4-9 Isometric Sketches
Isometric sketches are based on an isometric axis that contains three lines, 120° apart. See Figure 4-13. The isometric axis can also be drawn in a modified form that contains a vertical line and two 30° lines. The modified axis is more convenient for sketching and is the more commonly used form. See Figure 4-14.
The receding lines of an isometric sketch are parallel. This is not visually correct, as the human eye naturally sees objects farther away as smaller than those closer. In reality, railroad tracks appear to converge; however, it is easier to draw objects with parallel receding lines, and if the object is not too big, the slight visual distortion is acceptable. See Section 4-12 for an explanation of perspective drawings whose receding lines are not parallel, but convergent.
The three planes of an isometric axis are defined as the left, right, and top planes, respectively. See Figure 4-14. When creating isometric sketches, it is best to start with the three planes drawn as if the object were a rectangular prism or cube. Think of creating the sketch from these planes as working with a piece of wood, and trim away the unnecessary areas. Figure 4-15 shows an example of an isometric sketch. In the example, the overall proportions of the object were used to define the boundaries of the object, and then other surfaces were added as necessary.
Isometric sketches may be sketched in different orientations. Figure 4-16 shows six possible orientations. Note how orientation 1 makes the object look as if it is below you, and orientation 6 makes it look as if it is above you. Orientation can also serve to show features that would otherwise be hidden from view. The small cutout is clearly visible in only one of the orientations. Always try to orient the isometric sketch so that it shows as many of the object’s features as possible.
Figure 4-17 shows an isometric drawing of a cube that has a hole in the top plane. The hole must be sketched as an ellipse to appear visually correct in the isometric drawing. The axis lines for the ellipse are parallel to the edge lines of the plane. The proportions of the ellipse are defined by four points equidistant from the ellipse center point along the axis lines. The ellipse is then sketched lightly, checked for accuracy, and darkened.