What is Engineering Drawing basics

how to read engineering drawings pdf and how to draw engineering drawings basics, what are engineering drawing conventions and what is engineering drawing and graphics
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Published Date:03-08-2017
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5 Engineering drawing When you have read this chapter you should understand how to: • Interpret (read) drawings in first and third angle projection. • Sketch and dimension mechanical components in first and third angle projection. • Sketch mechanical components in isometric and oblique projection. 5.1 Engineering drawing Figure 5.1(a) shows a drawing of a simple clamp. This is a pictorial (introduction) drawing. It is very easy to see what has been drawn, even to people who have not been taught how to read an engineering drawing. Unfortunately such drawings have only a limited use in engineering. If you try to put Figure 5.1 Clamp: (a) pictorial drawing; (b) orthographic drawing; (c) fully dimensioned in millimetres124 Engineering Fundamentals all the information that is required to make the clamp onto this drawing it would become very cluttered and difficult to interpret. Therefore we use a system called orthographic drawing when we make engineering drawings. An example of an orthographic drawing of our clamp is shown in Fig. 5.1(b). We now have a collection of drawings each one looking at the clamp from a different direction. This enables us to show every feature of the clamp that can be seen and also some things that cannot be seen (hidden details). Features that cannot be seen are indicated by broken lines. Finally we can add the sizes (dimensions) that we need in order to make the clamp. These are shown in Fig. 5.1(c). A drawing that has all the information required to make a component part, such as Fig. 5.1(c), is called a detail drawing, but more of that later. 5.2 First angle There are two systems of orthographic drawing used by engineers: orthographic drawing • First angle or English projection. • Third angle or American projection. In this section we are going to look at first angle projection. We are again going to use the clamp you first met in Fig. 5.1(a). We look at the clamp from various directions. • Look down on the top of the clamp and draw what you see as shown in Fig. 5.2(a). This is called a plan view. • Look at the end of the clamp and draw what you see as shown in Fig. 5.2(a). This is called an end view. • Look at the side of the clamp and draw what you see as shown in Fig. 5.2(a). Although this is a side view, it is given a special name. It is called an elevation. • You can now assemble these views together in the correct order as shown in Fig. 5.2(b) to produce a first angle orthographic drawing of the clamp. As well as the things that can be seen from the outside of the clamp, we also included some ‘hidden detail’ in the end view and elevation. Hidden detail indicates a slot through the clamp in this example. The slot is shown by using broken lines. We did this because if we had shown the slot as an oval in the plan view it could have meant one of two things: • A slot passing right through the clamp. • A slot recessed part way into the clamp. It could not have been an oval-shaped lump on top of the clamp as this would have shown up in the end view and in the elevation. Sometimes only two views are used when the plan and elevation are the same. For example, a cylindrical component such as a shaft. Figure 5.3Engineering drawing 125 Figure 5.2 Principles of drawing in first angle projection: (a) plan view, end view, and elevation (side view); (b) collected views together make up an orthographic drawing Figure 5.3 First angle drawing of a cylindrical component: the elevation and plan views are the same and need only be drawn once126 Engineering Fundamentals Figure 5.4 Making a drawing in first angle projection: (a) ground line and planes; (b) initial construction lines; (c) line in the outline (outline is twice the thickness of the construction lines)Engineering drawing 127 shows that an elevation and an end view provide all the information we require. Finally let’s see how a first angle orthographic drawing is constructed. ◦ • First draw the ground lines and a plane at 45 as shown in Fig. 5.4(a). • Then start to draw in the construction lines faintly using lines that are half the thickness of the final outline. Figure 5.4(b) shows the construction lines in place. • Then follow each construction line round all the views in order to avoid confusion. • Finally, we ‘line in’ the outline so that it stands out boldly as shown in Fig. 5.4(c). 5.3 Third angle To draw the clamp in third angle (American) orthographic projection, you orthographic drawing merely have to rearrange the relative positions of the views. Each view now appears at the same side or end of the component from which you are looking at it. This is shown in Fig. 5.5(a). That is: • Look down on the clamp and draw the plan view above the side view or elevation. • Look at the left-hand end of the clamp and draw the end view at the same end. (a) Figure 5.5 (a) Principles of drawing in third angle projection; (b) projection symbol128 Engineering Fundamentals (b) Figure 5.5 (continued) So what is the advantage of third angle projection? Consider the general arrangement drawing for an airliner drawn to a fairly large scale so that fine detail can be shown. In first angle projection, the end view looking at the nose of the aircraft would be drawn somewhere beyond the tail. An end view looking at the tail of the aircraft would be drawn somewhere beyond the nose. It is much more convenient to draw the end view of the nose of the aircraft at the nose end of the elevation. Also, it is more Figure 5.6 Auxiliary view: EL = elevation; EV = end view; PL = plan; AV = auxiliary viewEngineering drawing 129 convenient to draw an end view of the tail of an aircraft next to the tail of the elevation. To avoid confusion, always state the projection used on the drawing. Sometimes the projection used is stated in words, more usually it is indi- cated by the use of a standard symbol. Figure 5.5(b) shows the combined projection symbol and how it is used. So far we have only considered features that are conveniently arranged at right angles to each other so that their true shape is shown in the plan, elevation or the end view. This is not always the case and sometimes we have to include an auxiliary view. This technique is important in the production of working drawings so that the positions of features on the surface that is inclined not only appear undistorted but can also be dimensioned. Figure 5.6 shows a bracket with an inclined face. When it is drawn in first angle projection, it can be seen that the end view showing the inclined surface and its features is heavily distorted. However, these features appear correct in size and in shape in the auxiliary view (AV) which is projected at right angles (perpendicular) to the inclined face. 5.4 Conventions An engineering drawing is only a means of recording the intentions of the designer and communicating those intentions to the manufacturer. It is not a work of art and, apart from the time spent in its preparation, it has no intrinsic value. If a better and cheaper method of communication could be discovered, then the engineering drawing would no longer be used. We are already part way along this road with CAD where the drawings are stored digitally on magnetic or optical disks and can be transmitted between companies by the internet. However, hard copy, in the form of a printed drawing, still has to be produced for the craftsperson or the technician to work to. As an aid to producing engineering sketches and drawings quickly and cheaply we use standard conventions. These are recognized internationally and are used as a form of drawing ‘shorthand’ for the more frequently used details. In the UK we use the British Standard for Engineering Drawing Prac- tice as published by the British Standards Institute (BSI). This stan- dard is based upon the recommendations of the International Standards Organization (ISO) and, therefore, its conventions and guidelines, and drawings produced using such conventions and guidelines are accepted internationally. 5.4.1 Types of line Figure 5.7 shows the types of line recommended by BS 308, together with some typical applications. The following points should be noted in the use of these lines. • Dashed lines should consist of dashes of consistent length and spac- ing, approximately to the proportions shown in the figure.130 Engineering Fundamentals Figure 5.7 Types of line and their applications • Thin chain lines should consist of long dashes alternating with short dashes. The proportions should be generally as shown in the figure, but the lengths and spacing may be increased for very long lines. • Thick chain lines should have similar lengths and spacing as for thin chain lines. • General. All chain lines should start and finish with a long dash. When thin chain lines are used as centre lines, they should cross one another at solid portions of the line. Centre lines should extend only a short distance beyond the feature unless required for dimensioning or other purposes. They should not extend through the spaces between the views and should not terminate at another line of the drawing. Where angles are formed in chain lines, long dashes should meet at the corners and should be thickened as shown. Arcs should join at tangent points. Dashed lines should also meet at corners and tangent points with dashes.Engineering drawing 131 5.4.2 Abbreviations for written statements Table 5.1 lists the standard abbreviations for written statements as used on engineering drawings. Some examples of their use are shown in Fig. 5.8. Some further examples will be given when we discuss the dimensioning of drawings. TABLE 5.1 Abbreviations for written statements Term Abbreviation Term Abbreviation Across flats A/F Hexagon head HEX HD British Standard BS Material MATL Centres CRS Number NO. Centre line CL or C Pitch circle diameter PCD L Chamfered CHAM Radius (in a note) RAD Cheese head CH HD Radius (preceding a dimension) R Countersunk CSK Countersunk head CSK HD Screwed SCR Counterbore C’BORE Specification SPEC Diameter (in a note) DIA Spherical diameter or radius SPHERE Ø or R Diameter (preceding a dimension) Ø Spotface S’FACE Drawing DRG Standard STD Figure FIG. Undercut U’CUT Hexagon HEX Figure 5.8 Examples of the use of standard abbreviations: (a) counterbored hole; (b) countersunk hole132 Engineering Fundamentals 5.4.3 Conventions Figure 5.9 shows some typical conventions used in engineering drawings. It is not possible, in the scope of this book, to provide the full set of conventions or to provide detailed explanations of the use. For this it is necessary to consult texts specializing in engineering drawing together with British Standard 308. The full standard is expensive but you should find the special abridged edition, BS PP7308: 1986: Engineering Drawing Practice for Schools and Colleges, adequate for your needs. This edition is published at a very affordable price. Figure 5.9 Typical conventions for some common featuresEngineering drawing 133 5.5 Redundant views It has been stated and shown earlier that where a component is symmet- rical you do not always require all the views to provide the information required for manufacture. A ball looks the same from all directions, and to represent it by three circles arranged as a plan view, an elevation and an end view would just be a waste of time. All that is required is one circle and a note that the component is spherical. The views that Figure 5.10 Redundant views: (a) first angle working drawing of a sym- metrical component (plan view redundant); (b) symmetrical component reduced to two views; (c) working drawing reduced to a single view by using revolved sections and BS convention for the square flange134 Engineering Fundamentals can be discarded without loss of information are called redundant views. Figure 5.10 shows how drawing time can be saved and the drawing sim- plified by eliminating the redundant views when drawing symmetrical components. 5.6 Dimensioning So far, only the shape of the component has been considered. However, in order that components can be manufactured, the drawing must also show the size of the component and the position and size of any features on the component. To avoid confusion and the chance of misinterpretation, the dimensions must be added to the drawing in the manner laid down in BS 308. Figure 5.11(a) shows how projection and dimension lines are used to relate the dimension to the drawing, whilst Fig. 5.11(b) shows the correct methods of dimensioning a drawing. Figure 5.11 Dimensioning: (a) projection and dimension lines; (b) correct and incorrect dimensioning 5.6.1 Correct dimensioning • Dimension lines should be thin full lines not more than half the thick- ness of the component outline. • Wherever possible, dimension lines should be placed outside the out- line of the drawing.Engineering drawing 135 • The dimension line arrowhead must touch but not cross the projec- tion line. • Dimension lines should be well spaced so that the numerical value of the dimension can be clearly read and so that they do not obscure the outline of the drawing. 5.6.2 Incorrect dimensioning • Centre lines and extension lines must not be used as dimension lines. • Wherever possible dimension line arrowheads must not touch the out- line directly but should touch the projection lines that extend from the outline. • If the use of a dimension line within the outline is unavoidable, then try to use a leader line to take the dimension itself outside the outline. 5.6.3 Dimensioning diameters and radii Figure 5.12(a) shows how circles and shaft ends (circles) should be dimen- sioned. It is preferable to use those techniques that take the dimension Figure 5.12 Dimensioning – diameters and radii: (a) dimensioning holes; (b) dimensioning the radii of arcs which need not have their centres located; (c) use of notes to save full dimensioning136 Engineering Fundamentals outside the circle, unless the circle is so large that the dimension will neither be cramped nor will it obscure some vital feature. Note the use of the symbol to denote a diameter. Figure 5.12(b) shows how radii should be dimensioned. Note that the radii of arcs of circles need not have their centres located if the start and finish points are known. Figure 5.12(c) shows how notes may be used to avoid the need for the full dimensioning of certain features of a drawing. Leader lines These indicate where notes or dimensions are intended to apply and end in either arrowheads or dots. • Arrowheads are used where the leader line touches the outline of a component or feature. • Dots are used where the leader line finishes within the outline of the component or feature to which it refers. 5.6.4 Auxiliary dimensions It has already been stated that, to avoid mistakes, duplicated or unnecess- ary dimensions should not appear on a drawing. The only exception to this rule is when auxiliary dimensions are used to avoid the calculation of, 25±0.1 50 ±0.1 50 ±0.1 (a) 125±0.1 75±0.1 25±0.1 (b) Figure 5.13 Cumulative error: (a) string (incremental) dimension- ing – cumulative tolerance equals sum of individual tolerances; (b) dimen- sioning from one common datum (absolute dimensioning) to eliminate cumulative effect (dimensions in millimetres)Engineering drawing 137 say, overall dimensions. Such auxiliary dimensions are placed in brackets as shown in Fig. 5.13. Auxiliary dimensions are also sometimes referred to as non-functional dimensions. 5.7 Toleranced It is true to say that if ever a component was made exactly to size no dimensions one would ever know because it could not be measured exactly. Having calculated the ideal size for a dimension, the designer must then decide how much variation from that size he will tolerate. This variation between the smallest and the largest acceptable size is called the tolerance. When toleranced dimensions are used, cumulative errors can occur wherever a feature is controlled by more than one toleranced dimension as shown in Fig. 5.13(a). It can be seen that chain dimensioning gives a build-up of tolerance that is greater than the designer intended. In this example the maximum tolerance for the right-hand hole centre is three times the individual tolerances. That is, the sum of the individual toler- ances is (±0.1) + (±0.1) + (±0.1) mm=±0.3 mm from the left-hand datum edge. This cumulative effect can be eliminated easily by dimen- sioning each feature individually from a common datum as shown in Fig. 5.13(b). It is not usually necessary to tolerance every individual dimension, only the important ones. The rest can be given a general tolerance in the form of a note in the title block as shown in Fig. 5.14. This general statement may refer either to open dimensions or it may say except where otherwise stated. In the examples shown, the general tolerance is 0.5 mm Figure 5.14 General tolerances138 Engineering Fundamentals with the limits stated as +0.3 mm and −0.2 mm in the example shown in Fig. 5.15(a) and with limits stated as ±0.2 mm in the example shown in Fig. 5.14(b). Both examples mean the same thing. Applied to an open dimension of 12 mm, the actual size is acceptable if it lies as shown in Fig. 5.14(c). 5.8 Sectioning Sectioning is used to show the internal details of engineering components that cannot be shown clearly by other means. The stages of making a sectioned drawing are shown in Fig. 5.15. It should be realized that the steps (a), (b) and (c) are performed mentally in practice and only (d) is actually drawn. X X (a) (b) (c) (d) Figure 5.15 Section drawing: (a) the clamp is to be sectioned along line X–X; (b) the cutting plane is pos- itioned on the line X–X as shown; (c) that part of the clamp that lies in front of the cutting plane is removed leaving the sectioned component; (d) sectioned orthographic elevation of the clamp shown in (a) – note that ◦ section shading lines lie at 45 to the horizontal and are half the thickness of the outline The rules for producing and reading sectioned drawings can be sum- marized as follows. • Drawings are only sectioned when it is impossible to show the internal details of a component in any other way. • Bolts, studs, nuts, screws, keys, cotters and shafts are not usually sectioned even when the cutting plane passes through them.Engineering drawing 139 • Ribs and webs are not sectioned when parallel to the cutting plane. • The cutting plane must be indicated in the appropriate view. • Hidden detail is not shown in sectioned views when it is already shown in another view. ◦ • The section shading (hatching) is normally drawn at 45 to the outline of the drawing using thin, continuous lines that are half the thickness ◦ of the outline. If the outline contains an angle of 45 then the hatching angle can be changed to avoid confusion. • Adjacent parts are hatched in opposite directions. To show more than two adjacent parts, the spacing between the hatched lines can be var- ied. A practical example of sectioning is shown in Fig. 5.16. Figure 5.16 Practical sectioning140 Engineering Fundamentals 5.9 Machining symbols Machining symbols and instructions are used to: • specify a particular surface finish; • determine a machining process; • define which surfaces are to be machined. Figure 5.17(a) shows the standard machining symbol (BS 308) and the proportions in millimetres to which it should be drawn. When applied to views of a drawing, as shown in Fig. 5.17(b), the symbol should be drawn as follows (in this context ‘normal’ means ‘at right angles to’): • Normal to a surface. • Normal to a projection line. • Normal to an extension line. • As a general note. Because a machining symbol is interpreted as a precise instruction, its form should be drawn carefully. Figure 5.17(c) shows three fundamental variations of the symbol. Figure 5.17 The machining symbol: (a) drawing a machining symbol; (b) applying the machining symbol as an instruction; (c) machining sym- bols; (d) specifying surfaces texture on a casting – dimensions omitted for clarityEngineering drawing 141 These symbols must be used carefully; one incorrect symbol or incor- rect application of a symbol can result in unnecessary manufacturing costs or even the scrapping of a component. 5.10 Types of To save time in the drawing office, most companies adopt a standardized engineering drawings and pre-printed drawing sheet as shown in Fig. 5.18 if manual draw- ing is still used. The layout and content will vary from company to company but, generally, such sheets will provide the following infor- mation: • The drawing number and title. • The projection used (first angle or third angle). • The scale. • The general tolerance. • The material specification. • Warning notes. Figure 5.18 Layout of drawing sheet142 Engineering Fundamentals • Any corrections or revisions, the date these were made, and the zone in which they occur. • Special notes concerning, for example, heat treatment, decorative, cor- rosion resistant or other surface finishes. If manual drawing is to be used then a pre-printed tracing sheet on trac- ing paper or plastic sheet will be used. The latter is more expensive but it is more durable if many copies of the drawing are required over an extended period of time. Modern drawing offices now use CAD systems. The standard layout is saved in the memory of the computer as a ‘tem- plate’ and can be called up by a keystroke whenever a drawing is to be made. 5.10.1 General arrangement drawings An example of a general arrangement (GA) drawing is shown in Fig. 5.19. It shows all the components correctly assembled, and lists all the parts required. For those parts that will be ‘bought in’, it will state the maker and catalogue reference for the benefit of the purchasing department. For those parts to be made in the factory, the detail drawing numbers will be provided together with the material specification and the quantity of parts required. General arrangement drawings do not Figure 5.19 General arrangement drawing

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