Textbook of Engineering Drawing, Second Edition

_____ Orthographic Projections 5.9

2. Looking from the front, the point lies 40 mm above H.P. A-al is the projector perpendicular to
   V.P. Hence al is the front view .of the point A and it is 40 mm above the xy line.


3. To obtain the top view of A, look from the top. Point A is 60mm in front ofV.P. Aa is the
   projector prependicular to H.P Hence, a is the top view of the point A and it is 60 mm in front
   ofxy.


4. To convert the projections al and a obtained in the pictorial view into orthographic projections
   the following steps are needed.
   (a) Rotate the H.P about the xy line through 90° in the clock wise direction as shown.
   (b) After rotation, the fIrst quadrant is opened out and the H.P occupies the position verically
   below the V.P line. Also, the point a on H.P will trace a quadrant of a circle with 0 as
   centre and o-a as radius. Now a occupies the position just below o. The line joining al
   and a, called the projector, is perpendicular to xy (Fig.5.9b).


5. To draw the orthographic projections.

Note:

(a) Front view : Draw the xy line an? draw ~ectior at any point on it. Mark al40mm _"

above xy on the projector.-.

(b) Top view: on the same projector, mark a 60 mm below xy. (Fig.5 .9c)

1. xy line represents H.P in the front view and v.p in the top view. Therefore while drawing the
   front view on the drawing sheet, the squares or rectangles for individual planes are not
   necessary.


2. Only the orthographic projections shown in FigA.9( c) is drawn as the solution and not the
   other two fIgures.

Probl~m : Draw the projections of a point A lying on HP and 25mm in front of v.P.

Solution: (Fig.5.10)

1. Point A is lying on H.P and so its front view allies on xy line in Fig.5.1 Oa. Therefore, mark a
   line xy in the orthographic projeciton and mark on it al (Fig.5.1 Ob).


2. Point A is 25mm in front ofV.P and its top view a lies on H.P itself and in front of xy.


3. Rotate the H.P through 90° in clock wise direction, the top view of the point a now comes
   vertically below al.


4. In the orthograpl;tic projection a is 25 mm below xy on the projector drawn from al.

~ Problem: Draw the projections of a point A lying on v.p and 70 mm above HP.

Solution: (Fig.5.lI)

1. Looking at the pictorial view from the front (Fig.5.11a) the point A is 70 mm above H.P and
   so al is 70 mm above xy. Hence, mark al the orthographic projection 70 mm above xy
   (Fig.5.11b).