4.34 Textbook of Englnnering Drawing------------------
1 at PI both above and below the axis of the conic.- Similarly, mark points 2, 3, 4, etc., as described above.
- Draw a smooth curve passing through the points V, PI' P 2' etc., which is the required ellipse.
7. Mark the centre, C of the ellipse and draw a perpendicular GH to the axis. Also mark the
other focus P such that CF = CP.
Fig. 4.50 Construction ofan Ellipse (given focus and directrix) Dl- Tangent at any point P on the ellipse can be drawn, by joining P P and by drawing PT
perpendicular to PP. Join TP and extend. Draw NP perpendicular to TP. Now, TPT and
NPN are the required tangent and normal at P respectively.
Problem: The foci of an ellipse are 90 mm apart and the major axis is 120 mm long. Draw the'
ellipse by using four centre method.
Solution: (Fig. 4.51) - Draw the major axis AB = 120 mm. Draw a perpendicular bisector COD. Mark the foci F
and P such that FO = PO = 45 mm. - With centre F and radius = AO = 60 mm draw arcs to cut the line COD at C and D as shown
in Fig. 4.51. Now, CD is the minor axis. - Join AC .With ° as centre and radius = OC draw an arc to intersect the line AB at E.
- With C as centre and AE as radius draw an arc to intersect the line AC at G
- Draw a perpendicular bisecator of the line AG to intersect the a.xis AB at ° 1 and the axis
CD (extended) at 02' Now ° 1 and 02 are th~ centres of the two arcs. The other two
centres 03 and 04 can be located by taking 003 = 0° 1 and 004 = 002' Also locate the
points 1,2,3 and 4 as shown.