First, Second, Third and Fourth Angle projections

Third Angle

Starting with a drawing like the one at top, you can break out the angles to arrive at projections like the one of the Third Angle above.

The next step after gaining approval of a proposal drawing (see Chapter 6, Lesson 1) is to make a working drawing, and that involves using a drawing system called Orthographic Projection. The system has lines that delineate shapes, symbols that convey precise information, dimensions that are clearly stated by numbers and words that express anything that lines, symbols and numbers cannot.

Orthographic Projection specifies where each part of the drawing is placed. It helps to think of whatever project you’re designing as being boxlike or as fitting in a box. The first thing to note is that, although the “box” has six sides, we can see only three of them at any one time. These visible faces are typically referred to as the front, side and top. The most efficient way to draw the box to record its shape and dimension is to draw each of the faces. To avoid confusion, the faces must be placed in a consistent relationship to each other. This relationship is called the layout. It seems simple enough until you realize that there is more than one way to position the front, side and top views in relation to one another.

To understand the different possibilities, imagine that you have the apparatus drawn at the top right. The back is a fixed plane to which the vertical and horizontal planes are hinged so you can fold them flat as if they were a sheet of drawing paper.

Place the same box in each quadrant and project the three images onto the planes. When you fold the vertical and horizontal planes flat, you will see how the images relate to one another in each layout. The four layouts are named First Angle, Second Angle, Third Angle and Fourth Angle. The diagram above shows the Third Angle, which is commonly used in the United States.

The diagram to the right shows the Third Angle, which is commonly used in the United States.

The image of the box is projected onto three surfaces. The two on the walls (the vertical planes) are called front elevation and end or side elevation. The image on the floor or ceiling (the horizontal plane) is called the plan.

The images on all three planes are what would be seen if you, the viewer, were standing outside the apparatus. Don’t be confused by thinking that the projected image is the one facing the plane: It’s the opposite side.

On a simple drawing, it may seem obvious which image is which elevation or plan. Nonetheless, each must be named. The name is normally placed below, with the first letter aligned with the first line of the drawing.

Of the four layouts available in the system, the First Angle and Third Angle are the two commonly used, the difference being the location of the plan above or below the elevations. That being said, drawings are occasionally positioned out of sequence, usually in the cause of showing an awkward detail. This underlines the importance of naming every element to avoid confusion.

Features of a Formal Drawing

To more clearly express the design of this conference table, a detail is drawn full-size. If limited to the small scale of the large table, it would be lost.

Lines: Three line weights are used: bold, regular and light. Bold lines are used for outlines and baselines. They make the piece of work stand out. Regular lines delineate the parts and the details. Light lines normally don’t represent parts that you could touch on the real thing, and for this reason they need to be in the background. Typically, dimension, break and hatch lines are all light lines.

Dimension lines are placed close enough that the relationship is obvious. The measurement may be placed on, over or under the continuous dimension line. Feet and inch symbols are not used.

Witness lines are an extension of the dimension they record. The preferred terminal is a 45° slash drawn freehand or with a drafting triangle — simple and quick, unlike arrowheads, which tend to be overdrawn.

Break lines indicate that the part continues beyond a particular point but that the continuation is not pertinent; they’re also used to compress a long element, with the dimension giving the true length.

Hatch lines indicate the cut surface of a section. They’re drawn at 45°.

Broken lines, sometimes inaccurately called dotted lines, show concealed parts.

Center lines are used when you want to increase the scale of a symmetrical drawing by drawing only half of the object; they’re marked with the initials CL.

Words: There are three places on the drawing that require words. First, each view and section must be named. Next, there’s a “specification column” that says what the piece is made of, what finish is used, what hardware and so on. Finally, there’s a “title block” that contains the name of the piece, whom it’s for, the scale, the date of the drawing and who drew it. Putting words and numbers on a drawing is called “lettering.” Always letter between light guidelines drawn 1/8" to 3/16" apart.