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Chapter 2 - Part 1
The Language and Terminology
of Technical Drawing
Chapter Objectives - at the completion of this chapter students will be able to:
•Define the terminology used to describe the geometry of multiview drawings
•Describe points, lines, angles
•Describe polygons
•Describe 3D objects
•Describe projection planes
•Describe normal, inclined and oblique surfaces
Supplemental Files
•Describe line types
•Describe line weights
•Interpret the multiviews of graphic primitives
•Describe orthographic projection including miter line technique
© Technical Drawing 101 with AutoCAD
Smith & Ramirez – All rights reserved.
Supplemental files are available for
download inside the Chapter 2 folder
of the book’s file downloads. Please
see the inside front cover for further
details.
2.1 THE TERMINOLOGY OF MULTIVIEW DRAWING
Multiview drawings are created by configuring the points, lines and planes of an object to create views that
represent the object’s features as they would appear if viewed from different points of view.
For example, a mechanical drafter might construct the front, top, and side views of a machine part. An
architectural drafter may draw the front, sides, and rear views of a building.
Before learning the techniques involved in the creation of the multiviews of an object, students should be familiar
with the terminology used to describe the features of an object.
© Technical Drawing 101 with AutoCAD
Smith & Ramirez – All rights reserved.
2.2 POINTS, PLANES, COORDINATE SYSTEMS, AND LINES
Points
A point is an exact “location” in space that is defined by coordinates that are located relative to a known origin point.
Points are often represented in technical drawings by a visible “dot”, and while the dot representing a point is visible, the point has no
dimensional size.
Locating Points in Two Dimensional (2D) Coordinate Systems
In a two dimensional (2D) coordinate system, points are defined on a 2D flat surface that represents a plane. The coordinates of the
point are located by measuring from two perpendicular lines that represent the X (horizontal) and Y (vertical) axes. The intersection
where the X and Y axes meet is called the origin. In technical drawings, the X and Y axes represent a 2D area referred to as the “XY
plane”.
In the example below, the origin point’s value would be stated as “zero comma zero”, or (0,0), which means the location of the origin
is zero units (0) on the X axis and zero units (0) on the Y axis. A point, represented by a green dot, is located at coordinate 2,3. This
means that the point’s location is 2 units to the right of the origin on the X axis and 3 units above the origin on the Y axis. Because
points 0,0 and 2,3 are both located on the XY plane they are considered to be coplanar.
Point located
at 2,3
2.1 Two Dimensional
Coordinate System with X
and Y Axes Noted
Note: Coordinates that are
defined relative to a 0,0
origin are also referred to as
absolute coordinates.
Origin (0,0)
© Technical Drawing 101 with AutoCAD
Smith & Ramirez – All rights reserved.
In the example below, two points (in red) are defined on the XY plane at coordinates 1,2 and 4,3 respectively. Points that lie on the
same plane are referred to as coplanar. Points that share the same location are referred to as coincident.
2.2 Points on a Two Dimensional Coordinate System
Origin Point (0,0)
Points that lie on the same plane are referred to as coplanar.
Points that share the same location are referred to as coincident.
© Technical Drawing 101 with AutoCAD
Smith & Ramirez – All rights reserved.
Negative Values in Two Dimensional (2D) Coordinate Systems
Points that are located below the X axis, or to the left of the Y axis, are described with negative coordinate values (a minus sign
precedes the coordinate).
In the example below, two points are defined at coordinates -3,1 and -1.5,-2.5 respectively.
Origin Point (0,0)
© Technical Drawing 101 with AutoCAD
Smith & Ramirez – All rights reserved.
Three Dimensional (3D) Coordinate Systems
In a three dimensional (3D) coordinate system, a Z axis is added to the X and Y axes. Using this system, points can be located relative
to the origin along the X, Y and Z axes. The Z axis represents the height of the point above or below the X,Y plane (see figure below).
For example, a 3D coordinate might be defined with the coordinates 1,1,1. This coordinate would lie one unit to the right of the origin
along the X axis, one unit from the origin along the Y axis, and one unit above the X,Y plane.
2.3 Three Dimensional Coordinate System
Origin Point (0,0,0)
of a 3D coordinate
system
© Technical Drawing 101 with AutoCAD
Smith & Ramirez – All rights reserved.
Lines
To a mathematician, a line is a set of continuous points that extend indefinitely in either direction. In technical drawing terminology, a line
is a segment defined by two points, the start point and the endpoint. These endpoints are defined with coordinates.
In technical drawing terminology, a line is a segment defined by two endpoints. The endpoints are defined with coordinates.
Points that lie on the same line are referred to as collinear.
Noncollinear points do not lie on the same line.
In the example below, a red line begins at a start point located at coordinate 2,2 and
ends at a point located at coordinate 8,7 (relative to the origin). These points are
collinear.
A point located exactly halfway between the start and end points would be the line’s
midpoint.
2.4 Coordinates for the Start and
End Points of a Two-dimensional
Line Drawn on the X-Y Plane
Parallel Lines
Parallel lines run side by side at a uniform
distance and never intersect, even if
extended.
Two or more planes can be parallel relative
to each other. Lines can be parallel to
planes.
Spline
A smooth curve that passes through, or near,
specified points.
© Technical Drawing 101 with AutoCAD
Smith & Ramirez – All rights reserved.
Angles
2.5 Two Lines Meeting to Form
an Angle
Angle
An angle is formed when two, noncollinear lines have the same endpoint. The angle at right is formed by
sides BA and BC. The angle formed by these lines is referred to as angle ABC.
Vertex
In 2D space, the common point where two lines meet is called a vertex. The plural of vertex is vertices.
In the example at right, angle ABC’s vertex is point B and the its sides are lines BA and BC.
Note: When specifying an angle using letters or numbers, the vertex should be the middle letter in the series. The angle formed by the
lines above is referred to as angle ABC.
Note: The point where two lines cross is
referred to as an intersection - as
opposed to a vertex.
In 3D objects, a vertex is where the object’s edges
meet.
In the object at right, all of the vertices have been
assigned a number.
This object has a total of 17 vertices.
© Technical Drawing 101 with AutoCAD
Smith & Ramirez – All rights reserved.
2.6 Three Dimensional Object with
Multiple Vertices
Types of Angles
Right Angle
Acute Angle
Obtuse Angle
The angle between the sides
measures exactly 90 degrees.
The angle between the sides
measures less than 90
degrees.
The angle between the sides
measures greater than 90 degrees
but less than 180 degrees.
2.7(a) Right Angle
2.7(b) Acute Angle
Perpendicularity
When two lines meet to form a right angle,
they are perpendicular. In the example
above, line BA is perpendicular to line BC.
Planes that meet at right angles to
each other are considered to be
perpendicular (see example at right),
Lines can also be perpendicular to
planes.
© Technical Drawing 101 with AutoCAD
Smith & Ramirez – All rights reserved.
2.7(c) Obtuse Angle
Types of Angles
Complementary Angles
Supplementary Angles
If two angles have a total measurement of 90 degrees
they are complementary angles.
If two angles have a total measurement of 180 degrees
they are supplementary angles.
2.8(b) Supplementary
Angles
2.7(c) Obtuse Angle
Opposite Angles
Adjacent Angles
When two lines cross, they form 4 angles. The
opposite angles have the same measure.
Where two lines cross, angles that share a common
side and common vertex, are called adjacent angles.
The sum of any two adjacent angle equals 180
degrees.
Therefore: Angle A = Angle B and Angle C = Angle D
2.9(a) Opposite Angles
Therefore, Angles A + C = 180 degrees and Angle C
+ B = 180 degrees, Angle B + D = 180 degrees and
Angle D + A = 180 degrees.
2.9(b) Adjacent Angles
© Technical Drawing 101 with AutoCAD
Smith & Ramirez – All rights reserved.
2.3 TERMINOLOGY OF GEOMETRIC SHAPES
Tangent
When a line touches a circle at only one point.
Circles
2.11(a) Lines Tangent to a Circle
A circle can be defined by its center point and
either a diameter or radius (diameter/2)
Tangency Point
Diameter Symbol
Radius
Center Point
2.10 Circle Terminology
Two circles that touch at
only one point are tangent.
Flat surfaces can also be
tangent to curved surfaces.
2.11(b) Circle Tangent to a Circle
Concentricity
When two or more circles share a common center
point they are concentric.
2.12(a) Concentric Circles
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Eccentricity
When two or more circles do not share a common
center point they are eccentric.
2.12(b) Eccentric Circles
Polygons
Polygons are multi-sided, 2D figures composed of straight line segments. The polygon’s start and end points meet at the same point which creates a
“closed” figure. Polygons are classified by the number of sides they contain.
Triangles - Three sided polygons.
Quadrilaterals - Four sided polygons.
Right
Triangle
Rectangle
Square
One right angle.
2.13(b) Quadrilaterals
Equilateral
Three equal angles.
Scalene
No equal angles.
2.13(a) Triangles
Hexagons - Six sided polygons.
Hexagon constructed by
inscribing it within a circle.
Hexagon constructed by
circumscribing it around a circle.
Pentagons - Five sided polygons.
Heptagon - Seven sided polygons.
Octagons - Eight sided polygons.
Nonagons - Nine sided polygons.
Decagons - Ten sided polygons.
Dodecagons - Twelve sided polygons.
2.13(c) Hexagons
© Technical Drawing 101 with AutoCAD
Smith, Ramirez & Schmidt
Cylinders
A cylinder is a three-dimensional object that is defined by its the
diameter or radius, its length, and the location of its center axis.
See Figure below.
Flat surfaces can also be tangent to curved surfaces.
Plane tangent to a cylinder
2.14(a) Cylinder
Coaxial
When two or more
cylinders are aligned
along the same center
axis they are coaxial.
Center planes meeting
along center axis.
2.14(b) Coaxial Cylinders
© Technical Drawing 101 with AutoCAD
Smith & Ramirez – All rights reserved.
3D SHAPES
Rectangular Prism
Rectangular Prism
Shown with a center plane.
SymmetryWhen the features of an
object are exactly the same
on both sides of the object’s
center plane, the object is
symmetrical.
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Smith & Ramirez – All rights reserved.
3D SHAPES
Triangular Prism
Triangular Prism
Triangular Prism
Shown with center
plane.
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Smith & Ramirez – All rights reserved.
Sometimes referred to
as a wedge.
3D SHAPES
Sphere
Sphere
Shown with 2 center planes.
Shown with 3 center planes.
Sphere
Cone
Shown with center plane.
Torus
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Cone
Truncated Cone
Open Chapter 2 Lecture –
Part 2
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