Interpretation of Drawings, Specifications and Technical Documentation
Drawing interpretation is the domain where most marks are quietly lost, because one misread symbol changes every dimension that follows. The exam tests projection, GD&T, tolerance and fit systems, surface texture, and how to reconcile drawings with specifications, bills of material, and revision notes.
Projection and views
Third-angle projection (common in Canada and the US) places the top view above the front view and the right-side view to the right. First-angle, used on ISO drawings, places the top view below and the right-side view to the left. The truncated-cone symbol in the title block distinguishes them. Auxiliary views show inclined surfaces true size; a feature shown in a foreshortened view must never be measured from the drawing.
GD&T essentials
A feature control frame reads left to right: geometric symbol, tolerance zone, material condition modifier, then datum references in order of precedence. Datum A in the frame is the primary datum and orients the part first. Basic dimensions (boxed) have no tolerance of their own; the position or profile control supplies it. At MMC (circle M), bonus tolerance equals the departure from MMC: a Ø10.00 +0.20/−0.00 hole produced at Ø10.15 gains 0.15 of extra positional tolerance.
Fits, tolerances and surface texture
Hole-basis fits are the default. H7/g6 is a sliding clearance fit; H7/h6 is a locational clearance; H7/p6 is an interference fit requiring press or heat. Surface texture uses Ra in micrometres (or µin); an Ra callout is not RMS, although older drawings may show RMS values. Lay symbols indicate the direction of the dominant pattern.
Specifications and documentation
Purchase specifications (ASTM, SAE/AMS, ISO) define chemistry and mechanical properties that the drawing may only reference. Always check the revision block against the part you are making; a superseded revision is scrap waiting to happen.
Worked example
A print shows Ø12.00 ±0.10 with position Ø0.20 at MMC to A|B|C. At MMC (Ø12.00) the allowed shift is Ø0.20. Machining the hole to Ø12.08, the bonus is 0.08, so the total positional tolerance is Ø0.28. Reasoning: bonus is always the amount the feature has departed from MMC toward LMC, applied to the stated tolerance.
Traps
Mixing first- and third-angle; treating basic dimensions as toleranced; applying bonus in the wrong direction; reading Ra as RMS; and ignoring notes such as break all edges or do not scale drawing.