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04-BS-15 · Undated paper

Question 3 of 5: Basic hole system and basic shaft system

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

Basic Studies / 04-BS-15 — Engineering Graphics & Design Process, undated. Closed-book, no calculator, 3 hours, 100 marks; five questions constitute a complete exam paper (answer ALL five). All sketches are freehand technical drawings following third-angle projection (CSA B78.1 / ASME Y14.5) unless the question calls for isometric pictorial or sectioning conventions.

Reference texts: Bertoline & Wiebe, Technical Graphics Communication (4th ed.) — orthographic projection, isometric pictorials, auxiliary/section views, third-angle projection; Giesecke, Technical Drawing / Engineering Graphics (15th ed.) — ASME Y14.5 dimensioning, CSA B78.1 drafting practice, sectioning conventions, glass-box projection theory.

Question 3: Basic hole system and basic shaft system (10 marks)

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.

A "fit" between a shaft and a hole is controlled by choosing tolerance zones for both parts relative to a common basic (nominal) size. Rather than picking the two zones completely independently every time, standards (ISO 286 / ASME B4.1) fix ONE of the two parts' tolerance zone against the basic size and vary only the other, giving two systems:

Basic hole system: the hole's tolerance zone is held fixed with its LOWER limit exactly at the basic size (fundamental deviation "H", zero fundamental deviation, tolerance added entirely in the positive/oversize direction). The desired fit — clearance, transition, or interference — is then obtained purely by selecting the SHAFT's tolerance-zone letter and grade (e.g. H7/g6 for a clearance running fit, H7/k6 for transition, H7/p6 for a press/interference fit).

Basic shaft system: the shaft's tolerance zone is held fixed with its UPPER limit exactly at the basic size (fundamental deviation "h", zero fundamental deviation, tolerance added entirely in the negative/undersize direction). The fit is then obtained by selecting the HOLE's tolerance-zone letter and grade (e.g. H7/h6, F8/h7, U7/h6).

BASIC HOLE SYSTEM Hole H Shaft g (example) Hole's LOWER limit is fixed at the basic size (H); the fit is set by the shaft's zone. Used for standard reamers / broaches / plug gauges (fixed-size hole tooling). BASIC SHAFT SYSTEM Shaft h Hole H7 (example) Shaft's UPPER limit is fixed at the basic size (h); the fit is set by the hole's zone. Used with off-the-shelf precision-ground stock shafting.
Fig. Q3 — tolerance-zone diagrams for the basic hole system (hole zone H fixed at the zero line) and the basic shaft system (shaft zone h fixed at the zero line); example mating zones shown for illustration.

When each is used. The basic hole system is the default in general mechanical design and is used whenever the HOLE is the harder/more expensive feature to produce to a variable size — standard-size reamers, broaches, and plug gauges each cut one fixed hole size, so it is cheapest to hold the hole at that one standard size and vary only the (easily-turned-to-any-diameter) shaft to obtain different fits. The basic shaft system is used instead when the SHAFT is the constrained feature — most commonly when off-the-shelf precision-ground shafting (cold-finished bar stock, standard bearing shaft sizes) is being used unmodified, so it is cheaper to bore each mating hole to whatever size gives the required fit than to turn a custom shaft diameter for every mating part. A common concrete example: one line-shaft (basic shaft, h6) carrying several different components (a gear needing a transition fit, a pulley needing a clearance fit, a coupling needing an interference fit) each bored to a different hole class on the same shaft diameter.