DRAWINGS IN ENGINEERING DESIGN
Introduction
Engineering drawing is not only the province of the draftsperson. It is the language of the engineer. It is their means of developing and recording their ideas, and conveying them to others. Every engineer will be using and referring to some form of drawings almost daily. They will often be producing or directing the preparation of drawings. Usually, they make the preliminary sketches and design drawings in accordance with principles of engineering drawing. Because this is the most unambiguous way of to convey and record information. It is also likely that every engineer at sometime will be checking the work of designer drafters and approving drawings before they are sent to manufacturing. When engineers sign off the final approval of a drawing, they take responsibility for it. An overlooked error in the drawing could be costly.
Words are not the natural language of engineers. Drawings are their prose, mathematics their grammar and differential equations their poetry.
Glegg
Ideally, then, engineers should be good draftspersons. They can constructively criticize the work of inexperienced drafters. However, with the limited time available at the University it is not possible to get the necessary proficiency. At the university you are given the fundamentals, and it is up to you to improve your knowledge and skill as required.
This course will emphasize design procedures. However, the design drawings which you will be making must be properly executed.
Development and Production Drawings
In their everyday work mechanical engineers must be familiar with production drawings. The function of the production drawing is to impart descriptions, specifications, and instructions to the shop so that three-dimensional objects and systems may be manufactured and assembled in their correct location with respect to other components of a machine.
Where do the ideas for the creation of the object originate, and how are these ideas developed? The form of a design is progressively developed graphically. For example, much of the original thinking is involved in the technical sketch made by the engineer or designer. Many calculations are done at this stage. As further confirmation of the practicability of the design an accurately made scaled drawing called a layout is made. The layout shows the overall dimensions and will show several critical elements assembled in their functional relationships. Detail drawings are then made. Usually one drawing is made for each part, showing complete details and instructions necessary for its manufacture. Finally, subassembly and assembly drawings are made to show how the detail parts are to be assembled and to show general dimensions.
Specifications and the Proposal Drawing
Layout representation begins with the interpretation of design specifications by making up proposal drawing ( Exhibit 1a , Exhibit 1b , Exhibit 1c ).
Suppose a Space Agency wants to purchase a new attitude control system. From preliminary studies they have found the flight characteristics of their vehicle. This gives them the control requirements for their system. A set of specifications is drawn up and requests for proposals are issued to the companies from which they wish to receive quotations. The design engineers at these companies on receiving copies of these specifications will begin rough designs. The designers will roughly design the components that will make up the system, sensors, actuators, computers, programs, etc. so that a proposal drawing can be made. These drawings show the general design that will best fulfil functional requirements. They show general dimensions, areas, weights and other basic design and manufacturing information. From these proposal drawings a preliminary estimate of engineering, tooling and production costs are made. The estimated cost and the proposal drawing are sent to the sales department who add a factor for profit and establish a selling price which is to be quoted. The price, drawings, and much other descriptive information are then submitted as a proposal and tender.
The proposal including the proposal drawings become an essential part of a design contract, and it is the basis of the eventual design, the drawings are not used for fabrication. When a complicated product is being considered, proposal drawings with the written text indicate only the method to be employed, in obtaining basic the functional requirements. They emphasize engineering principles to be used in design. The bulk of the minor design work is generally suggested but not completed. It is expected, therefore, that the final product although constructed according to the principles set forth in the proposal drawing, may differ considerably from it.
The degree of completeness of proposal drawings is inversely proportional to the complexity of the product. Thus, for a less complicated product the proposal drawing may also suffice as the layout, and occasionally even as a working drawing. The reason behind this is that involved systems require many specialists who must expend much time and effort to arrive at detailed solutions. The development procedure is expensive and can be justified only when an organization has received a contract to carry a design to its completion. Proposal drawings supply only enough information for contract acceptance.
The major product design work begins after the company has received the order and the proposal drawing has been accepted. Meanwhile many revisions may have been made to the proposal drawing to suit the customer's requirements before it is accepted. The principal component parts or sections of the product are assigned to specialized design groups, and each group might be headed by an engineer. A project engineer will be in charge of the complete product design. This however will vary greatly with the type of organization.
The proposal may include an outline drawing ( Exhibit 2 ), at the time of submission or shortly after the order is confirmed. Outline drawings become part of the contract obligation. Its purpose is to provide the customers sufficient information about the product that they can go on with the rest of their design. Therefore the outline drawing gives all the dimensions of the finished device requited to attach it and to connect it to the equipment it will work with. And it must also give the overall dimensions of the system so that the space it will occupy. Outline drawings are sometimes required to be certified. A responsible officer of the company, typically the Chief Engineer, must sign the drawing guaranteeing that the system will be in accordance with it.
Technical Sketch
One of the most useful procedures in all stages was the use of freehand sketches to represent all alternatives in a 3D arrangement.
Neri et al, ICED 83
The designers interpret the requirements shown on the proposal drawing, study the accompanying specifications, and begin thinking out solutions. The solutions are recorded in technical sketches. In the technical sketch ( Exhibit 3a, Exhibit 3b, Exhibit 3c ) the designer puts down the important factors - general shapes, clearances to be checked, structural investigations, functional requirements and basic manufacturing processes that may be used. The designer must exercise ingenuity in making approximations before an accurate stress analysis is made to decide actual sizes. Technical sketches are not discarded, they are valuable because they record most of the ideas and the directions that contribute to the final design. As much thinking and planning as possible should be shown in the rough sketches. This expedites a more direct solution and lessens the possibility of having to change design principles completely on the carefully drawn layout.
The Layout
A layout drawing (Exhibit 4a , Exhibit 4b , Exhibit 4c ) by the designer is an exact graphical representation of the design. It is intended for engineering rather than manufacturing use, although sometimes a layout drawing is used for experimental production. The layout is an accurate development of the conception of the design, or the placement of units. Essential elements are developed, and the geometry of the machine or structure is dimensionally defined taking into consideration its function, manufacture and other requirements. The layout is a key drawing from which production drawings are made. Several layouts may be required for one machine. For instance the steering mechanism in a car would require a layout drawing. The rear end would require another. In making the layout, the basic reference lines and center lines are located. Adjacent or existing parts are drawn in phantom lines. This conveniently defines the space available to work. The general shape of each component member is approximated and calculations are carried out simultaneously which finally determines the actual sizes. Sometimes the layout, the design sketches and calculations are made simultaneously because each provides information that is needed for the other.
Layout drawings are always drawn to scale, full scale if possible. CAD is extremely useful this way. Layout drawing can also be done rapidly on squared paper to give the scale. The prime consideration is accuracy - only a minimum of necessary essential graphical information is presented. Layout drawings are similar to assembly drawings, except that cross hatching is confined to the boarders and may be done free hand. Symbols may be used for standard components unless details are required for clarification.
For stress calculations freehand sketches may be used also. The sketches and calculations are filed for reference and checking purposes. The coordination of stress analysis, function, manufacturing, and clearance factors are all embodied in the layout.
The Production Detail Drawing
Detail drawings ( Exhibit 5 ) represent single elemental components. The drawing contains complete information for manufacturing the part.
Accepted drafting practice in industries engaged in mass production calls for a separate drawing for each cast, machined, or forged part. These detail drawings are made by detail draft-persons. They usually obtain the basic information required for the part from the layout drawing.
The person making the layout is usually the engineer or a senior designer. They will be responsible for several draft-persons of different grades. The detail drawing is critically important because when it is released for manufacture it must be a document that has only one interpretation. Once released, the responsibility for the accuracy of the drawing rests not with the draft-person, who produced it, but with the designer and/or engineer who produced the layout and approved the drawing. They therefore have a critical interest in the production drawing and should assure that it has been checked carefully before it is released for manufacture.
There are many reasons for separate layout and production drawings. Sometimes it may be possible to trace or modify a layout drawing as a basis for a detail drawing or more usually for an assembly drawing, but generally the layout drawing is used for design purposes only.
Assembly and Installation Instruction Drawings
After the detail production drawings have been made, the assembly drawing (Exhibit 6 ) is prepared. The purpose of the assembly drawing is to give all data required to assemble two or more parts together by bolting, press fitting, welding, riveting or some other process.
Assembly drawings may or may not provide information for making any or all the component parts. The assembly drawing is sometimes represented as a final installation drawing, or sometimes these drawings may be called subassemblies if they represent the assembly of a component of a machine which is made up of several parts.
The assembly drawing generally contains the Bill of Material. Although, for large and complicated machines each production detail drawing has its own Bill of Material.
Division of labor between Product and Tool Design
Modern design practice in industry has two component parts:
product design (product engineering or engineering)
tool design (tool engineering or tool design).
When modern mass production methods were beginning, design and manufacture were combined. Henry Ford, for example, created his design and developed it on paper and in machines. Engineering his product to the point where quantity manufacturing was feasible. He then facilitated designs of the tools, methods, and processes required for production and took a hand in the supervisory planning of manufacture.
As mass production techniques rapidly developed, significant divisions of labor became necessary. The first division occurred between engineering design and manufacture. The need for special engineering skills was recognized and engineers were trained for creative, or initial product design. When a successful experimental design has been developed, the engineer is presented with an entirely new set of problems concerning the mass production of the item. At this point a second division of labor occurred within the engineering design ranks. Mechanical engineers specialized in manufacturing know-how or tool engineering. The tool engineer assumed the responsibility of planning the production methods of the product with minimum cost of labor, materials and time. They also concerned themselves with the quality control of the product. Today in industry there is a further breakdown and tool design, manufacturing methods, and quality control are usually separate departments.
In the past production drawings were completed then sent to the production department for planning and in turn they initiated the tool design. Today's competitive environment has fostered the idea of "concurrent engineering". In concurrent engineering the product design, production, and tooling design are brought together and work together not sequentially but concurrently so that the tooling and production requirements influence the product design from the start. This desirable procedure has been facilitated by CAD since several departments can be working from the same set of data as it is being developed.
Tool Design
Too often the product engineer does not appreciate the many steps through which a design must go before it becomes an actual interchangeable part. On the other hand the tool engineer is not always sympathetic with many specialized problems that confront the product designer creating the initial design. An understanding and appreciation of tool problems result in a more efficient operation.
The tool designers must concern themselves with the following factors:
Analysis of the complete manufacture of the part;
Design and manufacture of tools and accessories;
Gauging and inspection of the finished part.
Obviously the tool designer must have a thorough knowledge of machine tools including the various standard small tools and accessories. The knowledge of machine tools and standard small tools must be supplemented with the ability for carefully designing special tools such as jigs, fixtures, gauges, punches and dies.
Although tools facilitate mass production, they themselves are custom-made single elements. Tool design principles and drafting practices, therefore, vary from production design and drawing techniques. Since tools usually represent one-off manufacture, tool drawings may contain all the detail drawings on the one drawing. The drawing may also show in phantom lines or color the outline and location of the production part it is associated with.
However, the tool designer still uses the standard stages of development in drawing a new tool, i.e., the idea sketch, the layout, the production drawing and assembly drawing.
Designing With the Layout Drawing
Generally designing takes place in at least two stages: draft and operational. During the draft stage the main arrangement and general design of a given unit are established (sometimes in several versions). After evaluation and discussion of the draft, the working operational arrangement is produced, it defines more accurately the details of the system and serves as the starting point for completing the project.
For the designer the purpose of a drawing is two fold. Although it may eventually convey information to others, it is first of all an aid to thought.
Albert Leyer
During these design stages it is important to identify and establish the principal components, and to find the correct order of design and development.
Attempting to design the whole system with all its elements at once is a typical error characteristic of novice designers. Having received the assignment which presents the purposes and the performance parameters of the project, the novice designer often tries to calculate and complete the design in all its details. then they try to draw all the elements to produce a picture as if it were a finished assembly drawing of the project. Such a procedure is an irrational one, and results in a string of poorly arranged constructional elements and units.
It is preferable to begin the design with a solution of principal design constraints, i.e., the selection of kinematics, the power sources and flow, or the correct sizes and shapes of the main components and of their most preferred relative positions (design sketches). Any attempt to completely describe parts in detail at this stage is not only useless, but harmful. It draws attention away from the main problems of the design and confuses the logical development the design.
Another important procedure for design is to first develop several design alternatives concurrently, analyze them and then select of the best. It is a mistake to set the direction of the design by accepting the first idea which arises, or to follow an obvious solution. The designer must analyze carefully all feasible solutions and choose the one most suitable for the given requirements. This requires deliberate effort, the problem is not at once solved, but sometimes only after long investigation.
Full development of each alternative is not necessary. Usually hand pencil sketches or overlays are sufficient to establish the advantages and limitations of an alternative and to decide whether it is advisable to continue with that particular alternative.
The drawing and the calculations must be carried out in a complementary manner, each contributing to the other. The initial calculations need only be tentative approximations. Main design elements should be evaluated not only for strength, but also for rigidity.
The designer cannot rely on solely selecting dimensions and shape of parts by eye. Of course, there are very skillful designers who almost without mistakes can establish sizes and cross-sections assuring stress levels acceptable for the given branch of engineering. Alternately they cannot rely on calculations alone, sketching or drawing the part to scale can uncover unsatisfactory dimensions and configurations. Remember, " If it looks wrong, it probably is wrong". Similarly copying trite shapes and keeping to traditional stress levels, will not create better designs.
To only depend upon calculations is also wrong. In the first place, the existing methods of strength calculations do not consider many factors that influencing the suitability of a design. Secondly, there are some parts and configurations (e.g., housings) which cannot be conveniently calculated at all, or the effort cannot be justified. Thirdly, other factors besides strength affect the sizes of parts. For example; the design of cast parts is dependent largely on casting technology, parts being machined must resist the cutting forces and be sufficiently rigid, heat treated parts should be large enough to avoid buckling.
Thus, besides calculations, the designer must be aware of existing design practices and regulations and follow them, if warranted.
Another prerequisite for good design practice is a continuous consideration of the manufacturing problems; from the very beginning ever component should be given a technologically reasonable shape. A skilled designer from the beginning considers how the part will be produced. Novice designers should constantly consult with the production and test engineers.
The design should be perused on the basis of standard dimensions (fitting diameters, sizes of keyed and spline connections, diameters of threads, etc.) where possible. At the same time maximum use of standard elements should be sought. If specific elements are necessary in one part of the system, the same element should be used elsewhere in the design as much as possible, the objective being to reduce the number of different parts.
In doing the design the designer must take into account all the conditions defining the operate ability of the machine, develop the systems of lubrication and cooling, assembly and disassembly, and attachment of adjacent parts (drive shafts, piping, electric wires, etc.); provide for convenient maintenance, inspection and adjustment of the mechanism; choose correct materials for the main components; think of methods for improving the machine's durability and wear-resistance of rubbing surfaces, and methods of corrosion protection; investigate and determine the limits of the machine when operating under forced conditions.
Composition does not always proceed smoothly. Often during designing some small defects, overlooked in the first estimates, are revealed. For their elimination it sometimes turns out necessary to return to schemes rejected earlier or to develop new ones.
Some units are not always successfully designed from the first. This should not discourage the designer--they have to devise some "tentative" alternatives and raise the design to the required level in the process of further activities. In such cases it is useful, to take a breathing space, after which, as a result of subconscious thought, the problem is often solved. After a while the designer looks at the outline drawing in another light, and sees the mistakes made at the first stage of the development of the main design idea.
Sometimes the designers unintentionally lose their objectiveness and do not see the drawbacks of their favorite variant or the potentialities of other versions. In such cases impartial opinions of outsiders, the advice of seniors and co-workers should be sought. Their fault-finding and criticism could turn out to be useful. Moreover, the sharper the criticism, the greater is the benefit derived.
At all stages of design manufacturers and operators should be consulted.
As general rule, the wider the consultation on the design, and the more attention the designer pays to the advice, the better will be the design.
The cost of designing is only a small portion of the machine manufacturing expenditures (excluding one-off or small-batch production products). In the final analysis, the greater the development work on the design, the more are the savings in the machine cost, time of manufacture and finishing, the better its quality and the greater the economic gains over the machine's service life.
If possible the layout is best drawn to a 1:1 scale. This enables a realistic presentation of machine proportions, and facilitates the selection of required dimensions and sections, their strengths and rigidities. In addition full scale, removes the necessity for a large number of dimensional specifications and simplifies later stages of design, in particular detailing, since dimensions can be taken directly from the full scale layout drawing.
Layouts on a reduced scale, particularly less than ½ scale, strongly impedes the design process, it distorts the proportions and reduces the clarity of the representation.
If a 1:1 scale is not practical, then at least critical parts and groups should be drawn full scale .
The design of simple systems may be developed in one projection if the drawing is sufficiently clear. The cross-sectional drawing can be interpreted in three-dimensions by ones imagination. However, with more sophisticated systems, this may cause serious errors; therefore, in such cases the design must be developed in several projections for clarity.
The development of a lay-out drawing is, a continuous process of search, trial, approximation, seeking alternatives. Alternatives are compared and the unsuitable rejected. Alternatives should be lightly added to the drawing and corrected when necessary, which means that an eraser is used more often than a pencil.
Cross sections can be left unhatched, or if hatched, only free hand. Time is not wasted on drawing standard parts in detail. Typical components and units (fasteners, packing, springs, antifriction bearings, etc.) should be depicted simply.
Contour outlining, hatching, listing and particulars of standard small parts are made at the final stage, when the layout is ready for discussion.
Often development drawings are free hand, the design drawn with a pencil on a sheet of squared paper. Such drawings have great advantages as to capacity, flexibility and easiness of introducing corrections.
This method is especially useful for showing smooth outlines characteristic of modern designs.
The method is convenient for designers having certain aptitudes for drawing. Some designers are capable, when applying this method, of preparing in a few hours complete arrangements, which can be handed over for detailing.
