Engineering was a craft before the Industrial Revolution but then much changed. Is this a “statement of the bleedin’ obvious” as Basil Fawlty would have said. No. I am not referring to the invention of the Steam Engine and the various weaving machines in themselves.
Simply put, it was the knock on effect, of interchangeable spares for those and other machines. Before the Industrial Revolution each machine was made by hand. Each of its parts was made to fit sufficiently accurately and smoothly with only the other parts of that particular machine. As more machines went into service and, with time, they needed replacement parts. It was expensive and a pain to have to go to the machine in question and make a new part to fit. Not only a pain: possibly in a working assembled machine it would be impossible to ‘find the sizes’.
Someone had the brilliant idea to make all the parts sufficiently accurately that any part would fit into any machine. The phrase “sufficiently accurately” is the secret here. All sorts of machine tools and techniques had to be developed to do the job but The Lad describes here only one.
Much increased accuracy was needed compared to woodwork. Before, wood could accomodate some inaccuracy due to its flexibility, lesser rigidity, and its usual static not dynamic function. Before you can make something accurately to size, you have to know what that size is [See my later post on the micrometer]. Then you have to mark the material accurately to that size. One component of the marking out process was actually a pair of items. The surface table and the scribing block.
The material that you are trying to make into an accurate component must be marked acurately, much more highly accurately than had been done previously. They need very fine lines permanently marked to a very accurate position. Then you can remove material to the marks. So: you stand the component on a very flat table usually made of cast iron, sometimes granite. If it is already machined, you may coat the surface with blue dye or copper sulphate to give a blue or coppery surface to make any scribed marks easy to see. A hard point mounted on a heavy block with a flat surface can then be set to a given, accurate height. As it smoooooooothly glides, steel on cast iron, over the table’s flat surface, it scribes a fine line on the material. Turn the component through a right angle, reset the scriber to a different height perhaps and scribe another line crossing the first. There you have it. At the intersection, is the place to drill a hole accurately in position.
The surface table is too big to form part of the current collection of The Lad. But the scribing block is not. Here it is in pictures.
As well as the heavy base, note the heavy sections that give us a rigid structure. Under the small scribing load, this rigidity leads to almost zero deflection and thus mark-up accuracy.
Sometimes the engineer is scribing several parallel fine lines on a horizontal surface rather than a vertical one. If so, she can use the machined straight edge of the table using the pins pushed down as shown to run along the edge. Even a cylindrical guide to the scribing can be used if she finds it necessary by mounting the vee in the base on the cylinder.
The whole unit weighs 1.1kg. The makers say that this one was made around 1965. The surface finish shows a slightly raddled beauty: it must have been very striking when it was new. I asked the Wolfson Heat Treatment Centre what the effect was. Derek Close and his colleagues suggest that it was made by steam blueing, a process where a component at greater than 600 degC is immersed in dry steam.
It certainly has some corrosion resistant properties. I say this because all but the, ground, corroded, lower surface of the base is steam blued and is not corroded.
This is one of the artefacts that The Lad collects. The essence of it is that it and the others in his collection are all things that The Lad has used albeit briefly. A scribing block has been around for a long time and its pedigree is tightly intertwined with engineering history.
This tool was called a “surface gauge” in the Buck and Hickman 1958 catalogue at £2.60. This catalogue is a weighty gem from The Lad’s 1960’s collection of engineering books. The Lad has always called it a scribing block so he will stick to that. Moore and Wright still exist by the way in Sheffield and Hampshire but are owned by Bowers Metrology Group [http://www.moore-and-wright.com/]. As an indication of a classic tool, the identical design of scribing block can still be bought new from Buck and Hickman that is also still thriving in 2010, http://www.buckandhickman.com/ . The current price is instructive though.