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14773 chapter 07

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14773 chapter 07Presentation Transcript

• Mechanical Sciences-I
Chapter 7: Orthographic Projections
• Introduction to Orthographic Projections
Introducing Orthographic Projections as the language of engineering designers
• Orthographic Projections
• Orthographic projections are an engineer’s language for conveying the shape and size information about the products he designs.
• An orthographic projection consists of the view obtained view when the object is viewed from very far away, so that the resulting rays are all parallel.
• The parallel rays that are used for constructing the views are called projectors.
Vijay Gupta
• Orthographic Projections
• The three principal views are take on picture planes which are held parallel to the three principal faces of the object, the front, the top and the side.
• The intersections of the projectors with the picture plane are the projections of the points from which the projectors emanate.
• The points are joined to obtain the views.
Vijay Gupta
• We consider here the development of the orthographic views of a simple object.
Object
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• For the Top view we view from the top!
Viewing
Direction
Picture Plane
Vijay Gupta
• Viewing
Direction
Projectors
Perpendicular to picture plane
Point of intersection with picture plane

Vijay Gupta
• Viewing
Direction

Intersections of all
extreme points
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• Top View
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• Similarly, viewing from the front with parallel projectors
Front View
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• Top & Front Views
on opening up the page
Notice the interrelation
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• Similarly, the
Right Side View
Again notice the interrelation
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• Mitre
The third view can also be obtained by taking projections from the two views, using the mitre line, a line at 450
Vijay Gupta
• Two types of projections commonly used: I & III angle
I-Angle
In third angle, picture planes in between the viewer & object
In first angle, picture plane behind the object
III-Angle
Vijay Gupta
• Opening up of the box with the various views in III angle
Vijay Gupta
• The relationship on plane paper of the various views in III angle
III Angle
Top
View
Front
View
Right
View
Left View
Vijay Gupta
• Vijay Gupta
• I-Angle
The relationship on plane paper of the various views in I angle
Front
View
Left View
Right
View
Top
View
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• Vijay Gupta
• Mitre
Front
Top View
Front View
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• Top View
Front
Front View
Vijay Gupta
• Drawing three views in III angle
Mitre
Top View
Front
Front View
Vijay Gupta
• A Video
Engg_graphics.mpg
• A demonstration
Vijay Gupta
Gboxw31.exe
• Front
Vijay Gupta
• Front
X
X
X
X
X
X
X
X
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• Notice that the oblique face of the cylinder appears as an ellipse in right-side view, but as lines in the front view.
Front
Vijay Gupta
• Hidden Features
• Hidden Features
Shown by dashed lines
• Hidden Features
• Hidden Features
• Hidden Features
• Hidden Features
• Lines and Areas
• Projections of lines and areas
• Meaning of lines and areas in orthographic projections
• Projection of Lines
A
B
• Projection of Lines
• A line may be projected in its true length
• A line may be fore-shortened
• A line may have a point as its projection
• Meaning of Areas in Orthographic Views
1. A surface in true shape
2. A foreshortened surface
3. A smoothly curved surface
4. A combination of tangent surfaces
• Meaning of Areas in Orthographic Views
B
B
B
Foreshortened Surface
Surface in True shape
• Meaning of Areas in Orthographic Views
C
C
D
D
C
D
Curved Surface
Tangent Surfaces
• Projections of Areas
Some areas are projected in true shapes, while others are distorted.
Areas parallel to picture planes are in true shapes
Four types of Areas
1.A surface in true shape
2. A foreshortened surface
3. A smoothly curved surface
4. A combination of tangent surfaces
• A plane surface will always appear in a principal view as a line or an area
• An plane surface that appears as a line in one view is normal to that view. It may or may not appear its true shape in the other views.
• An plane surface that appears as a line in two of the principal viewsappears as a true shape in the third view.
• A plane surface that appears as an area in two of the principal views can not be in true shape in any view.
• Any view that shows a plane surface as area shows it in a like shape
Adjacent Areas lie in different planes. If two areas were in the same plane, there will not be any boundary between the two.
Oblique surfaces appear as areas of like shape in all views
• Meaning of Lines in Orthographic Views
Three possible interpretations:
An edge view of a surface
An intersection of two surfaces
A surface limit - reversal of direction of a curved surface
(Surface Limit)
• Meaning of Lines in Orthographic Views
• An edge view of a surface
• An intersection of two surfaces
• A surface limit - reversal of direction of a curved surface
• Meaning of Lines in Orthographic Views
Another Example
• We next illustrate how to read the orthographic drawings. This is done by interpreting the three view to ‘draw’ the represented by those view.
3
6
2
7
1
Right front corner is cut away to represent surface 12345
3
1
2
Top front of the upper step is removed to reconcile the slope of 23 in side view.
4
5
Front top is cut away to create a step 1267
• Interpretation of Hidden Lines
• Draw the pictorial views of the object whose three views are shown.
• Draw the pictorial views of the object whose three views are shown.
• Missing Line Exercises
In the examples that follow, one or more lines may be missing in (only) one view. Try constructing a pictorial view to determine what line(s) are missing.
• Missing Line Exercises
One or more lines may be missing in (only) one view. Try constructing a pictorial view to determine what line(s) are missing.
• Missing Line Exercises
• Missing Line Exercises
• Missing Line Exercises
• Missing Line Exercises
• Missing Line Exercises
• Missing Line Exercises
• Missing Line Exercises
• Missing Line Exercises
• Missing Line Exercises
• Missing Line Exercises
• Missing Line Exercises
• Missing Line Exercises
• Missing Line Exercises
• Missing Line Exercises
• Missing Line Exercises
• Missing Line Exercises
• Missing Line Exercises
?
• Missing Line Exercises
• Missing Line Exercises
• Missing Line Exercises
• 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
• Sectional Views
A
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
• 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
5/4 ream
Clarify the view using sections.
• Sectional Practices
In the following slides we show some sectioning practices. The principle involved in these practices is to reduce the drawing effort as much as possible while maintaining clarity as much as possible.
• Try reducing the number of views required.
• Draw as few hidden lines as possible. Use a variety of sections as required.
• Offset Sections
Note that the sectioning plane is offset to bring out both the hidden features in one view
• Full Sections
• Half 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.