How do designers use scale, ratio and tolerance calculations in drawings and manufacture?
Scale, ratio and tolerance calculations: scale factors and reading scale drawings, ratio and proportion, tolerance limits and bands, and the use of these in technical drawings and dimensioning, with worked calculations.
A focused answer to OCR A-Level Product Design on scale, ratio and tolerance calculations: scale factors and reading scale drawings, ratio and proportion, tolerance limits and bands, and their use in technical drawings and dimensioning, with worked calculations.
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What this dot point is asking
OCR wants you to calculate with scale factors, ratio and proportion, and tolerance limits and bands, and to apply these to technical drawings and dimensioning. These are the everyday maths skills a designer uses to draw, proportion and dimension a product, and they carry marks in Component 01.
Scale and scale factor
The exam trap is the direction: at the real object is bigger, so multiply drawing-to-actual and divide actual-to-drawing.
Ratio and proportion
Tolerance limits and the tolerance band
Using these in drawings and dimensioning
Scale, ratio and tolerance come together on a working drawing: the drawing is at a stated scale, features are dimensioned at true (actual) size, and critical dimensions carry tolerances so the manufacturer knows the acceptable range. Reading a scale drawing correctly, proportioning a design and stating tolerances are the practical reasons these calculations are examined.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20204 marksA drawing is at a scale of 1:5. A feature measures 38 mm on the drawing. Calculate its actual size, and state the size on the drawing of an actual 250 mm dimension.Show worked answer →
A Component 01 scale calculation. Marks for each conversion.
At a scale of , the actual size is the drawing size times : mm. For an actual dimension of mm, the drawing size is the actual size divided by : mm.
A common dropped mark is multiplying when you should divide, or vice versa. At the real object is five times larger than the drawing, so multiply drawing-to-actual and divide actual-to-drawing.
OCR 20224 marksTwo ingredients in a mix are in the ratio 3:2. If 750 g of the mixture is made, calculate the mass of each ingredient. A part is dimensioned 40 mm with a tolerance of plus or minus 0.2 mm; state its limits and tolerance band.Show worked answer →
A Component 01 ratio and tolerance calculation. Marks for the ratio split and the tolerance limits and band.
Ratio: the total number of parts is . One part g. So the first ingredient g and the second g (check: g). Tolerance: the upper limit mm, the lower limit mm, and the tolerance band mm.
A common dropped mark is forgetting to find the value of one part first (total divided by the sum of the ratio numbers), or giving the limits but not the band.
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