Technical Drawing and Graphical Expression: Standardisation, Views, Dimensioning and CAD
Drawing questions are quick marks if you know the conventions and expensive if you mix them up. The PAU paper shows a sketch or describes a part in words and asks for correct views, a section, a scale reading or a properly dimensioned drawing.
Projection systems
- First-angle projection (ISO-E, used in Europe): the object sits between the observer and the plane, so the view from the left is drawn on the right and the top view goes below the front view.
- Third-angle projection (ISO-A, used in the USA): the plane is between the observer and the object, so views appear on the same side as the observer.
- The truncated-cone symbol distinguishes the two systems on a drawing frame.
- Six views are possible (front, top, bottom, left, right, rear); only the minimum needed to describe the part is drawn.
Lines and sections
- Continuous thick: visible edges. Dashed: hidden edges. Dash-dot thin: axes and symmetry planes. Continuous thin: dimension and hatching lines.
- A full section cuts the whole part; a half section shows one half in section; an offset section follows a stepped cutting plane. Hatching at 45° marks the cut material and is omitted on shafts, bolts and pins cut longitudinally.
Dimensioning (ISO 129)
- Every dimension appears once, on the view that shows the feature most clearly; never dimension a hidden edge if a visible one exists.
- Functional dimensions locate mating features; reference dimensions are shown in parentheses and not used for manufacture.
- Dimension lines are parallel to the measured edge, with arrows touching the extension lines; small dimensions place arrows outside with the value between them.
- Units are millimetres by default, with no unit symbol written.
- Threads: M20×2.5 means metric thread, nominal diameter 20 mm, pitch 2.5 mm. Tolerances follow the hole-basis system, and surface roughness uses the Ra symbol in µm.
CAD
- Work with layers for outlines, hidden lines, dimensions and text; use blocks for repeated symbols.
- Parametric modelling ties dimensions to constraints, so a change of one parameter updates the geometry.
- DXF and DWG are the standard exchange formats; a drawing must be printed at its stated scale.
Worked example
A part is drawn at 1:5. A feature measures 40 mm on the paper, so its real size is 40 × 5 = 200 mm, and the dimension written on the drawing is 200, never 40. If the same part is later issued at 2:1, the paper measurement becomes 400 mm but the dimension stays 200.
Traps that cost marks
- Measuring the paper and entering that value as the dimension.
- Placing the left view on the left in a first-angle drawing.
- Dimensioning to hidden lines or repeating the same dimension on two views.
- Omitting the section hatching or hatching parts that are not cut.