Introduction

One of the best ways to communicate one's ideas is through some form of picture or drawing. This is especially
true for the engineer. The purpose of this guide is to give you the basics of engineering sketching and drawing.
We will treat "sketching" and "drawing" as one. "Sketching" generally means freehand drawing. "Drawing"
usually means using drawing instruments, from compasses to computers to bring precision to the drawings.
This is just an introduction. Don't worry about understanding every detail right now - just get a general feel for
the language of graphics.
We hope you like the object in Figure 1, because you'll be seeing a lot of it. Before we get started on any
technical drawings, let's get a good look at this strange block from several angles.

Figure 1 - A Machined Block.

Isometric Drawing
The representation of the object in figure 2 is called an isometric drawing. This is one of a family of three-
dimensional views called pictorial drawings. In an isometric drawing, the object's vertical lines are drawn
vertically, and the horizontal lines in the width and depth planes are shown at 30 degrees to the horizontal.
When drawn under these guidelines, the lines parallel to these three axes are at their true (scale) lengths. Lines
that are not parallel to these axes will not be of their true length.

Figure 2 - An Isometric Drawing.

Any engineering drawing should show everything: a complete understanding of the object should be possible
from the drawing. If the isometric drawing can show all details and all dimensions on one drawing, it is ideal.
One can pack a great deal of information into an isometric drawing. However, if the object in figure 2 had a
hole on the back side, it would not be visible using a single isometric drawing. In order to get a more complete
view of the object, an orthographic projection may be used.
Orthographic or Multiview Drawing
Imagine that you have an object suspended by transparent threads inside a glass box, as in figure 3.

Figure 3 - The block suspended in a glass box.

Then draw the object on each of three faces as seen from that direction. Unfold the box (figure 4) and you have
the three views. We call this an "orthographic" or "Multiview" drawing.

Figure 4 - The creation of an orthographic Multiview drawing.

Figure 5 - A Multiview drawing and its explanation.

Which views should one choose for a Multiview drawing? The views that reveal every detail about the object.
Three views are not always necessary; we need only as many views as are required to describe the object fully.
For example, some objects need only two views, while others need four. The circular object in figure 6 requires
only two views.

Figure 6 - An object needing only two orthogonal views.

Dimensioning

Figure 7 - An isometric view with dimensions.

We have "dimensioned" the object in the isometric drawing in figure 7. As a general guideline to dimensioning,
try to think that you would make an object and dimension it in the most useful way. Put in exactly as many
dimensions as are necessary for the craftsperson to make it -no more, no less. Do not put in redundant
dimensions. Not only will these clutter the drawing, but if "tolerances" or accuracy levels have been included,
the redundant dimensions often lead to conflicts when the tolerance allowances can be added in different ways.
Repeatedly measuring from one point to another will lead to inaccuracies. It is often better to measure from one
end to various points. This gives the dimensions a reference standard. It is helpful to choose the placement of
the dimension in the order in which a machinist would create the part. This convention may take some
experience
Difference between First Angle Projection and Third Angle Projection
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The orthographic projection system is used to represent a 3D object in a 2D plane. The orthographic projection
system utilizes parallel lines, to project 3D object views onto a 2D plane. According to the rule of orthographic
projection. To draw a projection view of a 3D object on a 2D Plane. The horizontal plane is rotated in the
clockwise direction.
Types of Orthographic projection systems are first angle and third angle projection.
1. First Angle Projection:
In the first angle projection, the object is placed in the 1st quadrant. The object is positioned at the front of a
vertical plane and top of the horizontal plane. First angle projection is widely used in India and European
countries. The object is placed between the observer and projection planes. The plane of projection is taken
solid in 1st angle projection.
Symbol –

2. Third Angle Projection:

In the third angle projection, the object is placed in the third quadrant. The object is placed behind the vertical
planes and bottom of the horizontal plane. Third angle projection is widely used in the United States. The
projection planes come between the object and observer. The plane of projection is taken as transparent in 3rd
angle projection.
Symbol –
Difference between First Angle Projection and Third Angle Projection:

SR.N
O First Angle Projection Third Angle Projection

1 The object is placed in the first quadrant. The object is placed in the third quadrant.

The object is placed between the plane of The plane of projection is placed between
2
projection and observer. the object and observer.

3 The plane of projection is opaque. The plane of projection is transparent.

Front view is at the top of the horizontal Front view at the bottom of the horizontal
4
axis. axis.

Top view at the bottom of the horizontal

5 Top view at the top of horizontal axis.
axis.

Right view is at the right side of vertical

6 Right view is at the left side of vertical axis.
axis.

7 Left view is at the right side of vertical axis. Left view is at the left side of vertical axis.

It is widely used in United State and

8 It is widely used in Europe, India, Canada.
Australia.

First Angle dimension

Concentric Circles Definition

Circles that have the same center point are known as concentric circles, they fit within each other and have the
same distance apart from each way around. Having different radius, the region between two concentric circles is
known as the annulus. According to Euclidean Geometry, the condition for two circles being concentric is that
they must not have the same radii.

Difference between concentric and eccentric.

Using Scale Factor to Find Actual Dimensions

People usually confuse between these two, so to avoid that, we must understand what an eccentric circle is. The
circles which are usually contained within another circle are called eccentric circles, given that they may or may
not have the same center. When they share the same center, they can be called as concentric circles.

A scale factor is somewhat like a unit scale. It is a ratio that compares scale dimensions to actual dimension. It
differs because it does not give any specific units.
A scale factor for a scale drawing might be 1/200, which means that the actual size is 200 times the drawing.
Scale factors can be used to determine both scale and actual sizes or dimensions.
Example
Earlier, you were given a problem about the model train displays at the fair.
The smallest one is an "N" gauge, which is a scale of 1:160. If the display area is 4 feet by 8 feet, how big an
area does the display represent?
First, write the scale factor.
Scale/actual=1/160
Next, write ratios to represent the unknown actual length and width of the display to the scale length and width
of the display.
Length=scale/actual=4/l
Width=scale/actual=8/w
Then, write two proportions by setting the two length ratios equal to one another and the two width ratios equal
to one another. The given unit is feet.
Length =4/l=1/160
Width= 8/w=1/160
Next, cross multiply each equation.
L=4×160
L=640 feet
W=8×160
W=1,280 feet
Then, to find the area of the display, multiply length times width.
640 ft.×1,280 ft.=819,200 sq.ft.
The answer is 819,200 square feet.

Type of line and thickness

Traditional drafters need to be able to create several different line widths because different line widths convey
different information.

The first step in creating a traditional technical drawing is to align the paper so that it will be positioned square
to the parallel bar.