Is it not simply a graphic representation like any other? How does it differ from a technical drawing?

These are just some of the many questions which need to be answered when first encountering the world of Technical Illustration. In discussions with specialists who are familiar with this subject it is important to know the specialist terms and how to use them correctly. Such specialists will often react with dismay at the incorrect use of terms.

The correct use of such terms indicates that a person has the necessary expertise and know-how of the industry. Wrong usage has the opposite effect.

The following sections will use examples to explain the key terms encountered in Technical Illustration.

Definition

Technical illustrations is a term which generally covers the figures used in technical documentation. However, a technical illustration can just as easily appear on a display at a trade fair or in an encyclopedia.

The following definition formulates the term more precisely:

This definition naturally requires explanation. Depending on the intended use of the illustration, e. g. in a spare-parts catalogue or a repair manual, it will need to perform different functions. When used in a spare-parts catalogue, it will need to depict a unit or a part of the unit in a way which allows it to be identified with ease. When used in a repair manual on the other hand, its task will be to show activities relating to the unit. In such instances, it will generally illustrate a particular situation, e. g. the installation or removal of a part.

In all instances, however, it does not have the goal of showing every last design detail of the unit. Such details are often not only superfluous, but actually detract the reader from the matter at issue. To reinforce this effect, the important parts are often highlighted by the use of thicker lines or by being depicted in a larger scale than the rest of the illustration.

Technical drawing and technical illustration

There is often some uncertainty as to whether differences exist between technical drawings and technical illustrations and, if so, what these differences consist of.

Technical drawings are required for designing and manufacturing a machine. Technical illustrations differ from this quite markedly and are used instead for depicting a machine as clearly and simply as possible and without any details.

A technical drawing is a scale representation of a unit or part, with the views which are required for an accurate description of this part. These views may include front-view, top-view and lateral-view representations of the part. Perspective representations are generally not employed for technical drawings.

Technical drawings are the device which the drawing office and workshop use to communicate. They are used to describe machines or installations with sufficient precision to allow them to be constructed on the basis of these specifications. Even the smallest detail must be defined with exact precision to ensure that the drawing is unambiguous. The specifications will include details of form, dimensions, surface quality, materials etc. which are entered on the drawing in accordance with established drawing rules and standards. Reading a technical drawing can be a very complex process since a wealth of information will need to be interpreted.

Technical illustrations

are used to deprict a part in a way which makes it clearly recognisable. This does not require the inclusion of every detail. In most cases, parts are represented in perspective form since this is the form the human eye is more familiar with. Consequently, technical illustrations can also be comprehended by persons who have not undergone any special training as draughtsmen.

Views and perspectives

A drawing or illustration is created by drawing a workpiece in the way it appears when viewed from a certain direction.

Technical drawings use technical views to depict a workpiece. These represent an unambiguous and complete image of the workpiece. Technical illustrations use perspective representations which are easier for the reader to follow. Different perspectives are employed depending on the usage and type of the illustration.

The technical views include e. g. front view, lateral value and top view. They show the details visible from a given direction without any perspective effect. Concealed edges are shown dashed.

Technical illustrations, on the other hand, use perspective representations. In simplified terms, the more natural the perspective is, the more complex it is to include it in the illustration.

The reason for this lies in perspective reduction which is used. Everyone will know from their own experience that an object appears smaller at a distance than it does close up. It is also true that all lengths are reduced in size the further away they are from the observer's eye.

This reduction results in what is known as a vanishing point perspective. This perspective corresponds to the way in which a human eye or a camera sees. The disadvantage of this perspective, however, is precisely the natural way it presents an object, since it requires parts to be depicted differently as their position changes relative to the observer. This means that a part in an illustration will need to be redrawn even if it is merely shifted to another position on the drawing area.

This work can be reduced by selecting a different form of presentation which, while being slightly less natural in its effect, is easier to create. Parallel perspective representations offer a solution to this. Unlike the vanishing point perspective, two lines which are parallel in reality also remain parallel in the illustration. The direction from which the observer views the workpiece is unimportant in the first instance.

The advantage is that a part which has been drawn using the parallel perspective can be moved around at will without having to be redrawn.

The parallel perspectives include trimetry, dimetry and isometry. There are also various oblique projections e. g. cavalier perspectives, though these are seldom used.

The difference between these perspectives lies in the direction the workpiece is viewed from. Trimetry is the most general form and allows every direction of observation. To illustrate the differences to dimetry or isometry, let us first take a look at the problems associated with perspective reduction. The illustration opposite shows a cube with an edge length of 100 mm, i.e. each line you can see is 100 mm long in reality. If you measure the lines on the page, however, you will discover that the individual lines are different lengths. This is the result of perspective reduction which is dependent on the angle at which the cube is viewed.

The edges of the cube can also be viewed as the major axes of a coordinate system. Each axis has its own reduction factor. There are a number of special cases which simplify working with these reduction factors. It is of course a very complex business if a dimension of 100 mm gives rise to different lengths depending on which edge of the cube it is located. If the same reduction factors were applied on different axes, this problem would be resolved with ease.

One of these special cases is dimetry. The term dimetry is applied to those perspectives where two of the three reduction factors are the same. There are a whole range of angles for which this condition applies. To achieve a measure of uniformity here, the dimetric projection based on DIN 6 is generally selected in which the projection of the major axes in the illustration forms angles of 7° or 42° with the horizontal.

There is one case, however, where all three reduction factors are the same - namely isometry. In isometry, the major axes in the illustration form angles of 30° with the horizontal. Isometry thus forms the simplest type of perspective drawing from the actual drawing aspect. This has meant that isometry has become the standard perspective for technical documentation.

It is used primarily in the spare-parts sector. What is important in such applications is the recognisability of the figures rather than their naturalness. It is also used often in repair and operating manuals, though these applications also make use of the central perspective. This is due to the fact that frequent use is made of photographs which are simply traced over and already incorporate the perspective.

Ellipses

Closely linked to the problem of perspectives is the question of ellipses. These form the key element of Technical Illustration.

The simplest way of examining this matter is to observe a cylinder which is tilted stage by stage out of a horizontal orientation and into vertical position. The cross-section of the cylinder initially appears as a circle and from one stage to the next gradually changes into an ellipse which becomes ever shallower until finally only a line remains as it assumes a vertical position.

The perspective accurately defines which ellipse matches which angle of inclination. With manual drawing, gauges and computation tables help the user determine what is known as the ellipse angle. This value defines the 3D orientation of the ellipse.

A second example shows how a cylinder in rotated around an axis in isometry. Here, too, the changes occurring in the ellipses should be observed. At this point you should consider the possibilities you know for generating ellipses with a program. Programs generally only provide functions for creating circles or arcs which are compressed either horizontally or vertically. A number of graphics programs offer functions which allow the subsequent rotation of such ellipses. Any further editing, e. g. precise cutting at selected points or performing changes to the diameter, can then only be performed with difficulty and generally without any possibility of entering dimensions.

The situation is further aggravated by the need to satisfy graphic requirements in the form of different line thicknesses and gradual transitions between these (see following section). It is also worth pointing out that the task of determining ellipses and working with ellipse templates was the primary reason why many draughtsmen failed to come to terms with the manual drawing of perspectives.

Line thicknesses

In most cases, different line thicknesses are employed in Technical Illustration for design considerations and to reinforce the perspective effect. The most important rule for the application of different line thicknesses lies in the correct assignment of "thick" and "thin" line widths for the outer and inner edges of the object being drawn. An edge is drawn thick if it has nothing behind it but "air". Consequently, inner edges are those which, from the angle of vision used for the drawing, do not represent a break between the object and its surroundings.

Or expressed in simpler terms:

If I can place a finger behind the line it is drawn thick, otherwise it will be thin.

The ratio of thick lines to thin ones should be approx 2:1.

Ellipses require special attention. Half on an ellipse will be drawn as an outer edge and the other half as an inner edge. In such cases, a thick line will meet a thin line. In order to create a good optical impression, the thick line is "tapered" down to the thickness of the thinner line in the zone where the two meet. All centre lines are drawn with dot-dash lines and are underlaid with a shadow (i.e. isolated from the background).

The line thicknesses of the various line types depend on the size of the illustration when printed out and on thew intended application. The lines must be wide enough to prevent them breaking up during filming. However, they should not be so thick that close, adjacent lines run into each other. This is particularly important if the illustration is to be reduced substantially in size for the final application (e.g. microfilming). If a section of a large drawing is selected, i.e. a section is greatly enlarged, all line thicknesses must become thicker.

For certain types of illustration, the basic principles governing line thicknesses are disregarded:

Ghost representations

This term refers to representations of components or subassemblies which are included in the illustration to enhance understanding. Take, for example, an installation environment in a spare-parts catalogue. In this instance, all lines are drawn "thin". The ghost representation is easily recognised and yet merges into the background when viewed against the spare parts which are drawn in the "standard" style.

Emphasising a part

Where components are to be emphasised (e.g. when specifying setting distances between two parts or identify a wear part), all lines can be drawn with a thick line. The parts in question are then immediately noticeable.

Scale

Unlike the technical drawing, the primary function of the technical illustration is to ensure ease of recognition - including for users without any specialist training.

It is often neither important nor possible to maintain a uniform scale. The following example will illustrate this point:

Where an illustration shows very small parts alongside large parts, there is a risk that the smaller parts will merge together during the print or will simply not be recognisable. For this reason, the bolt in the above illustration, for example, is shown much larger than it is in real life. In most cases, the observer will not register this manipulation and will merely absorb the content of the information presented to him.

A further reason for imprecise and out-of-scale drawing is the time factor. As already mentioned several times above, the creation time is an important factor in Technical Documentation. If everything were drawn with great precision, this would involve very considerable drawing and computation work, which in turn would cost valuable time. In most cases, only key reference dimensions are used to ensure the correct proportions of an object. Details such as drill holes are then often inserted by eye.

Tracing templates

This practice of producing illustrations has been adopted from traditional manual techniques for use in electronic creation. There are various reasons for this:

Suppose, for example, that existing stocks of drawings are required in data form in the computer. The high output quality and the capability to incorporate changes cannot be achieved by simple scanning alone. The drawings need to be traced using a vector-oriented illustration program.

A further reason is to be found in the tracing technology itself. If an illustrator has a template, this means he does not have to rely on voluminous technical documentation. Consequently, even a person who is not experienced in illustration work can acquire this skill with ease. Through constantly working with this tracing technology (particularly with photo templates), an illustrator can produce line drawings very quickly and thus - particularly where large quantities are involved - very cost-effectively. This work is also suitable for assigning to service providers. Perhaps the most important reason why today's automotive industry in particular makes use of tracing photographs is the ease with which, by employing a low abstraction level and depicting only the essential points, it enables a user to recognise a real-life situation. The illustration in the car owner's operating manual will allow him to comprehend the situation clearly and to apply it to his own vehicle.

A special feature of tracing photographs is given by the tempate itself. Every photograph has been shot from a different angle of vision, i.e. features a different perspective. The representation is not isometric. This fact is particularly significant for the most important element, the ellipse, since constant use is made of different ellipse angles.

Experienced illustrators are able to achieve very short creation times using manual techniques. It is therefore important that electronic techniques are able to attain at least the same speed.

The traditional method

Which tools are required for manual tracing?

• Tracing paper
• Pencil, ink pen
• Ellipse template, ruler
• Eraser, scalpel
• Light table
• Photocopier with enlargement/reduction
• Repro camera
• Paper, photographic paper
•  Finished text pages and adhesive

Where an illustration is sizable or exploded drawings extend over several sheets, a rough scribble is first made of the page layout. A pencil drawing is then made on the tracing paper. Experienced illustrators will restrict themselves to the key outlines and lines. After a check, the drawing proper is made using different thicknesses of ink pens. In some instances, a new sheet of tracing paper will be laid over the pencil drawing and the latter traced. To make the template easier to trace, a light table is often used as a drawing surface. Photographs are also laid under tracing paper for tracing.

Finally, the text or reference numbers are entered onto the drawing using stencils. Corrections can be made during the drawing stage by carefully removing the ink with a scalpel.

Changes are time-consuming, particularly where there are several per sheet, and are restricted by the thickness of the tracing paper. To avoid holes appearing in the paper, an entire section of a sheet will be cut out in some cases and a newly drawn part fitted in its place.

If drawings are to be reused, either an existing drawing will be copied directly onto tracing paper and drawing work added or a (possibly) scaled paper copy is placed under the tracing paper for tracing.

Once the drawing is finished, a paper print is produced with a repro camera, cut to size and assembled (pasted) into the graphic frame of the documentation page.

This then is the series of steps from the first pencil stroke to the final illustration in a manual using the traditional drawing method. The time required (two drawing stages plus pasting) and the additional costs involved are naturally crucial factors even for a new illustration. The major advantage of electronic creation lies in the possibilities it offers for further re-use and updating. This advantage can be further exploited by the use of libraries (standard parts, recurring components) and a database.

One important difference between the two methods is worthy of further emphasis:

With manual methods, however, the illustrator is left to his own devices and skills. His tools are very limited and, in the case of ellipse templates for example, call for a high level of skill in selecting the correct ellipse angle (see Tracing of Photo Templates).

Since training in the profession of technical illustrator is not available in most countries (except at private training establishments), there has always been a scarcity of trained illustrators. In the U.K. specialised colleges exist for technical illustrators. This is one of the primary reasons why so may Britons are encountered in this branch of industry.

2D - 3D representations

A 2D representation is perfectly adequate for producing illustrations for technical documentation. Once a view has been drawn, the information of the areas lying behind it are no longer required. A 3D file naturally allows the user to rotate the part to another angle of vision for use in another application. Set against this, however, is the extra work entailed and the need for technical documentation to be available quickly and in up-to-date form. Creating a 3D drawing involves considerable extra time (particularly for changes) and also requires the illustrator to have additional mathematical training. Moreover, a CAD program is unable to satisfy the needs of an illustration or can do so only with difficulty (e.g. line thicknesses, out-of-scale representation, graphic attributes, data formats). The greater storage capacity required for 3D data only mentioned besides.

Customer wishes

Everyone using a computer and software programs for the first time to perform work he is familiar with will initially attempt to apply his usual method of working. And that is also the case with the illustrator.

Once he has become familiar with the possibilities opened up by electronic creation, however, his thirst for even greater ease and speed of operation and for automatic functions will grow immensely. This is particularly true for less creative work.

Another factor is the head of the TD department who is looking to cut costs, save time and streamline operations.

An important aspect in this regard is the use of existing data and in particular data from CAD programs (see separate chapter on this subject).

The ideal solution of course would be to simply press a button and watch the CAD data flow automatically into the documentation where they would adopt the relevant graphic style elements and cause no problems in the process. Of course, graphic changes would be updated automatically in all relevant documentation, but only in those parts of the documentation you wanted updated.