14773 chapter 07

Vijay Gupta

Chapter 7: Orthographic Projections

Mechanical Sciences-I

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Introduction to Orthographic Projections

Introducing Orthographic Projections as the language of

engineering designers

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• 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.

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Orthographic Projections

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• 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.

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Orthographic Projections

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Object

We consider here the development of the orthographic views of a simple object.

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ViewingDirection

Picture Plane

For the Top view we view from the top!

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ViewingDirection

Point of intersection with picture plane

Projectors

Perpendicular to picture plane

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ViewingDirection

Intersections of allextreme points

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Top View

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Front View

Similarly, viewing from the front with parallel projectors

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Top & Front Viewson 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

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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

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Opening up of the box with the various views in III angle

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The relationship on plane paper of the various views in III angle

TopView

FrontView

RightView

Left View

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The relationship on plane paper of the various views in I angle

TopView

FrontView

Left View

RightView

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Principles of Multi-View Drawings

1. The Front and the Top views are aligned vertically.

2. The Front and the Side views are aligned horizontally.

3. The front of the object in the top view faces the front view.

4. The front of the object in the side view faces the front view.

5. The depth of top view is the same as the depth of side view.

6. The width of top view is same as the width of front view.

7. The height of side view is same as height of front view.Vijay

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Front View

Top View

Mitre

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Front

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Front View

Top View

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Front

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Drawing three views in III angle

Front View

Top ViewMitre

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Front

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A Video

Engg_graphics.mpg

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A demonstration

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Gboxw31.exe

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Front

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XX X

X

XX

X

X

Front

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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

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Hidden Features

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Shown by dashed lines

Hidden Features

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Hidden Features

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Lines and Areas

• Projections of lines and areas• Meaning of lines and areas in

orthographic projections

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Projection of Lines

A

B

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• A line may have a point as its projection

• A line may be projected in its true length

• A line may be fore-shortened

Projection of Lines

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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

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Surface in True shapeForeshortened Surface


B

B

B

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Curved Surface

C

C

C

Tangent Surfaces

D

D

D


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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 shape2. A foreshortened surface3. A smoothly curved surface4. A combination of tangent surfaces

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Reading Areas

• 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.

• 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 two of the principal views appears as a true shape in the third view.

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Reading Areas

• Any view that shows a plane surface as area shows it in a like shape

• A plane surface that appears as an area in two of the principal views can not be in true shape in any view.

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Oblique surfaces appear as areas of like shape in all views

Reading Areas

Adjacent Areas lie in different planes. If two areas were in the same plane, there will not be any boundary between the two.

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Reading Areas

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(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

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• An edge view of a surface

• An intersection of two surfaces

• A surface limit - reversal of direction of a curved surface

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Another Example


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We next illustrate how to read the orthographic drawings. This is done by interpreting the three view to ‘draw’ the represented by those view.

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1 2

3

45

1

2

3

6

7Start with a cuboid

Reading Lines & Areas

Right front corner is cut away to represent surface 12345

Front top is cut away to create a step 1267

Top front of the upper step is removed to reconcile the slope of 23 in side view.

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Interpretation of Hidden Lines

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Draw the pictorial views of the object whose three views are shown.

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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.

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One or more lines may be missing in (only) one view. Try constructing a pictorial view to determine what line(s) are missing.


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?

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Sectional Views

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Sectional Views

Whenever a representation becomes confused due to too many essential hidden details that it is difficult to interpret, sectional views are employed

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Too many hidden lines

Too complicated to interpret

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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

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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

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Cutting Plane

The details of the hub are now clearer.

Sectional Views

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A sectional view makes things much clearer.

Sectional Views

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Sectional Views

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This does not differentiate cut and uncut portions

Note that the cutting plane line is long dash – two short dashes line

Sectional Views

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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

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5/4 ream

Sectional Views

Clarify the view using sections.

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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.

Sectional Practices

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Note that the sectioning plane is offset to bring out both the hidden features in one view

Offset Sections

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Full Sections

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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

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