Creating Machine Drawing Projections

In technical drawings, the view of an object is called a projection. A projection is made by extending lines of sight, known as projectors, from the observer’s eye through the edges and points of an object to a flat surface, called the projection plane. This process creates the kind of view shown in Image 1-1.

This type of projection is called central projection because the projectors meet at a single point: the observer’s eye. The size of the projected view can change a lot, depending on where the object and the projection plane are positioned. It also depends on how far the observer is from the object and the plane. Because of these variations, central projection isn’t used very often in technical drawings.

If the observer is far enough away from both the object and the projection plane, the projectors won’t meet at a point anymore. Instead, they’ll run parallel to each other. This method is called parallel projection, and it’s the one most commonly used in technical drawings because it’s simpler. As shown in Image 1-3, view B, when the projectors are at right angles to the projection plane, the parallel projection will keep the object’s true size. This stays the same no matter where the object and the projection plane are or how far the observer is.

Orthographic and Oblique Projection

An orthographic projection is a type of parallel projection where the projectors are at right angles (perpendicular) to the projection plane, as shown in Image 1-2. An oblique projection is different because the projectors hit the projection plane at an angle other than 90 degrees.

Image 1-3 shows the same object using both orthographic and oblique projections. In the example, the block’s front surface—facing the projection plane—is placed parallel to it. In the orthographic projection, only the front surface of the block is visible, showing just two dimensions: length and width.

The oblique projection, however, shows both the front surface and the top surface of the block. This means it includes all three dimensions: length, width, and height. Because of this, oblique projection is one way to show all three dimensions of an object in a single view. Another method is called axonometric projection, which we’ll cover later in this lesson.

Principal Planes of Projection

When you look at an object through a projection plane from a point far away (essentially at infinity), you get an accurate outline of the visible face of the object, as shown in Image 1-4. However, seeing just one face doesn’t usually give you a complete idea of the object. To fully describe it, you need to use other projection planes.

To figure out an object’s true height, width, and depth, you’ll need to look at it from three main views: the front, the top, and the side. These are called the principal planes of projection. The three principal planes, shown in Image 1-5, are the vertical plane, the horizontal plane, and the profile plane (side view).

The angles formed between the horizontal and vertical planes are divided into four quadrants called the first, second, third, and fourth angles. For practical reasons, only the first-angle and third-angle projections are commonly used today.

First-Angle Projection

First-angle projection is shown using a cube in Image 1-6. In this method, the front of the cube faces the vertical plane of projection. The result is a front view on the vertical plane, a left side view on the profile plane, and a top view on the horizontal plane.

To transfer these views onto drafting paper, all the views must be placed in the same plane. Start by imagining the vertical plane of projection is already flat on the paper. Then, rotate the profile plane counterclockwise and the horizontal plane clockwise. When you do this, the views line up as shown in Image 1-7.

In European drafting, this first-angle projection is commonly used and considered acceptable. However, in the United States, it’s seen as confusing. This is because the top view appears below the front view, the left side view seems to show the right side of the object, and the bottom of the object is shown at the top of the layout. For these reasons, first-angle projection is rarely used in American drafting.

Third-Angle Projection

In third-angle projection, as shown with a cube in Image 1-8, you get a front view on the vertical plane, a right side view on the profile plane, and a top view on the horizontal plane. The front view, located on the vertical plane, is already placed on your drawing paper. To bring the other views into the same plane, you rotate them both clockwise. This creates a third-angle projection, as seen in Image 1-9.

In this layout, the top view is positioned above the front view, the right side of the object (as shown in the front view) aligns with the right side view, and the top from the front view is positioned above the top view.

Here’s how to make the third-angle projection shown in Image 1-9:

1. Draw a horizontal line (AB) and a vertical line (CD) that intersect at O (Image 1-10).
   • Line AB marks the joint between the horizontal and vertical planes.
   • Line CD marks the joint between these two and the profile plane.
2. You can start by drawing any of the three views, with the other views projected from it. Let’s assume you start with the front view based on the given dimensions.
3. Draw the front view, then project it upward with vertical projection lines to create the top view.
4. To get the side view, project the top view over to the CD line with horizontal projection lines.
5. Use O as the center and a compass to extend these projection lines to the AB line.
6. Finally, draw the right side view by extending the vertical projection lines downward from AB, and project the right side of the front view horizontally to the right.

Use of a Miter Line

A miter line (Image 1-11) can help you draw a third view while you’re still working on the other two views, as shown in Image 1-11, view A.

To use a miter line:

1. Place the miter line (Image 1-11, view B) to the right of the top view at a comfortable distance, ensuring the drawing looks balanced.
2. Draw light projection lines from the top view to the miter line (Image 1-11, view C).
3. Then, draw vertical projection lines downward (Image 1-11, view D).
4. Use the front view to draw horizontal projection lines (Image 1-11, view E) toward the right, making them intersect with the vertical projection lines.

Following these steps will outline and position the right side view (Image 1-11, view F).

In some drawings, projection lines from the top view are extended to help create the right side view.

Isometric Projection

Isometric projection is the most commonly used type of axonometric projection, a method that shows an object in all three dimensions in one view.

Axonometric projection is a form of orthographic projection, where the projectors are always perpendicular to the projection plane. However, the object itself is angled to the projection plane, not the projectors.

In isometric projection, as shown with a cube in Image 1-12, the cube is angled so that all its surfaces make the same angle with the projection plane. As a result, the edges of the cube in the projection appear shorter than they are in real life. This shortening is called foreshortening.

Since all the surfaces are at the same angle to the projection plane, the edges foreshorten at the same ratio. Because of this, only one scale is needed for the entire drawing. That’s why it’s called isometric, which literally means “one scale.”

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Images and some concepts are from the “NONRESIDENT TRAINING COURSE” U.S. Navy dated September 2015. The remainder is my own descriptions and narrations.