3 Freehand Sketching

Freehand Sketch

PROJECTION METHOD

Perspective

Parallel

Oblique

Orthographic

Axonometric

Multiview

PROJECTION THEORY

The projection theory is used to graphically represent 3-D objects on 2-D media (paper, computer screen).

The projection theory is based on two variables:

Line of sight
Plane of projection (image plane or picture plane)

Line of sight is an imaginary ray of light between an observer’s eye and an object.

There are 2 types of LOS : parallel

and

converge

Parallel projection

Perspective projection

Line of sight

Plane of projection is an imaginary flat plane which

the image is created.

The image is produced by connecting the points where the LOS pierce the projection plane.

Parallel projection Perspective projection

Plane of projection Plane of projection

Disadvantage of Perspective Projection

Width is distorted

Perspective projection is not

used by engineer for manu- facturing of parts, because

It is difficult to create.
It does not reveal exact shape and size.

Oblique projection of an object may be obtained by

projecting the object with parallel projections.
- In oblique projection, the front face of the object appears in its true size and shape, as it is placed parallel to the picture plane.

Orthographic Projection

Orthographic projection is a parallel projection technique in which the parallel lines of sight are perpendicular to the projection plane MEANING

Object views from top

ORTHOGRAPHIC VIEW

Orthographic view depends on relative position of the object

to the line of sight.

More than one view is needed to represent the object.

Multiview drawing

Three dimensions of an object is shown.

Axonometric drawing ORTHOGRAPHIC VIEW

NOTES

Multiview drawing

Orthographic projection technique can produce either

Axonometric drawing

that show all three dimensions of an object in one view.

Both drawing types are used in technical drawing for communication.

Axonometric (Isometric) Drawing

Advantage

Easy to understand

Disadvantage

Shape and angle distortion

Example

Distortions of shape and size in isometric drawing

Circular hole becomes ellipse.

Right angle becomes obtuse angle.

Multiview Drawing

Advantage

It represents accurate shape and size.

Disadvantage

Require practice in writing and reading.

Example

Multiviews drawing (2-view drawing)

Freehand Sketching

Straight Line

Hold the pencil naturally.
Spot the beginning and end points.
Swing the pencil back and forth between the points, barely touching the paper until the direction is clearly established.
Draw the line firmly with a free and easy wrist-and-arm motion

Horizontal line Vertical line

Nearly vertical inclined line

Nearly horizontal inclined line

Small Circle

Method 1 : Starting with a square
1. Lightly sketching the square and marking the mid-points.
2. Draw light diagonals and mark the estimated radius.
3. Draw the circle through the eight points.

Step 1 Step 2 Step 3

Small Circle

Method 2 : Starting with center line

Lightly draw a center line.
Add light radial lines and mark the estimated radius.
Sketch the full circle.

Step 1 Step 2 Step 3

Large Circle

Place the little finger (or pencil’ s tip) at the center as a pivot, and set the pencil point at the radius-distance from the center.
Hold the hand in this position and rotate the paper.

Method 1 : Starting with a square

Method 2 : Starting with a center line

Steps in Sketching
1. Block in main shape.
2. Locate the features.
3. Sketch arcs and circles.
4. Sketch lines.

Example

Drawing Standard

Standards are set of rules that govern how technical drawings are represented.

Introduction

Drawing standards are used so that drawings convey the same meaning to everyone who reads them.

Partial List of Drawing Standards

Code number	Contents
JIS Z 8311	*Sizes and Format of Drawings*
JIS Z 8312	*Line Conventions*
JIS Z 8313	*Lettering*
JIS Z 8314	*Scales*
JIS Z 8315	Projection methods
JIS Z 8316	Presentation of Views and Sections
JIS Z 8317	Dimensioning

Drawing Sheet

Trimmed paper of

a size A0 ~ A4.

Standard sheet size

(JIS) A2

A4 210 x 297

A3 297 x 420 A1

A2 420 x 594

A1 594 x 841

A0 841 x 1189

(Dimensions in millimeters) A0

Orientation of drawing sheet

Sheet size	c (min)	d (min)
A4	10	25

A3	10	25
A2	10	25
A1	20	25
A0	20	25

Drawing Scales

Scale is the ratio of the linear dimension of an element of an object shown in the drawing to the real linear dimension of the same element of the object.

Size in drawing Actual size

Drawing Scales

Designation of a scale consists of the word “SCALE“

followed by the indication of its ratio, as follow

SCALE 1:1 for full size

SCALE X:1 for enlargement scales (X > 1) SCALE 1:X for reduction scales (X > 1)

Dimension numbers shown in the drawing are correspond to “true size” of the object and they are independent of the scale used in creating that drawing.