Pp chapter 5

Technical Sketching and Shape
Description

Technical Sketching

Projections
Projections
Behind every drawing of an object is a space
relationship involving four imaginary things:
1. The observer’s eye or the station point
2. The object
3. The plane of projections
4. The projectors or visual rays or lines of
sight

In the diagram, (efgh) is the projection of the object (ABCD) on the plane of projections (A) as
viewed by the observer whose eye is at the station point (O).
The image on the plane is produced by the points at which the projectors pierce the plane of
projection.
The projectors for a “cone” of projectors resulting in a foreshortened image known as a
perspective.

Projections
If the observer’s eye is imagined as infinitely
distant from the object and the plane of
projection, the projectors will be parallel.
This type of projection is know as a parallel
projections.
If the projectors are also perpendicular to the
plane of projection the result is an orthographic
or right-angle projections.

Projections
Classification of projections:
There are two main types of projection:
Perspective (central projection)
Parallel Projection

Multiview Projection
The method of viewing an object to obtain a multiview projection is illustrated in figure a. Between
the observer and the object a transparent plane is located parallel to the front view. The view is
obtained by drawing perpendicular lines (projectors) from all points of the edges of the object to the
plane of projection (figure b). The piercing points of these projectors form lines on the projection
plane (figure c)

A similar procedure can be used to obtain the top view (figure a) and the
right-side view (figure b).

If planes of projection are placed parallel to the principal faces of the object, they form a “glass
box” as shown in figure a. Since the glass box has six sides, six views of the object can be
obtained.
To show the views on a flat sheet of paper it is necessary to unfold the planes so that they will all
lie in the same plane. All planes except the rear plane are hinged to the frontal plane (figure b).

The positions of the six
planes after they have
been revolved are shown.
The front, top, and
bottom views all line up
vertically and are the
same width.
The rear, left-side, front,
and right-side views all
line up horizontally and
are the same height.

The front, top, and right-side views of the object are shown with folding lines between the views.
These folding lines correspond to the hinge lines of the glass box (figure a).
The H/F folding line is between the top and front views.
The F/P folding line is between the front and right-side views.
Folding lines are useful in solving graphical problems in descriptive geometry. As a rule folding
lines are omitted in industrial practice (figure b).

Views of an Object
A pictorial drawing shows an object as it
appears to the observer but cannot describe the
object fully because it does not show the exact
shapes and sizes no matter which direction it is
viewed from.
Industry requires a more complete and clear
description of an object to make certain the
object is manufactured exactly as intended by
the designer or engineer.
To accurately describe an object a number of
systematically arranged views are used. This
system is called multiview projection.
To obtain a view the observer is looking
perpendicularly toward one of the faces of the
object to obtain a true view of the shape and
size of that side.

Views of an Object
Views of an object can be
obtained by revolving the object.
To obtain the top view, hold the
object in the front view position.
Revolve the object to bring the top
of the object up and toward you.
To obtain the right side view, hold
the object in the front view
position. Revolve the object to
bring the right side toward you.
The front, top, and right side views
are arranged as shown and are
called the three regular view
because they are the views most
frequently used.

Views of an Object
Any object can be viewed
from six mutually
perpendicular directions.
The six views are always
arranged as shown.
The three principal
dimensions of an object
are Height, Width, and
Depth. Any one view can
only show two
dimensions. The third
dimension is found in an
adjacent view.

Views of an Object
The front view of an object should show the
object in its operating position. The front
view should also show the best shape of the
object and the most detail.
In the example the side of the automobile
was selected as the front view of the
drawing rather than the actual front of the
automobile.
Machine parts are often drawn in the
position that it occupies in the assembly
drawing.

Views of an Object
A production drawing should show only those views needed for a clear and complete shape
description of the object. Often only two views are needed to clearly describe the shape of an
object.
In selecting the views, show only those that best show the essential contours or shapes and
have the lease number of hidden lines.
Unnecessary or duplicate views are eliminated or not shown. In the example, the left side,
rear, and bottom views are eliminated.

If three views of an object are drawn using the conventional arrangement of views a large
wasted space is left on the paper (figure a). In such cases the profile plane may be considered
hinged to the horizontal plane instead of the frontal plane which results in better spacing of the
views (figure b).

Technical Sketching
Freehand sketches are of great value to
designers and engineers in organizing their
thoughts and recording their idea.
The term “freehand sketch” does not mean a
crude or sloppy freehand drawing.
A freehand sketch should be made with care
and with attention to proportion, clarity, and
correct line technique.
A freehand sketch only requires a pencil, paper,
and eraser.

Technical Sketching
Soft pencils, such as HB or F, should be used
for freehand sketching.
A sharp point is used to produce thin lines for
drawing center lines, hidden lines, and
dimension and extension lines. These lines
should be thin and dark.
A rounded point is used to produce visible
object lines that are thick and dark.
A sharp point is also used to draw construction
lines. Construction lines are drawn thin and
light.

Technical Sketching
A good freehand line is not expected to be as
rigidly straight or exactly uniform as a line drawn
with instruments.

Technical Sketching

One method of sketching circles is to lightly
sketch the enclosing square, mark the
midpoints of the sides, draw arcs tangent to the
sides of the square, then heavy in the final
circle.

Another method is to sketch the two center
lines, add light radial lines, sketch light arcs
across the lines a the estimated radius distance
from the center, then heavy in the final circle.

Sketching Three Views
Steps in Making a Sketch

Step 1:
block in the enclosing rectangle
for the three views using
construction lines.

Step 2:
Block in all details using
construction lines.

Step 3:
Sketch all arcs and circles using
construction lines.

Step 4:
Lighten all construction lines.

Step 5:
Darken in all final lines.

Hidden lines
Hidden lines are used to show hidden features. They are made thin and dark (dense black).
A hidden line is a dashed line consisting of 1/8” dashes with 1/32” spaces.

Correct and incorrect
practices in drawing
hidden lines.

Center Lines
Center lines are used to indicate axes of symmetry, bolt circles, paths of motion and in
dimensioning. They are made thin and dark (dense black).
A center line consists of a long line, short dash, and a long line.
Center lines extend 1/4” past the feature for which they were drawn.

Examples of center line
applications.

Precedence of lines:
Visible object lines, hidden lines, and center
lines often coincide on a drawing. The
drafter must determine which lines to show
and which ones to eliminate.
A visible object line always takes
precedence over hidden lines and center
lines (A) & (B).
A hidden line always takes precedence over
a center line (C).

No line should be drawn where a curved surface is tangent to a plane surface. When a curved
surface intersects a plane surface a definite edge is formed. Show are examples of
intersections and tangencies.

The correct method of representing fillets in connection with plane surfaces tangent to
cylinders is shown in figure a and figure b. These small curves are called runouts. Runouts
have a radius equal to that of the fillet and a curvature of one eighth of a circle (figure c).

Examples of
typical filleted
intersections.

The Glass Box Method of Projection
The Glass Box:
If planes of projection are placed
parallel to the principal faces of an
object, they form a “glass box” as
shown.
Notice that the observer is always on
the outside looking in so that the
object is seen through the planes of
projection.
Since the glass box has six sides, six
views of the object are obtained.

The Glass Box:
Since it is required to show the views
of a three-dimensional object on a
flat sheet of paper, it is necessary to
unfold the planes so that they will all
lie in the same plane.
All planes except the rear plane are
hinged to the frontal plane, the rear
plane being hinged to the side plane.

Each plane revolves outwardly from
the original box position until it lies in
the frontal plane.

The Glass Box:
Alignment of the six principal views

The Glass Box: Planes of Projection
Frontal plane of projection – the plane upon
which the front view is projected.
Horizontal plane of projection – the plane upon
which the top view is projected.
Profile plane of projection – the plane upon
which the side view is projected.

The Glass Box: Fold Lines
Fold lines are imaginary lines separating the
planes of projection corresponding to the hinged
lines of the glass box.

The Glass Box:
Any one view of an object can only show two
principal dimensions, the third dimension must
be found in an adjacent view.

Pp chapter 5

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