Editing Engineering drawing (section)

== Common features ==
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 angles, such as front, top, side, etc.
* ''Dimensions'' – the size of the object is captured in accepted units.
* ''[[Tolerance (engineering)|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.

=== Line styles and types ===
[[File:Line types.svg|right|thumb|210px|Standard engineering drawing line types]]
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 [[cross section (geometry)|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".
* ''phantom'' – (not shown) are alternately long- and double short-dashed thin lines used to represent a feature or component that is not part of the specified part or assembly. E.g. billet ends that may be used for testing, or the machined product that is the focus of a tooling drawing.

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 pencil.
* '''Type C''' lines are used for breaks when the whole object is not shown. These are freehand drawn and only for short breaks. 2H pencil
* '''Type D''' lines are similar to Type C, except these are zigzagged and only for longer breaks. 2H pencil
* '''Type E''' lines indicate hidden outlines of internal features of an object. These are dotted lines. 2H pencil
* '''Type F''' lines are Type E lines, except these are used for drawings in electrotechnology. 2H pencil
* '''Type G''' lines are used for centre lines. These are dotted lines, but a long line of 10–20&nbsp;mm, then a 1&nbsp;mm gap, then a small line of 2&nbsp;mm. 2H pencil
* '''Type H''' lines are the same as type G, except that every second long line is thicker. These 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. These are drawn with a long line of 10–20&nbsp;mm, then a small gap, then a small line of 2&nbsp;mm, then a gap, then another small line. 2H pencil.

===Multiple views and projections===
{{Main|Graphical projection}}
[[File:First angle projection.svg|thumb|right|Image of a part represented in ''first-angle projection'']]
[[File:Conventions of placing vues in technical drawings.svg|thumb|right|Symbols used to define whether a projection is either ''first-angle'' (left) or ''third-angle'' (right)]]
[[File:Graphical projection comparison.png|thumb|right|Several types of graphical projection compared]]
[[File:Various projections of cube above plane.svg|thumb|Various projections and how they are produced]]
[[File:Engineering drawing isometric.svg|thumb|right|Isometric view of the object shown in the engineering drawing [[#Example|below]]]]
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:

==== Multiview projection ====
A ''[[multiview projection]]'' is a type of [[orthographic projection]] that shows the object as it looks from the front, right, left, top, bottom, or back (e.g. the ''primary views''), and is typically positioned relative to each other according to the rules of either [[Multiview projection|first-angle or third-angle projection]]. The origin and vector direction of the projectors (also called projection lines) differs, as explained below. 
* In ''first-angle projection'', the parallel projectors originate as if radiated ''from behind the viewer'' and pass through the 3D object to project a 2D image onto the orthogonal plane ''behind'' it. The 3D object is projected into 2D "paper" space as if you were looking at a [[radiograph]] of the object: the top view is under the front view, the right view is at the left of the front view. First-angle projection is the [[ISO 128|ISO standard]] and is primarily used in Europe.
* In ''third-angle projection'', the parallel projectors originate as if radiated ''from the far side of the object'' and pass through the 3D object to project a 2D image onto the orthogonal plane ''in front of'' it. The views of the 3D object are like the panels of a box that envelopes the object, and the panels pivot as they open up flat into the plane of the drawing.<ref name="French_Vierck_1953_pp99-105">{{Harvnb|French|Vierck|1953|pp=99–105}}</ref> Thus the left view is placed on the left and the top view on the top; and the features closest to the front of the 3D object will appear closest to the front view in the drawing. Third-angle projection is primarily used in the United States and Canada, where it is the default projection system according to [[ASME]] standard ASME Y14.3M.

Until the late 19th century, first-angle projection was the norm in North America as well as Europe;<ref name="French1918p78">{{Harvnb|French|1918}}, [https://books.google.com/books?id=6R5DAAAAIAAJ&pg=PA78 p. 78].</ref><ref name="French_Vierck_1953_pp111-114">{{Harvnb|French|Vierck|1953|pp=111–114}}</ref> but circa the 1890s, third-angle projection spread throughout the North American engineering and manufacturing communities to the point of becoming a widely followed convention,<ref name="French1918p78"/><ref name="French_Vierck_1953_pp111-114"/> and it was an ASA standard by the 1950s.<ref name="French_Vierck_1953_pp111-114"/> Circa World War I, British practice was frequently mixing the use of both projection methods.<ref name="French1918p78"/>

As shown above, the determination of what surface constitutes the front, back, top, and bottom varies depending on the projection method used.

Not all views are necessarily used.<ref name="French_Vierck_1953_pp97-114">{{Harvnb|French|Vierck|1953|pp=97–114}}</ref> Generally only as many views are used as are necessary to convey all needed information clearly and economically.<ref name="French_Vierck_1953_pp108-111">{{Harvnb|French|Vierck|1953|pp=108–111}}</ref> The front, top, and right-side views are commonly considered the core group of views included by default,<ref name="French_Vierck_1953_p102">{{Harvnb|French|Vierck|1953|p=102}}.</ref> but any combination of views may be used depending on the needs of the particular design. In addition to the six principal views (front, back, top, bottom, right side, left side), any auxiliary views or sections may be included as serve the purposes of part definition and its communication. View lines or section lines (lines with arrows marked "A-A", "B-B", etc.) define the direction and location of viewing or sectioning. Sometimes a note tells the reader in which zone(s) of the drawing to find the view or section.

==== Auxiliary views ====
An ''auxiliary view'' is an orthographic view that is projected into any plane other than one of the six ''primary views''.<ref>Bertoline, Gary R. ''Introduction to Graphics Communications for Engineers (4th Ed.).'' New York, NY. 2009</ref> These views are typically used when an object contains some sort of inclined plane. Using the auxiliary view allows for that inclined plane (and any other significant features) to be projected in their true size and shape. The true size and shape of any feature in an engineering drawing can only be known when the Line of Sight (LOS) is perpendicular to the plane being referenced.
It is shown like a three-dimensional object. Auxiliary views tend to make use of [[axonometric projection]]. When existing all by themselves, auxiliary views are sometimes known as ''pictorials''.

==== Isometric projection ====
An [[isometric projection]] shows the object from angles in which the scales along each axis of the object are equal. Isometric projection 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". One of the things that makes isometric drawings so attractive is the ease with which 60° angles can be constructed with only a [[Compass-and-straightedge construction|compass and straightedge]].

Isometric projection is a type of [[axonometric projection]]. The other two types of axonometric projection are:
* [[Dimetric projection]]
* [[Trimetric projection]]

==== Oblique projection ====
An [[oblique projection]] is a simple type of graphical projection used for producing pictorial, two-dimensional [[image]]s of three-dimensional objects: 
* it projects an image by intersecting parallel rays (projectors)
* from the three-dimensional source object with the drawing surface (projection plan).
In both oblique projection and orthographic projection, parallel lines of the source object produce parallel lines in the projected image.

==== Perspective projection ====
[[Perspective (graphical)|Perspective]] is an approximate representation on a flat surface, of an image as it is perceived by the eye. The two most characteristic features of perspective are that objects are drawn:
* Smaller as their distance from the observer increases
* Foreshortened: the size of an object's dimensions along the line of sight are relatively shorter than dimensions across the line of sight.

==== Section views ====
Projected views (either ''auxiliary'' or ''multi view'') which show a cross section of the source object along the specified cut plane. These views are commonly used to show internal features with more clarity than regular projections or hidden lines, it also helps reducing number of hidden lines.In assembly drawings, hardware components (e.g. nuts, screws, washers) are typically not sectioned. Section view is a half side view of object.

=== Scale ===
{{Main|Architect's scale|Engineer's scale|Metric scale}}
Plans are usually "scale drawings", meaning that the plans are drawn at specific [[ratio]] relative to the actual size of the place or object. Various scales may be used for different drawings in a set. For example, a floor plan may be drawn at 1:50 (1:48 or {{frac|4}}″ = 1′ 0″) whereas a detailed view may be drawn at 1:25 (1:24 or {{frac|2}}″ = 1′ 0″). Site plans are often drawn at 1:200 or 1:100.

Scale is a nuanced subject in the use of engineering drawings. On one hand, it is a general principle of engineering drawings that they are projected using standardized, mathematically certain projection methods and rules. Thus, great effort is put into having an engineering drawing accurately depict size, shape, form, [[aspect ratio]]s between features, and so on. And yet, on the other hand, there is another general principle of engineering drawing that nearly diametrically opposes all this effort and intent—that is, the principle that ''users are not to scale the drawing to infer a dimension not labeled.'' This stern admonition is often repeated on drawings, via a boilerplate note in the title block telling the user, "DO NOT SCALE DRAWING."

The explanation for why these two nearly opposite principles can coexist is as follows. The first principle—that drawings will be made so carefully and accurately—serves the prime goal of why engineering drawing even exists, which is successfully communicating part definition and acceptance criteria—including "what the part should look like if you've made it correctly." The service of this goal is what creates a drawing that one even ''could'' scale and get an accurate dimension thereby. And thus the great temptation to do so, when a dimension is wanted but was not labeled. The second principle—that even though scaling the drawing ''will'' usually work, one should nevertheless ''never'' do it—serves several goals, such as enforcing total clarity regarding who has authority to discern design intent, and preventing erroneous scaling of a drawing that was never drawn to scale to begin with (which is typically labeled "drawing not to scale" or "scale: NTS"). When a user is forbidden from scaling the drawing, they must turn instead to the engineer (for the answers that the scaling would seek), and they will never erroneously scale something that is inherently unable to be accurately scaled.

But in some ways, the advent of the [[computer-aided design|CAD]] and [[model-based definition|MBD]] era challenges these assumptions that were formed many decades ago. When part definition is defined mathematically via a solid model, the assertion that one cannot interrogate the model—the direct analog of "scaling the drawing"—becomes ridiculous; because when part definition is defined this way, it is not ''possible'' for a drawing or model to be "not to scale". A 2D pencil drawing can be inaccurately foreshortened and skewed (and thus not to scale), yet still be a completely valid part definition as long as the labeled dimensions are the only dimensions used, and no scaling of the drawing by the user occurs. This is because what the drawing and labels convey is in reality a ''symbol'' of what is wanted, rather than a true ''replica'' of it. (For example, a sketch of a hole that is clearly not round still accurately defines the part as having a true round hole, as long as the label says "10mm DIA", because the "DIA" implicitly but objectively tells the user that the skewed drawn circle is a symbol ''representing'' a perfect circle.) But if a mathematical model—essentially, a vector graphic—is declared to be the official definition of the part, then any amount of "scaling the drawing" can make sense; there may still be an error in the model, in the sense that what was ''intended'' is not ''depicted'' (modeled); but there can be no error of the "not to scale" type—because the mathematical vectors and curves are replicas, not symbols, of the part features.

Even in dealing with 2D drawings, the manufacturing world has changed since the days when people paid attention to the scale ratio claimed on the print, or counted on its accuracy. In the past, prints were plotted on a plotter to exact scale ratios, and the user could know that a line on the drawing 15&nbsp;mm long corresponded to a 30&nbsp;mm part dimension because the drawing said "1:2" in the "scale" box of the title block. Today, in the era of ubiquitous desktop printing, where original drawings or scaled prints are often scanned on a scanner and saved as a PDF file, which is then printed at any percent magnification that the user deems handy (such as "fit to paper size"), users have pretty much given up caring what scale ratio is claimed in the "scale" box of the title block. Which, under the rule of "do not scale drawing", never really did that much for them anyway.

=== 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.
* [[Geometric dimensioning and tolerancing]] is a method of specifying the functional geometry of an object.

=== Sizes of drawings ===
{{Main|Paper size}}
[[File:A size illustration.svg|thumb|ISO paper sizes]]
[[File:ANSI_size_illustration.svg|thumb|ANSI paper sizes]]

Sizes of drawings typically comply with either of two different standards, [[ISO standard|ISO]] (World Standard) or [[ANSI/ASME Y14.1]] (American).

The metric drawing sizes correspond to international [[paper size]]s. 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 pen]]s were chosen in sizes so that one could add detail or drafting changes with a pen width changing by approximately a factor of the [[square root of 2]]. A full set of pens would have the following nib sizes: 0.13, 0.18, 0.25, 0.35, 0.5, 0.7, 1.0, 1.5, and 2.0&nbsp;mm. However, the International Organization for Standardization (ISO) called for four pen widths and set a colour code for each: 0.25 (white), 0.35 (yellow), 0.5 (brown), 0.7 (blue); these nibs produced lines that related to various text character heights and the ISO paper sizes.

All ISO paper sizes have the same aspect ratio, one to the square root of 2, meaning that a document designed for any given size can be enlarged or reduced to any other size and will fit perfectly. Given this ease of changing sizes, it is of course common to copy or print a given document on different sizes of paper, especially within a series, e.g. a drawing on A3 may be enlarged to A2 or reduced to A4.

The US 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.

[[American Society of Mechanical Engineers]] (ASME) [[ANSI/ASME Y14.1]], Y14.2, Y14.3, and Y14.5 are commonly referenced standards in the US.

=== Technical lettering ===
[[Technical lettering]] is the process of forming letters, numerals, and other [[character (computing)|characters]] in technical drawing. It is used to describe, or provide detailed specifications for an object. With the goals of [[legibility]] and uniformity, styles are standardized and lettering ability has little relationship to normal writing ability. Engineering drawings use a [[sans-serif|Gothic sans-serif]] script, formed by a series of short strokes. Lower case letters are rare in most drawings of [[machine]]s. ISO Lettering templates, designed for use with technical pens and pencils, and to suit ISO paper sizes, produce lettering characters to an international standard. The stroke thickness is related to the character height (for example, 2.5&nbsp;mm high characters would have a stroke thickness - pen nib size - of 0.25&nbsp;mm, 3.5 would use a 0.35&nbsp;mm pen and so forth). The ISO character set (font) has a seriffed one, a barred seven, an [[open four]], six, and nine, and a round topped three, that improves legibility when, for example, an A0 drawing has been reduced to A1 or even A3 (and perhaps enlarged back or reproduced/faxed/ microfilmed &c). When CAD drawings became more popular, especially using US software, such as AutoCAD, the nearest font to this ISO standard font was Romantic Simplex (RomanS) - a proprietary shx font) with a manually adjusted width factor (override) to make it look as near to the ISO lettering for the drawing board. However, with the closed four, and arced six and nine, romans.shx typeface could be difficult to read in reductions. In more recent revisions of software packages, the [[TrueType]] font ISOCPEUR reliably reproduces the original drawing board lettering stencil style, however, many drawings have switched to the ubiquitous Arial.ttf.