Engineering Drawing
What this domain tests
Reading orthographic views, identifying line types and their meaning, applying dimensioning rules, and calculating limits and fits from toleranced sizes.
Core rules
- Projection. First-angle projection is standard Indian workshop practice (BIS drawing practice, SP 46): the object lies between the observer and the plane, so the top view is drawn below the front view and the left side view on the right. Third-angle reverses both.
- Line types. Continuous thick = visible edge; short dashes = hidden edge; long-short chain = centre line and pitch circle; continuous thin = dimension and extension line; double-dash chain = cutting plane; wavy thin = break line.
- Dimensioning. Aligned system, figures above the dimension line, millimetres implied, no unit symbol repeated, never dimension the same feature twice, dimension to visible lines rather than hidden ones.
- Limits and fits. Hole-basis is preferred. For the 18–30 mm size range, hole 25 H7 = +0.021/0 and shaft 25 g6 = −0.007/−0.020 mm.
- Symbols. ∅ diameter, R radius, surface roughness Ra in micrometres, tolerance grade numbers IT01–IT18.
Worked example
Find the maximum and minimum clearance of a 25 H7/g6 sliding fit. Hole limits: 25.021 max, 25.000 min. Shaft limits: 24.993 max, 24.980 min. Maximum clearance = hole max − shaft min = 25.021 − 24.980 = 0.041 mm. Minimum clearance = hole min − shaft max = 25.000 − 24.993 = 0.007 mm. Because the minimum clearance is positive, the fit never interferes; it is a clearance fit, and the H7/g6 combination is the classic close-running fit.
Traps
Placing the top view above the front view out of third-angle habit; mistaking a chain centre line for a hidden line; writing 'mm' after every dimension; adding tolerances instead of subtracting to find minimum clearance.