KEYCREATOR 3D Direct Modeling Software
KeyCreator Lesson KC9601
Mechanical Drawing Projection
The widespread use of CAD
(Computer Aided Design) tools has
revolutionized the creation of
mechanical drawings throughout every
area in industry.
A CAD user who is well-trained in the
use of a particular software package
can create a detailed drawing in a
fraction of the time that was required
when drawings were made with
mechanical pencils and drafting
machines on a traditional drafting
table.
Unfortunately, many of the senior managers and professionals who attend CAD training
classes complain that while new hires are almost always quite adept at manipulating a
computer, in many cases, they really don’t understand how to create a world-class
mechanical drawing
In the rush to embrace technology it is easy to forget that mechanical drawings are a
language with rules and structure. As a communication tool, the mechanical drawing
greatly simplifies the conveyance of extensive details required to create a given part.
The old adage that “A picture is worth a thousand words” is really brought home when
you look at a drawing for almost any mechanical part.
A mechanical drawing is most effective when it is
created following generally accepted conventions. For
instance, a common convention is to show a feature
that is hidden in a particular view with a dashed line.
So, when you see two, parallel lines close together that
extend across the entire span of a part, you
immediately think that this might represent a hole that
goes through the part.
Next, you look at a companion view that is oriented at
right angles to this first view. If you see a circle that
corresponds to those two parallel lines, you confirm
the fact that there is, indeed, a hole in the part.
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Learning the language of mechanical drawing requires
practice. Just as you improve reading and writing skills
by reading books and writing stories and themes of
your own, you can master the creation and use of
mechanical drawings by looking at correctly executed
drawings and creating drawings of your own.
In these lessons, we’ll start with the basic
principles of representing a threedimensional object in a two-dimensional
drawing.
We’ll review the rules of projection and then
proceed to cover numerous ancillary topics,
such as the creation of sectional views, detail
views, and the proper ways to dimension and
add tolerances to a part.
It is equally useful if you decide to create a
drawing the old fashioned way, with paper
and pencil! If you’re new to mechanical
drawing and want to get a quick start in the
right direction, these lessons are meant for
you. So grab your mouse or pencil and let’s
start on our journey.
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Basic Drawing Views
Let’s start by talking about some very basic drawing conventions. For our initial
discussion, well work with the simple part illustrated to the right.
Now you can immediately see that trying to
describe this part by using just words and
numbers would be pretty difficult, even though
the part only has a few features. Providing a
picture of the part like this one helps, but we still
have to add actual sizes for the holes, etc.
Now, a typical simple drawing of the part
illustrated above might look like the one
illustrated below:
We call this type of drawing a Third Angle Projection View Orthographic Drawing. It is
the most common form of drawing standard used in the United States. All of the drawing
development presented in this book will follow this convention. In a moment, we’ll look
at how the views are developed in detail.
THIRD ANGLE PROJECTION ORTHOGRAPHIC DRAWING
TOP
VIEW
FRONT
VIEW
ISOMETRIC
VIEW
RIGHT
SIDE
VIEW
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Another drawing projection system, used in some other countries, but rarely seen in the
United States, is the First Angle Orthographic Projection System.
A drawing of our same part done using this system would look like the one illustrated
below:
FIRST ANGLE PROJECTION ORTHOGRAPHIC DRAWING
TOP
VIEW
ISOMETRIC
VIEW
FRONT
VIEW
RIGHT
SIDE
VIEW
In this projection system, the views are projected from the part surfaces out to the faces
of an imaginary glass box that surrounds the part. If you look at the top view, it looks like
you are underneath the part, looking up at the counter-bored hole from underneath in the
top view. That is why the counter-bore is shown as a hidden style circle.
Let’s return to our third angle
projection drawing from the previous
page.
We’ll start by looking at the top view.
Even to an untrained person, it is easy
to see that the two, concentric circles
at the right end of the view represent
the top rim of the counter-bore and
circular edge of the through hole that
lies beneath it.
You can also easily make out the
outline of the rest of the body.
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Looking at the left end of the view,
you see two, parallel, dashed lines.
These represent the through hole that
traverses from the front surface of the
cylindrical boss to the rear face of the
part.
A great way to visualize the projections of the orthographic views is to think of a glass
box that completely surrounds the object that you are documenting. The top-facing
surfaces of the object project up to the top plane of the box. Similarly, the right side
facing and front facing surfaces project to their respective planes on the box. I’ve
illustrated this below.
PROJECTION TO TOP PLANE
OF SURROUNDING BOX
CREATES TOP VIEW
PROJECTION TO FRONT
PLANE OF SURROUNDING
BOX CREATES FRONT VIEW
PROJECTION TO RIGHT SIDE
PLANE OF SURROUNDING
BOX CREATES RIGHT VIEW
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You can then unfold the
planes of the surrounding
box to create the third angle
projection drawing.
TOP SIDE OF GLASS
BOX UNFOLDS OUT
TO FRONT PLANE
Now you can actually
project to each of the six
faces of the surrounding
box, and in some very
complicated parts, you
might actually use more
than the three views that we
show here.
However, for the majority of
basic parts, the normal
convention is to show a
front view, a right side view,
and a top view.
RIGHT SIDE OF
GLASS BOX
UNFOLDS OUT TO
FRONT PLANE
FRONT PLANE
An isometric view in the upper, right corner of the drawing is also desirable since it
provides a really good concept of the part. In the old days, when we created drawings by
hand with pencil and paper, we often skipped the isometric view because it required some
extra skill and labor to create. Modern CAD systems can generate an isometric view (or
any view of a part) with a simple click of the mouse. So the current de facto standard is to
include one on most drawings.
The decision to add views to a drawing is
governed primarily by the need to provide
enough information so that someone else (The
manufacturer or the end customer.) can
completely understand what the part looks like.
Let’s look at a simple example to illustrate how
multiple views help to completely document a
part.
We’ll start with the simple part illustrated to the
right. Now, this part is basically a block with a
slot that ends in a full radius fillet.
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A front view of the part would look like this:
With dimensions we could determine the width and height of
the block and the width and height of the slot.
However, we have no way of knowing how deep the slot or
block is. There is also no indication that there is a blend
surface at the bottom of the slot.
FRONT VIEW
Adding a top view gives us the ability to quickly
provide dimensions on the depth of the block and
the slot.
TOP
VIEW
We still, however, have no clear indication of the
blend at the bottom of the slot. In fact, there are
many variations of our block that would have this
particular combination of top and front views.
FRONT
VIEW
I’ve illustrated a few to the left just to
get you thinking. (OK, I smell smoke
so I guess you’re getting a message
here!)
To completely define this part, we
really need to add a right side view.
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For this part, a four view
drawing makes it easy to
define.
THE ISOMETRIC
VIEW GIVES A
QUICK CONCEPT
OF THE PART
Adding a few dimensions
to the four view drawing
gives you a compact
description of the exact
part.
THE RIGHT SIDE VIEW
SHOWS THE EXACT
PROFILE OF THE SLOT
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