In Mechanical Engineering,drawings are made either in first angle projection or in third angle projection.We do not make drawings in second or fourth angle projection.We will discuss the reason behind doing so in this article..
Let us first understand the concept behind the angle projection system. Imagine you have Vertical Plane (VP) and Horizontal Plane (HP) (Perpendicular to each other),now we have 4 quadrants. For 1st angle projection system, we place object in 1st quadrant, for 2nd angle projection system, we place object in 2nd quadrant and so on.
In Second quadrant, Vertical plane (VP) lies in between object and observer. Therefore front view of the object will lie on vertical plane whereas top view will lie on horizontal plane. As per rule of projection when horizontal plane is rotated 90 degree in clockwise direction, top and front view will overlap.
Overlapping projection views create confusion in the drawing. Therefore 2nd angle projection system is not used.
Similarly when object is placed in 4th quadrant both top and front view will overlap. Therefore fourth angle projection is also not used.
To sum up, because of overlapping of front and top views 2nd and 4th angle projections are not used. First and third angle projection systems are recommended for orthographic projections
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