, A type of technical drawing, is used to fully and clearly define requirements for engineered items.
Engineering drawing (the activity) produces engineering drawings (the documents). More than just the drawing of pictures, it is also a language—a graphical language that communicates ideas and information from one mind to another. Most especially, it communicates all needed information from the engineer who designed a part to the workers who will make it.
Overview
Relationship
to artistic drawing
Engineering drawing and artistic
drawing are both types of drawing, and either may be called simply "drawing"
when the context is implicit. Engineering drawing shares some traits with
artistic drawing in that both create pictures. But whereas the purpose of
artistic drawing is to convey emotion or artistic sensitivity in some way
(subjective impressions), the purpose of engineering drawing is to convey
information (objective facts).
One of the corollaries that follows from this fact is that, whereas anyone can
appreciate artistic drawing (even if each viewer has his own unique
appreciation), engineering drawing requires some training to understand (like
any language); but there is also a high degree of objective commonality in the
interpretation (also like other languages).
In fact, engineering drawing has evolved into a language that is more precise
and unambiguous than natural languages;
in this sense it is closer to a programming language in its communication ability. Engineering drawing uses an
extensive set of conventions to convey information very precisely, with very
little ambiguity.
Relationship
to other technical drawing types
The process of producing engineering
drawings, and the skill of producing those, is often referred to as technical
drawing or drafting (also spelled draughting), although
technical drawings are also required for disciplines that would not ordinarily
be thought of as parts of engineering (such as architecture, landscaping, cabinet making,
and garment-making).
Cascading
of conventions by specialty
The various fields share many common
conventions of drawing, while also having some field-specific conventions. For
example, even within metalworking, there are some process-specific conventions
to be learned—casting, machining, fabricating, and assembly all have some special drawing conventions,
and within fabrication there is further division, including welding, riveting, pipefitting, and erecting. Each of
these trades has some details that only specialists will have memorized.
Legal
instruments
An engineering drawing is a legal
document (that is, a legal instrument),
because it communicates all the needed information about "what is
wanted" to the people who will expend resources turning the idea into a
reality. It is thus a part of a contract; the purchase order
and the drawing together, as well as any ancillary documents (engineering
change orders [ECOs], called-out specs), constitute the contract. Thus, if the resulting product
is wrong, the worker or manufacturer are protected from liability
as long as they have faithfully executed the instructions conveyed by the
drawing. If those instructions were wrong, it is the fault of the engineer.
Because manufacturing and construction are typically very expensive processes
(involving large amounts of capital and payroll), the question of liability for errors has great legal implications as each party tries to blame the other and
assign the wasted cost to the other's responsibility. This is the biggest
reason why the conventions of engineering drawing have evolved over the decades
toward a very precise, unambiguous state.
Media
For centuries, until the post-World
War II era, all engineering drawing was done manually by using pencil and pen
on paper or other substrate (e.g., vellum, mylar). Since the advent of computer-aided design (CAD), engineering drawing has been done more and more in
the electronic medium with each passing decade. Today most engineering drawing
is done with CAD, but pencil and paper have not disappeared.
Some of the tools of manual drafting include pencils, pens and their ink, straightedges,
T-squares,
French curves,
triangles, rulers, protractors,
dividers, compasses, scales, erasers, and tacks or push pins. (Slide rules
used to number among the supplies, too, but nowadays even manual drafting, when
it occurs, benefits from a pocket calculator
or its onscreen equivalent.) And of course the tools also include drawing
boards (drafting boards) or tables. The English idiom "to go back to the
drawing board", which is a figurative phrase meaning to rethink something
altogether, was inspired by the literal act of discovering design errors during
production and returning to a drawing board to revise the engineering drawing. Drafting machines
are devices that aid manual drafting by combining drawing boards,
straightedges, pantographs, and other tools into one integrated drawing environment.
CAD provides their virtual equivalents.
Producing drawings usually involves
creating an original that is then reproduced, generating multiple copies to be
distributed to the shop floor, vendors, company archives, and so on. The
classic reproduction methods involved blue and white appearances (whether white-on-blue
or blue-on-white),
which is why engineering drawings were long called, and even today are still
often called, "blueprints" or "bluelines",
even though those terms are anachronistic
from a literal perspective, since most copies of engineering drawings today are
made by more modern methods (often inkjet or laser printing)
that yield black or multicolour lines on white paper. The more generic term
"print" is now in common usage in the U.S. to mean any paper copy of
an engineering drawing. In the case of CAD drawings, the original is the CAD
file, and the printouts of that file are the "prints".
Relationship
to model-based definition (MBD/DPD)
For centuries, engineering drawing
was the sole method of transferring information from design into manufacture.
In recent decades another method has arisen, called model-based definition (MBD) or digital product definition (DPD). In MBD, the
information captured by the CAD software app is fed automatically into a CAM
app (computer-aided
manufacturing), and is translated via
postprocessor into other languages such as G-code, which is executed by a CNC machine tool (computer numerical control).
Thus today it is often the case that the information travels from the mind of
the designer into the manufactured component without having ever been codified
by an engineering drawing. In MBD, the dataset, not a
drawing, is the legal instrument. The term "technical data package"
(TDP) is now used to refer to the complete package of information (in
one medium or another) that communicates information from design to production
(such as 3D-model datasets, engineering drawings, engineering change orders
(ECOs), spec revisions and addenda, and so on). However, even in the MBD
era, where theoretically production could happen without any drawings or humans
at all, it is still the case that drawings and humans are involved. It still
takes CAD/CAM programmers, CNC setup workers, and CNC operators to do
manufacturing, as well as other people such as quality assurance staff
(inspectors) and logistics staff (for materials handling,
shipping-and-receiving, and front office
functions). These workers often use drawings in the course of their work that
have been produced by rendering and plotting (printing) from the MBD dataset.
When proper procedures are being followed, a clear chain of precedence is
always documented, such that when a person looks at a drawing, s/he is told by
a note thereon that this drawing is not the governing instrument (because the
MBD dataset is). In these cases, the drawing is still a useful document,
although legally it is classified as "for reference only", meaning
that if any controversies or discrepancies arise, it is the MBD dataset, not
the drawing, that governs.
Systems
of dimensioning and tolerancing
Almost all engineering drawings
(except perhaps reference-only views or initial sketches) communicate not only
geometry (shape and location) but also dimensions and tolerances for those characteristics. Several systems of dimensioning
and tolerancing have evolved. The simplest dimensioning system just specifies
distances between points (such as an object's length or width, or hole center
locations). Since the advent of well-developed interchangeable manufacture, these distances have been accompanied by tolerances of the
plus-or-minus or min-and-max-limit types. Coordinate dimensioning
involves defining all points, lines, planes, and profiles in terms of Cartesian
coordinates, with a common origin. Coordinate dimensioning was the sole best
option until the post-World War II era saw the development of geometric
dimensioning and tolerancing
(GD&T), which departs from the limitations of coordinate dimensioning
(e.g., rectangular-only tolerance zones, tolerance stacking) to allow the most
logical tolerancing of both geometry and dimensions (that is, both form
[shapes/locations] and sizes).
Engineering
drawings: 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.
- 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
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."
- 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.
- 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 F[typo] 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 mm, then a gap, then a small line of 2 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 mm, then a small gap, then a small line of 2 mm, then a gap, then another small line. 2H pencil.
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
The orthographic projection shows 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
origin and vector direction of the projectors (also called projection lines)
differs, as explained below.
- In first-angle projection, the projectors originate as if radiated from a viewer's eyeballs and shoot through the 3D object to project a 2D image onto the 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 standard and is primarily used in Europe.
- In third-angle projection, the projectors originate as if radiated from the 3D object itself and shoot away from the 3D object to project a 2D image onto the 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. 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;
but circa the 1890s, the meme of third-angle projection spread throughout the
North American engineering and manufacturing communities to the point of
becoming a widely followed convention,
and it was an ASA standard by the 1950s.
Circa World War I, British practice was frequently mixing the use of both
projection methods.
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.
Generally only as many views are used as are necessary to convey all needed
information clearly and economically.
The front, top, and right-side views are commonly considered the core group of
views included by default,
but any combination of views may be used depending on the needs of the
particular design. In addition to the 6 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
projection
An auxiliary view is an orthographic
view that is projected into any plane other than one of the six principal
views.
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.
Isometric
projection
The isometric projection show 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 degree angles can be
constructed with only a compass and straightedge.
Isometric projection is a type of axonometric projection. The other two types of axonometric projection are:
Oblique
projection
An oblique projection is a simple type of graphical projection used for producing
pictorial, two-dimensional images 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
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
Orthographic) 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 may be available using regular projections or hidden
lines. In assembly drawings, hardware components (e.g. nuts, screws, washers)
are typically not sectioned.
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 1/4"=1'-0") whereas a detailed view may
be drawn at 1:25 (1:24 or 1/2"=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 ratios
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, s/he must turn instead to the engineer (for
the answers that the scaling would seek), and s/he will never erroneously scale
something that is inherently unable to be accurately scaled.
But in some ways, the advent of the CAD and 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 15mm long corresponded to a 30mm 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
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 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 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 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.
American
Society of Mechanical Engineers
(ASME) Y14.2, Y14.3, and Y14.5 are commonly referenced standards in the U.S.
Technical
lettering
Technical lettering is the process of forming letters, numerals, and other 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 Gothic sans-serif
script, formed by a series of short strokes. Lower case letters are rare in
most drawings of machines. 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, 2.5mm high characters would have a stroke thickness - pen
nib size - of 0.25mm, 3.5 would use a 0.35mm 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 American 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 (over ride) 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.
Conventional
parts (areas) of an engineering drawing
Title
block
The title block (T/B, TB) is an area
of the drawing that conveys header-type information about the drawing, such as:
- Drawing title (hence the name "title block")
- Drawing number
- Part number(s)
- Name of the design activity (corporation, government agency, etc.)
- Identifying code of the design activity (such as a CAGE code)
- Address of the design activity (such as city, state/province, country)
- Measurement units of the drawing (for example, inches, millimeters)
- Default tolerances for dimension callouts where no tolerance is specified
- Boilerplate callouts of general specs
- Intellectual property rights warning
Traditional locations for the title
block are the bottom right (most commonly) or the top right or center.
Revisions
block
The revisions block (rev block) is a
tabulated list of the revisions (versions) of the drawing, documenting the revision control.
Traditional locations for the
revisions block are the top right (most commonly) or adjoining the title block
in some way.
Effectivity
block
The effectivity block provides a
(usually tabular) list of the effectivity of the part design, that is, which
higher assemblies it is used in, and thus which models of machine the part is
used in.
Notes
list
The notes list provides notes to the
user of the drawing, conveying any information that the callouts within the
field of the drawing did not. It may include general notes, flagnotes, or a
mixture of both.
Traditional locations for the notes
list are anywhere along the edges of the field of the drawing.
General
notes
General notes (G/N, GN) apply
generally to the contents of the drawing, as opposed to applying only to
certain part numbers or certain surfaces or features.
Flagnotes
Flagnotes or flag notes (FL,
F/N) are notes that apply only where a flagged callout points, such as to
particular surfaces, features, or part numbers. Typically the callout includes
a flag icon. Some companies call such notes "delta notes", and the
note number is enclosed inside a triangular symbol (similar to capital letter delta,
Δ). "FL5" (flagnote 5) and "D5" (delta note 5) are typical
ways to abbreviate in ASCII-only contexts.
Field
of the drawing
The field of the drawing (F/D, FD)
is the main body or main area of the drawing, excluding the title block, rev
block, and so on.
List
of materials, bill of materials, parts list
The list of materials (L/M, LM,
LoM), bill of materials (B/M, BM, BoM), or parts list (P/L, PL) is a (usually
tabular) list of the materials used to make a part, and/or the parts used to
make an assembly. It may contain instructions for heat treatment, finishing,
and other processes, for each part number. Sometimes such LoMs or PLs are
separate documents from the drawing itself.
Traditional locations for the
LoM/BoM are above the title block, or in a separate document.
Parameter
tabulations
Some drawings call out dimensions
with parameter names (that is, variables, such a "A", "B",
"C"), then tabulate rows of parameter values for each part number.
Traditional locations for parameter
tables, when such tables are used, are floating near the edges of the field of
the drawing, either near the title block or elsewhere along the edges of the
field.
Views
and sections
Each view or section is a separate
set of projections, occupying a contiguous portion of the field of the drawing.
Usually views and sections are called out with cross-references to specific
zones of the field.
Zones
Often a drawing is divided into
zones, with labels along the margins, such as A,B,C,D up the sides and
1,2,3,4,5,6 along the top and bottom. Names of zones are thus, for example, A5,
D2, or B1.
Abbreviations
and symbols
As in many technical fields, a wide
array of abbreviations and symbols have been developed in engineering drawing
during the 20th and 21st centuries. For example, cold rolled steel
is often abbreviated as CRS, and diameter is often
abbreviated as DIA,
D, or ⌀.
Example
of an engineering drawing
Here is an example of an engineering
drawing (an isometric view of the same object is shown above). The different
line types are colored for clarity.
- Black = object line and hatching
- Red = hidden line
- Blue = center line of piece or opening
- Magenta = phantom line or cutting plane line
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