An artefact or system can be represented in a variety of ways. Engineering drawing is but one of the ways. Figure 1.7 shows some of the ways that products or systems can be represented.
Verbal or written instructions take the form of words describing something. If the words take the form of a set of instructions for doing something, they are ideal. If the words are used to tell a story, then they can paint beautiful pictures in the imagination. However,
Imagined and variable Defined and having legal status
Figure 1.7 Engineering drawing representation should be specification
Imagined and variable Defined and having legal status
Figure 1.7 Engineering drawing representation should be specification words are clumsy with respect to transmitting information about an engineering artefact. Perhaps a chair or a table could be described without too much difficulty but for anything very much more complex, words become inadequate. Hence, the expression, 'a picture says a thousand words'! Painting or sketching can certainly convey visual information. However, it is also open to artistic interpretation and licence. Ancient pictures of kings and queens often did more credit to them than was justified! Several paintings by Constable of the Dedworth area show the church at different locations because it adds to the artistic balance. Three-dimensional models can certainly be made of engineering artefacts and structures. Indeed, the use of rapid prototyping for the construction of feasibility models is a fast-growing industry. Clay and plastic models have been around for years and mock-ups of new engineering designs for style-based design give the designer a new level of understanding and interpretation. However, three-dimensional models cannot be posted to somebody or sent via the Internet!
All the above techniques can certainly represent an artefact and enable it to be visualized. However, they all fall short of providing a specification that would allow something to be made accurately by a manufacturer speaking a different language to that of the designer. Specification of an artefact is of a higher order than visualization. Specification is needed for engineering artefacts because the instruction to manufacture something given to a subcontractor has financial as well as legal implications. Engineering drawing is a form of engineering representation along with all the others but it is the only one that provides a full specification which allows contracts to be issued and has the support of the law in the 'servant-master' sense. To put it in the words of a BSI drawing manual: 'National and international legal requirements that may place constraints on designs and designers should be identified. ... These may not only be concerned with the aspect of health and safety of material but also the avoidance of danger to persons and property when material is being used, stored, transported or tested' (Parker, 1991).
In Section 1.3, it was stated that engineering drawing was the equivalent of a language. A language has to have a set of rules and regulations for it to operate correctly. The same is true of engineering drawing. In the English language, there are two basic rules. The first is the word order that gives information on subject and object. The second is spelling, which gives information on the words themselves in terms of the spelling, i.e. the nouns, verbs, etc. Considering the word order, the phrase 'the cat sat on the mat' is very different from the phrase 'the mat sat on the cat'. All that has happened is that the words 'cat' and 'mat' have been swapped. Previously, the phrase described a perfectly feasible situation whereas it now describes an impossible situation. Thus, the word order gives information on which is the subject and which is the object. The second set of rules concern
* third (or first) angle projections
* sections and cutting planes
Typical standards: IS0128-1982, IS0128-34:2001, ISO-44,2001, IS0128-50:2001, IS07573:1983.
size and shape
* surface finish
Typical standards: IS015768:2001, IS0129-1.2:2001, IS0128-22:1999, ISO406:1987, IS01101:1983, !S01302:2001.
Figure 1.8 Representation, visualization and specification spelling and thus the phrase 'the cat sat on the mat' is very different from 'the bat sat on the mat' and yet this difference is the result of only one letter being changed!
In engineering drawing, there are similarly two sets of rules. The first also concerns order but in this case the order of the different orthographic views of an engineering artefact. The second is concerning how the individual views are drawn using different line thicknesses and line types, which is the equivalent of a spelling within each individual word. These are shown in Figure 1.8. The first set are the 'drawing layout rules', which define information concerning the projection method used and therefore the arrangement of the individual views and also the methodology concerning sections. The second set of rules is the 'manufacturing rules', which show how to produce and assemble an artefact. This will be in terms of the size, shape, dimensions, tolerances and surface finish. The drawing layout rules and the manufacturing rules will together make a legal specification that is binding. Both sets of rules are defined by ISO standards. When a contractor uses these two sets of rules to give information to a subcontractor on how to make something, each party is able to operate because of the underpinning provided by ISO standards. Indeed, Chapter 4 will give information concerning a legal argument between a contractor and a subcontractor. The court awarded damages to the subcontractor because the contractor had incorrectly interpreted ISO standards. In another dispute with which the author is familiar, the damages awarded to a contractor because of poor design bankrupted a subcontractor.
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