hatched when cut ALONG its length, but IS hatched when cut ACROSS it, because then the section genuinely is a small solid rectangle and no false impression arises.