Basics of Engineering Working Drawings
Basics of Engineering Working Drawings
Engineering drawing is graphics. It is also known as technical drawing. It is an effective way of communicating technical ideas and is a necessary tool in engineering design where the majority of the design processes are graphically based. Engineering drawings are used in the design process for visualization, communication, and documentation.
Engineering drawing is a two-dimensional representation of three-dimensional objects. In general, it provides necessary information about the shape, size, surface quality, material, and manufacturing process, etc. of the object. It is the graphic language from which a trained person can visualize objects. Engineering drawing is called the universal language of engineers.
Graphics or drawing is the language of the designer, technician, and engineer. It is used for the communication of the designs and construction details to others. The language of graphics is written in the form of drawing which represents the shape, size, and specifications of a physical object. The language is read by interpreting drawing so that the physical object can be constructed exactly as originally conceived by the designer.
Graphics is a visual communication language which include images, text, and numeric information. Graphics communication using engineering drawings and models is a clear and precise language with definite rules which are to be followed for successful engineering design. Graphic communication is used in every phase of engineering design starting from concept design to the production stage.
Drawing is one of the oldest forms of communication, dating back even farther than verbal communication. Engineering drawing has its origin sometime in 500 BCE in the regime of King Pharos of Egypt when symbols were used to convey the ideas among people.
There are two major types of drawings namely (i) artistic drawings, and (ii) technical drawings. Artistic drawings are a form of free-hand representation which makes use of pictures to provide a general impression of the object being drawn. There are no hard rules or standards in the preparation of artistic drawings. They are simply drawn by the artists, based more or less on their talent and skills. Although these drawings are frequently very attractive, they find very limited use in the world of science. On the other hand, technical drawings are detailed drawings drawn accurately and precisely. They are pictures which have been prepared with the aid of mathematical instruments or with the aid of computer in order to record and transmit technical information. They provide an exact and complete description of things which are to be built or produced. Technical drawings do not portray the objects the way they directly appear to the eye. Instead, they make use of several specialized symbols and conventions in order to transmit technical information clearly and exactly. For understanding and correctly interpreting the technical drawings, people need to acquaint themselves with the fundamentals of the technical drawings.
Technically, a drawing can be defined as ‘a graphic representation of an idea, a concept, or an entity which actually or potentially exists in life’. The drawing itself is a way of communication all necessary information about an abstract, such as an idea or concept or a graphic representation of some real entity, such as a spare part, an equipment, an assembly, a system, a facility, a tool, or a structure. The drawings prepared by any technical person is to be clear, unmistakable in meaning, and there is not to be any scope for more than one interpretation.
A technical person can use the graphic language as powerful means of communication with others for conveying ideas on technical matters. However, for effective exchange of ideas with others, the engineer is required to have proficiency in (i) language, both written and oral, (ii) symbols associated with basic sciences, and (iii) the graphic language. Engineering drawing is a suitable graphic language from which any trained person can visualize the needed object. Since an engineering drawing displays the exact picture of an object, it obviously conveys the same ideas to every trained eye. Irrespective of language barriers, the drawings can be effectively used in other countries, in addition to the country where they have been prepared. Hence, the engineering drawing is the universal language of all engineers.
Engineering drawing is a technical language, which is well standardized. Engineering drawings are prepared as per the standards. There are national and international standards available for the preparation of drawings. ISO (the International Organization for Standardization) has several standards. The first is ISO 128. ISO 128 consists of a large number of parts. Other ISO standards related to technical drawing are ISO 216, ISO 129 along with its parts, ISO 406, ISO 1660, ISO 2203, ISO 3040, ISO 3098, ISO 4172, ISO 5261, ISO 5455, ISO 5456, ISO 5457, ISO 5459, ISO 5845 along with its parts, ISO 6410, ISO 6411, ISO 6412, ISO 6413, ISO 6414, ISO 6433, ISO 7083, ISO 7200, ISO 7437, ISO 7518, ISO 7519, ISO 8015, ISO 8048, ISO 8560, ISO 8826 along with its parts, ISO 9222 along with its parts, ISO 9958-1, ISO 9961, ISO 10209, ISO 10579, ISO 13567, ISO 13715, and ISO 15786.
ISO standards for technical drawings are available in a two volumes hand book namely (i) ISO Standards handbook: Technical drawings, Volume 1: Technical drawings in general, and (ii) ISO Standards handbook: Technical drawings, Volume 2: Mechanical engineering drawings; Construction drawings; Drawing equipment.
The ISO most recommended paper sizes for technical drawings are known as A-format series. Other series, like the B-series, are of lesser importance. In the A-format series, the largest size is A0. The size of an A1 paper is half the size of A0, while A2 is half the size of A1, and so forth. It is to be noted that a higher order paper size (which is always smaller in size) is obtained by simply halving the preceding size along its longer side. For technical drawings A4 is considered to be the smallest paper size. Smaller-sized A-format series papers (i.e., A5, and A6, etc.) are very rarely used for technical drawings.
The discrepancy between the actual sizes of objects and the size of the papers which is used for drawing necessitates to prepare drawings which are either smaller or bigger in size than the actual objects. This is only possible through the use of scales. A scale is simply the ratio of the linear dimension appearing on the drawing compared to the corresponding linear dimension on the object. A scale has no units as it is simply a ratio (i.e., dimension on drawing: dimension on object). Scales are used either for enlargements or reductions. The scale of 1:1 implies the object has been drawn to true size. A scale of say 2:1 implies that the object has been enlarged twice its true size. A scale of 1:2 implies that the object has been reduced to its half size.
The drawings can be made free-hand, by mechanical tools, or by computer methods. Working drawings are the set of technical drawings used for the production of the product. They contain all the necessary information needed to produce and assemble a product. They are a means of clearly and concisely communicating all of the information necessary to transform an idea or a concept in to reality. Hence, a technical drawing frequently contains more than just a graphic representation of its subject. It also contains dimensions, notes, and specifications.
CAD (computer-aided design) is the use of computer-based software to aid in design processes. CAD software is frequently used by different types of engineers and designers. CAD software can be used to create two-dimensional (2-D) drawings or three-dimensional (3-D) models.
Presentation of engineering drawings
Technical drawings are based on the fundamental principle of projection. A projection is a drawing or representation of an entity on an imaginary plane or planes. An object has three dimensions, namely length, width and thickness. The projections of the object convey all the three dimensions, along with other details of the object on a sheet of paper. The projection is defined as a representation of an object on a two-dimensional plane.
A projection involves four components consisting of (i) the actual object which the drawing or projection represents, (ii) the eye of the viewer looking at the object, (iii) the imaginary projection plane, and (iv) imaginary lines of sight called projectors. It can be obtained by viewing the object from the point of sight and tracing in correct sequence, the points of intersection between the rays of sight and the plane on to which the object is projected.
The two broad types of projections, both with several sub-classifications, are orthographic (parallel) projection, and perspective projection. Orthographic projection, common method of representing three-dimensional objects, normally by three two-dimensional drawings in each of which the object is viewed along parallel lines that are perpendicular to the plane of the drawing. Perspective projection is a non-linear projection where three-dimensional objects are projected on a picture plane. This has the effect that distant objects appear smaller than nearer objects.
Perspective projections are axonometric (pictorial) projections. These are drawings in which the object is drawn in three dimensions (3-D), i.e., three sides of the object appear in one drawing. Normally only one drawing is prepared / used. They are used extensively in artistic drawing. A three-dimensional view shows length, width, and height of the object simultaneously. It provides only a general impression of the shape of the object by allowing the observer to see three of its sides as well as its three overall dimensions. In these drawings, an exact and complete description of its shape, particularly as applied to its slots on the underside is lacking.
Two standards are presently used for axonometric projections. These are namely (i) diametric projection, and (ii) isometric projection. In diametric projection, all dimensions along two axes are drawn to ‘true size’ The dimensions along the third axis are halved. This projection is preferred when one view of the object is to be emphasized than the other two views (i.e., when one view is of more interest than the other views). In isometric projection, all dimensions along all the three axes are drawn to ‘true size’. Isometric projection is preferred when the three views of the object are of equal importance for accurate presentation of the object.
A projection is called orthographic projection when the point of sight is imagined to be located at infinity so that the rays of sight are parallel to each other and intersect the plane of projection at right angle to it. The principles of orthographic projection can be followed in four different angles or systems namely, first, second, third, and fourth angle projections. A projection is said to be first, second, third, or fourth angle when the object is imagined to be in the first, second, third, or fourth quadrant respectively
For the presentation of an object in a unique way, normally more views (and sometimes sections) are needed. In orthographic projection, the views are seen in directions which make right angles (i.e., 90-degree) with each other. The number of views needed are to be sufficient to represent the object completely and conveniently, but it is to be kept to the minimum. For the majority of the purposes, three views are normally sufficient.
Engineering drawings normally utilize orthographic views rather than axonometric views. Orthographic view helps to record the shapes of objects exactly and completely. It is a two-dimensional (2-D) drawing. It shows only one side of an object and two of its overall dimensions. A minimum of two views are needed to show the three dimensions of any object and hence to describe its shape completely. Some features of the object which do not directly appear on viewing the object from any specific direction (known as hidden details) are shown on the drawing as dotted lines.
A sectional view obtained by assuming that the object is completely cut by a plane is called a full section or sectional view. This sectioned view provides all the inner details, better than the not-sectioned view with dotted lines for inner details. The cutting plane is normally represented by its trace (vertical trace) in the view from the front and the direction of sight to obtain the sectional view is represented by the arrows.
It is to be noted that, in order to obtain a sectional view, only one half of the object is imagined to be removed, but is not actually shown removed anywhere except in the sectional view. Further, in a sectional view, the portions of the object which have been cut by the plane are represented by section lining or hatching. The view also contains the visible parts behind the cutting plane. Sections are used primarily to replace hidden line representation, hence, as a rule, hidden lines are omitted in the sectional views.
A half sectional view is preferred for symmetrical objects. For a half section, the cutting plane removes only one quarter of an object. A half sectional view is used to indicate both interior and exterior details in the same view. Even in half sectional views, it is a good practice to omit the hidden lines. It is to be noted that a centre line is used to separate the halves of the half section.
Auxiliary sections can be used to supplement the principal views used in orthographic projections. A sectional view projected on an auxiliary plane, inclined to the principal planes of projection, shows the cross-sectional shapes of features such as arms, ribs, and so on.
For describing any object completely through its orthographic projections, it is important to select a number of views. The number of views needed to describe any object depends upon the extent of complexity involved in it. The higher the symmetry, the lesser is the number of views needed.
The views of a given object are to be positioned on the drawing sheet so as to give a good and balanced appearance. Keeping in view, (i) number of views, (ii) scale, and (iii) space between the views, the person preparing the drawing is to decide about the placement of views on the drawing sheet. Sufficient space between the views is to be provided to facilitate placement of dimensions, nd notes etc., on the drawing without over-crowding.
In orthographic projection, three views are normally drawn. The three chosen views can be any of the six hypothetical faces of the object. The front view which is the view of highest importance in representing the object (normally the most complicated of all the views) as seen when the object is placed directly in front of the viewer. This view normally serves to represent the object (e.g., a work piece) in the most common position in which it is used. It is normally the first view to be drawn with other views following.
The rear view is directly opposite to the front view and is at the back of the object. The right-hand side view and the left-hand side view appear on the right side and left side of the object respectively. The top and bottom views are at the top side and bottom side of the front view. These six views are at right angles to one another.
Auxiliary view utilizes an additional projection plane other than the common planes in a multi-view. Since the projections of an object need to show the true shape and size of the object, the projection plane is to be parallel to the object surface. Hence, any surface which is not in line with the three major axis needs its own projection plane to show the features correctly.
A detail view is a separate large-scale drawing view of a small section of another view. It is normally used to show features which are small relative to the size of the part or assembly being depicted by the drawing. A detailed view is designated by a letter and its scale is indicated.
Broken views are normally used for parts having one of its dimensions considerably larger than the other dimensions (such as long shafts) in order to make it possible to display the drawing view in a larger scale on a smaller size drawing sheet. In a broken view, portions of the view which do not contain any details are broken out (removed) and the remaining portions of the view are brought closer together.
Two standards are normally in use in orthographic projection of drawings, namely (i) the first angle projection, and (ii) the third angle projection. It is to be noted that corresponding views are identical in both methods of projection except for their relative positions on the drawing paper. In the first angle projection, the front view is the basis (reference) and the other views are drawn as ‘shadows’ of that view. That is, the left- hand side view for example is drawn on the right side of the front view. Similarly, the top view (plan) is drawn at the bottom of the front view etc. In the third angle projection the front view is the basis (just as before) but the other views are drawn as ‘reflections’ of that view. The left-hand side view is drawn on the left-hand side of the front view. Similarly, the top view (plan) is drawn at the top of the front view. Fig 1 shows orthographic projections.
Fig 1 Orthographic projections
Orthographic views when carefully selected, can reveal the external features of even the most complicated objects. However, there are objects with complicated interior details and when represented by hidden lines, cannot effectively reveal the true interior details. This can be overcome by representing one or more of the views ‘in section’. A sectional view is obtained by imagining the object, as if cut by a cutting plane and the portion between the observer and the section plane being removed. Fig 2 shows the projection symbols.
Fig 2 Projection symbols
Classification of drawings
Equipment drawing – It is pertaining to equipment parts or components. It is presented through a number of orthographic views, so that the size and shape of the component is fully understood. Part drawings and assembly drawings belong to this classification. Fig 3a shows example of equipment drawing.
Fig 3 Examples of equipment and production drawings
Production drawing – It also referred to as working drawing. A component or part drawing is termed as a production drawing, if it facilitates its manufacture. It is an authorized document to produce the component on the shop floor. It furnishes all the dimensions, limits, and special finishing processes such as heat treatment, honing, griding, lapping, and surface finish etc., to guide the technician on the shop floor in producing the component. The title also mentions the material used for the product, and number of parts needed for the assembled unit etc. Since a technician normally makes one component at a time, it is advisable to prepare the production drawing of each component on a separate sheet. However, in some cases the drawings of related components can be given on the same sheet. Fig 3b shows an example of production drawing.
Component or part drawing – It is a detailed drawing of a component to facilitate its production. All the principles of orthographic projection and the technique of graphic representation are followed to communicate the details in a part drawing. A part drawing with production details is rightly called as a production drawing or working drawing.
Assembly drawing – It is a drawing which shows the different parts of an equipment in their correct working locations. Assembly drawings demonstrate how a number of separate sub-assemblies, parts, standard components, and specifications come together in a unified assembly. Normally speaking, an assembly drawing is used to show fit and function, and verify how a product is put together. In order to fulfill its purpose, assembly drawings are to provide sufficient information to enable the assembly of a component. Fig 4 shows example of an assembly drawing.
Fig 4 Example of assembly drawing
The important features of assembly drawings include (i) they have a number of views to show how parts fit together, (ii) they have normally section views to show how parts fit and to eliminate hidden details, (iii) they typically have dimensions to indicate range of motions and / or overall size of assembly for reference purposes, (iv) they have leader lines and balloons to identify individual components, (v) they have parts list (bill of materials, BOM) which is related to balloon numbers on drawing, and (vi) they can need multiple 3-D views (in different orientations) on a separate page for very large assemblies. There are several types of assembly drawings.
The first is the design assembly drawing. When an equipment is designed, an assembly drawing or a design layout is first drawn to clearly visualise the performance, shape, and clearances of different parts comprising the equipment.
The second is the detailed assembly drawing. It is normally made for simple equipments, comprising of a relatively smaller number of simple parts. All the dimensions and information necessary for the construction of such parts and for the assembly of the parts are given directly on the assembly drawing. Separate views of specific parts in enlargements, showing the fitting of parts together, can also be drawn in addition to the regular assembly drawing.
The third is sub-assembly drawing. Several equipments are assembled with many pre-assembled components as well as individual parts. These pre-assembled units are known as sub-assemblies. A sub-assembly drawing is an assembly drawing of a group of related parts, which form a part in a more complicated equipment. Example of such drawings is the tail-stock in a lathe.
The fourth is the installation assembly drawing. On this drawing, the location and dimensions of few important parts and overall dimensions of the assembled unit are indicated. This drawing provides useful information for assembling the equipment, as this drawing reveals all parts of an equipment in their correct working position.
The fifth is the assembly drawing for catalogues. Special assembly drawing is prepared for the equipment catalogue. This drawing shows the overall and principal dimensions and only those pertinent details and dimensions which are of interest to the potential purchaser of the equipment. Fig 5 shows example of catalogue drawing.
Fig 5 shows example of catalogue drawing
The sixth is the assembly drawing for instruction manuals. These drawings in the form of assembly drawings, are to be used when an equipment, shipped away in assembled condition, is knocked down in order to check all the parts before reassembly and installation elsewhere. These drawings have each component numbered on the job. Fig 6 shows example of assembly drawing for instruction manuals.
Fig 6 Example of assembly drawing for instruction manuals
The seventh is the exploded assembly drawing. In some cases, exploded axonometric views are supplied to meet instruction manual requirements. These drawings normally find a place in the parts list section of the instruction manual for the equipment. Drawings of this type can be easily understood even by those with lesser experience in the reading of drawings, since in these exploded views, the parts are positioned in the sequence of assembly, but separated from each other.
Exploded views are typically used in assembly drawings in order to show the relationship or order of assembly of the different parts. An exploded view shows the components of an object slightly separated by distance, or suspended in surrounding space in the case of a three-dimensional exploded diagram. In mechanical systems, normally the component closest to the centre is assembled first, or is the main part in which the other parts get assembled. Exploded views can also help to represent disassembly of parts, where the parts on the outside normally get removed first. Fig 7 shows example of exploded assembly drawing.
Fig 7 Example of exploded assembly drawing
The eighth is the schematic assembly drawing. It is very difficult to understand the operating principles of complicated equipment, merely from the assembly drawings. Schematic representation of the unit facilitates easy understanding of its operating principle. It is a simplified illustration of the equipment or of a system, replacing all the elements, by their respective conventional representations. Fig 8a shows example of schematic assembly drawing.
Fig 8 Examples of schematic assembly and machine shop drawings
The ninth is machine shop drawing (Fig 8b). Rough castings and forgings are sent to the machine shop for finishing operation. Since the machinist is not interested in the dimensions and information of the previous stages, a machine shop drawing frequently gives only the information necessary for machining. Based on the same principle, one can have forge shop drawing, pattern shop drawing, and sheet metal drawing etc.
The tenth is patent drawing. When new equipments or devices are invented, patent drawings come into existence, to illustrate and explain the invention. These are normally axonometric drawings and are to be self-explanatory. It is necessary that the patent drawings are mechanically correct and include complete information of every detail of the invention. However, these drawings are not useful for production purposes. The salient features on the drawing are numbered for identification and complete description.
Drawing sheet layout
Standard layouts of drawing sheets are available in different national and international standards. Fig 9 shows typical standard layout of a drawing sheet, showing the drawing frame, a typical title block, part list (BOM) and revision block table.
Fig 9 Typical standard layout of a drawing sheet
It is a standard practice for a drawing frame to be printed on each sheet, defining a margin around the outside of drawing area. The margins of the drawing frames are standardized for each size of paper.
In every engineering drawing, a title block is included at the bottom right-hand corner. The title blocks are locally standardized but are to be designed in such a way that it can be easily understood.
The information needed in any standard title block is normally (i) name of the organization along with its logo, (ii) name of the object, (iii) number of the drawing (particularly useful for reference where more than one drawing are concerned, typically in assembly drawings), (iv) revision number, (v) format of the paper used (paper size), (vi) scale used, (vii) dimensioning unit (normally millimetres, mm), (viii) symbol for the method of projection used, (ix) date when the drawing was finished, (x) name of the person who has prepared the drawing, , (xi) name of the person who has checked the drawing, and (xii) remarks. Fig 10 shows examples of title and revision blocks.
Fig 10 Examples of title and revision blocks
In addition to the information above, for drawings produced using CAD software, it is highly desired to have some additional information in the title block which typical consists of (i) the name of CAD software used and its version, (ii) the name of the drawing file, (iii) the name of the source part or assembly line, and (iv) the units of dimensions (if the drawing is mistakenly printed on a different paper size, the scale becomes meaningless).
A revision table is normally located in the upper right of the drawing frame. All the modifications to the drawing are documented here.
The parts list is a necessary component in any assembly drawing. It is normally drawn on top of the title block. The parts list normally has the same width as the title block. The height depends on the number of items to be included. The information which is normally included in the parts list is (i) part reference number, (ii) name of the part, (iii) number of parts needed in an assembly, (iv) material used to manufacture the part, (v) indication of standard or dimension, and (vi) drawing number. When the parts list is very large, a separate drawing sheet can be used for the parts list alone.
A drawing can be divided up into a grid using letters and numbers. When zoning is used, it is located inside the drawing frame. Zoning is normally used for large size drawing sheets where it allows easy reference to different parts of the drawing by referencing a coordinate such as A3, or B2 etc.
Information other than pictorial views and dimensions necessary for completing a drawing is classified as ‘notes’. Notes are normally placed in the lower left corner of the drawing sheet and they are numbered consequently downwards.
Preparation of engineering drawings
Engineering drawings of real-life objects are normally prepared in three stages namely (i) sketches, (ii) hand drafts, and (iii) detail drawings. This sequence is not very binding but majority of the persons find it very useful to work in that order.
Sketching is almost always the first step in the preparation of engineering drawings. The work-piece (object) is carefully studied and all the necessary dimensions are measured. The views which are necessary to completely describe the object are very roughly drawn (free hand). All dimensions are indicated on the sketch as deemed necessary. In majority of the situations, axonometric views of the object are drawn.
Sketches are never submitted anywhere. They are just drawn to assist the person at a later time during the preparation of the standard drawings. That is, the only purpose of preparing sketches is to enable the person to transfer useful information from the spot to a convenient place where the person can actually prepare acceptable engineering drawings. For this reason, there are no hard rules or general guidelines with respect to sketches. However, sketches are to be drawn accurately so that they can prove useful at a later stage.
Hand drafts are actually proper engineering drawing drawn free hand. The similarity between a sketch and a hand draft is that they are both drawn free-hand. But unlike sketches, hand drafts are drawn following all the rules and guidelines governing engineering drawings. Hand drafts are drawn roughly to scale and all the necessary dimensions are indicated. Only those views, which are necessary to completely describe the object, are presented. The hand draft is then given to the person who ‘copies’ the same into a proper engineering drawing. This ensures that the person produces an engineering drawing in exactly the same way as the object appears. In short, a hand draft saves the time and effort which is otherwise needed to prepare a proper engineering drawing.
Detail drawings (also known as working drawings) are engineering drawings presenting single items (object / equipment component / work-piece etc.). They are meant to enable the person in a workshop to produce (by machining / casting / forging / fabricating etc.) the desired item. Such details as dimensional tolerances, surface finish, special treatments, material to be used for the component etc. are specified. The number of views to be presented depends on the complexity of the item. In several situations, sectional views are included to show hidden details which cannot conveniently and explicitly appear in any external view.
The scale used for the detail drawing is to allow a clear understanding of the drawing (i.e., use enlargements and / or reductions as one finds it appropriate). Sometimes just a small portion of the drawing is enlarged to show all the details. Such partial enlargements are normally included in the same drawing. When the item is drawn much enlarged, it is desired to add a picture (drawing) to true size for more information.