Textbook of Engineering Drawing, Second Edition

_____ Geometrical Contructions 4.15

c

D

50

VF =1

VA

Fig. 4.22 Construction of a Parabola -Eccentricity Method

To draw a normal and tangent through a point 40mm from the directrix.

To draw a tangent and normal to the parabola. locate the point M which is at 40 mm from the
directQx. Thenjoin M to F and draw a line through F, perpendicular to MF to meet the directrix at
T. The line joining T and M and extended is the tangent and a line NN, through M and perpendicular
to TM is the normal to the curve.

To draw an Ellipse with eccentricity equal to 2/3 for the above problem (Fig. 4.23).

Construction is similar to the one in FigA.22 to draw an ellipse including the tangent and normal.
only the eccentricity is taken as 2/3 instead of one.

Draw a hyperbola with eccentricity equal to 3/2 for.the above problem (Fig. 4.24).

The construction ofhyperobola is similar to the above problems except that the eccentricity ratio
VFNA = 3/2 in this case.

Note: The ellipse ,is a closed curve and has two foci and two directrices. A hyperbola is an open
curve.