Basic workshop calculations and technical drawing
Basic workshop calculations and technical drawing tests unit conversion, area and volume, ratios, percentages, tolerances and the reading of engineering drawings.
Calculations
Work in consistent units: convert mm to m by dividing by 1,000. Area of a circle = πr²; volume of a cylinder = πr²h. Ratio and proportion solve mixture, gear and scale problems. Percentage error = (measured − true) ÷ true × 100.
Drawing
Orthographic projection shows front, plan and end views. First-angle projection places the view of the left side on the right of the front view; third-angle places it on the left. A scale of 1:5 means the drawing is one-fifth of full size. Dimensions carry tolerances such as 25.00 ± 0.05, meaning acceptable limits of 24.95 to 25.05 mm. Surface finish and welding symbols add manufacturing instructions.
Worked example. A drawing at 1:20 shows a component 38 mm long. Actual length = 38 × 20 = 760 mm. If the drawing tolerance is ±0.3 mm, the real tolerance is ±0.3 × 20 = ±6 mm. If the specified length is 760 ± 6 mm, the limits are 754 mm and 766 mm.
Traps
Mixing first- and third-angle views. Multiplying when converting mm to m. Applying the scale in the wrong direction. Forgetting that a tolerance applies to both directions. Adding dimensions that are already given by a chain.