Stock Forms, Sizes & Quantity Calculations
Stock Forms, Sizes and Calculating Material Quantities
Every material used in design and technology — metal, timber, polymer, paper, textile or electronic component — is manufactured and sold in standard stock forms and sizes. Knowing these forms lets you calculate exactly how much material a design needs, minimise waste, and choose the most efficient way to manufacture a prototype. A significant number of marks in the exam are awarded for applying maths to design contexts, so being confident with stock sizes and simple calculations is essential.
Metals and Timbers: Common Stock Forms
Ferrous and non-ferrous metals are supplied as bar, sheet, plate, pipe/tube, castings, wire, and as powder for powder metallurgy. Some sections are made by extrusion. Sheet metal thickness is described using gauge, while thicker or irregular sections are described by cross-sectional area, diameter, or the wall thickness of tubes.
Natural and manufactured timbers are supplied as regular sections, mouldings, dowels and sheets. Manufactured boards such as plywood and MDF come in standard sheet sizes — a full sheet is typically 2440 mm × 1220 mm, with a half sheet at 1220 mm × 1220 mm and a quarter sheet at 1220 mm × 610 mm. Timber can also be bought PAR (planed all round) or PSE (planed square edge), and sizes are given in either imperial or metric measurements.
A full sheet of manufactured board such as MDF or plywood is 2440 mm × 1220 mm. A half sheet is 1220 mm × 1220 mm and a quarter sheet is 1220 mm × 610 mm. Knowing these standard sizes lets you calculate how many components can be cut from one sheet with the least waste.
Papers, Boards and Polymers: Standard Sizes
Paper and board are supplied by weight — grams per square metre (gsm) — with board thickness also given in microns, and as laminates. Sizes follow the A-series (A0 down to A4 and smaller), the B series, foolscap, letter and envelope sizes, or are simply specified by area and diameter for circular or irregular stock.
Thermoforming and thermosetting polymers are supplied as bar, sheet, pipe/tube, mouldings, resin, granules or powder, and film. Sizes are given as thickness, cross-sectional area, standard sheet size or diameter — the same principles used for metal, applied to a different material.
A maker needs six identical rectangular polymer panels, each 150 mm × 100 mm, cut from a sheet of acrylic. Area of one panel = 150 × 100 = 15,000 mm². Total area needed = 15,000 × 6 = 90,000 mm². If the standard stock sheet is 600 mm × 400 mm (240,000 mm²), there is enough material with some left over to allow for waste from cutting and finishing. Always show each step of a calculation like this in an exam answer, even if you are unsure of the final figure — method marks are available for the correct formula and correct substitution.
Textiles: Rolls, Denier and Standard Widths
Textiles are supplied as rolls, blocks, single or double weights, and laminates. Standard fabric roll widths are 90 cm, 115 cm and 150 cm (wider upholstery and furnishing rolls of 200 cm and 240 cm are also available). The fineness of a yarn is measured in denier, which affects both the fabric’s weight and how it drapes when made up into a product.
Standard fabric roll widths are 90 cm, 115 cm and 150 cm. Choosing a pattern layout that matches the closest standard width minimises fabric waste when marking out and cutting.
Systems: Components, Tolerance and Electrical Calculations
Electronic components have their own stock forms and sizes. Resistors are supplied in standard value ranges such as the E12 series, each with a stated tolerance. Components can be through-hole, with wire leads that pass through a printed circuit board, or surface-mount technology (SMT), which sits directly on the board’s surface without drilled holes.
Three key units let you calculate how a circuit behaves: current is measured in amps, resistance in ohms and potential difference in volts. These three quantities are linked by Ohm’s Law:
$$V = I \times R$$
When resistors are connected in series, their values simply add together:
$$R_{total} = R_1 + R_2 + R_3$$
When resistors are connected in parallel, the total resistance is always lower than the smallest individual resistor. It is calculated using:
$$\frac{1}{R_{total}} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3}$$
For example, three 100 \(\Omega\) resistors connected in series give a total of 300 \(\Omega\). The same three resistors connected in parallel give a total of roughly 33 \(\Omega\). Always lay out each stage of a calculation like this clearly — the formula, the substitution and the final answer — so you can pick up method marks even if your final figure is wrong.
Choosing the Right Stock Form
Selecting the correct stock form and size is not just a maths exercise. It affects cost — buying a size closer to what you actually need reduces waste and expense — manufacturing capability, since some processes only work with certain stock forms (extrusion, for example, needs a shaped die), and sustainability, because efficient cutting layouts reduce the amount of scrap material sent to landfill.