04-BS-15 · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams, 04-BS-15 Engineering Graphics & Design Process, 2018-May. Closed-book, no calculator; five questions constitute a complete exam paper, and all sketches must be freehand (no straightedges).
Reference texts: Bertoline & Wiebe, Technical Graphics Communication (4th ed.) — orthographic projection, isometric pictorials, auxiliary and section views, first/third-angle projection, dimensioning; Giesecke et al., Technical Drawing / Engineering Graphics (15th ed.) — ASME Y14.5 / CSA B78.1 dimensioning and tolerancing, ISO fit systems, constructive solid geometry (Boolean primitives); Ulrich & Eppinger, Product Design and Development — Quality Function Deployment / House of Quality.
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
The table lists the standard ISO preferred fits between a hole and a shaft of the same nominal (basic) size, grouped into three families — clearance, transition, and interference — and gives each fit's symbol in two equivalent forms: a hole-basis symbol (e.g. H7/p6) and the matching shaft-basis symbol (e.g. P7/h6).
Each symbol is a letter-plus-number IT (International Tolerance) grade pair for the hole, followed by the same for the shaft: the capital letter (H, C, D, F, G, K, N, P, S, U…) is the fundamental-deviation letter that sets where the tolerance zone sits relative to the basic size, and the number (6, 7, 8, 9, 11…) is the IT grade that sets how wide that zone is (a smaller number is a tighter, more precisely toleranced zone). A capital letter always denotes a hole, a lower-case letter always denotes a shaft. "H7/p6" therefore means: the hole is toleranced H at grade 7 (zone starts exactly at the basic size and extends only upward, i.e. the hole can only be at or above nominal), and the shaft is toleranced p at grade 6 (a small positive deviation, so the shaft is always slightly larger than nominal) — together giving the table's locational interference fit. The asterisk footnote adds that for very small basic sizes (0–3 mm) the same H7/p6 pair only reaches a transition fit, because the p deviation there is smaller than the IT7 hole tolerance.
The sketch makes the table concrete for a 20 mm basic size. H7 lets the hole be 0 to 21 µm over size. An f7 shaft is 20–41 µm under size, so it always clears. A k6 shaft (+2 to +15 µm) may clear or interfere depending on the actual sizes: a transition fit. A p6 shaft (+22 to +35 µm) always interferes, if only by 1 µm at the extreme. The fit class is a property of how the two zones sit, not of either tolerance alone.
In the hole-basis system (the convention the printed table itself is built around — every row's first symbol starts with H), the hole's tolerance zone is fixed at H for every fit; only the shaft's letter/grade changes to move from a loose running fit (H11/c11) through a snug locational fit (H7/h6) to a force fit (H7/u6). In the shaft-basis system (right-hand sketch, the table's second symbol column), the shaft is held at h and the hole's letter/grade changes instead. Hole-basis is used far more often in practice because a hole's finished size is fixed by the reamer, broach or drill-and-ream tool used to make it (a discrete, standard tool size), while a shaft's size is easily adjusted continuously on a lathe or grinder to hit any target diameter — so it is cheaper to standardize the hole and vary the shaft than the reverse.
A designer selects the fit class from the table by matching the intended function to a description column: a rotating shaft in a plain bearing needs a running clearance fit (e.g. H8/f7, close running); a locating dowel or bearing that must stay put but be removable for service needs a locational clearance or transition fit (H7/h6 or H7/k6); a press-fit gear hub or bearing race that must never rotate relative to its shaft needs an interference fit (H7/s6 or H7/u6). The designer then calls out the fit symbol directly on the drawing (e.g. øxx H7 on the hole, øxx p6 on the shaft) instead of specifying two independent plus/minus tolerances, which guarantees the two mating parts will always assemble with the intended clearance or interference regardless of exactly where each part's actual size falls within its own tolerance zone.
