Engineering Drawing
What is tested
Engineering Drawing tests your ability to read views, line types, dimensions, tolerances, fits, surface finish and assembly information. You must extract the manufacturing requirement from a drawing without guessing.
Core rules
- In first-angle projection, the view from above is placed below the front view and the view from the left is placed on the right. In third-angle projection the top view is above the front view.
- Visible outlines are continuous thick lines; hidden edges are dashed; centre lines are long-short chain lines; dimension lines are thin and terminate with arrowheads.
- Dimension from a datum or functional face where possible, do not duplicate a dimension, and do not dimension to a hidden line if a visible feature can be used.
- A limit fit is described by the hole and shaft tolerance zones. H is a hole with lower deviation zero; g is a shaft with negative upper deviation; H7/g6 therefore gives clearance.
- Geometric symbols control form, orientation, location and runout. A boxed tolerance needs a symbol, value and datum where applicable.
Worked example
A drawing specifies a hole Ø40 H7 and a shaft Ø40 g6. The H hole zone starts at the basic size and goes positive; the g shaft zone lies below the basic size. Therefore the smallest hole is larger than the largest shaft, so the pair is a clearance fit. If the shaft were p6, the zones would overlap or interfere, giving an interference fit. The reasoning is based on tolerance-zone position, not on the nominal size alone.
Traps
Mixing first-angle and third-angle views, treating a hidden line as a visible edge, ignoring a general tolerance note, and assuming a surface-finish symbol applies to every face. Another common error is adding chain dimensions when a datum dimension is required, which stacks tolerances and causes scrap.