Rib Hatching and Sectioning Conventions on a Cast Bracket
A pattern shop is checking a drawing of a cast-iron bracket before making the pattern. The bracket is a 160 × 90 × 20 flat base, a vertical plate 20 mm thick standing on the base at one end, and a 12 mm thick triangular rib bracing the plate at mid-length. Two Ø18 holes pass through the vertical plate, and two 22 mm wide slots are milled through the base from the underside, either side of the centreline. The drawing gives a full section A–A on the vertical centreline, cutting lengthwise through the rib and through one of the Ø18 holes. In that section the drafter has hatched the rib with the same 45° iron hatching as the plate, carried hatching straight across the Ø18 hole, and drawn the base slots as hidden lines. The patternmaker says the section 'shows a solid wall where the rib is' and still cannot find the rib thickness anywhere on the sheet.
1. Justify the convention that governs the rib and show how the rib thickness should be conveyed instead.
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The convention. Ribs, webs, spokes, gear teeth and similar thin features are not hatched when the cutting plane passes longitudinally through them; they are outlined and left clear. The rule is a deliberate exception to 'cut metal is hatched', and it exists because the section is meant to convey the true mass of material at the cut. A 12 mm rib cut along its length occupies a triangular area on the sheet that is large in projection but thin in fact; hatched, it reads as a solid wall of the full projected thickness, which misleads anyone judging weight, cooling, or strength. That is exactly the patternmaker's complaint.
What the section should show. The rib outline - two long lines converging from the base to the plate - with no hatching inside, and no cut line where it meets the plate beyond the normal visible edge.
How to convey thickness. The thickness is a dimension, not a section property:
- dimension '12' on the plan or front view, ideally as '12 THK';
- or add a removed section B–B taken at right angles across the rib, where the cut is genuinely across the 12 mm and hatching is correct and informative;
- or use a revolved section drawn on the rib in the plan view.
A removed section across the rib is the cleanest answer: it shows the 12 mm directly, hatched correctly, without creating a false impression anywhere else.
2. Explain what is wrong with the hatching across the Ø18 hole and how a correctly sectioned hole should look.
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The error. Hatching denotes material removed by the cutting plane - solid metal. A hole is a void: there is no material there to cut. Hatching drawn across the hole therefore states that the casting is solid where the drawing elsewhere shows a drilled hole, and a pattern made from this sheet would cast the boss solid and scrap the part at the drill. It also defeats the purpose of taking the section through the hole at all, since the section was presumably placed on that centreline to show the hole clearly.
Correct treatment.
- The hole outline is drawn as a continuous visible line where the cutting plane passes through its axis;
- the interior is left clear - no hatching, no hidden lines for the far side of the bore;
- hatching resumes on the far side of the hole and the hatch lines on each side should align in direction and spacing, so the part reads as one component rather than two;
- the hole is dimensioned on the section or in the view where it is clearest, not both.
Related convention worth stating. Solid cylindrical standard parts cut lengthwise - bolts, studs, pins, rivets, shafts, keys, and similar - are not sectioned; they are drawn in outside view even when the cutting plane passes along their axes. The same reasoning applies: the section is there to show the surrounding material, and sectioning a standard part adds nothing and clutters the view.
3. Deal with the hidden lines for the base slots and state the viewing/annotation requirements that a valid A–A section must carry.
Read the worked answer
Hidden lines in a section. Hidden lines are normally omitted from sectional views, because the section already exposes the interior and hidden detail on top of hatching produces an unreadable drawing. Here the slots are off the centreline, so the cutting plane misses them entirely; drawing them as hidden lines in A–A is both against convention and uninformative - it clutters the section without locating anything.
Where the slots belong.
- Show them in the plan or end view, where their 22 mm width and position can be dimensioned once;
- or, if the section must show them, use an offset section: step the cutting plane so it passes through one slot, with the step shown and the arrows still indicating one viewing direction;
- state the slot note once: '2 SLOTS 22 WIDE THRU, UNDERSIDE'.
Requirements for a valid A–A.
- Cutting-plane line shown as a thick chain line with its ends and any changes of direction clearly marked;
- an arrow at each end at 90° to the plane, indicating the direction of sight - the section shows what lies beyond the plane in that direction, not what is in front of it;
- matching capital letters at both ends and the title 'SECTION A–A' under the view;
- hatching at a consistent angle and pitch across all parts of the same component, with adjacent separate components hatched at different angles;
- scale and projection symbol consistent with the rest of the sheet.