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Engineering drawing

An engineering drawing is a type of drawing that is technical in nature, used to fully and clearly define requirements for engineered items, and is usually created in accordance with standardized conventions for layout, nomenclature, interpretation, appearance (such as typefaces and line styles), size, etc.

Engineering drawings are often referred to as "blueprints." However, the term is an anachronism, and is due to the fact that most copies of engineering drawings were formerly made using a chemical printing process that yielded graphics on blue-colored paper.

Common features of engineering drawings

A variety of line styles are used to graphically represent physical objects. Types of lines include the following:

  • visible - are continuous lines used to depict edges directly visible from a particular angle.
  • hidden - are short-dashed lines that may be used to represent edges that are not directly visible.
  • center - are alternately long- and short-dashed lines that may be used to represent the axes of circular features.
  • cutting plane - are thick, medium-dashed lines, or thick alternately long- and double short-dashed that may be used to define sections for section views.
  • section - are thin lines in a parallel pattern used to indicate surfaces in section views resulting from "cutting." Section lines are commonly referred to as "cross-hatching."

In most cases, a single view is not sufficient to show all necessary features, and several views are used. Types of views include the following:

  • orthographic projection - show the object as it looks from the front, right, left, top, bottom, or back, and are typically positioned relative to each other according to the rules of either first-angle or third-angle projection. The former is primarily used in Europe and Asia, the latter is primarily used in the United States and Canada. Not all views are necessarily used, and determination of what surface constitutes the "front," etc., varies from object to object. "Orthographic" comes from the Greek for "straight writing (or drawing)."
  • section - depict what the object would look like if it were cut perfectly along cutting plane lines defined in a particular view, and rotated 90 degrees to directly view the resulting surface(s), which are indicated with section lines. They are used to show features not externally visible, or not clearly visible.
  • detail - show portions of other views, "magnified" for clarity.
  • auxiliary projection - similar to orthographic projections, however the directions of viewing are other than those for orthographic projections.
  • isometric - show the object from angles in which the scales along each axis of the object are equal. It corresponds to rotation of the object by +/- 45 degrees about the y-axis, followed by rotation of approximately +/- 35.264 degrees [= arcsin(tan(30 degrees))] about the x-axis, starting from an orthographic projection view. "Isometric" comes from the Greek for "same measure."

The required sizes of features are conveyed through use of dimensions. Distances may be indicated with either of two standardized forms of dimension: linear and ordinate.

  • With linear dimensions, two parallel lines, called "extension lines," spaced at the distance between two features, are shown at each of the features. A line perpendicular to the extension lines, called a "dimension line," with arrows at its endpoints, is shown between, and terminating at, the extension lines. The distance is indicated numerically at the midpoint of the dimension line, either adjacent to it, or in a gap provided for it.
  • With ordinate dimensions, one horizontal and one vertical extension line establish an origin for the entire view. The origin is identified with zeroes placed at the ends of these extension lines. Distances along the x- and y-axes to other features are specified using other extension lines, with the distances indicated numerically at their ends.
Sizes of circular features are indicated using either diametral or radial dimensions. Radial dimensions use an "R" followed by the value for the radius; Diametral dimensions use a circle with forward-leaning diagonal line through it, called the diameter symbol, followed by the value for the diameter. A radially-aligned line with arrowhead pointing to the circular feature, called a leader, is used in conjunction with both diametral and radial dimensions. All types of dimensions are typically composed of two parts: the nominal value, which is the "ideal" size of the feature, and the tolerance, which specifies the amount that the the value may vary above and below the nominal.

Notes--textual information--are also typically included in drawings, specifying details not otherwise conveyed. Notes are almost always in completely uppercase characters, for uniformity and maximal legibility after duplication of the drawing, which may involve substantial reduction in size. Leaders may be used in conjunction with notes in order to point to a particular feature or object that the note concerns.

Sizes of drawings typically comply with either of two different standards, metric or U.S. customary, according to the following tables:

Metric Drawing Sizes (mm)
A4210 X 297
A3297 X 420
A2420 X 594
A1594 X 841
A0841 X 1189

U.S. Customary Drawing Sizes
A8.5" X 11"
B11" X 17"
C17" X 22"
D22" X 34"
E34" X 44"

The metric drawing sizes correspond to international paper sizes. The U.S. customary "A-size" corresponds to "letter" size, and "B-size" corresponds to "ledger" size.

ANSI Y14.2, Y14.3, and Y14.5 are standards that are commonly used in the U.S.

See descriptive geometry[?].

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