15774_1st angle 3rd angle
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Basics of Orthographic projections Basics of Orthographic projections
Department of Mechanical EngineeringLOVELY PROFFESIONAL UNIVERSITY,
JALANDHAR
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Orthographic Projections2
Drawing is the one of the language of communicating our views and ideas in the form of picture.
As like the all language this also have its grammar and own vocabulary .
Lines are the words for this language and set of lines forms the sentence which conveys the our idea .
So for that it is important to study the different types of lines used in drawing.
In engineering drawing, the word ‘projection’ means an image or the act of obtaining the image of an object.
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Types of line
Types of lines used in ENGG. Drawing
LINES
Lines are like the alphabet of a drawing language. Each line in a drawing is used in a specific sense.
Pencil Grades
An H grade pencil is advised for THICK and MEDIUM lines. THIN lines may be drawn by a 2H grade pencil.
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Orthographic Projections6
The projection system used in engineering drawing, is depicted in above Fig. The lines of sight of the observer create the view of the object on the screen. The screen is referred as plane of projection (POP). The lines of sight are called projection lines or projectors
Viewing
Direction
Picture Plane
For the Top view we view from the top!
Viewing
Direction
Point of intersection with picture plane
Projectors
Perpendicular to picture plane
Viewing
Direction
Intersections of allextreme points
Top View
Front View
Similarly, viewing from the front with parallel projectors
Top & Front Viewson opening up the page
Notice the interrelation
Similarly, the
Right Side View
Again notice the interrelation
Orthographic Projections14
Defining the Six Principal Views or Orthographic Views
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Principal planes in drawing
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The three RPs required to obtain the views in multi-view projections are the HP, the VP and the PP, in shown above Fig.
The HP and the VP make four quadrants.
The position of an object in space can be determined by these quadrants, i.e., the object can be in the first quadrant or in the second quadrant or in the third quadrant or in the fourth quadrant.
The line at which the HP and the VP meet is called horizontal reference line and denoted by XY.
The line at which the HP (or the VP) and the PP meet is called the profile reference line and is denoted by X1Y1.
After the views are obtained, the HP is rotated about XY in the clockwise direction to bring it in plane with the VP.
The PP is rotated about X1Y1 away from the object.
Two types of projections commonly used: I & III angle
In third angle, picture planes in between the viewer & object
In first angle, picture plane behind the object
The relationship on plane paper of the various views in III angle
TopView
FrontView
RightView
Left View
The relationship on plane paper of the various views in I angle
TopView
FrontView
Left View
RightView
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Front View When the observer looks at the object from the front, the view obtained is called the front view (FV) or Elevation. FV is seen on the VP.
Top View When the observer looks at the object from above, the view obtained is called top view (TV) or plan. TV is seen on the HP.
Side Views When the observer looks at the object from side, i.e., from his left-hand side or right hand side, the view obtained is called side view (SV). SV is seen on the PP.
Left-Hand Side View When the observer views the object from his left-hand side, the view obtained is called left-hand side view (LHSV).
Right Hand Side View When the observer views the object from his right-hand side, the view obtained is called as right-hand side view (RHSV).
Bottom View When the observer looks to the object from below, the view obtained is called bottom view (BV) or bottom plan.
Rear View When the observer looks to the object from back, the view obtained is called rear view (RV) or back view or rear elevation.
ORTHOGRAPHIC VIEWS
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PROJECTION SYSTEMS
1. First angle system
2. Third angle system
First Quadrant
ThirdQuadrant
- European country
- ISO standard
- Canada, USA,
Japan, Thailand
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ORTHOGRAPHIC PROJECTION
1st angle system 3rd angle system
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ORTHOGRAPHIC VIEWS
1st angle system 3rd angle system
Foldingline
Foldingline
Foldingline
Foldingline
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ORTHOGRAPHIC VIEWS
1st angle system 3rd angle system
Front View
Front View
Right Side View
Right Side View
Top View
Top View
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First angle system Third angle system
PROJECTION SYMBOLS
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First-angle Projection Method
In first-angle projection, an object is placed in the first quadrant, i.e., above the HP and in front of the VP. (shown in NEXT fig.)
The object lies in between the observer and the plane of projection.
The plane of projection is assumed to be non transparent and views drawn on it.
Front view is above the reference axis and top view lies below the reference axis exactly bellow the front view.
Right hand side view is drawn to left of front view and Left hand side view is drawn to the right of front view.
First angel projection symbol:
METHODS OF MULTIVIEW PROJECTION
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First angle projection method
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Third -angle Projection Method
In third-angle projection, an object is placed in the third quadrant, i.e., below the HP and behind the VP (shown in NEXT fig.)
The plane of projection lies in between the observer and the object.
The plane of projection is assumed to be transparent and views drawn on it.
Front view is below the reference axis and top view lies above the reference axis exactly above the front view.
Right hand side view is drawn to right side of front view and Left hand side view is drawn to the left side of front view.
Third angel projection symbol:
METHODS OF MULTIVIEW PROJECTION
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Third angle projection method
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Difference in first and third angel projection
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The second and fourth angel methods are not used in practice because after rotating the horizontal plane by 90° in clockwise direction the front view and top are overlapping. So we can not differentiate the front and top vies.
Why we are not using second and fourth angle projections ?
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To obtain the projections of various faces of an object, the following rules must be observed:
1. If a face is perpendicular to the direction of viewing, its true shape and size will be seen in that view.
2. If a face is parallel to the direction of viewing, it is seen as a line in that view. This view is called the line view or edge view.
3. If a face is inclined to the direction of viewing, its true shape and size will not be seen in any view.
4. If an edge of the object is perpendicular to the direction of viewing, its actual length will be seen in that view.
5. If an edge of the object is parallel to the direction of viewing, it is seen as a point in that view. This view is called point view.
6. If an edge of the object is inclined to the direction of viewing, its foreshortened length will be seen in that view. The foreshortened length is obtained by locating the end points of the edge.
Some guide lines to draw the views of object
Glass Box Approach
Place the object in a glass box
Freeze the view from each direction (each of the six sides of the box) and unfold the box
Glass Box Approach35
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Hidden Features
Hidden Features
Hidden Features
Hidden Features
(Surface Limit)
Three possible interpretations:An edge view of a surfaceAn intersection of two surfacesA surface limit - reversal of direction of a curved surface
Meaning of Lines in Orthographic Views
Sectional Views
Sectional Views
Whenever a representation becomes confused due to too many essential hidden details that it is difficult to interpret, sectional views are employed
Too many hidden lines
Too complicated to interpret
Sectional Views•A portion of the part is cut away to reveal the interior.•For this purpose a cutting plane is employed. The shape of the object is clarified by distinguishing between the areas where the cutting plane actually cuts the solid material and the areas where it meets voids.•Wherever the cutting plane cuts the solid material, the area is hatched
The structure of this pulley becomes clearer if we imagine the pulley is cut at the meridian plane, the material to the left of the cutting plane is removed and a projection viewing from the left is drawn.
A
A
Sectional Views
Cutting Plane
The details of the hub are now clearer.
Sectional Views
A sectional view makes things much clearer.
Sectional Views
Sectional Views
This does not differentiate cut and uncut portions
Note that the cutting plane line is long dash – two short dashes line
Sectional Views
Hatch the solid portions which are exposed freshly by the cutting plane
These areas not hatched because the cutting plane does not cut any material here. These represent holes.
Sectional Views
Note that the sectioning plane is offset to bring out both the hidden features in one view
Offset Sections
Full Sections
In many symmetrical objects one can show the internal & the external feature in the same view by considering a plane which cuts only one half the object.
Half Sections
Third-angle Projection
First-angle Projection
First and Third Angle Projections
First Angle Third Angle
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