Engineering Drawing
Engineering Drawing tests drawing interpretation: projection method, line conventions, dimensioning, sectioning, limits and fits, and surface finish. The aim is to extract the manufacturing information without seeing the object.
Reading rules
- First-angle projection: the top view is placed below the front view and the left-side view on the right. Third-angle projection places each view on the same side as the observer.
- Visible edges are continuous thick lines; hidden edges are dashed; centre lines are chain thin.
- Dimensions state the true size, not the measured or scaled size.
- Limits and fits: uppercase letters denote holes, lowercase denote shafts. H7/g6 is a clearance fit in the hole-basis system.
- A surface-finish symbol gives the required roughness; the tick points to the surface.
Worked example
A hole is 25 H7 (+0.021/0) and the shaft is 25 g6 (−0.007/−0.020). 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. Both values are positive, so the fit is clearance. Reason: pair the largest hole with the smallest shaft for maximum clearance, and the smallest hole with the largest shaft for minimum clearance.
Traps
Reading a first-angle layout as third-angle; using nominal size instead of the limits; dropping the minus signs on shaft deviations; confusing hidden lines with centre lines; assuming a section view shows all hidden detail behind the cutting plane.