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Engineering Drawing
By: Abdul Rehman 1317 Page 2
INTRODUCTION
A graphical language used by engineers and other technical personnel associated
with the engineering profession. The purpose of engineering drawing is to convey
graphically the ideas and information necessary for the construction or analysis of
machines, structures, or systems.
Engineering drawings are usually created in accordance with standardized
conventions for layout, nomenclature, interpretation, appearance (such as typefaces
and line styles), size, etc. One such standardized convention is called GD&T
The drawings are still often referred to as "blueprints" or "blue lines", although
those terms are anachronistic from a literal perspective, since most copies of
engineering drawings that were formerly made using a chemical-printing process
that yielded graphics on blue-colored paper or, alternatively, of blue-lines on white
paper, have been superseded by more modern reproduction processes that yield
black or multicolor lines on white paper.
The basis for much engineering drawing is orthographic representation
(projection). Objects are depicted by front, top, side, auxiliary, or oblique views, or
combinations of these. The complexity of an object determines the number of
views shown. At times, pictorial views are shown.
Engineering drawings often include such features as various types of lines,
dimensions, lettered notes, sectional views, and symbols. They may be in the form
of carefully planned and checked mechanical drawings, or they may be freehand
sketches. Usually a sketch precedes the mechanical drawing.
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Many objects have complicated interior details which cannot be clearly shown by
means of front, top, side, or pictorial views. Section views enable the engineer or
detailer to show the interior detail in such cases. Features of section drawings are
cutting-plane symbols, which show where imaginary cutting planes are passed to
produce the sections, and section-lining (sometimes called cross-hatching), which
appears in the section view on all portions that have been in contact with the
cutting plane.
In addition to describing the shape of objects, many drawings must show
dimensions, so that workers can build the structure or fabricate parts that will fit
together. This is accomplished by placing the required values (measurements)
along dimension lines (usually outside the outlines of the object) and by giving
additional information in the form of notes which are referenced to the parts in
question by angled lines called leaders.
A set of working drawings usually includes detail drawings of all parts and an
assembly drawing of the complete unit. Assembly drawings vary somewhat in
character according to their use, as design assemblies or layouts; working drawing
assemblies; general assemblys; installation assemblies; and check assemblies.
Schematic or diagrammatic drawings make use of standard symbols which indicate
the direction of flow. In piping and electrical schematic diagrams, symbols are
used. The fixtures or components are not labeled in most schematics because the
readers usually know what the symbols represent
Structural drawings include design and working drawings for structures such as
building, bridges, dams, tanks, and highways. Such drawings form the basis of
legal contracts. Structural drawings embody the same principles as do other
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engineering drawings, but use terminology and dimensioning techiques different
from those shown in previous illustrations.
Drawing Instruments
Pencils
The pencils we normally use in engineering drawing are H1, H2 & H3
Triangle
Two medium sized, 6 to 12 inches long, one of 45 degrees, the other of 30/60
degrees
.
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Scale
A regular scale and a T-square.
Compass
A compass is used for drawing circles with a diameter not on the templates. It can
also be used as a divider.
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Drawing board
A small drawing board with drafting linoleum will be useful for work at home.
Line Styles And Types
Standard engineering drawing line types
A variety of line styles graphically represent physical objects. Types
oflines include the following:
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Visible Lines
These are continuous lines used to depict edges directly visible from a
particular angle.
Hidden Lines
These are short-dashed lines that may be used to represent edges that are not
directly visible.
Center Lines
These are alternately long- and short-dashed lines that may be used to represent
the axes of circular features.
Cutting plane
These are thin, medium-dashed lines, or thick alternately long- and double
short-dashed that may be used to define sections for section views.
Section Lines
These are thin lines in a pattern (pattern determined by the material being "cut"
or "sectioned") used to indicate surfaces in section views resulting from
"cutting." Section lines are commonly referred to as "cross-hatching."
Phantom - (not shown)
These are alternately long- and double short-dashed thin lines used to represent
a feature or component that is not part of the specified part or assembly. E.g.
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billet ends that may be used for testing, or the machined product that is the
focus of a tooling drawing.
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Lines can also be classified by a letter classification in which each line is given a
letter.
Type A
Lines show the outline of the feature of an object. They are the thickest lines on
a drawing and done with a pencil softer than HB.
Type B
Lines are dimension lines and are used for dimensioning, projecting, extending,
or leaders. A harder pencil should be used, such as a 2H.
Type C
Lines are used for breaks when the whole object is not shown. They are
freehand drawn and only for short breaks. 2H pencil
Type D
Lines are similar to Type C, except they are zigzagged and only for longer
breaks. 2H pencil
Type E
Lines indicate hidden outlines of internal features of an object. They are dotted
lines. 2H pencil
Type F
Lines are Type F[typo] lines, except they are used for drawings in
electrotechnology. 2H pencil
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Type G
Lines are used for centre lines. They are dotted lines, but a long line of 10
20 mm, then a gap, then a small line of 2 mm. 2H pencil
Type H
Lines are the same as Type G, except that every second long line is thicker.
They indicate the cutting plane of an object. 2H pencil
Type K
Lines indicate the alternate positions of an object and the line taken by that
object. They are drawn with a long line of 1020 mm, then a small gap, then a
small line of 2 mm, then a gap, then another small line. 2H pencil.
Lettering
Lettering is the process of forming letters, numerals, and other characters in
technical drawing. It is used to describe, or provide detailed specifications for, an
object. With the goals of legibility and uniformity, styles are standardized and
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lettering ability has little relationship to normal writing ability. Engineering
drawings use a Gothic sans-serif script, formed by a series of short strokes. Lower
case letters are rare in most drawings of machines.
Orthographic projection
The orthographic projection shows the object as it looks from the front, right, left,
top, bottom, or back, and are typically positioned relative to each other according
to the rules of either first-angle or third-angle projection.
First angle projection is the ISO standard and is primarily used in Europe. The3D object is projected into 2D "paper" space as if you were looking at an X-ray
of the object: the top view is under the front view, the right view is at the left of
the front view.
Third angle projection is primarily used in the United States and Canada, whereit is the default projection system according to BS 8888:2006, the left view is
placed on the left and the top view on the top.
Not all views are necessarily used, and determination of what surface constitutes
the front, back, top and bottom varies depending on the projection used.
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Isometric projection
The isometric projection show the object from angles in which the scales along
each axis of the object are equal. Isometric projection corresponds to rotation of
the object by 45 about the vertical axis, followed by rotation of approximately
35.264 [= arcsine(tan(30))] about the horizontal axis starting from an
orthographic projection view. "Isometric" comes from the Greek for "same
measure". One of the things that makes isometric drawings so attractive is the ease
with which 60 degree angles can be constructed with only a compass and
straightedge.
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The Isomeric view of the object
Perspective
Perspective is an approximate representation on a flat surface, of an image as it is
perceived by the eye. The two most characteristic features of perspective are that
objects are drawn:
Smaller as their distance from the observer increases Foreshortened: the size of an object's dimensions along the line of sight are
relatively shorter than dimensions across the line of sight.
Scale
Plans are usually "scale drawings", meaning that the plans are drawn at
specific ratio relative to the actual size of the place or object. Various scales may
be used for different drawings in a set. For example, a floor plan may be drawn at
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1:50 (1:48 or 1/4"=1'-0") whereas a detailed view may be drawn at 1:25 (1:24 or
1/2"=1'-0"). Site plans are often drawn at 1:200 or 1:100.
Showing dimensions
The required sizes of features are conveyed through use ofdimensions. Distances
may be indicated with either of two standardized forms of dimension: linear and
ordinate.
With lineardimensions, two parallel lines, called "extension lines," spaced atthe distance between two features, are shown at each of the features. A line
perpendicular to the extension lines, called a "dimension line," with arrows at
its endpoints, is shown between, and terminating at, the extension lines. The
distance is indicated numerically at the midpoint of the dimension line, either
adjacent to it, or in a gap provided for it.
With ordinate dimensions, one horizontal and one vertical extension lineestablish an origin for the entire view. The origin is identified with zeroes
placed at the ends of these extension lines. Distances along the x- and y-axes to
other features are specified using other extension lines, with the distances
indicated numerically at their ends.
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Parallel Dimensioning
Parallel dimensioning
consists of several
dimensions originating from
one projection line.
Superimposed Running
Dimensions
Superimposed running
dimensioning simplifies
parallel dimensions in order
to reduce the space used on adrawing. The common origin
for the dimension lines is
indicated by a small circle at
the intersection of the first
dimension and the projection
line. In general all other
dimension lines are broken.
The dimension note can
appear above the dimension
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line or in-line with the
projection line
Chain Dimensioning
Chains of dimension should
only be used if the function
of the object won't be
affected by the accumulation
of the tolerances. (A
tolerance is an indication of
the accuracy the product has
to be made to. Tolerance will
be covered later in this
chapter).
Combined Dimensions
A combined dimension uses
both chain and parallel
dimensioning.
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Dimensioning by Co-
ordinates
Two sets of superimposed
running dimensions running
at right angles can be used
with any features which need
their centre points defined,
such as holes.
Simplified dimensioning by
co-ordinates
It is also possible to simplify
co-ordinate dimensions by
using a table to identify
features and positions.
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Dimensioning Small Features
When dimensioning small features, placing the dimension arrow between
projection lines may create a drawing which is difficult to read. In order to clarify
dimensions on small features any of the above methods can be used.
Dimensioning circles
All dimensions of circles are proceeded by this symbol; . There are several
conventions used for dimensioning circles:
(a) Shows two common methods of dimensioning a circle. One method dimensions
the circle between two lines projected from two diametrically opposite points. The
second method dimensions the circle internally.
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(b) Is used when the circle is too small for the dimension to be easily read if it was
placed inside the circle. A leader line is used to display the dimension.
(c) The final method is to dimension the circle from outside the circle using an
arrow which points directly towards the centre of the circle.
The first method using projection lines is the least used method. But the choice is
up to you as to which you use.
Dimensioning Holes
When dimensioning holes the method of manufacture is not specified unless they
necessary for the function of the product. The word hole doesn't have to be added
unless it is considered necessary. The depth of the hole is usually indicated if it is
isn't indicated on another view. The depth of the hole refers to the depth of the
cylindrical portion of the hole and not the bit of the hole caused by the tip of the
drip.
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Dimensioning Radii
All radial dimensions are proceeded by
the capital R. All dimension arrows
and lines should be drawn
perpendicular to the radius so that the
line passes through the centre of the
arc. All dimensions should only have
one arrowhead which should point to
the line being dimensioned. There are
two methods for dimensioning radii.
(a) shows a radius dimensioned with the centre of the radius located on thedrawing.
(b) shows how to dimension radii which do not need their centres locating.
Spherical dimensions
The radius of a spherical surface (i.e. the top of a drawing pin) when dimensioned
should have an SR before the size to indicate the type of surface.
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Tolerancing
It is not possible in practice to manufacture products to the exact figures displayed
on an engineering drawing. The accuracy depends largely on the manufacturing
process used and the care taken to manufacture a product. A tolerance value shows
the manufacturing department the maximum permissible variation from the
dimension.
Each dimension on a drawing must include a tolerance value. This can appear
either as:
A general tolerance value applicable to several dimensions. i.e. a notespecifying that the General Tolerance +/- 0.5 mm.
or a tolerance specific to that dimensionThe method of expressing a tolerance on a dimension as recommended by the
British standards is shown below:
Note the larger size limit is placed above the lower limit.
All tolerances should be expressed to the appropriate number to the decimal points
for the degree of accuracy intended from manufacturing, even if the value is limit
is a zero for example.
45.25 should not be expressed as 45.25
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44.80 44.8
Sectioning
Many times when the interior details of an object cannot be seen from the outside
An isometric drawing that does not show all details
We can get around this by pretending to cut the object on a plane and showing the
"sectional view". The sectional view is applicable to objects like engine blocks,
where the interior details are intricate and would be very difficult to understand
through the use of "hidden" lines (hidden lines are, by convention, dotted) on an
orthographic or isometric drawing.
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Imagine slicing the object in the middle
"Sectioning" an object
Sectioning the object in figure above
Take away the front half above figure and what you have is a full section view
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Sectioned isometric and orthogonal views
Working DrawingA working drawing is a type of technical drawing, which is part of the
documentation needed to build an engineering product or architecture. Typically in
architecture these could include civil drawings, architectural drawings, structural
drawings, mechanical drawings, electrical drawings, and plumbing drawings
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