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Pictorial drawings are a type of technical illustration that shows several faces of an object at once. Such drawings are used by any industry that designs, sells, manufactures repairs, installs or maintains a product. Axonometric and oblique pictorial drawings use a parallel projection technique and are frequently used in technical documents, sales literature, maintenance manuals, and documentation supplements in engineering drawings. Perspective drawings use a converging projection technique and are more commonly found in architectural drawings.
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AXONOMETRIC DRAWINGS |
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| 9.1 The Greek word axon means axis and metric means to measure. Axonometric projection is a parallel projection technique used to create pictorial drawings of objects by rotating the object on an axis relative to a projection plane to create a pictorial view. |
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| 9.2 Axonometric projection is one of four principle projection techniques: Multiview, axonometric, oblique, and perspective. In multiview, axonometric, and oblique projection the observer is theoretically infinitely far away from the projection plane. The differences between a multiview drawing and an axonometric drawing are that, in a multiview, only two dimensions of an object are visible in each view and more than one view is required to define the object; whereas, in an axonometric drawing, the object is rotated about an axis to display all three dimensions, and only one view is required. |
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| 9.3 Axonometric drawings are classified by the angles between the lines comprising the axonometric axes. When all three angles are unequal the drawing is classified as a trimetric. When two of the three angles are equal the drawing is classified as a dimetric. When all three angles are equal the drawing is classified as an isometric. |
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| 9.4 Although there are an infinite number of positions that can be used to create such a drawing only are few are used. |
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ISOMETRIC AXONOMETRIC PROJECTIONS |
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| 9.5 An isometric view of an object is created by rotating it 45 degrees about a vertical axis, then tilting forward until the body diagonal of the cube (A-B) appears as a point in the front view. The angle the cube is titled forward is 35 degrees 16 minutes. The three corners meet to form equal angles of 120 degrees which is called the isometric axis. All the edges of the cube are parallel to the edges that make up the isometric axis since projections of parallel lines are parallel. Any line that is parallel to one of the legs of the isometric axis is called an isometric line. The planes of the faces of the cube and all planes parallel to them are called isometric planes. |
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| 9.6 The forward tilt of the cube causes the edges and planes of the cube to become foreshortened as it is projected onto the picture plane. Thus the projected lengths are approximately 80% of the true lengths and an isometric projection ruler must be used. If the drawing is drawn at full scale it is called an isometric drawing. Isometric drawings are almost always preferred over isometric projection for engineering drawings, because they are easier to produce. |
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| 9.7 The development of an isometric scale produced on paper using a regular scale. |
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| 9.8 Size comparison of isometric drawing and true isometric projection. |
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ISOMETRIC AXONOMETRIC DRAWINGS |
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| 9.9 Isometric axes can be positioned in a number of different ways to create different views of the same object. |
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| 9.9A Regular isometric looking down on the top of the object. |
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| 9.9B Reversed axis isometric is developed by looking up on the bottom of the object. |
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| 9.9C Long axis isometric is developed by looking from the right with one axis drawn at 60 degrees to the horizontal. |
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| 9.10 Any line that runs parallel to any of the isometric axes is called an isometric line. Any line that does not run parallel to isometric axes is called a non-isometric line. Non-isometric lines are inclined and oblique object lines that cannot be measured directly when creating an isometric drawing but must be created by locating two endpoints. |
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| 9.11 A cube is represented in an isometric drawing. The three faces on the isometric cube are called isometric planes. Isometric planes are surfaces which are parallel to the isometric surfaces formed by any two adjacent isometric axes. |
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| 9.12 Planes which are not parallel to any isometric plane are called non-isometric planes. |
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STANDARDS FOR ISOMETRIC DRAWINGS |
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| 9.13 In isometric drawings hidden lines are omitted unless absolutely necessary to completely describe the object. Normally, most isometric drawings will not have any hidden lines. You can avoid using hidden lines if the most descriptive viewpoint is chosen. However, there are times when the object has some features which cannot be described no matter which isometric viewpoint is taken. |
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| 9.14 In isometric drawings center lines are drawn if symmetry must be shown or for dimensioning. Normally, center lines are not used on isometric drawings. |
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| 9.15 Dimensioned isometric drawings used for production purposes must be ANSI standard, with dimension and extension lines and lines to be dimensioned lying in the same plane. |
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| 9.16 Dimensioned drawings used for illustration purposes may use the aligned method. |
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| 9.17 Creating an isometric drawing using the boxing-in method. |
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| 9.18-19 Constructing non-isometric lines by locating points in isometric. |
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| 9.20 Creating an isometric view of an object with an oblique plane. |
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| 9.21 To draw an angle in an isometric drawing, locate the endpoints of the lines that form the angle and draw the lines between the endpoints. |
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| 9.22 Irregular curves are drawn in isometric by constructing points along the curve in the multiview drawing which are then located in the isometric view. These points are then connected with an irregular curve drawing instrument. |
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| 9.23 Circles that lie on any face of an isometric cube will appear as ellipses. |
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| 9.24 The location of center lines and the major and minor axes of isometric ellipses. |
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| 9.25 Constructing a true isometric ellipse. |
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| 9.26 Constructing an ellipse using the four-center ellipse construction method. |
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| 9.27 Four-center ellipses drawn on the three faces of an isometric cube. |
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| 9.28 The comparison between a true ellipse and one constructed by the four-center method. |
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| 9.29 Application for which four-center technique is not used because of accuracy concerns. |
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| 9.30-31 Drawing isometric ellipses using an isometric ellipse template. |
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| 9.32 An ellipse is drawn on an inclined plane of an isometric drawing by plotting a points on a grid that is on the non-isometric plane. |
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| 9.33 Since arcs are partial circles, they appear in isometric drawings as partial isometric ellipses. |
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| 9.34 Constructing a curved intersection on an isometric drawing. |
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| 9.35-36 The isometric development of a sphere. |
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| 9.37 Section views are used to reveal interior features of objects. |
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| 9.38 Development of a full-section isometric view. |
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| 9.39 Development of a half-section isometric. |
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| 9.40 Screw threads are represented by a series of equally spaced isometric ellipses whose major diameter is equal to the diameter of the screw. |
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| 9.41-42 Representation of holes and rounds in isometric drawings. |
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| 9.43-44 Isometric assembly drawings used for production purposes normally have circles, called balloons, that contain numbers and are attached to leader lines, point to the various parts. |
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| 9.45 An isometric grid is a grid paper set using the isometric axes with vertical and diagonal lines. |
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DIMETRIC PROJECTION |
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| 9.46 Dimetric projection is an axonometric representation in which two of the axes are at equal angles to the plane of projection and two of the angles are equal and total more than 90 degrees. |
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| 9.47-8 It is worth noting that when laying out a dimetric drawing, the two equal axes can be laid out on either side of the vertical to create a symmetric dimetric projection. If one of the equal axes and the third axis are placed on either side of the vertical, then an asymmetric dimetric projection is created. |
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| 9.49 The odd axis can either have more or less exposure than the other two. For example, if the object has a single prominent face, then the odd axis should have more exposure. |
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| 9.50 Laying out a dimetric drawing is more complicated than an isometric because two different scales must be used. Dimetric scales and ellipse templates can be of great assistance. |
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TRIMETRIC PROJECTION |
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| 9.51-4 Trimetric projection is the most realistic of the axonometric projections, but is also the hardest to draw. In a trimetric projection, all three angles and scales are unequal. |
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OBLIQUE DRAWINGS |
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Oblique drawings are a form of pictorial drawings in which the most descriptive or natural front view is placed parallel to the plane of projection. |
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| 9.55 Oblique projection is used as the basis for both oblique drawing and oblique sketching. However, oblique projection and to a large extent oblique drawing, is not as commonly used as other types of pictorials because of the excessive distortion that occurs. Because of their simplicity, many times oblique sketches are used to communicate ideas. |
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| 9.56 Oblique projection is a unique form of parallel projection. As the name indicates, oblique projection results when the projectors are parallel to each other but at some angle other than perpendicular to the projection plane. If the principal view of the object is placed such that its surfaces are parallel to the projection plane, the resulting projection is an oblique pictorial. Historically, the most descriptive face of an object in oblique projection has been placed parallel to the frontal plane. |
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| 9.57-8 A comparison of orthographic projection and oblique projection is illustrated. The actual angles that the projectors make with the plane of projection in oblique projection is not significant, thus different angles can be used. However, angles for receding edges of between 30° and 60° are preferable because they offer minimum distortion of the object. |
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| 9.59 The cavalier oblique is drawn true length along the receding axis. The cabinet oblique is drawn half the true length along the receding axis. The general oblique can be drawn anywhere from full to half length along the receding axis. |
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| 9.60 The half-size receding axis on the cabinet oblique reduces the amount of distortion. |
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| 9.61 The various angles for a cavalier oblique drawing. |
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| 9.62 Any face of an object that is placed parallel to the frontal plane in oblique projection will be drawn true size and shape. Thus, the first rule in creating an oblique drawing is to develop the drawing so that cylinders or irregular surfaces are placed parallel to the frontal plane. This allows these features to be drawn quicker and without distortion. |
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| 9.63 A second rule in developing oblique drawings is that the longest dimension of an object should be located parallel to the frontal plane. |
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| 9.64 If there is conflict between these two rules, always draw the cylindrical or irregular surfaces parallel with the frontal plane because representing this geometry without distortion is more advantageous. |
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| 9.65 Constructing an oblique drawing using the box technique. |
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| 9.66 If an object is comprised mostly of full or partial cylindrical shapes, place these shapes in the frontal plane so that they will be drawn true size and shape. |
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| 9.67 It is not always feasible to locate an object so that all of its cylindrical features are parallel to the frontal plane. |
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| 9.68 Constructing an oblique ellipse using the alternate-four center method. |
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| 9.69 Cylinders, circles, arcs, and other curved or irregular features can be drawn point by point using offset coordinates. |
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| 9.70 Construction of an ellipse using the offset coordinate method. |
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| 9.71 Normal angular measurements can only be made in oblique drawing when the plane that contains the angle is parallel to the frontal plane. If the angle lies in one of the oblique receding planes, it is necessary to convert the angle into linear measurements. |
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| 9.72 The same conventions used to develop different section view types in isometric drawing will be used in oblique drawing. |
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| 9.73 Screw threads must be equally spaced along the center lines of the thread, but the spacing need not equal the actual pitch. |
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| 9.74 In oblique drawings, dimensions lie in the plane of the surface to which they apply, and unidirectional text placement is used. |
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SUMMARY |
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The three classifications of pictorial drawings are axonometric, oblique, and perspective. Isometric drawings are the most popular among the various axonometric drawings, because they are the easiest to create. Both axonometric and oblique drawings use parallel projection. As the axis angles and view locations are varied, different pictorial views of an object can be produced. Perspective drawings use converging lines to produce a pictorial view. The converging lines recede to vanishing points which produce a realistic looking image. Perspectives are commonly used in architectural work to create realistic scenes of buildings and structures. |