Engineering project and technical drawing: representation, dimensioning and design process
What the exam tests
Orthographic projection, dimensioning to ISO, sections, tolerances, scales, and the design process from brief to solution.
Projection and views
First-angle projection (ISO, Europe): the object is between observer and plane; the front view is placed above the top view, and the left view is placed on the right. Third-angle (US) reverses this. Use the minimum number of views; hidden edges are dashed lines; centre lines are chain lines. Isometric drawings use 30° axes and true lengths along the axes.
Dimensioning
Dimension lines with arrows and extension lines; no duplicate dimensions. Place dimensions outside the outline where possible, in mm without units. Give diameter with Ø, radius R, spherical SR, countersink ⌵. Do not dimension to hidden lines if avoidable. Use baseline or chain dimensioning; avoid cumulative tolerance build-up by using baseline dimensions.
Sections and details
Cutting-plane line with arrows; hatching at 45°; different hatch patterns for adjacent parts. Omit hidden lines in sections unless needed. Detail views at enlarged scale; scale 1:1, 1:2, 2:1.
Tolerances and fits
Tolerance = max − min. ISO 286: hole H7 with shaft g6 gives clearance; H7/p6 gives interference. Geometric tolerances include straightness, flatness, circularity, position and parallelism, referenced to datums.
Design process
Need → specification → concept generation → evaluation matrix → detailed design → prototype → test → manufacture. Design for manufacture and assembly (DFM/DFA), safety factors, standards and cost are part of the decision.
Worked example
A bracket is drawn at scale 1:2. A dimension on the drawing measures 35 mm. Actual length = 35 × 2 = 70 mm. If the drawing specifies 70 ±0.1 mm, a part at 70.15 mm is out of tolerance. On the drawing, the tolerance band is 0.2 mm; at 1:2 it appears as 0.1 mm, so never scale the drawing for inspection.
Traps
- Mixing first- and third-angle view placement.
- Dimensioning the same feature twice.
- Using a scaled drawing for measurement.
- Forgetting units or using commas incorrectly.
- Omitting datums for geometric tolerances.