Stock forms, standard sizes and standard components

A builder in North America asks a timber yard for a two-by-four. Take it home and measure the end. How many inches across will it be? Two inches by four is the obvious answer, and it is wrong. A finished two-by-four measures about one and a half inches by three and a half, which is thirty-eight by eighty-nine millimetres. The name comes from the rough size the timber was first sawn to, before drying and planing made it smaller. So why is it still sold under a name that is not its size, and why does nobody mind? The two-by-four's name gets settled once standard sizes are in hand.

A maker buys three different kinds of thing, and an answer has to keep them apart. A material is what something is made of, like MDF, polypropylene or cotton. A stock form is the shape and size that material is sold in, usually with numbers and units. An MDF sheet two thousand four hundred and forty by one thousand two hundred and twenty millimetres, and eighteen millimetres thick, is a stock form. A standard component is a ready-made part you fit, like a screw, a hinge, a zip or a resistor. You do not make it. You buy it finished, and fit it. One test sorts them. A material is what it is made of. A stock form is how you buy that material, with a size and a unit. A standard component is a finished part you fit. Here are nine things to sort into material, stock form or standard component. MDF, an MDF sheet eighteen millimetres thick, a zip, polypropylene, a twenty-five kilogram sack of polypropylene granules, a hinge, cotton, a fifty metre roll of cotton calico, and a box of wood screws. Which of the nine are materials, which are stock forms, and which are components? Materials: MDF, polypropylene and cotton. Stock forms: the MDF sheet, the sack of granules and the roll of calico, each with a size and a unit. Standard components: the zip, the hinge and the screws. Two examiner's reports show the mix-up from both directions. The first is on a question that asked for a stock form. Some students just named a material, for example, MDF, which was incorrect. Some students had read the question too quickly and gave a standard component rather than material stock form as an answer. The second report covers the reverse case, where a single standard component was wanted. Some students named a material stock form which was incorrect. The fix is to run the test before you write. Made of, bought as, or fitted: then answer the one the question asked for.

Stock forms come in standard shapes and sizes in every material area. Your course asks for at least one area in depth, so find your own row, and treat the others as ones to recognise. Timbers come as planks and boards, often planed all round, PAR for short, which means planed smooth on every face. There is also dowel, mouldings, and manufactured boards like MDF in large sheets, all sized in millimetres. Which would a chair maker buy for round leg rails: planks, dowel, or MDF sheet? Dowel. It is already a round rod of a standard diameter, so it only needs cutting to length. Metals come as sheet, strip, bar, rod, tube and angle, and as ingots for casting. Sizes are in millimetres, like a six millimetre round bar or a two millimetre sheet. Polymers come as granules, pellets or powder for moulding, and as sheet, rod, film and foam. Papers and boards come in sheets of standard A sizes, like A four, and in rolls. They are sold by thickness, and by weight in gsm, which stands for grams per square metre. Textiles come in rolls, or bolts, of fabric of a known width, sold by the metre. Thread comes on reels, and yarn in hanks and balls. Now a quick one: what unit would you buy polymer granules by: metres, kilograms or A sizes? By the kilogram. Granules have no length or sheet size, so they are weighed into sacks. Stock comes in fixed sizes, so a maker cannot buy exactly the piece a design needs. Draw a shelf on a standard MDF sheet, and everything left round it is offcut. So the size you buy sets the waste. Knowing the stock sizes is what lets a designer calculate how much to buy, and that sum is taught in the video on material costs, in the maths skills chapter.

Standard components come by area too. Timber and metal products use screws, nuts and bolts, hinges, rivets and knock-down fittings. Polymer products use caps, clips and fasteners. Textiles use zips, buttons, poppers and thread. Papers and boards use clips, fasteners and bindings. And systems products use resistors, LEDs, switches, batteries, gears and pulleys. Now the benefits, starting with stock forms. A small workshop bends coat hooks from steel. What stock form should the workshop buy its steel in? Mild steel round bar, or rod, of a set diameter, such as six millimetres, sold in standard lengths. Mild steel alone is only the material, and the hook is the product. That stock form helps the workshop in two ways. The bar arrives straight and already the right diameter, so the workshop only cuts and bends it, with nothing to prepare first. And six millimetre bar is a standard size, so many suppliers sell it. The workshop can compare prices, reorder quickly, and know every batch is the same, so the hooks all match. Known lengths also let it plan how many hooks come from each bar, which keeps waste down, and standard bars stack neatly on a rack. One examiner's report records this about stock forms. Students discussed with knowledge and understanding many correct reasons why stock forms are beneficial to designers and manufacturers. Now standard components. A company makes bicycle lights. It buys in the LEDs, the push switches, the battery clips and the small screws that hold each case together. Write down two things the company gains by buying these in, rather than making them. Pause it there and work it out for yourself. First, there is no need for machines, skills or time to produce those parts. A specialist turns out millions, so each one is very cheap when ordered in bulk. Second, standard parts are made to agreed sizes and ratings, and checked by the supplier. So every light in a batch works the same, and a faulty switch can be swapped for an identical one from stock. Another answer says this: the rider can buy spare batteries anywhere. Why does that point not count as a gain for the company? It helps the rider, who is the user, not the business that makes the lights. The question was about the maker. A different examiner's report, on a suitcase, describes the same wrong turn. Some candidates did not describe how these facts benefitted the manufacturer. Other candidates described benefits to the user of the suitcase rather than the manufacturer. The way to stay on track: write the company at the top of your answer, before the first point, and check every point is a gain for them. On a question asking for two advantages of standard components to the manufacturer, one report records this. Many detailed responses were able to access all four marks available.

Now back to the two-by-four from the start of this video. Its name is not its size, and builders order it every day without a tape measure. Why does a name that is not the real size cause nobody any trouble? Because it is a standard. Every mill working to it dries and planes it to the same finished size, so whatever merchant you buy from, the same piece arrives, and any one fits a design drawn for another. That is the whole point of a stock size, and of a standard component like an AA battery. Everyone agrees the size first, so a part from one maker fits a product made by another.

To close, three things this video has not sorted yet, and then the two-by-four once more.

A three metre length of aluminium tube: material, stock form or standard component? Stock form. Aluminium is the material, and tube of a set size and length is the shape it is bought in. Now a popper on a baby's vest. Which of the three is that? The popper is a standard component: a finished part that the maker fits, not makes. Here's a line from an answer: 'standard parts keep repair costs low for the owner.' Who gains? The owner, the user. Asked about the manufacturer, it would not count. Now back to the two-by-four: why can a builder order one without measuring it? A two-by-four is a standard stock size. Every merchant working to that standard sells the same finished piece under the same name.

If the three-way sort is sorted for you now, a thumbs up records it, and with that, every video in this chapter can be marked off. If it still blurs, keep the video saved, and know that nobody gets the material, stock form and component split first time.

That completes our chapter on Materials and their properties. Next chapter: Choosing and justifying materials and components. Hopefully that's next in your feed, but if not, search for ScholaFly D T oh four, oh oh.

Every other video is on scholafly.com too, easy to search by video code or by keyword. Whatever you're revising, go there and have a look, or, watch the next video.