BASIC ENGINEERING DRAWING Prepared By: Syed Basharat Ali
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Basic Engineering Drawing Contents Ortho Graphic Projection
Lines Sectioning Terminology Abbreviations Conventional
Representation Of Common Features Pictorial Drawing Dimensioning
Limits And Fits Threads Assembly Drawing
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Ortho Graphic Projection In the engineering industry
communication between the drawing office and the work shop is
achieved mainly by means of engineering drawings. The principal
method used to prepare these drawings is known as Ortho Graphic
Projection. Basically, Orthographic Projection is the
representation of a three dimensional component on a flat surface
(the drawing sheet) in two dimensional form. At least two
orthographic views, therefore, are required to indicate fully the
shape and size of a component. If the component is a complicated
one then usually more then two views are shown to aid
understanding. In this country two methods of a Orthographic
Projection are used. One is known as First Angle Orthographic
Projection (often referred to as English Projection), the other as
Third Angle Orthographic Projection (American Projection). Both
methods of representation are illustrated and explained in this
section
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First Angle Orthographic Projection The pictorial drawing
opposite indicates the shape of the component with a single view.
An orthographic drawing indicates the Shape of a component by using
a number of views each looking at a different face of the
Component. At least two views are necessary to fully represent the
component. Usually, however, three views are shown in order to
clarify internal And external detail. A Front View (F) A Plan View
(P) A side View (L&R)
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Front View The front view or front elevation represents what is
seen when looking at the front of the component in the direction of
arrow F.
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Plan View A plan view represent what is seen when looking at
the top of the component in the direction of arrow P.
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Side View The side view or side elevation represents what is
seen when looking at the side of the component in the direction of
either arrow R or arrow L. These arrows are at 90 to both arrow F
and arrow P. View looking in direction of arrow R. Right- Hand Side
View (R) View looking in direction of arrow L. Left- Hand Side View
(L)
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In First Angle Ortho Graphic Projection The Front View is Above
the Top view. The Right-hand side view is on the Left-hand side of
the front view. The left hand side view is on the Right-hand side
of the front view.
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Third Angle Orthographic Projection When representing a three
dimensional component in Third Angle Orthographic Projection, the
basic views are exactly the same as those shown when using First
Angle Orthographic Projection. The difference between First Angle
and Third Angle is in the positioning of the views relative to each
other. In Third Angle Orthographic Projection the individual views
are placed on the drawing sheet in projection with each other as
shown :
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Point For Third Angle Orthographic Projection The plan is
always projected ABOVE the front view. The right-hand side view is
shown on the RIGHT-Hand side of the front view. The left-hand side
view is shown on the LEFT-Hand side of the front view.
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A Comparison Of First And Third Angle Ortho Graphic Projection
The plan is BLOW the front view. The Right-hand side view is on the
Left- hand side of the front view. The left hand side view is on
the Right-hand side of the front view. The plan is ABOVE the front
view. The right-hand side view is on the Right- hand side of the
front view. The left-hand side view is on the Left-hand side of the
front view.
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Kind Of Lines Kinds Of Lines Line Group (Intensity measured in
mm) Typical application 1,20,80,50,3 Solid 1,2 0,8 0,5 0,3 Visible
edge of parts; contours 0,4 0,3 0,2 0,1 Dimension lines, extension
lines, hatching lines, cross section lines, reference line, surface
line, contour lines of adjacent parts. Broken (dashed) 0,6 0,4 0,3
0,2 Invisible edges Alternate long dashes with dots 1,2 0,8 0,5 0,3
Lines indicating section planes. Center lines, Circular pitches,
index circle, finished parts down machine allowance, ultimate lever
position.
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Common Lines Used In Engineering Drawings
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Sample Drawing
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Sectioning Drawings of the outside of sample components are
often sufficient to convey all the information necessary to make
the component. More complicated components, however may require
sectional views to clarify internals details. A sectional view is
obtained when one imagines the component to be cut through a chosen
section plane often on a center line. If the vee-block is cut on
section plane C-C as shown the resulting sectional view projected
from the plan replaces the usual front view of the block. Sectional
Front View looking on cutting plane C-C End View
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Sectional views are drawn only when it is necessary to explain
the construction of a complex object or assembly. Some of the
examples used in the next few slides have been chosen to illustrate
the rules of sectioning although in practice, as in the case of the
vee-block drawn above a sectional view may not have been necessary.
The draftsman has to decide how a component or assembly should be
sectioned in order to provide the fullest possible information. The
recommendations of BS 308 enable him to do this in a way that is
understood by all engineers. Rules Of Sectioning A sectioned object
is shown by lines drawn preferably at 45. Thin lines touch the
outline. Size of sectioned part determines line spacing preferably
not less than 4 mm. If two adjacent parts are sectioned, the
section lines are drawn in opposite directions. Lines are staggered
where the parts are in contact. Where more then two parts of an
assembly are to be sectioned, the lines cannot all be opposite.
Sectional lines are closer together on the third area usually the
smallest
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The sectional view of a symmetrical object is obtained when the
section plane cuts through the obvious centre line. Hatching may be
omitted if the meaning is clear without it. If an object is NOT
symmetrical the section plane chosen should be clearly stated.
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Sectioning Exceptions There are a number of a features and
parts which are not normally sectioned even though they may lie in
the section plane. A good way to accept these exceptions to be
general rule is to imagine how complicated the drawing would look
if they were sectioned. They are sectioned, however, when they lie
across the section plane.
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Staggered Section Planes Section C-C Revolved Section D-D
Revolved Section A-A Realigned Each Part of the section plane is
swung to the vertical before projecting to the sectional End view.
By using the convention the draftsman avoids using too many
auxiliary views. A staggered section plane should only be used when
there is a resulting gain in clarity.
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TERMINOLOGY Communication between the drawing office and the
work shop is mainly achieved via the engineering drawing
orthographic or pictorial. In order to reduce drafting time a
number standard parts are abbreviated. Before this engineers
shorthand can be correctly it is necessary to understand the terms
used to describe features of engineering components. This
terminology is common to both drawing office and workshop and is
often used when discussing the various manufacturing and machining
processes used in engineering. Many different types of holes may be
seen on engineering drawings. The more common ones, associated with
drilling, reaming and tapping. The name and where appropriate the
application of each is indicated.
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1.A drilled hole or, if grater accuracy is required, a reamed
hole. 2.A blind tapped hole i.e. a threaded hole which passes only
a part way through the plate. 3.A countersunk hole provides a
mating seat for a countersunk head screw or rivet. 4.A counter bore
provides a housing for the heads of cap screw, bolts, etc. 5.A spot
face a much shallower circular recess. Provides a machined seat for
nuts, bolt heads, washers, etc.
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Abbreviations Many terms and expressions in engineering need to
be written on drawings so frequently as to justify the use of
abbreviations which help to reduce drafting time and costs. A
selection of the more commonly used ones are stated and clarified
in the following table. AbbreviationMeaningSketch/Notes A/CAcross
corners A/FAcross flats Hex HDHexagon head ASSYAssembly CRSCenters
CLCenter line CHAMChamfered CH HDCheese head CSKCountersunk C
BORECounter bore
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AbbreviationMeaningSketch/Notes CYLCylinder or Cylindrical
DIADiameter (In a note) Diameter (preceding a dimension) RRadius
(preceding a dimension, Capital only) DRG.Drawing FIG.Figure LHLeft
hand LGLong MAT:Material NO.Number PATT NO.Pattern number PCDPitch
circle diameter I/DInside diameter O/DOut side diameter
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AbbreviationMeaningSketch/Notes RHRight hand RD HDRound head
SCRScrewed SPECSpecifications S FACESpot face SQSquare Square
(preceding dimension) STDStandard U CUTundercut M/CDMachined
mmMillimeter NTSNot to be scale RPMRevolution per minuteSI symbol:
rev/min SWGStandard wire Gauge TPIThreads per Inch
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Conventional Representation of Common Features Screw Threads
There are many components commonly used in engineering which are
complicated to draw to full. In order to save drawing time, these
parts are shown in a simplified, conventional form.
SubjectConvention The screw thread is represented by two parallel
lines. The distance between these lines is approximately equal to
the depth of thread. The inside line is THIN and the circle is
broken
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Springs A spring is designated by stating the diameter of the
wire, the coil diameter (inside or outside), the form of the spring
ends, the total number of the coils and its free length. in the
case of compression spring, the pitch of the coil may be deduced
from its free length and number of coil.
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Shaft Details it is frequently necessary to fix a component to
one end of a shaft or spindle so that a torque may be transmitted.
ConventionSubject Side View Square on the end of a long Shaft
Splined Shaft
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Knurling Knurling is a common method of providing a roughened
to aid tightening or slackening of a screw by hand. This is formed
by pressing special rollers against the surface of the component
whist it revolves in lathe. Diamond Knurl on a machine screw head
Straight Knurl on a circuit terminal SubjectConvention
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Long Components There are occasions when bars, shafts, spindles
or tubes may be too long to be drawn to a reasonable scale. In such
cases the elevation may be interrupted. Subject Convention Hollow
Shaft OR Tube Rectangular Bar Circular Shaft OR Spindle
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Multiple Holes When a large number of holes of equal diameter
are equally spaced around a diameter or a line, only one hole need
be drawn in full with the reminder marked with a short center line.
That circle is called the pitch circle diameter or PCD
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Gears Before gears be drawn a great deal of background
knowledge about their nomenclature and construction must be
acquired. Subject Convention Side view of gear wheel is in section
Spur Gear Worm And Wheel
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A good example of a how a complex component maybe drawn
relatively simply is the bevel gear. The assembly shown blow is of
a pair of gear of equal size, the direction of motion being changed
through an angle 90. In the arrangement he gears are often referred
to mitre wheel. The gares ma be of differing sizes of course and
the angle between the shaft may be other then 90. In this letter
case, the side view of the gear assembly would have to show one
gear as then ellipse.
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Pictorial Drawing A component may be represented graphically in
various ways. An Orthographic Drawing, for example, requiring a
minimum of two views to fully communicate the size and the shape of
a component, is used in engineering mainly to convey manufacturing
instruction from the designer to the craftsman. On the other hand a
well executed Pictorial Drawing adequately representing all but the
most complicated components using one view only, is used mainly as
an aid to visualization of the shape of a component rather then for
communication detailed instruction for manufacture. A pictorial
drawing, generally, is a quickly produced approximately scaled
representation of a component a picture rather then an accurately
scaled line drawing. There are many different types of pictorial
representation. Two of the most commonly used ones are known as
Isometric Drawing and Oblique Drawing.
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Isometric All receding lines are drawn at 30
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Oblique An oblique pictorial drawing presents with the
component with one of its faces as a true shape. This shape is
drawn on the front face of the oblique box as shown below. The
longest face is usually drawn on the front of the oblique box with
receding lines between and full size.
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Methods Of Construction Of Oblique Drawing There are many
variations in angle, length of receding lines, and directions from
which a component may be viewed in order to produce an oblique
drawing as can be seen by examples on the previous slide. Different
oblique drawings of the same component may each provide the details
required. The receding lines may be drawn at any angle to the
horizontal but an angle of 30, 45, 60 is proffered as lines can be
drawn with set squares. Receding lines may be any proportion of
heir true length. A good pictorial representation is obtained if
lengths from to actual length is used.
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Dimensioning A number of the basic rules of dimensioning can be
explained by reference to the above drawing of a thin plate. The
sides marked A and B are known as DATUM faces. They are used as
reference edges from which dimensions are drawn. Datum's may or may
not be machined. Even if they are not machined it is good practice
to choose reference edges in order to simplify the layout of
dimensions. 1.Dimension Line: Thin full lines placed outside the
component where possible and spaced well away from the out lines.
The longer dimension lines are placed outside shorter ones.
2.Projection Lines: Thin full lines which extend from the view to
provide a boundary for the dimension line. Drawn at 90 to the out
line. 3.Arrowheads: Drawn with sharp strokes which must touch the
extension lines. 4. A Leader line is a thin full line which is
drawn from a note, a dimension or, in this case, a balloon and
terminates in an arrowhead or a dot. 5. Relatively small gap. 6.
Relatively short tail.
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7. Crossing extensions lines usually a break to ensure clarity.
8. Dimension placed above the dimension line. This is preferred to
the alternative method of placing the dimension in a gap in the
line. Avoid using both methods on the same drawing if possible. 9.
Dimension placed so that it may be read from bottom or right hand
side of the drawing sheet.
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Arrangement Of Dimensions Dimensions should be placed so that
they may be read from either the bottom or right-hand side of a
drawing, for example: Various methods of dimensioning narrow spaces
or width are shown above.
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Dimensioning Circles 1.The way a circle is dimensioned the
dimension always refers to the diameter and NOT the radius. 2.A
circle is never dimensioned on a center line. 3.The conventional
symbol for diameter is . The leader line must be drawn in line with
the center of the circle. In the example it is preferable to
dimension the side view even tough the cylindrical shape is not
apparent. Dimension in this view, however, must always be preceded
by symbol .
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Dimensioning Radii A radius should be dimensioned by a
dimension line which passes through, or is in line with, the center
of the arc. The dimension line should have one arrowhead which
should be placed at the point of contact with the arc. The
abbreviation R should always precede the dimension.
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Dimensioning Angles Angles are Expressed in: 1. Degrees e.g. 90
2. Degrees and minutes e.g. 27 30 The placing of the angular
dimension depends on the position of the angle in relation to the
bottom and/or the right-hand side of the drawing sheet and the size
of the angle.
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Dimensioning Chamfers 45 chamfer should be specified by one of
the methods below:
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Location Dimensions The features can be located from a machined
surface or center line. Such s surface or line is known as DATUM.
Examples on pervious slides have been shown components and features
may be dimensioned when size is the main consideration. Spigot
located from two reference edges ( R). Both holes located from two
reference edges ( R). Both holes located from two reference edges (
R) then hole B related to hole A.
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The simple bearing bracket casting on the left shows both size
and location dimensions. Reference surface are marked with
machining symbol:- This is placed so that it may be read from the
right of the sheet. It is preferable to place the symbol on the
appropriate projection line rather than as show on the left. No
symbol is required where the machine is specified i.e. in the case
of the drilled holes, the reamed holes and the spot-face. The
location dimension are those show with letter L and size dimensions
by a letter S. Some of the size dimensions are less accurate then
other e.g. the thickness of the rib is fixed during the casting
process whistle the 11 mm diameter holes is accurately reamed. The
20 mm diameter hole located by the dimension from the machined base
to the center line of the hole.
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Threads A screw thread, often shortened to thread, is a helical
structure used to convert between rotational and linear movement or
force. A screw thread is a ridge wrapped around a cylinder or cone
in the form of a helix, with the former being called a straight
thread and the latter called a tapered thread. A screw thread is
the essential feature of the screw as a simple machine and also as
a fastener.
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Pipe Threads G Series These are parallel pipe threads having
thread angle of 55 and are used where pressure-tight joints are not
made on threads. Taper Pipe Threads Whitworth Form These threads
have a taper of 1 in 16 and a thread angle of 55 and are used where
pressure tight joints are made on threads.
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American Pipe Threads These threads have a taper of 1 in 16 and
a thread angle of 60. The types of threads include NPT, NPTF &
ANPT. ACME Threads Acme screw threads are mainly used for the
purpose of producing traversing motions on machines, tools etc. The
multi-start threads are used to provide fast relative traversing
motion.
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Stub ACME The Stub Acme screw threads are generally confined to
those unusual applications like transmission of power and motion
where a coarse pitch thread of shallow depth is required due to
mechanical or metallurgical considerations. Trapezoidal Thread
Trapezoidal threads are used for transmission of power and motion
and are nearly similar to ACME threads, but are made to metric
dimensions and standards. The most commonly used class of threads
are 7e for external threads & 7H for internal threads.
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Buttress Screw Thread These are asymmetrical threads and are
used for transmission of power in one direction. The most common
thread profile is 7 / 45. Round Threads These threads are also
known as Knuckle threads and are insensitive to dirt and damage due
to their round shape and are used in fastening screw threads in
clutch of railway cars and for large valves and gates, for bottle
caps etc.
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Parts of Thread
Slide 53
PITCH The axial distance between threads. Pitch is equal to the
lead in a single start screw. LEAD The axial distance the nut
advances in one revolution of the screw. The lead is equal to the
pitch times the number of starts. LEAD = PITCH x STARTS For
example: 1/4" 4 RH requires four turns for one inch of travel. A
1/4 4 RH has two starts and a 0.125" pitch. 0.125" pitch X two
starts = 0.250 lead. SCREW STARTS The number of independent threads
on the screw shaft; example one, two or four.
Slide 54
Slide 55
Right Hand And Left Hand Threads Right Hand Thread A type of
thread that is screwed in by rotating it clockwise. Left Hand
Thread A type of thread that is screwed in by rotating it
anti-clockwise.
Slide 56
Assembly Drawings The purpose of an assembly drawing is to
provide visual information about the way in which parts of machine
or structure fit together. There are several types of assembly
drawings and the differences in presentation depend on the uses for
which they are intended. They are: 1. Layout Assemblies in which
the designer places together all the various parts in order to
established overall sizes, distances, etc. and as a result the
feasibility of this design. 2. Outline Assemblies these gives
general information about a machine or a group of components, for
example, main sizes and centre distances which would show how the
unit would be installed. This type of assembly is often used in
catalogues giving details of the range of units offered for sale.
3. General Assemblies or Arrangement Drawings shows clearly how
components fit together and more important how the assembled unit
functions'. Outside views, sectional and part sectional views may
be used but dimensions are rarely needed. The various parts may be
labeled by ballooning and parts list would complete the drawing. 4.
Sub-Assembly are drawings which show only one unit of a multi unit
component. One more complicated or multiple part components it may
first be necessary to arrange parts into sub assemblies which are
then built up into the main assembly.
Slide 57
5. Sectioned Assemblies a simple assembly may be drawn with out
the need for sectional views and clearly understood. On more
complex assembly drawings, however, too many hidden detail lines
tend to confuse and a sectional view of the assembled parts conveys
the information more clearly.