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Figure 5.17B An example of the use of isometric drawings in architecture and engineering.

### Comparison of Isometric and Orthographic Drawings

Compare the simple rectangular block shown in the orthographic representation (third-angle projection) in Figure 5.18 and the three-dimensional isometric representation. Notice that the vertical lines of the orthographic and isometric drawings (views A and B) remain vertical. The horizontal lines of the orthographic drawing are not horizontal in the isometric drawing but are projected at 30- and 60-degree angles; the length of the lines remains the same in the isometric and in the orthographic drawings.

Purpose of Isometric Drawing

The task of an isometric drawing is primarily to show a three-dimensional picture in one drawing. It is like a picture that lacks artistic details. Many utilities workers have difficulty in clearly visualizing a piping or ducting installation when they are working from a floor plan and an elevation drawing. The isometric drawing facilitates understanding by combining the floor plan and the elevation. It clearly communicates the details and clarifies the relationship of the pipes in an installation. Although isometric drawings are not normally drawn to scale on blueprints, some architects and engineers prefer drawing them to scale. Isometric drawings, like other types of drawings, follow certain rules and conventions to show three dimensions on a flat surface.

### Dimensioning Isometric Drawings

An isometric drawing, or sketch, is dimensioned with extension and dimension lines in a manner somewhat similar to that of a two-dimensional drawing. The extension lines extend from the drawing, and the dimension lines are parallel to the object line and of equal length to it. Dimensioning the isometric drawing is more difficult because it consists of a single view, with less room available than on three separate views.

Circles or holes will be skewed or drawn within an isometric square. For example, a circle will appear elliptical in shape and is actually drawn by connecting a series of four arcs, drawn from the centerlines of the isometric square. The ellipses may also be drawn with the use of templates. Curved or round

corners are drawn in the same manner by locating the end of the radius on the straight line and then connecting the two points to form a triangle. The third point of the arc is actually the center of the triangle. Connect the three points with a freehand arc.

In isometric projections the direction of viewing is such that the three axes of space appear equally foreshortened. The displayed angles and the scale of foreshortening are universally known. However, in creating a final, isometric instrument drawing, a full-size scale—i.e., without the use of a foreshortening factor—is often employed to good effect because the resultant distortion is difficult to perceive.

Isometric drawing render a three-dimensional view of an object in which the two sets of horizontal lines are drawn at equal angles and all vertical lines are drawn vertically. In the resulting drawing all three angles are equally divided about a center point, and all three visible surfaces have equal emphasis. Orthographic techniques cannot be used in isometric drawings.

Any angle can be used to draw an isometric view, but the most common is 30 degrees because it can be drawn with a standard triangle and gives a fairly realistic view of an object. Today, CAD programs are the easiest way to draw isometric projections, but isometrics are also quick to draw manually and can be measured at any convenient scale. To manually draw in isometric, you will need a 30/60-degree set square.

There are four simple steps to manually draw a 5-inch box in isometric (Figure 5.19):

1. Draw the front vertical edge of the cube.

2. The sides of the box are drawn at 30 degrees to the horizontal to the required length.

3. Draw in the back verticals.

4. Drawn in top view with all lines drawn 30 degrees to the horizontal.

When you first start working with isometric techniques, use a simple box as a basic building block or guide to help you draw more complicated shapes. Figure 5.20 shows how to use such a simple box to accurately draw a more complicated L shape.

The first step is to lightly draw a guide box. This box is the size of the maximum dimensions. In this case, it measures 5 inches in length, 2.5 inch in width, and 5 inches in height. To achieve the L shape, we need to remove an area from this box. Draw a second box measuring 4 x 1 x 5 inches, the shape that needs to be removed from the first box to create the shape we require. For the finished shape, draw in the outline of the object using a heavier line. By using this technique complex shapes can be accurately drawn.

Circles in isometric do not appear circular. They appear skewed and are actually elliptical. There are several methods of constructing circles in isometric drawing. For many manual tasks the easiest method is to use an isometric circle template, which can be bought at most good art shops. These templates contain a number of isometric circles of various sizes.

Isometric circles can also be drawn manually using the following method:

1. Draw an isometric square and then draw in the diagonals, a vertical, and a line at 30 degrees from the midpoint of the sides, as illustrated in Figure 5.21.

2. Place your compass point on the intersection of the horizontal and the vertical lines and draw in a circle that touches the edges of the box

3. For the next section of the isometric circle place your compass point on the corner of the isometric square and draw in the arc as shown in the illustration.

4. Complete the circle by repeating the process for the other parts, using the appropriate techniques.

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