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The objects we look, around us, are in 3-Dimensional form. When we try to communicate the structure of objects to others then we take the help of pictures / pictorial drawings. These pictorial drawings are 'one plane' drawings because our mode of communication is paper which has only two dimensions and these drawings show the object approximately as it appears to the viewer.
In engineering, one plane drawings are extensively used in addition to the orthographic views of an object to give the best understanding. So the practice of drawing the objects in one plane, pictorial view, from the orthographic views is essential. There are three methods to draw the pictorial drawings i.e.
1. Perspective Projection
2. Oblique Projection
3. Axonometric Projection
Perspective projection is mostly used by the artists, professional designers and architects to show the views as it appears to the human eye. It appears to converge at a point, called vanishing point. The Oblique projection is mostly used by the mathematicians and furniture manufacturers. They impart third dimension at an angle to the two dimensional images, to show the depth. The Axonometric projection differs from the other one plane views on the basis of rotation angle along one or more of its axes relative to the plane of projection. It is extensively used in mechanical engineering to show the blocks, machine parts, assemblies etc. It shows an image of an object from a skew direction.
On the basis of inclination angle of the three principal axes to the plane of projection, the axonometric projection is classified among, isometric projection, diametric projection antrimetric projection.In isometric projection, all the angles between principal axes are equalwhile in diametric projection, only two angles between three principal axes are equal and over90°and in trimetric projection, all the three angles are unequal and not less than 90°. As the principal axes are inclined to the plane of projection so the measurement along them are also foreshortened. But the most advantageous point of isometric projection is that it needs a single scale to measure along each of the three axes. So in general, we use only isometric projection in engineering practice.
1.2 ISOMETRIC PROJECTION
The isometric projection of an object is a one plane view drawn with the object so placed with respect to the plane of projection that all the three principal axes appear to be inclined to each other at an equal angle of 120°.
The isometric scale is used to measure the foreshortened length of dimensions of any object to draw the isometric projection. The steps of construction of isometric scale are given below ; refer
(i) Draw a horizontal line PQ.
(ii) Draw the true lengths on a line PM inclined at 45° to the horizontal line (say up to 70 mm )
(iii) Draw another line PA at 30° to the horizontal line.
(iv) D r a w t h e v e r t i c a l
projection of all the points of true length
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