Engineering Drawing, Schematics and Technical Documentation
Domain 3 tests your ability to read and produce the drawings and schematics used in repair: orthographic views, sections, dimensioning with limits and fits, surface-finish symbols, and hydraulic/electrical circuit diagrams.
Core rules
- India follows first-angle projection: the view from the front is placed above the top view, and the view from the left is placed on the right.
- Hidden detail uses dashed lines; centre lines use long-short dashes; section hatching shows solid material.
- A limit dimension such as 40 +0.05/−0.02 gives maximum 40.05 mm and minimum 39.98 mm; tolerance is 0.07 mm.
- Schematic symbols are functional, not pictorial: a hydraulic pump is a circle with a filled triangle, a relief valve is a square with an arrow, a filter is a diamond.
- Service manuals use exploded views with item numbers; the parts list gives the item, description and quantity.
Worked example
A hole is dimensioned 25.000 to 25.021 mm and the shaft 24.980 to 24.993 mm. Maximum clearance = largest hole − smallest shaft = 25.021 − 24.980 = 0.041 mm. Minimum clearance = smallest hole − largest shaft = 25.000 − 24.993 = 0.007 mm. Reasoning: clearance is always hole minus shaft; both extremes must be checked, not just the nominal 25 mm. This is a clearance fit.
Traps
Placing a first-angle view in the third-angle position; treating the nominal size as the actual size; reading a section view as an external view; ignoring notes such as typical or both sides.