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Lecture 2: Engineering Curves
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Engineering Curves
• used in designing certain objects
Conic Sections
• Sections of a right circular cone obtained by
cutting the cone in different ways
• Depending on the position of the cutting plane
relative to the axis of cone, three conic sections
can be obtained
– ellipse,
– parabola and
– hyperbola
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Conic Sections
• An ellipse is obtained when a section
plane A–A, inclined to the axis cuts all
the generators of the cone.
• A parabola is obtained when a section
plane B–B, parallel to one of the
generators cuts the cone. Obviously, the
section plane will cut the base of the
cone.
• A hyperbola is obtained when a section
plane C–C, inclined to the axis cuts the
cone on one side of the axis.
• A rectangular hyperbola is obtained
when a section plane D–D, parallel to
the axis cuts the cone.
A
A
B
B
C
C
D
D
O
O
3
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Conic is defined as locus of a point moving in a plane such that the
ratio of its distance from a fixed point (F) to the fixed straight line is
always a constant. This ratio is called as eccentricity.
Ellipse: eccentricity is always <1
Parabola: eccentricity is always=1
Hyperbola: eccentricity is >1
The fixed point is called the Focus
The fixed line is called the Directrix
Axis is the line passing though the
focus and perpendicular to the
directrix
Vertex is a point at which the conic
cuts its axis VC
VF
e =
5
• Eccentricity is less than 1.
• Closed curve.
• The fixed points represent the foci.
• The sum of the distances of a point on the
ellipse from the two foci is equal to the major
axis
• The distance of any end of the minor axis
from any focus is equal to the half of the
major axis
Relationship between Major axis, Minor axis and Foci
• If major axis and minor axis are given, the
two fixed points F1 and F2 can be located
with the following fact
• If minor axis is given instead of the distance between the foci, then locate the foci F and
F’ by cutting the arcs on major axis with C as a center and radius= ½ major axis= OA
Ellipse
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An ellipse has two foci (F and F’), two directrices (AB and A’B’ ), two
axes (V–V’ and V 1–V 1’) and four vertices (V, V’, V 1 and V 1’ ). The
two axes are called the major axis and minor axis.
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Methods for Generating Ellipse
1. Focus-Directrix Or Eccentricity Method
– General method of constructing any conics when the
distance of the focus from the directrix and its
eccentricity are given.
2. Concentric Method
– This method is applicable when the major axis and
minor axis of an ellipse are given.
3. Oblong Method
– This method is applicable when the major axis and
minor axis or the conjugate axes with the angle
between them is given.
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Types of Problems
• Focus-Directrix Or Eccentricity Method
– Draw an ellipse if the distance of the focus from the
directrix 50 mm and the eccentricity is 2/3
– Draw a parabola if the distance of the focus from the
directrix is 55 mm
– Draw a hyperbola of e = 4/3 if the distance of the focus
from the directrix = 60 mm
• Concentric Method
– Draw an ellipse having the major axis of 60 mm and the
minor axis of 40 mm
• Oblong Method
– Draw an ellipse having conjugate axes of 60 mm and 40
mm long and inclined at 750 to each other
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F1
V1
D
D
1 2 3 4 5
11
21
31
41
51
C
V2
Focus-Directrix or Eccentricity Method
10
45
Slope of line is e

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Focus-Directrix or Eccentricity Method
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Q.1: Draw an ellipse if the distance of focus from the directrix is 70 mm and the
eccentricity is 3/4.
1. Draw the directrix and axis as shown.
2. Mark F on axis such that CF 1= 70 mm.
3. Divide CF into 3 + 4 = 7 equal parts and mark V at the fourth division from C.
Now, e = FV/ CV = 3/4.
4. At V, erect a perpendicular VB = VF. Join CB.
5. Through F, draw a line at 45° to meet CB produced at D. Through D, drop a
perpendicular DV’ on CC’. Mark O at the midpoint of V– V’.
6. Mark a few points, 1, 2, 3, … on V– V’ and erect perpendiculars though them
meeting CD at 1’, 2’, 3’…. Also erect a perpendicular through O.
7. With F as a centre and radius = 1–1’, cut two arcs on the perpendicular through 1
to locate P1 and P1¢. Similarly, with F as a centre and radii = 2–2’, 3–3’, etc., cut
arcs on the corresponding perpendiculars to locate P/2 and P/2’, P/3 and P/3’,
etc. Also, cut similar arcs on the perpendicular through O to locate V1 and V1’.
Steps for Focus-Directrix or Eccentricity Method
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A B
C
D
O
1’ 2’ 3’
1
3
2
P1
P2
P3
Oblong Method
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Steps for Oblong Method
Draw an ellipse with a 70 mm
long major axis and a 45 mm
long minor axis.
or
Draw an ellipse circumscribing
a rectangle having sides 70 mm
and 45 mm.
1. Draw the major axis AB = 70 mm and minor axis CD = 45 mm, bisecting each other
at right angles at O.
2. Draw a rectangle EFGH such that EF = AB and FG = CD.
3. Divide AO and AE into same number of equal parts, say 4. Number the divisions as
1, 2, 3 and 1’, 2’, 3’, starting from A.
4. Join C with 1, 2 and 3.
5. Join D with 1’ and extend it to meet C–1 at P1. Similarly, join D with 2’ and 3’ and
extend them to meet C–2 and C–3 respectively to locate P/2 and P/3. 14

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Concentric Circle Method
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A B
C
D
Draw an ellipse having the major
axis of 70 mm and the minor axis
of 40 mm.
Draw the major axis AB = 70 mm
and minor axis CD = 40 mm,
bisecting each other at right angles
at O.
Draw two circles with AB and CD
as diameters. Divide both the
circles into 12 equal parts and
number the divisions as A, 1, 2, 3, …
10, B and C, 1’, 2’, 3’ … 10’, D.
Through 1, draw a line parallel to CD. Through 1’, draw a line parallel to AB. Mark P1 at
their intersection.
Obtain P/2, P4, P5, etc., in a similar way.
Draw a smooth closed curve through A– P1–P/2– C– P4– P5– B– P6– P7– D– P/9– P10–
A.
Concentric Circle Method
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Tangent and Normal at any point P
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Draw ellipse using Focus-Directrix or Eccentricity Method
F
P
Q
Tangent
Normal
1. Mark the given point P and join
PF1 .
2. At F1 draw a line perpendicular to
PF1 to cut DD at Q.
3. Join QP and extend it. QP is the
tangent at P
4. Through P, draw a line NM
perpendicular to QP. NM is the
normal at P
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Tangent and Normal at any point P when Focus and Directrix are
not known
1. First obtain the foci F and
F′ by cutting the arcs on
major axis with C as a
centre and radius =OA
2. Obtain NN, the bisector of
∠FPF′. N-N is the
required normal
3. Draw TT perpendicular to
N-N at P. T-T is the
required tangent
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Few Applications of Ellipse
Elliptical gear
Arch
Bullet nose
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Parabola
• A parabola is a conic whose eccentricity is equal to 1. It is an open-
end curve with a focus, a directrix and an axis.
• Any chord perpendicular to the axis is called a double ordinate.
• The double ordinate passing through the focus . i.e LL’ represents the
latus rectum
• The shortest distance of the vertex from any ordinate, is known as the
abscissa.
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Methods for Generating Parabola
1. Focus-Directrix Or Eccentricity Method
– General method of constructing any conics when the distance of the
focus from the directrix
– For example, draw a parabola if the distance of the focus from the
directrix is 55 mm.
2. Rectangle Method and Parallelogram Method
– This method is applicable when the axis (or abscissa) and the base ( or
double ordinate) of a parabola are given or the conjugte axes with the
angle between them is given
– For example, draw a parabola having an abscissa of 30 mm and the
double ordinate are 70 mm, or
– Draw an parabola having conjugate axes of 60 mm and 40 mm long
and inclined at 750 to each other.
3. Tangent Method
– This method is applicable when the base and the inclination of
tangents at open ends of the parabola with the base are given
– For example, draw a parabola if the base is 70 mm and the tangents at
the base ends make 60° to the base..
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Focus-Directrix Or Eccentricity Method
• Distance of the focus from the directrix is known.
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F
C C’
A
B
V
CV = VF
E EV = VF
D
Slope of CD is e = 1
1 2 3 4
1’
2’
3’
4’
Center = F
Radius = 1-1’
`
`
`
`
1. Draw directrix AB and axis CC’ as shown.
2. Mark F on CC’ such that CF = 60 mm.
3. Mark V at the midpoint of CF. Therefore, e = VF/
VC = 1.
4. At V, erect a perpendicular VB = VF. Join CB.
5. Mark a few points, say, 1, 2, 3, … on VC’ and erect
perpendiculars through them meeting CB
produced at 1’, 2’, 3’, …
6. With F as a centre and radius = 1–1’, cut two arcs
on the perpendicular through 1 to locate P1 and
P1’. Similarly, with F as a centre and radii = 2–2’,
3–3’, etc., cut arcs on the corresponding
perpendiculars to locate P2 and P2’, P3 and P3’,
etc.
7. Draw a smooth curve passing through V, P1, P2,
P3 … P3
Draw a parabola if the distance of the focus from the directrix is 60 mm.
Steps for Focus-Directrix or Eccentricity Method
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Rectangle Method
• Abscissa and ordinate are known.
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A B
AB-Ordinate
C
D
CD-Abscissa
1 2
1’
2’
1. Draw the double ordinate RS = 70
mm. At midpoint K erect a
perpendicular KV = 30 mm to
represent the abscissa.
2. Construct a rectangle RSMN such
that SM = KV.
3. Divide RN and RK into the same
number of equal parts, say 5.
Number the divisions as 1, 2, 3, 4
and 1’, 2’, 3’, 4’, starting from R.
4. Join V–1, V–2, V–3 and V–4.
Q.1: Draw a parabola having an abscissa of 30 mm and the double ordinate of 70 mm.
5. Through 1’, 2’, 3’ and 4’, draw lines parallel to KV to meet V–1 at P1, V–2 at P2, V–3 at P3 and V–4 at
P4, respectively.
6. Obtain P5, P6, P7 and P8 in the other half of the rectangle in a similar way. Alternatively, these
points can be obtained by drawing lines parallel to RS through P1, P2, P3 and P4. For example, draw
P1– P8 such that P1– x = x– P8.
7. Join P1, P2, P3 … P8 to obtain the parabola.
Steps for Rectangle Method
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Steps for Tangent and Normal at a
point p on parabola
1. Join PF. Draw PQ parallel to the axis.
2. Draw the bisector T– T of – FPQ to represent the required tangent.
3. Draw normal N– N perpendicular T– T at P.
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Steps for Parallelogram Method
Q.1: Draw a parabola of base 100 mm and axis 50 mm if the axis makes 70°
to the base.
1. Draw the base RS = 100 mm and through its midpoint K, draw the axis KV
= 50 mm, inclined at 70° to RS.
2. Draw a parallelogram RSMN such that SM is parallel and equal to KV.
3. Follow steps as in rectangle method
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Tangent Method
• Base and inclination of tangents are known.
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R S
L
RS-Base
RL and SL are tangents
1
2
3 1’
2’
3’
P
S
V
VR=VS
R
Tangent
Method to draw tangent at a point on parabola 1. First locate the point P on the curve
2. Draw the ordinate PS 3. On LK, mark T such that TV =VS
4. Join TP and extend to obtain tangent TT 5.Draw normal N-N perpendicular to T-T at P
1. Draw the base RS = 70 mm. Through R and
S, draw the lines at 60° to the base, meeting
at L.
2. Divide RL and SL into the same number of
equal parts, say 6. Number the divisions as 1,
2, 3 … and 1’, 2’, 3’, … as shown.
3. Join 1–1’, 2–2’, 3–3’, ….
4. Draw a smooth curve, starting from R and
ending at S and tangent to 1–1’, 2–2’, 3–3’,
etc., at P1, P2, P3, etc., respectively
Q. Draw a parabola if the base is 70 mm and the tangents at the base ends make 60° to
the base.
Steps for Tangent Method
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To find the focus and the directrix of a parabola given its axis
Tangent and Normal at any point P when Focus and
Directrix are not known
1. Draw the ordinate PQ
2. Find the abscissa VQ
3. Mark R on CA such that RV=VQ
4. Draw the normal NM perpendicular to RP at P
1. Mark any point P on the parabola
2. Draw a perpendicular PQ to the given axis
3. Mark a point R on the axis such that RV=VQ
4. Focus: Join RP. Draw a perpendicular bisector of RP
cutting the axis at F, F is the focus
5. Directrix: Mark O on the axis such that OV= VF. Through
O draw the directrix DD perpendicular to the axis 31
Few Applications of Parabola
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