Engineering drawing and reading
Engineering drawing and reading tests the ability to interpret views, lines, dimensions, tolerances, sections and symbols and to convert them into a correct workpiece.
Core rules
- Orthographic views show an object from front, top and side. First-angle projection places the view on the far side of the object; third-angle places it on the near side. Check the projection symbol.
- Visible outlines are continuous thick lines; hidden edges are dashed; centre lines are chain lines; dimension and projection lines are thin.
- Dimensions give size and location from datums. Tolerances give permitted variation; fits show clearance or interference between mating parts.
- Surface finish symbols specify roughness and machining method. Geometric tolerancing controls form, orientation, position and runout.
- Section views show internal detail; hatching must be consistent and not look like a separate part. Read the title block, scale and notes before dimensions.
Worked example
A dimension reads 25 plus or minus 0.05 mm. The upper limit is 25.05 mm and the lower limit is 24.95 mm, giving a tolerance zone of 0.10 mm. A measured 25.07 mm is out of specification even though it is close to nominal. The inspector should reject or rework it according to the drawing.
Traps
Assuming a view is third-angle when it is first-angle reverses features; ignoring hidden lines misses holes; scaling a drawing instead of reading dimensions is unreliable; and overlooking a general tolerance note causes false acceptance.