Technical Drawing, Measurement and Calculation
Technical Drawing, Measurement and Calculation
Drawings and measurements are the shared language between the design office, the workshop and the client. This domain tests reading orthographic views, interpreting dimensioning and tolerances, using scales, and taking reliable readings with rules, vernier calipers and micrometers.
Reading a drawing
- The title block gives part name, material, scale, projection symbol and general tolerance. Read it before anything else.
- Projection: under first-angle (ISO-E, the convention used in most CAP papers) the view seen from the left is placed on the right. Check the truncated-cone symbol if it is shown.
- Line types: thick continuous = visible outline; dashed = hidden detail; chain = centre line; thin continuous = dimension, hatching, leader.
- Dimensions are given once, in millimetres by default, from a datum; Ø means diameter, R means radius, and a tolerance such as 25 ±0.2 sets the permitted range.
Measurement discipline
- Read a steel rule at eye level to avoid parallax.
- Vernier caliper, resolution 0.02 mm: reading = main scale + (number of the coinciding vernier line × 0.02).
- Micrometer, resolution 0.01 mm. Check zero on both instruments before use and close the jaws on the work, not on air.
Worked example
A plan is drawn at 1:50. The measured length on paper is 64 mm and the width 42 mm. How many 30 × 30 cm tiles are needed, allowing 10 % for cutting?
Real length = 64 × 50 = 3 200 mm = 3.2 m; real width = 42 × 50 = 2 100 mm = 2.1 m. Area = 3.2 × 2.1 = 6.72 m². One tile = 0.30 × 0.30 = 0.09 m². Tiles = 6.72 ÷ 0.09 = 74.7, so 75 tiles; with 10 % waste, 75 × 1.10 = 82.5, so order 83 tiles. Reasoning: scale up first, convert to metres, divide areas, then round up and add waste.
Traps that cost marks
- Dividing by the scale when converting a real size into a drawing size, and multiplying when going the other way.
- Reading the last vernier line instead of the coinciding one, or ignoring a worn jaw face.
- Confusing Ø with R, or dropping the tolerance range.
- Forgetting waste on cut materials.
- Mixing 1:50 with 1:5 when the plan states one scale.