Page 15 - CITS - ED - Mechanical

ENGINEERING DRAWING - CITS

Major axis: It is the longest distance which passes through the centre, at right angle to the fixed lines called the
directrix. AB is the major axis.
Minor axis: It is the maximum distance which bisects the major axis at right angle. It will be parallel to the direc-
trix. CD is the minor axis.
Directrix: It is a straight line perpendicular to the major axis.
Focus: When an arc is drawn with C or D as centre and radius equal to half the major axis i.e , it is cut at two
points F and F on themajor axis. F and F are the focal points of an ellipse F or F is the focus. The sum of
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the distances from F , F to any point on the curve i.e., F P + F P is always constant and equal to the major axis.
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Focal radii: The distances from point P on the curve to the focal points F and F are called focal radii. Sum of
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the focal radii is equal to the major axis.
Eccentricity: The ratio between the distances from the vertex to focus and vertex to the directrix is called the
eccentricity and is always less than one.
AF /A0 is less than one.
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It can also be stated as the ratio of the distance from focus onto any point on the curve, say P and the perpen-
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dicular distance of P from the directrix. i.e P F /P M is a constant.
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Vertex: The end points of the major axis on the curve are called vertex. (A, B)
Tangent and normal to an ellipse: Normal is the line bisecting the angle F PF in Fig 4. Tangent in a line at 90°
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to the normal and touching the ellipse.
Directrix, axis, focus, vertex and tangent are the elements common to ellipse, parabola and hyperbola.
All ellipse can be constructed in different methods:
– Rectangle method (oblong)
– Concentric circle method
– Arcs method
– String and pins method
– Paper trammel method
– 4 centre method
– Conjugate diameters method

– Eccentricity method
Practical applications: In general, a circle in a pictorial drawing is represented by an ellipse. The use of elliptical
shape is rarely used for engineering applications. Ellipse is dealt extensively in mathematical books. Elliptical
shape is adopted for better asthetics.

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CITS :Engineering Drawing (Mechanical) - Exercise 1