The Importance of Drawing in the Mechanical Design Process

This is a study on the importance of drawing (both formal drafting and informal sketching) during the process of mechanical design. Five hypotheses, focused on the types of drawings, their necessity in mechanical problem solving, and their relation to the external representation medium, are presented and supported. Support is through referenced studies in other domains and the results of protocol studies performed on five mechanical designers. Videotapes of all the marks-on-paper made by designers in representative sections of the design process were studied in detail for their type and purpose. The resulting data is supportive of the hypotheses. These results also give requirements for future computer aided design tools and graphics education, and goals for further studies.

I. INTRODUCTION

Let's study the importance of drawing (both formal drafting and informal sketching) in the process of mechanical design. This goal can be extended to state that we intend to show the necessity of drawing during all the developmental stages of a mechanical design. Through the information presented here, the requirements for future computer aided design tools, graphics education, and further studies will be developed.

All mechanical engineers are taught drafting. Thus, most engineers are skilled at making and interpreting these formal mechanical drawings. These drawings are representations of a final design (the end product of the design process) and they are intended to archive the completed design and communicate it to other designers and manufacturing personnel. Additionally, engineers are notorious for not being able to think without making "back-of-the-envelope" sketches of rough ideas. Sometimes these informal sketches serve to communicate a concept to a colleague, but more often they just help the idea take shape on paper. It is in considering how these sketches help an idea take form that gives a hint that drawing's role in engineering is more than just to archive a concept or to communicate with others.

Understanding the use of both drafting and sketching in design is important to help formulate the future development of Computer Aided Design or Drafting (CAD) systems. As CAD evolves and becomes more "intelligent," the question of what attributes these systems must have becomes more important. In the past CAD system attributes have primarily been driven from developments in the computer industry. It is only through understanding drawing's importance in the design process that these systems can be based on design needs. Additionally, the pressures of CAD tool development, faculty time demands, and course expenses cause academic institutions to reevaluate the content of their "graphics" courses. Understanding drawing's importance in the design process helps establish what skills need to be taught to engineers during their training.

This exposition is organized by first, in Section II, clarifying the types of drawings used in mechanical design. The hypotheses to be addressed in this paper are given in Section III. A discussion of research on the understanding of visual imagery to be used as a basis for arguments in support of the hypotheses is in Section IV. In Section V is a discussion of the results of data taken on how mechanical engineers use drawings during design. Lastly, in Section VI, is a discussion of how well the hypotheses have been supported and the implications of our findings on CAD development, educational requirements and future research directions.

II. TYPES OF DRAWINGS USED IN DESIGN

Engineers make many types of marks-on-paper. In research, to be described in Section V, we have broken down these marks into two main groupings: support notation and graphic representations. Support notation includes textual notes, lists, dimensions (including leaders and arrows) and calculations. Graphic representations include drawings of objects and their functions, and plots and charts.

Mechanical design graphic representations are often scale drawings made with mechanical instruments or CAD computer systems. These drawings, made in accordance with a set of widely accepted rules, are defined as having been drafted. Sketches, on the other hand, are defined as "free hand" drawings. They are usually not to scale and may use shorthand notations to represent both objects and their function.

A differentiation must be made between the act of graphic representation and the medium on which it occurs. The medium, whether it be paper and pencil, a computer stylus on a tablet, chalk on a blackboard or other medium may put interface restrictions on the representation. The following discussions are concerned with what is being represented, not with how the representation is made. However, the discussions point to the medium's restriction on representation and the need for improved interfaces.

Another aspect of drawings to be considered is the level of abstraction of the information to be represented. During the design process, the design is refined from an abstract concept to a final, detailed, drafted design.

Even though a good percentage of an engineer's graphic representation is informal sketching, drafting is the focus of most engineering training and the strength of CAD systems. On the other hand, most engineers receive no formal training in sketching. It is often assumed to be some natural ability. Additionally, CAD systems do not support sketching in any meaningful way.

For the purposes of this exposition, the term CAD is defined as the use of interactive computer graphics to help solve a mechanical design problem. Current CAD tools aid the mechanical design process in four ways: as an advanced drafting tool; through assisting in the visualization of hardware and data; by improving data organization and communication; and through being used as a pre- and post-processor for computer based analytical techniques such as finite element analysis, weight and mass properties, kinematic analysis, etc. For all these uses, the "design" must be refined to the point that a scale drawing of it can be made. Thus, for current systems, the "D" in CAD means drafting.

III. THE RELATION OF DRAWING TO PROBLEM SOLVING

1. To archive the geometric form of the design.

2. To communicate ideas between designers and between the designers and manufacturing personnel.

3. To act as an analysis tool. Often, missing dimensions and tolerances are calculated on the drawing as it is developed.

4. To simulate the design.

5. To serve as a completeness checker. As sketches or other drawings are being made, the details left to be designed become apparent to the designer. This, in effect, helps establish an agenda of design tasks left to accomplish.

6. To act as an extension of the designer's short term memory. Designers often unconsciously make sketches to help them remember ideas that they might otherwise forget.

Hypothesis 1. Drawing is the preferred method of external data representation by mechanical engineering designers.

Designers represent data both internally, in their minds, and externally on paper, a computer screen or other media. It is fairly obvious that designers like to draw in these mediums and prefer a picture to a written description of an object. It is important to understand why drawing representations are preferred over other forms such as text or propositions (if-then rules).

In Why a Diagram is (Sometimes) Worth Ten Thousand Words, by Larkin and Simon, the authors explore the use of diagrams in problem solving. Here a "diagram" is a drafted, schematic drawing representing the objects in a physics problem. In this paper sentential (textual, indexed by position in a list - a sentence) and diagrammatic (graphical) representations and their effect on problem solving are compared. In comparing these the authors conclude that:

1. Diagrams can group all information that is used together thus avoiding large amounts of search for needed elements. Text only indexes to the next element in the sentence list (the adjacent piece of information) while diagrams have many adjacent elements.

2. Diagrams explicitly preserve information about geometry and topology, whereas text is only serial in nature. This feature of diagrams allows for easy indexing of information to support computation processes. However, text preserves the temporal or logical sequence of information. This is lost in diagrams.

3. Diagrams use location to group information about a single element, avoiding the need to match symbolic labels. Diagrams automatically support a large number of perceptual inferences; the information can be indexed in a variety of manners.

It seems reasonable that these conclusions, made about diagrammatic representations, can be extended to all graphical representations. Based on Larkin and Simon's conclusions it is easy to see why, in the complexity of mechanical design, drawings are preferred over text.

Hypothesis 2. Sketching is an important form of graphical representation serving needs not supported by drafting.

Later in this paper we will analyze all the marks-on-paper made by a small group of engineering designers. Their drawing marks will be classified as either free hand (sketching) or drafting marks. The hypothesis above states that the sketches have a role that more formal drafting cannot fill. Dan Herbert in Study Drawings in Engineering Design: Applications for CAD Systems considers the use of sketches (study drawings) in the solution of architectural design problems. He defines "study drawings" as "informal, private drawings that architectural designers use as a medium for graphic thinking in the exploratory stages of their work." Architects and engineers often make these study drawings in the borders of or adjacent to their formal drawings. In his paper Herbert conjectures about the properties of sketches that affect the design process. These properties form the basis for his theory of the use of sketches in design.

In Herbert's theory, sketches are used because they provide an extended memory for the visual images in the mind of the designer. Since sketches can be made more rapidly than formal drawings, they allow for more facile manipulation of ideas. Furthermore sketches allow the information to be represented in various forms such as differing views or levels of abstraction. Thus he calls sketches graphic metaphors for both the real object and the formally drafted object under development. In fact Herbert claims that sketches are a principal medium of external thinking. Herbert's thoughts lead to the third hypothesis.

Hypothesis 3. Drawing is a necessary extension of visual imagery used in mechanical design. It is a necessary extension of a designer cognitive capability for all but the most trivial data representation, constraint propagation, and mental simulation.

This hypothesis states that without data representation on media external to the designer there can be no design of substantive problems. Anecdotal support for this hypothesis is evident in asking a designer to design something and observing him/her reach for a pencil or chalk. In the next section of this paper we will discuss a model of information processing in human problem solving that gives some scientific support to these anecdotal observations and to the hypothesis. In this model, drawings are an extension of the humans' limited ability to visualize objects in their mind.

The limitation of cognitive ability leads to the forth hypothesis.

Hypothesis 4. Drawings require transformation from the designer's memory to the extended memory medium. The nature of the transformation is dependent on the characteristics of the medium.

The manner in which humans represent information in their memory is still a subject of much debate and research. Whatever the form of this internal representation, it is potentially different from the representation made externally on paper, in a CAD system, or through some other media. The transformation between these two media is one of both correspondence and implementation. Correspondence is the transformation between the internal and the external vocabularies. If the designer has a visualized 3-D object in his/her mind and wants to represent the object externally, it can be transformed into an isometric, orthographic, or other 2-D representation and drawn on paper or with a 2-D CAD tool, or it can be transformed to boolean primitives and represented on a solid modeling tool.

Further, depending on the medium chosen, there is the additional necessity to transform the image to meet the requirements of the implementation. The cognitive process for drawing a line with a pencil is different from that for specifying the end points for a CAD representation of the line. Both the research of Larkin and Simon and of Herbert focus on correspondence. There is no literature known to the authors concerning the effect of implementation on the cognitive load of the designer.

Even though the exact form of human memory is still unknown, it is generally discussed by psychologists in terms of cognitive units or chunks of data.The nature of these cognitive units, which will be discussed in the next section, leads to the fifth and final hypothesis.

Hypothesis 5. Drawings both utilize and determine the cognitive units (design features) used in mental image formulation. Thus, designer's cognitive information organization is interdependent with drawing's characteristics.

This hypothesis is double edged. It seems obvious that the content and structure of drawings is dependent on the mental image and how it is formed (its cognitive chunks).

IV. A COGNITIVE MODEL OF MECHANICAL DESIGN

In an earlier description of designers as information processors, we this figure as the environment in which the design takes place.

This figure is based on the model developed by Newell and Simon and called the Information Processing System (IPS). The figure can be viewed as a "map" of the locations in which information about the design may be stored. It is divided into an internal work space (inside the mind of the designer) and an external workspace (outside the mind of the designer). Within the designer, there are two locations corresponding to the two different kinds of memory: short-term memory (STM) and long-term memory (LTM). There is also a "processor" that is responsible for applying operators and controlling the design process. In Ullman, Dietterich and Stauffer, ten operators were identified that characterize the problem solving in mechanical design. External to the designer there are many "design state storage locations" including graphical representation media such as pieces of paper and CAD tools, as well as other sources such as textual notes, handbooks and colleagues.

Thus, the design or some feature of the design can only be represented in three locations: STM, LTM, and the so-called, external memory. Each "location" has certain properties that affect how it can be used in design. In order to support the hypotheses about the importance of drawing in the mechanical design process, this model needs to be discussed in more detail.

To discuss drawing's role in the mechanical design process, the characteristics of the STM, the LTM, and the information flow between them and the external environment will be developed. This detail is based on Newell and Simon's model, the extensions of it to visual imagery by Kosslyn and the effort to codify it by Anderson. It must be realized that the contents of the model given here are not fully agreed to in the cognitive psychology community, but they are certainly secure enough to provide a basis for discussing the role of drawing in mechanical design.

4.1 Short Term Memory

Short-term memory is very fast and powerful. The contents of the STM comprises the information we are aware of, our conscious mind. All design operations (e.g. visual perception and drawing creation) are made on information that is brought into short-term memory. Unfortunately, STM has limited capacity. Studies have shown that it is limited to approximately seven cognitive units or "chunks" of information. Although limited in capacity, the STM is a fast processor with processing times on the order of 100 msec.

In the view of Kosslyn, one function of the short term memory is as a visual buffer. In this capacity it is considered a hard-wired, special purpose, short term buffer that evolved from the need to process information from the eyes. Thus, this buffer is viewed as a coordinate space with limited spatial extent, more clarity toward the center, and the image fading without regeneration effort. The visual buffer supports images derived from the eyes during perception, and from both the eyes and the long term memory during idea generation and manipulation.

4.2 Long Term Memory

The long-term memory, on the other hand, has essentially infinite capacity, but access is slow (from 2 to 10 seconds per chunk). Access to long-term memory is also not direct. Instead, memories must be triggered by some cue or retrieval strategy based on information in short-term memory. During design, parts of the design state are stored in long-term memory. These are relatively easy to cue because, at any time, currently important parts of the design state are in short-term memory and can act as pointers for the knowledge in the long-term-memory.

In terms of visual imagery, according to Kosslyn there are two different types of information stored in the long term memory: facts about objects (including size of objects, how they are put together, names of superordinate categories, their function, etc.) and encodings of the literal appearance of the object (list of coordinates where points should be placed in the visual buffer). Shepard argues that there may not be a concrete or "first order" isomorphism between an external object and the corresponding representation. He proposes a "second order" isomorphism in which functional relations among objects are modeled and stored. It is clear that the information about images contains more than the literal appearance and that a propositional memory must be included as part of the visual image.

4.3 Cognitive Units

The contents of the cognitive units processed in the LTM and STM are not exactly clear. Anderson, in his effort to build a computer simulation of human information processing, utilizes three types of data representations for these units: spatial images, textual data and propositions. It will be shown that the spatial representation view of memory is especially important in considering the form-oriented field of mechanical engineering design.

There is virtually no evidence in the literature about the way in which designers encode information about mechanical objects in their memory. There has been much conjecture about this however as the chunks of data, more commonly referred to in engineering design as the design features, are the basic building blocks of human design organization. Thus if a design is to be represented in a manner that is most easily communicated to/from a human designer, then the information should be encoded in features that are familiar to him. Current CAD systems use features that are made of geometric primitives such as lines, arcs, solids and icons. These are the features designers are taught to use, but it is not clear that these are the way the information is best organized in their memory and most easily indexed. This gives rise to the following questions. Where do these features come from? Is it that designers have a natural set of features in their heads or is the patterning developed through their education?

The only windows that exist to study the chunking of objects in a designer's head is through his/her representation of these features as written text, words, drawings or gestures. Features used by humans to represent geometry and topology are often not easily represented textually or verbally, but can be graphically represented quite easily. For example, novice chess players when asked to recall the position of chess men on a board did so on a man-by-man basis. However, experts recalled patterns of men, larger, more complex features than the novice. These chunks had no formal "names" that could be represented as a simple written or spoken term, but they could be graphically represented by the positions on the board quite easily. Similar experiments in the domain of architecture resulted in chunks that, although easily represented graphically, could only be awkwardly represented textually as, for example, "A string of exterior walls" or "Two wall segments with windows forming an exterior corner of a square space".

The only experiment of this nature performed on mechanical engineers was performed by Waldron and Waldron. In this experiment novice (undergraduate students), intermediate (graduate students) and experts (practicing engineers) were shown an assembly drawing for a short period of time. The drawing was then covered and the subjects asked to reproduce it. By observing the video taped protocols of the subjects performing this task it was evident that the novices remembered line segments, the intermediate subjects remembered objects such as gears and the experts remembered functional components embodying a large number of physical objects.

4.4 How the IPS Model Supports the Hypotheses

The above description is important to the understanding of the use of graphical representations in mechanical design as it gives insight into the correctness of the hypotheses put forward in Section III.

First it must be noted that all the tasks performed on a design require either recall of information from the long-term memory or manipulation in the short-term memory. Since the number of chunks of information the STM can work with is limited and mechanical designs get complicated very rapidly, the short term memory forms a critical bottleneck for human designers. Additionally, as the STM transforms information so rapidly and the long term memory so slowly, only external representations made through transforming the image to graphic or textual representation can serve as added memory. Since textual information is so limited in characterizing form, graphic representation is the only reasonable memory extender for mechanical designers. This supports the first hypothesis.

The second hypothesis, that sketches serve purposes not supported by drafting, has much to do with the speed of the representation. One purpose in making a graphic image in the external environment is so that it can be viewed, encoded in the STM, and parsed in a new way. In other words, rapid external image generation allows designers to "see" information differently than the way it was generated. Thus the method of generating the external image must be rapid and flexible or it will slow down the cognitive processing.

Since the STM is so limited in capacity and mechanical design is so complex, drawings are needed in design as an extension of the visual imagery capability. Thus the IPS model supports hypothesis three. Considering both hypotheses two and three, drawing both extends the capacity of the STM and gives the capability of reparsing the information for continued processing.

The forth hypothesis stated that graphical representation is a metaphor for visual imagery and it requires transformation dependent on the medium. It would be ideal if the medium image representation requirements exactly matched the image representation in the short term memory. In one sense this match always occurs in that, to some degree, the medium used formulates the image chunks that are stored in the long-term memory and later form a basis for generating and inspecting the image in short-term memory.

The fifth hypothesis states that graphical representation both utilizes and determines design features. We have historically been constrained to two-dimensional media (e.g. pencil and paper) and these have, to some degree, formed our representations. It is not clear how we encode and store three -dimensional objects. There is one argument that memory is object-centered and that the object can thus be manipulated as a solid model. This is countered by the argument that memory is viewer-centered and thus only the specific view stored in LTM can be used for generation or inspection. Thus, the current state of cognitive research is not yet refined enough to support this hypothesis.

VI. CONCLUSIONS AND IMPLICATIONS FOR RESEARCH, EDUCATION, AND CAD

The evidence both from research in cognitive psychology and from the protocol studies of designers points to the importance of drawing in the design process beyond the documentation of final designs. Not only are drawings the preferred form of data representations for the designer, but they are a necessary part of the design process. Sketching as a form of drawing has been shown to have properties that make its use important in design. Additionally, the medium for external representation plays an active role in the amount of transformation needed to draw images from the STM and in the content of the cognitive chunks used. Finally, although with weak support, it has been shown that drawings are a window to the cognitive process used in problem solving by design engineers.

The observations concerning the use of drawings in the design process indicate areas for CAD tool development. Since drawings play such an active role in the design process, CAD tools need to be more than tools to record well thought-out and structured concepts. Specifically:

CAD systems must allow for sketching input. This is necessary for a number of reasons. First, it is a rapid representation method. Rubber banding and select methods traditional to CAD systems are simply not fast enough. Second, the additional cognitive load to implement current systems is detrimental to the design process. Icon and menu selecting add an unneeded step to creating an image. Third, in conceptual design in particular, it is not necessary that all graphical representations be as refined as that demanded by current CAD systems.

In engineering education, it's crucial being able to represent design concepts graphically. It appears that the very design process itself is limited by the ability to use graphics as a cognitive extension. This implies the need for training not only in the standard drafting skills, but additionally in the ability to represent concepts that are more abstract and best represented as sketches. Thus, the training of good designers is somewhat dependent on training in graphic skills including both formal drawing and informal sketching.