Textbook of Engineering Drawing, Second Edition

--------------___ Geometrical Contructions 4.35

6. With centre 03 and radius = 03B draw an arc 4B3 and with centre 02 and radius = 02C
   draw an arc 1 C4.


7. Similarly draw arcs for the remaining portion and complete the ellipse.

Fig. 4.51 Construction ofan Ellipse (four-centre method)

Problem : Construct an ellipse when its major axis is 120mm and minor axis is 80mm.

Solution: (Fig.4.52)

1. Take a strip of paper (Tramel) and mark PQ = half the minor a.xis and PR = half the major
   as shown.


2. Draw AB = 120 mm to liepresent the major axis and bisect it at 0. Through ° draw a
   vertical CD = 80 mm to represent the minor axis.


3. Keep the trammel such that Q is lying on the major axis and R on the minor xis. Now the
   position of the point P is one of the points on the ellipse.


4. Then change the position of the trammel such that Q and R always lie on AB and CD
   respectively. Now the new position of the pint P is another point to construct the ellipse.


5. Repeat the above and rotate the trammel for 360°, always keeping Q along AB and R along
   CD.


6. For different positions ofQ and R, locate the positions of point P and draw a smooth ellipse.
   Problem: Construct an ellipse when its major axis is 90 mm and minor axis is 55 mm.

Solution : (Fig.4.53)

1. Draw the major axis AB = 90 mm and bisect it at 0. Through ° draw a vertical line
   CD=55mm.


2. To represent diameters draw two concentric circles.


3. Divide the circles into 12 number of equal parts and draw the radial lines. These radial lines