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. Its purpose is to accurately and unambiguously capture all the geometric
features of a product or a component. The end goal of an engineering drawing is to convey all the required information that will
allow a manufacturer to produce that component.
Engineering drawings are often referred to as "blueprints" or
"bluelines". However, the terms are rapidly becoming an anachronism, since most copies of engineering drawings that were formerly made using a chemical-printing
process that yielded graphics on blue-colored paper or, alternatively, of blue-lines on white paper, have been superseded by more
modern reproduction processes that yield black or multicolour lines on white paper.
The process of producing engineering drawings, and the skill of producing them, is often referred to as technical drawing, although technical drawings are also required for disciplines that would not
ordinarily be thought of as parts of engineering.
Common features of engineering drawings
Drawings convey the following critical information:
- Geometry – the shape of the object; represented as views; how the object will look when it is viewed from various
standard directions, such as front, top, side, etc.
- Dimensions – the size of the object is captured in accepted units.
- Tolerances – the allowable variations for each dimension.
- Material – represents what the item is made of.
- Finish – specifies the surface quality of the item, functional or cosmetic. For example, a mass-marketed product
usually requires a much higher surface quality than, say, a component that goes inside industrial machinery.
A variety of line styles 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 thin, 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 pattern (pattern determined by the material being "cut" or "sectioned") used to indicate
surfaces in section views resulting from "cutting." Section lines are commonly referred to as "cross-hatching."
Lines can also be classified by a letter classification in which each line is given a letter.
- Type A lines show the outline of the feature of an object. They are the thickest lines on a drawing and done with a
pencil softer than HB.
- Type B lines are dimension lines and are used for dimensioning, projecting, extending, or leaders. A harder pencil
should be used, such as a 2H.
- Type C lines are used for breaks when the whole object is not shown. They are freehand drawn and only for short
breaks. 2H pencil
- Type D lines are similar to Type C, except they are zigzagged and only for longer breaks. 2H pencil
- Type E lines indicate hidden outlines of internal features of an object. They are dotted lines. 2H pencil
- Type F lines are Type F[typo] lines, except they are used for drawings in electrotechnology. 2H pencil
- Type G lines are used for centre lines. They are dotted lines, but a long line of 10–20mm, then a gap, then a small
line of 2mm. 2H pencil
- Type H lines are the same as Type G, except that every second long line is thicker. They indicate the cutting plane of
an object. 2H pencil
- Type K lines indicate the alternate positions of an object and the line taken by that object. They are drawn with a
long line of 10–20mm, then a small gap, then a small line of 2mm, then a gap, then another small line. 2H pencil.
Here is an example of an engineering drawing. The different line types are colored for clarity.
Black = object line and hatching
Red = hidden line
Blue = center line
Magenta = phantom line or cutting plane
Sectional views are indicated by the direction of arrows, as in the example above.
The objects can be represented with different views (front, rear, top, bottom, left and right side). There are two ways to
place the different views on the drawing:
- The ISO standard considers a projection on the opposite direction, like
an X-ray radiography; the top view is under the front view, the right view is at the left of
the front view... This is called First Angle Projection.
- The American standard (called Third Angle Projection) places the left view on the left and the top view on the top.
The standard in use is represented by a truncated cone.
Multiple views and projections
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° to directly view the resulting surface(s), which are indicated with section lines. They 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° about the vertical axis, followed by rotation of
approximately ± 35.264° [= arcsin(tan(30°))] about the horizontal axis starting from an orthographic projection view. "Isometric"
comes from the Greek for "same measure."
Isometric projection
of the above example object.
Showing dimensions
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 value may vary above and below the nominal.
Sizes of drawings
-
Sizes of drawings typically comply with either of two different standards, ISO (World Standard) or U.S. customary, according to the following
tables:
ISO A Drawing Sizes (mm)
| A4 |
210 X 297 |
| A3 |
297 X 420 |
| A2 |
420 X 594 |
| A1 |
594 X 841 |
| A0 |
841 X 1189 |
U.S. Customary Drawing Sizes
| A |
8.5" X 11" |
| B |
11" X 17" |
| C |
17" X 22" |
| D |
22" X 34" |
| E |
34" X 44" |
Other U.S. Drawing Sizes
|
|
|
|
" |
| D1 |
24" X 36" |
| E1 |
30" X 42" |
The metric drawing sizes correspond to international paper sizes. These developed further
refinements in the second half of the twentieth century, when photocopying became cheap.
Engineering drawings could be readily doubled (or halved) in size and put on the next larger (or, respectively, smaller) size of
paper with no waste of space. And the metric technical pens were chosen in sizes so that
one could add detail or drafting changes with a pen of double (or half) the width to the copy.
The U.S. customary "A-size" corresponds to "letter" size, and "B-size" corresponds to "ledger" or "tabloid" size. There were
also once British paper sizes, which went by names rather than alphanumeric designations.
ANSI Y14.2, Y14.3, and Y14.5 are standards that are commonly
used in the U.S.
See also
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