It was years ago when The Lad first reached for the spanner for the usual 3/8 UN nut. It wasn’t there.
“What scruff [actually, a less repeatable name] has nicked the spanner?”
One of his compatriots had either left it lying around near where he used it last or nicked it for his tool box. In a busy workshop, it cannot be surprising if any tool goes walkabout. It happens even in the best, practical, production engineering school. But then everybody needed to use them so it was worth putting one that you have used back where it came from and everybody else could find it when they needed it. At least that’s what the instructors said – piously.
It was round about that time that The Lad first saw an advert for tool sets. The big ones with dozens of spanners within a steel tool box that attracted his covetous, young eye. The biggest and most complete ones even had castors as they were too heavy to carry. Have a look at one – product TKU 1014 at http://mdmetric.com/prod/kingdick/products/kd_toolkits.pdf ,
212 pieces and 67 kg! The Lad supposes that you could call it the secret of the engineer’s inner Nerd. But then who does not have such a secret?
Why so many pieces? Well, if the golden path and self-styled foundation of the modern world – Information Technology – can suffer from legacy systems even in its youth; then so can Engineering, that has been around many times longer. As well, the spanner has several forms for different jobs such as open jaw; socket; ring spanner etc.
The spanner is important to a mechanic. The screw thread has an ancient lineage and so has the regular shape of a nut or bolt head, most often a hexagon, that is required to manipulate it. Both are still ubiquitous in all engineering structures. The ancient lineage means that in its early days many different standard sizes of nut, bolt or screw were used. A major problem was that interchangeability of fasteners between manufacturers and machines was impossible.The thing about screw threads is that they are likely to be in use in some machines for decades or even longer. If you want to maintain one of those machines, you need a matching spanner.
It’s not just the diameter of the bolt that allows it to fit in a screw hole. Put simply it’s both the shape, usually a triangle, and also the depth of the spiral groove. The screw or bolt will not even begin to do its job if the bolt diameters match but both these features do not.
After an initial push in the early 19th Century when an accepted range screw thread designs that pairing a nominal screw or bolt diameter with a standard angle and depth of grooves was early seen to be useful subject for agreement between even competitive entrepreneurial engineering firms. A series developed by the great pioneer, Whitworth, and named after him became widely accepted. Around the same time also very widely used was the BA [British Association] series. This latter series had the advantage that the series went to much smaller diameters of screw which made it suitable for small instrument applications. The United States had its own, non-interchangeable series’s known as the US Standard developed by the engineer, Sellars. For pictures of any of these threads without The Lad infringing any copyright consult any engineering handbook.
It was only after the Second World War in the early 1940’s that further significant strides were made to reduce the still remaining variety of ‘standard’ designs. It was the unprecedented explosion of engineering production during and supporting the recovery after the war that led to the realisation of the serious inefficiencies and wasted costs were caused by the lack of an even more widely standardised, and interchangeable system of threads. At this the national engineering bodies of the USA and Canada and the UK came together to design a more rational series which they called the Unified series. Even this series was still restricted to the Imperial units of measurement. The final stage, to date, was to derive the ISO Metric series based upon the metric unit of length; that is the millimetre in the case of the thread. The Lad says the final stage but that will be completed only when everyone across the world uses the metric screw series. That’s certainly not easy and indeed he can’t say that it has yet happened. The USA still uses its standard AF [Across Flats] series widely.
The Lad has described a simple outline of the field. There’s a lot more to it of course: many professional engineering designers have to move, for good reasons, into much more detail such as fine and coarse thread series and limits and fits and indeed other more specialised thread forms such as ‘buttress’ and ‘knuckle’. Then of course there are the very different components called power screws……
As engineering is the most powerful and essential tool in the advance of human civilisation across the globe and the management of force is at its core; so the screw thread in its principal task of storing force grew to be and remains vital to most engineering structures and power plants. It is a most subtle adaptation of the wedge whose unknown inventor must be saluted as a genius on a par with Isaac Newton and above Leonardo da Vinci.