Introduction
1.
Engineering drawing is the application of the principles and methods of
geometry. Without the knowledge of geometry it is difficult to prepare a
drawing. All drawings have points, lines and angles. Geometrical drawing is the
foundation of all engineering drawing. Accuracy, neatness and legibility are of
great importance in engineering drawing.
2.
Geometrical drawing is the art of representation of geometrical objects such as
square, rectangle, cube, cylinder, cone, sphere, etc. Geometrical drawing
consists of:
(a)
Plane Geometrical Drawing.
It is the representation of objects on a drawing sheet, having two dimensions,
i.e. length, breadth and radius such as square, rectangle, parallelogram,
circle, etc.
(b)
Solid Geometrical Drawing.
It is the representation of objects on a drawing sheet, having three
dimensions, i.e. length, breadth and height such as cube, cylinder, cone,
sphere, etc.
Terms
3.
Point.
According to
Euclid, it is something that has position but no dimensions. A point is
marked by a dot in engineering drawing and denoted by capital letters A, B, C…
4.
Lines.
Lines are of various types as given below:
(a) Straight Line.
lt is the shortest distance between two points joined by a series of points
overlapping each other.
(b)
Curved Line.
A curved line is a line which continuously changes direction. It is neither
broken nor a straight line.
(c)
Horizontal Line. In a three
dimensional object, a horizontal line is a line which is parallel to the
surface of earth looked upon as a plane. In engineering drawing, a line drawn
parallel to base line is known as a horizontal line.
(d)
Vertical Line.
It is a straight line perpendicular to the horizontal line.
(e)
Oblique Line.
It is a straight line, which is neither vertical nor horizontal.
(f)
Parallel Lines.
Two lines are said to be parallel if these lie in the same plane and do
not meet however far they are extended.
5.
Angle.
Two straight lines meeting at a point make an angle. An angle is the
included inclination of the two lines. [Refer Fig 1(a) and (b)]
(a) Acute Anqle.
It is an
angle numerically smaller than a right angle.
(a)
Obtuse Angle. An obtuse angle is an
angle greater than a right angle and smaller than a straight angle.
(b)
Straight Angle. It is an angle whose sides lie on the same straight line, but extend in
opposite directions from the vertex. It is an angle of 180°.
(c)
Reflex Angle. It is an angle, which is greater than 180O and less than 360O.
Fig. 1(a) Various Angles
(d)
Adjacent Angles. Two angles having a common
side, a common vertex and lying on opposite sides of their common side are
termed as adjacent angles.
(e)
Complementary Angles. Two angles whose sum is a right angle are termed as
complementary angles. The two acute angles in a right triangle are always
complementary to each other.
(f)
Supplementary Angles. Two angles whose sum is 1800
are termed as supplementary angles.
Fig. 1(b) Various Angles
6.
Chord. It
is a straight line joining any two points on the circumference of a circle.
The longest chord of a circle is its diameter.
7.
Tangent to a Circle.
It is a straight line that passes through exactly one point on the
circumference of the circle. A tangent is always at right angle to the radius
passing through the point of contact.
Fig. 1(c) Chord
and Tangent
