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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