Chapter_09_Ortho

8/7/2019 Chapter_09_Ortho

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ORTHOGRAPHIC PROJECTIONS

Multiview orthographic projection is a method of drawing two or more views of

an object on the RPs placed at right angles to each other. The word µortho¶ means

perpendicular. In this projection, the projectors are perpendicular to the POP and

parallel to each other.

MULTIVIEW PROJECTION SYSTEM

The three RPs required to obtain the views in multiview projections are the HP,

the VP and the PP, Fig. 9.1. 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 X 1Y 1. 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 X 1Y 1 away from the object.





ORTHOGRAPHIC VIEWS

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 righthand 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).

For longer objects of uniform cross section (e.g., long pipe, spline shaft, etc.), the

SV is usually referred as end view.

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.The FV, TV and either LHSV or RHSV are usually drawn in orthographic

projection. The other views are added if they are extremely essential.

METHODS OF MULTIVIEW PROJECTION

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, Fig. 9.2(a). The observer looks at the object from the front

(i.e., direction X ) to obtain FV on the VP. Similarly, to get TV on the HP and LHSV

on the PP, the observer looks at the object from the above (i.e., direction Y ) and

from the left side (i.e., direction Z ) respectively. TV is placed below FV and LHSV isplaced on the right side of FV. RHSV is placed on the left side of the FV.





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, Fig. 9.3(a). The RPs are rotated as in first-angle

method, i.e., the HP in clockwise direction and the PP away from the object.

Obviously, TV is placed above FV, LHSV on the left side of FV and RHSV on the

right side of FV, Fig. 9.3(b).





PROJECTING THE SVs

The SVs are always placed alongside FV. They are obtained by drawing the

projectors from FV and TV.

ORTHOGRAPHIC VIEWS: SYSTEMATIC APPROACH

Often, we are given with the pictorial (usually isometric) view of an object with thefront direction of viewing, as shown in Fig. 9.4. The arrow marked X shows the

direction through which the object is viewed to obtain FV. Note that this direction

is perpendicular to the POP, i.e., the VP. Once the direction of viewing for FV is

known, directions for other views, viz. TV, LHSV and RHSV can be easily decided.

If the observer is facing the object, then his left-hand side and right-hand side will

indicate the directions for LHSV and RHSV respectively. The direction of viewingfor TV will obviously be from top of the object.



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 notbe 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.





Example 9.1 For the object shown in Fig. 9.6(a), draw FV, TV and LHSV.

Solution The object has 10 faces, marked as A, B, C, ..., J. Except face G, all otherfaces are perpendicular faces. The face G is inclined. Therefore, all the faces other

than face G will show their true shape and size in one of the orthographic views.

The faces H, I and J are the right face, bottom face and rear face respectively and

not visible.

Refer Fig. 9.6(b).

To obtain the FV, observer looks in the direction X. The faces which areperpendicular to the direction X will be seen in their true shape. The faces which

are parallel to the direction X will be seen as edge views. Thus, faces A, C and E will

be seen to their true shapes. The faces B, D, F and G will be seen as edge views.

To obtain the TV, the observer looks in the direction Y. The face F will be seen in

true shape. The face G will be seen foreshortened as it is inclined to the direction Y.

The faces A, B, C, D, E, H and J will be seen as edge views.

To obtain the LHSV, the observer looks in the direction Z. The faces B and D will be

seen as their true shapes. The face G will be seen as foreshortened. The faces A, C,

E, F, J and I will be seen as edge views.

The three orthographic views are shown in Fig. 9.6(c). The LHSV is drawn on the

right-hand- side of FV since we are following the first-angle method of projection.





Hidden Features

The features of the object not seen in a particular view are called as hidden

features. The hidden features, internal or external, are shown by drawing dashed

lines for the edges (or extreme generators in case of cylindrical or conical

features) forming the hidden feature in that particular view.

Example 9.3 Draw the RHSV of the object of Example 9.1, Fig. 9.6(a).

Solution It is clear that, in RHSV (i.e., the view in the direction W) the faces B,

G and D will not be seen. However, as these are the hidden features, they are

shown by dashed lines, Fig. 9.8.





Example 9.5 For the object shown in Fig. 9.10(a), draw FV (in direction X), TV,

LHSV and RHSV.

Solution With reference to Fig. 9.10(a), the faces that will be seen as true shapes,

apparent shapes and hidden faces in different views are shown in the following

table:

Figure 9.10(b) shows the required four views. Please note carefully how hidden

lines are drawn for the hidden faces.







Circular Features

The objects having circular faces (like cylindrical projections, holes, flanges, etc.)

are characterized by the centrelines of the circular features.

Example 9.6 Draw FV, TV and RHSV of the object shown in Fig. 9.11(a).

Solution The object has a cylindrical projection on the front face and a vertical

hole. Fig. 9.11(b) shows its three views. Note how centrelines are used to

indicate the circular features. Also observe the dashed lines for the hole.

Precedence of Lines

Wherever visible line, hidden line and centrelines overlap, the following

precedence rules should be observed:

1. A visible line has precedence over a hidden line and a centreline, i.e., visible

line should only be drawn if it overlaps with a hidden line and/or centreline.

2. A hidden line has precedence over a centreline, i.e., a hidden line should only

be drawn if it overlaps with a centreline.

3. If a visible line or a hidden line precedes a centreline, the ends of the

centreline should be drawn to show its existance.





SECTIONAL VIEWS

The internal hidden details of the object are shown in orthographic views bydashed lines. Obviously, the intensity of dashed lines in orthographic views

depends on the complexity of internal structure of the object. Therefore, the

general practice is to draw sectional views for complex objects in addition to or

instead of simple orthographic views. A sectional view, as the name suggests, is

obtained by taking the section of the object along a particular plane. An imaginary

cutting plane is used to obtain the section of the object.

Types of Cutting Planes and Their Representation

A cutting plane is represented by a cutting plane line as explained in Chapter 2,

Section 2.2.4. The cutting plane line indicates the line view of the cutting plane.

The two ends of the cutting plane line are made slightly thicker and provided witharrows. The direction of the arrow indicates the direction of viewing of the object.

In the first-angle method of projection, the direction of the arrows is toward the

POP, i.e., toward XY (or X 1Y 1). Different types of section planes are explained in

Fig. 9.25



Auxiliary Inclined Plane



Hatching of the Sections

The surface created by cutting the object by a section plane is called as section.

The section is indicated by drawing the hatching lines (section lines) within thesectioned area. The hatching lines are drawn at 45° to the principal outlines or

the lines of symmetry of the section, Fig. 9.26. The spacing between hatching lines

should be uniform and in proportion to the size of the section.



Various Sectional Views

Figure 9.27(a) shows an object with the vertical cutting plane A ± A, horizontal

cutting plane B ± B and profile cutting plane C ± C marked on it. The correspondingsectional views are shown in Fig. 9.27(b)



Methods of Sectioning

The sectioning should be made in such a way that all the complicated internal

features of the object will be as clear as possible. The various methods of sectioning

are explained below.

F ull Section The sectional view obtained after removing the front-half portion of

an object through its centre is known as a full section, Fig. 9.28(a).

Half Section The sectional view obtained after removing the front quarter portion

by means of two cutting planes at right angles to each other is known as half section,

Fig. 9.28(b).



O ffset Section The sectional view obtained by a cutting plane in a zigzag way so

as to reveal the maximum details of the object is known as an offset section, Fig.

9.29.



Revolved Section A revolved section is used to show the uniform shape of the object

from end to end, Fig. 9.30.

Removed Section A removed section is used to show the variable shape of the object

from end to end, Fig. 9.31.



Aligned Section An aligned section is used to show the shape of features that do not

align with the vertical and horizontal centrelines of the object, Fig. 9.32.

Ribs and Other Standard Parts in Section When a cutting plane passes longitudinally

through the centre of the ribs, spokes, webs or other standard parts, they are not

shown sectional because it gives a wrong impression of the thickness or of the other

details, Fig. 9.33.



Example 9.20 Draw the sectional FV, TV and SV of the object shown in Fig.

9.34(a).

Solution The object is cut by cutting plane A± A as shown. This will give a full

sectional view. As indicated by the direction of arrows on the cutting plane line,

the FV will be sectional. The three views of the object, namely, sectional FV, TV

and LHSV are shown in Fig. 9.34(b). The cutting plane line A± A is also shown in

TV. As already explained, in the first-angle method, the arrows of cutting plane

line are pointed toward XY, i.e., toward the other view. In the third-angle method,

the arrows on the cutting plane line in one view (say TV) will be pointing away

from other view (say FV).



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