82
Technology: Engineering Our World
A few extra rules apply to hidden lines, as shown in Figure 3-32. For
example, they almost always begin and end with a dash touching with
the line where they start and end (1). However, this rule is not followed
when the dash would continue a visible line (2). Dashes should join at
corners (3) and (4). The dashes of parallel hidden lines that are close
together should be staggered (5).
Dimensioning
Most drawings include two types of dimensions: overall dimensions
and detail dimensions. To fully describe the size and shape of the view in
Figure 3-33A, you need two overall and two detail dimensions.
If a hole is added to this view, then you must add the dimensions
shown in Figure 3-33B. One dimension shows the size, or diameter, of
the hole. The other two dimensions show the exact location of the center
of the hole.
Notice the position of the dimensions in Figure 3-33B. Smaller
dimensions are placed inside the larger, overall dimensions. This is the
preferred placement for dimensions.
Scale Drawings
Ideally, objects should be shown at their full size in an orthographic
projection. However, some objects are too large to fit on a sheet of paper.
Others are so small that if you show them at their actual size, the details
are too small to see clearly. These objects are represented on paper using
a scale drawing. The objects in a scale drawing are larger or smaller than
the object by a fixed ratio. Examples of scaled drawings are an architect’s
drawing of a building, a map, and an electronic engineer’s schematic of
a printed circuit.
If you wanted to draw a full-size front view of the skateboard in
Figure 3-34, you would need a piece of paper larger than the skateboard.
Full-size is a scale of 1:1. Each inch of the drawing paper represents 1″
of the actual object. If you are working in metric, each centimeter of the
drawing paper represents 1 centimeter of the actual object.
A drawing that is one-half of full size has a scale of 1:2. In this case,
each inch (or each centimeter) on the drawing paper represents 2″ (or 2 cm) ″
of the actual object. Thus, the actual object would be twice the size of the
views on the drawing paper.