Section through a ribbed bracket: hatch across ribs, bolt and hole; auxiliary view from the wrong parent
You are reviewing a cast-iron bearing bracket. Section A-A is taken on the vertical centre plane and cuts four triangular ribs lengthwise. The drafter has hatched ribs, bosses and main body with one continuous 45°/3 mm cast-iron pattern, and the hatch runs straight through the Ø18 bolt hole and across the bolt shank. A second sheet must show an inclined boss face at 25° to the vertical true shape; the drafter projected the auxiliary view from the top view, and the boss comes out as a foreshortened ellipse instead of a circle. A Ø6 oil hole in the hub has been drawn as a separate full section, which duplicates most of the main section and confuses the shop. The company works to ISO 128-40/44 and ISO 128-34 and wants fewer views, not more. The shop has to inspect the bracket from this sheet.
1. List the section errors in A-A and state the correct convention for each.
Read the worked answer
Errors and corrections:
- Ribs hatched. A rib, web or spoke cut longitudinally (the cutting plane running along its length) is conventionally not hatched - it is outlined with thick continuous lines only, and the adjacent body is hatched. This is not cosmetic: hatching the rib makes a thin cast rib read as solid material in section and hides the fact that the rib is a stiffener, and it misleads the shop about wall thickness. (ISO 128-44; the same convention applies to gear teeth and to spokes of a wheel cut by an aligned section.)
- Hole hatched. The Ø18 hole must be left clear - the hatch stops at the hole boundary, and the hole is outlined. Hatching through a hole implies solid material where a bolt must pass.
- Bolt hatched / no clearance. Shafts, bolts, nuts, washers, pins, keys and rivets are shown unsectioned when cut longitudinally, and drawn with their actual clearance in the hole. The bolt is drawn in outside view within the section, the hole is hatched around it, and the gap between shank and hole wall is visible.
- One hatch for everything. Adjacent parts in an assembly are hatched at different angles or spacings so the boundary between parts is unambiguous. A single continuous pattern across the whole bracket erases the part boundary at the boss.
- Section line and label. Verify the cutting-plane line is a chain line with thick ends and arrows showing the direction of sight, labelled A-A, and that the section view is labelled A-A. Arrow direction is a common inspection point.
Net result after correction: fewer ambiguous areas, the rib reads as a stiffener, and the bolt hole can be checked with a plug gauge from the section.
2. Explain why the auxiliary view of the 25° boss is foreshortened, and describe how to construct it correctly.
Read the worked answer
An auxiliary view gives true shape only when the new projection plane is parallel to the face and the lines of sight are perpendicular to it. That requires projecting from the view in which the face appears as an edge (a line).
Here the face is inclined 25° to the vertical, so it appears edge-on in the front view, not in the top view. Projecting from the top view means the projection plane is not parallel to the face, so the circle of the boss is compressed by cos25 = 0.906 along one axis - the ellipse the drafter produced. The major axis equals the true diameter and the minor axis is 0.906D, so a Ø60 boss would come out 60 × 54.4 mm.
Correct construction:
- In the front view, draw the inclined face as a line at 25° to vertical; this is the edge view.
- Draw projection lines from every feature on that edge, perpendicular to the edge.
- Set the auxiliary plane parallel to the edge at a convenient distance; transfer depths from the top (or side) view with dividers or a construction line, measured perpendicular to the front view's projection direction.
- The resulting view shows the boss as a true circle, the bolt circle as a true circle, and the face's true length dimensions.
- Label the view (e.g., 'View B') and show the viewing-direction arrow and label at the parent view, per ISO 128-30/34.
If the face also carries a bolt circle, this auxiliary view is the only place those holes can be dimensioned as a true-radius pattern; dimensioning them in the front view would repeat the same foreshortening error.
3. The drafter wants to show the Ø6 oil hole. Compare the broken-out section with a separate full section and specify the correct convention for the chosen one.
Read the worked answer
A separate full section for a Ø6 hole is the wrong tool: it costs a whole view, duplicates the main section, and gives the shop two sections to reconcile.
Broken-out section is the correct choice for a local feature such as this oil hole. The view stays whole and only the area around the hole is broken away to expose the wall.
Conventions to apply:
- The boundary of the broken area is a thin continuous irregular (freehand or spline) break line, not a straight line, and it must not coincide with a visible edge or a centre line of the part - if it does, the reader cannot tell where the break is.
- The broken-out area is hatched to the same standard as a normal section of that part, stopping at the hole boundary.
- The break must be large enough to show the hole's full depth and the wall it passes through, including the exit on the bore; nothing else in the view is disturbed.
- No cutting-plane line is needed for the broken-out section because it is local; the break line itself defines the cut.
The alternative is a removed section on an extension of the hole centre line, which works if the hole is on the axis and the wall section is simple, but it separates the hole from the view the shop is reading. For a hub oil hole I would use the broken-out section, keep the main A-A as the only full section, and delete the duplicate view - which also satisfies the shop's request for fewer views.
Check the result by asking whether the shop can measure the hole depth and confirm it breaks into the bore from the one view: if yes, nothing more is needed.