Technical drawing in engineering is the creation of a two-dimensional (2-D) graphical representation of a 3-D structure. It's used in a wide variety of industries, but especially in those needing precision in their manufacturing processes. Drafters use specific techniques to ensure their drawings will be understood by the crew that will construct the structure represented in the drawing. They usually apply these techniques using computers, but also sometimes use more traditional manual tools, including paper, ruler and templates.
One technique used by drafters is that of multi-view drawings, which show an object from different viewpoints. Three views are common, but some simple structures only need one, while complex ones need four or more.
Multi-view drawings consist of orthographic projections, which show a structure's height, width and depth. For example, the top view, one kind of orthographic projection, shows an object's length and width. The left orthographic view shows length and height, and front view shows height and width.
Dimensioning, another technique, shows the exact size and location of each part of an object. The result of this technique typically appears on a drawing as sets of arrows that indicate a part, and text that describes the part. (See the Resource link for an example of these particular graphics.)
Clear dimensioning requires a certain type of typography. Common handwriting, as legible as it might be, can vary from one drafter to another. Production crews require a specific typography, however, that's the same across all drawings. Therefore, drafters dimension with a special font, Gothic, that has no serifs, which are the small squiggles on characters in the common Times New Roman and Courier fonts.
Tolerancing is the indication of a range of acceptable values that an object's size or position can have. For example, a drawing may show that a doorjamb's height can be ten feet, plus or minus a quarter of an inch. That "quarter of inch" is the tolerance.
Tolerancing is needed because most real-world structures can't be manufactured to exact specifications. One reason is that physical things change in size, such as when heat expands them.
There are three ways of illustrating tolerances on a drawing: as limits, e.g., "20.2 to 20.4 inches"; as variation, e.g., "20.3 inches, plus or minus .1 inch"; and in notes, e.g., "tolerances are 0.2 inches unless otherwise specified."
Section views are those that let you see a structure's insides and require a conceptual "section plane" to create them. To create a section view of a human being's torso, you would pass a section plane through the torso, with the plane separating its left and right halves. The result would be a display of each internal organ and the part that the plane passed through: i.e., the rib cage, liver or intestines.
Pictorial drawings are those that appear to have three dimensions, so are more natural looking than the multi-view orthographic drawings described earlier. Pictorial drawings show the height, width and depth of an object all at once. If you were to make a drawing of an object while viewing it from above, and midway between its front and left sides, you'd get a perspective drawing, which is one type of pictorial drawing. Other types are axonometric, which shows an object's parallel lines as parallel, in contrast to perspective's converging lines; and oblique, which presents a 3-D view where only the object's front face is presented without distortion.
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