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22
Two-Dimensional Drawing
objectives
After completing this chapter, you should be able to
●
●
●
●
●
●
Identify manual drafting tools used in drawing creation
Apply the techniques of geometric construction in manual
drafting
Utilize basic CAD system commands and processes in creating a drawing
Create a manually drafted or CAD-based orthographic projection drawing
Develop a section and auxiliary view drawing using CAD
Identify several common issues that occur in CAD drafting
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22.01
introduction
T
22.02 Manual 2-D Drawings
This section provides a glimpse into the skills needed to create drawings manually.
Three components are required to create high-quality drawings. First, tools are needed
to create neat drawings and lettering. Second, dexterity and drawing technique play a
part in consistently developing quality work. Finally, practice and experience are
needed to create drawings in a timely and efficient manner.
22.02.01 Manual Drawing Tools
The process of creating a 2-D drawing manually is sometimes called drafting. The following list and Figure 22.01 contain some of the basic tools used in the creation of
2-D manual drawings. These tools are sometimes called drafting instruments
because they are made with great precision and are used to create accurate graphics.
Brush—used to keep the drawing area clean by removing dry erase material, eraser
shavings, and other debris.
Compass—used to create accurate circles and arcs and used in the layout of geometric shapes. To use a compass, open it to the desired radius using a scale as a guide.
Carefully place the pointed (anchor) end of the compass through the paper into the
drafting table. For good compass technique, tilt the compass to one side and allow
the lead to follow the compass around the circle. The lead in the compass should be
sharpened often to ensure clean, even arcs. To darken an arc, make multiple passes
instead of pressing down on the compass.
Dividers—used to lay out and transfer distances. They can also be used to break
longer object segments into equal distances. A scale can be used to help set divider
measurements.
Drafting Machine—a tool clamped to a drafting table that holds two straightedges
(used to draw and measure lines) usually at 90 degrees relative to each other. The
straightedges can be rotated to any angle desired and moved about the table while
the desired angle is maintained.
Drafting Table—a large, rectangular, flat table with a smooth surface and straight
sides. The sides of the table are parallel and perpendicular to each other to aid in
the drawing of horizontal and vertical lines with excellent precision.
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wo-dimensional (2-D) drawings have been the mainstay of graphical
communication for mechanical, civil, and other engineering fields for centuries. Although the use of computer-based three-dimensional (solid modeling) is predominant in the United States and Western Europe, most of the world
depends heavily on 2-D drawings for the transfer of engineering geometry information. Two-dimensional drawing, whether done by hand or with the assistance of
a computer, will be around for a very long time. In some areas of the world, it is
still the only way that engineering graphics are created and used. There is a
tremendous amount of legacy data in 2-D format that must be edited using the
same methods used to create the data. In some types of industries, 2-D drawing is
still the only acceptable means of presenting engineering graphics.
Two-dimensional drawings are still heavily utilized in some industries in the
United States. The breakdown is in two areas: manually drawn 2-D drawings and
2-D drawings developed through computer-aided design (CAD). Almost all 2-D
drawings that are created today to build or manufacture objects are done using
CAD tools; however, many basic skill sets can be replicated in both manual drawings and CAD drawings.
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FIGURE 22.01. Manual
drafting tools.
Drafting Vellum—a high-quality paper that is abrasion-resistant and stable over
long periods of time. It allows errantly drawn items to be erased with little ghosting.
Dry Erase Bag—holds an inert powder that is layered on a drawing to prevent
smudges. The bag is occasionally kneaded and tapped on the drawing so tools will
glide over the surface.
Eraser—used to remove errant lead marks. Drafting erasers are usually not the common pink rubber variety, but rather a soft white plastic type, which is less abrasive
to drawing surfaces.
Eraser Shield—used to protect areas of a drawing while an eraser is being used.
French Curve—used to create smooth curves through a set of points by aligning the
part of the curve that best fits the points.
Leads—thin rods of graphite bound in a clay base and sharpened for use in marking
lines, text, and other art on a drawing. Drawing leads vary in darkness based on the
grading, as shown in Figure 22.02. The familiar #2 yellow pencil, which contains
#2 grade graphite, draws a medium dark line. The higher the lead grade number, the
harder the lead and the lighter the line produced. As the lead grade number decreases,
the graphite composition gets softer, resulting in a darker line. Pencil lead grades and
their equivalent drafting lead grades are shown in Figure 22.03.
FIGURE 22.02. Various shades of lead marks used in drafting.
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Pencil
Grade
Drafting
Lead
#1
B
#2
HB
#2-1/2
F
#3
H
#4
2H
FIGURE 22.04. Drafting leads and a lead holder.
FIGURE 22.03. Equivalent
marking darkness of drafting
leads and pencil grades.
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FIGURE 22.05. Close-up view of
the graduation marks on an
engineering scale.
Lead Holders—used to hold pieces of pencil lead, as shown in Figure 22.04, that are
sharpened to provide the line width and darkness required for various features on the
drawing. Like a pencil, lines are drawn by moving the arm away from the body. This
technique provides the best consistency and prevents the hand from smudging lines
that have already been drawn. The lead holder can be slowly twirled about its length
axis while the line is created to maintain a uniform wear on the point of the lead. Today
mechanical lead holders are being replaced by mechanical pencils that have a standardized lead diameter (e.g., 0.5 mm, 0.7 mm, and 0.9 mm) and a specific lead hardness.
Lettering Guide—a template-like tool that helps create guidelines (like lines on notebook paper) to make it easier to create consistently sized lettering. The linear set of
holes on an Ames lettering guide, for example, is in increments of 1/16 of an inch.
Protractor—used to measure the angles between lines.
Scales—used to measure distances on drawings. Like rulers, scales provide graduated
marks that are used to determine the length of objects, as shown in Figure 22.05.
Scales, which are most commonly triangular in cross section to create six different
sets of graduation marks, provide the necessary means for appropriate scaling of
the entities to ensure that the drawing will fit on a standard-sized sheet of paper.
This tool is not used to draw lines since the edge of a scale may have nicks created
by the graduation marks. Measurement marks on a drawing are usually made with
small dots, and the entity is created with the proper drawing tool. For entity measurement, dividers can be set to the length of the entity for transfer to a scale.
T square—used (in the absence of a drafting machine) to draw horizontal lines. The
cross of the T is placed against an edge of the drafting table to provide horizontal
or vertical drawing edges across the table. Angled lines can be drawn by placing the
appropriate triangle(s) against the edge of the T square.
Templates—used to create common shapes such as small circles, ellipses, polygons,
arcs, arrowheads, and other geometric entities. A template used to make text and
numbers is also called a lettering guide.
Triangles—used to create the lines on a drawing. These are used in conjunction with
a straightedge drafting board or a T square. Fixed triangles typically have angles of
30-60-90 degrees (commonly called a 30-60 triangle) and 45-45-90 degrees (commonly
called a 45 triangle). Adjustable triangles are also available to create lines at
less common angles. By combining a 30-60 triangle and a 45 triangle, lines at other
common angles can be drawn, as shown in Figure 22.06. Adjustable triangles,
which can be set to any acute angle, are also available.
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75 deg.
165 deg.
REFERENCE LINE
REFERENCE LINE
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105 deg.
15 deg.
REFERENCE LINE
REFERENCE LINE
FIGURE 22.06. By placing the 30-60 and the 45 triangles in various orientations, lines at different angles can be drawn.
22.02.02 Preparation for Manual Drawing
Manual drafting tools aid in the creation of basic geometric elements such as lines,
arcs, and curves at almost any size and orientation. Your instructor is the best source
for the details of how to use the tools properly to create a drawing. This section briefly
discusses the basics of tool utilization and some “best practices.” The following steps
should be used to prepare for the production of a manual drawing:
1. Clean your tools before and after using them. Wash and dry them to keep your
tools, carrying case, and storage drawer free from graphite residue.
2. When placing vellum on your drafting table, align the edge of the vellum with the
straight edges of the drafting machine or T square. Tape down the corners of
the paper with drafting tape. When moving the T square or drafting machine, lift
it slightly so you do not snag and tear the vellum.
3. Use the dry erase bag to put a layer of a rubberlike powder on the vellum surface.
This powder bonds with loose graphite particles to help keep your tools clean and
eliminate smudges on your drawing. A small clean towel is also handy for keeping
your hands and tools clean.
4. Decide how much space on the vellum is to be occupied by the views of the object
to be drawn, including sufficient space between the views for any dimensions that
are to be included.
5. Select the best scale to use to produce the desired sizes of the views.
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22.02.03 Making a Simple Manual Drawing
Now that you have been introduced to the drawing preparation process and some of
the basic geometry creation skills, this section walks you through a basic drawing of the
flat gasket shown in Figure 22.07. Here is the equipment needed:
■ T square or drafting machine
■ Triangle
■ Scale
■ Circle template
■ Eraser
■ Eraser template
4.00
FIGURE 22.07. The goal is to
re-create a drawing of this
flat gasket.
2.00
R .50 TYP
Ø 1.00
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6. Once the view placement has been determined, create the exterior shape of the
object views using projection rules and proper dimension scaling.
7. Use triangles or a protractor to verify angles after they have been drawn.
8. Sharpen your drawing leads often as the accuracy of your drawing may depend on
a sharp lead.
9. Place an additional piece of paper or vellum under your drawing hand to avoid
smudges. Try not to touch the surface of the drawing with your hand or put fingerprints on the drawing surface. Oils from your skin may be absorbed by the vellum and leave a stain.
10. Use the eraser template when erasing. Lay the template on top of your drawing,
exposing the area that needs to be erased through the proper-sized opening. Erase
lightly through the opening to remove the errant line.
11. Use a circle template for small circles and arcs as this is faster and easier than
using a compass (assuming the proper size circle is available).
12. Use a template when creating small polygons (e.g., for hex head bolts) and ellipses as this
is the fastest way to draw these objects (assuming the proper size object is available).
13. Use special lettering templates, if necessary and if available, to assist in lettering.
Lettering is only similar to printing. You can be a poor printer but still have good
lettering.
14. When finished with your drawing, lightly rub the drawing with your dry erase bag
to pick up any loose graphite. Then clean your drawing using a brush to remove
all of the dry erase powder.
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You will begin with the basic steps of drawing straight horizontal and vertical lines
on a sheet of paper to outline the object. Use a lead grade that is harder than #4 (or
harder than 2H). If erasing is needed, use a light touch to prevent smearing. You will go
back over the final drawing with a softer lead grade to create the final darker image.
Whether you are using a T square or a drafting machine, the general process is the
same. If you are using a T square, make sure you hold the cross of the T firmly against
the edge of the drafting table. Large errors can occur when you try to use one hand to
hold a T square loosely in an approximate position while drawing lines with the other
hand. Align your T square or drafting machine and draw a horizontal line at least
4 inches long. Use the scale to make very light marks to indicate a length of 4 inches
along the line. These steps are shown in Figure 22.08.
FIGURE 22.08. Drawing and
marking a horizontal line,
which will be the bottom edge
of the gasket.
4 inches
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Set the triangle firmly against the straight edge of the T square or drafting machine.
To create the right edge of the gasket, place the perpendicular edge of the triangle
on the right end of the 4-inch line mark. Draw a light vertical line at least 2 inches long.
Use the scale to mark a 2-inch distance above the 4-inch line, as shown in Figure 22.09.
Create the left edge of the gasket in a similar fashion, as shown in Figure 22.10.
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2 inches
FIGURE 22.09. Drawing and marking a vertical line, which will be the right edge of the gasket.
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Using the T square or drafting machine, lightly draw the top edge of the gasket by
connecting the two vertical lines at their 2-inch marks using a horizontal line, as shown
in Figure 22.11.
The circle template will be used to create the rounded edges. Select the template circle with the diameter of the arc needed, which is 1 inch (to provide a radius
2 inches
FIGURE 22.10. Drawing and marking another vertical line, which will be the left edge of the gasket.
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FIGURE 22.11. Drawing a horizontal line, which will be the top edge of the gasket, at the marks.
of 0.5 inch at the corners of the gasket). Use the template to create a 0.5-inch radius
arc that is tangent to horizontal and vertical lines at each corner of the gasket, as
shown in Figure 22.12.
To locate the center hole of the gasket, use the scale to mark the center of the bottom horizontal line (i.e., 2 inches from the left mark on this line). Lightly draw a vertical line through the outline of the gasket at this 2-inch location. Then use the scale to
mark the center of the right vertical line (i.e., 1 inch from the bottom horizontal line).
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Use the 1-inch
diameter size
FIGURE 22.12. Using a circle template to add the rounded corners.
Lightly draw a horizontal line through the outline of the gasket at this 1-inch location,
as shown in Figure 22.13. The center of the hole will be located at the intersection of
the horizontal and vertical lines just drawn through the center of the gasket outline.
The circle template will have centerlines on the edges of the 1-inch hole. Align
these centerlines to the vertical and horizontal lines, locating the center of the circle,
and draw the 1-inch circle as shown in Figure 22.14. Using the marks on the on the
1" diameter circle template, again position the template so the marks line up on
the lightly created lines; then draw the circle.
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2 inches
1 inch
FIGURE 22.13. Locating and marking the center of the hole.
Use the eraser and eraser template to remove the corners of the rectangle back
to the radius locations, as shown in Figure 22.15. A softer lead is then used to trace over
the lightly created lines to complete the construction of the gasket.
Finally, annotations are added. Annotations include the numbers, lines, and arrows
used to show the sizes of various features on the part. The numbers, or dimensions, are
added first, as shown in Figure 22.16. Lettering should be between 1/8 inch and 1/4 inch
in height. If your lettering is neat, the numbers can be drawn freehand. Otherwise, a lettering template should be used.
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FIGURE 22.14. Aligning the circle template and making the center hole.
The extension lines and dimension lines are then added as shown in Figure 22.17
with the assistance of the drafting machine or T square and triangle. Remember to
leave a small space of about 1/16 inch between the extension lines and the features they
define. This space helps distinguish between lines that are a part of the object and lines
that are a part of the annotation. This small amount of space between the dimension
lines and the extension lines allows neater arrowheads to be drawn, as shown in
Figure 22.18. Arrowheads can be drawn freehand if it can be done neatly or drawn with
the assistance of an arrowhead template.
Lines with arrowheads that point from a dimension directly to a feature, as shown
for the circle and arcs in Figure 22.19, are called leaders.
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FIGURE 22.15. Erasing the
construction lines and darkening
the outline.
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4.00
FIGURE 22.16. Dimensions
are added.
2.00
R .50 TYP
Ø 1.00
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4.00
FIGURE 22.17. Extension lines,
dimension lines, and centerlines
are added.
2.00
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R .50 TYP
Ø 1.00
4.00
FIGURE 22.18. Arrowheads are
added to the dimension lines.
2.00
R .50 TYP
Ø 1.00
4.00
FIGURE 22.19. Leaders are added.
2.00
R .50 TYP
Ø 1.00
22-15
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22.03 Two-Dimensional CAD Drawings
Two-dimensional CAD drawings are similar to 2-D manually created drawings in that
both are a collection of lines, arcs, circles, and other 2-D geometric elements that are
grouped to represent the image of an object. With 2-D CAD, however, the manual
drawing tools are replaced by a computer, as shown in Figure 22.20, and the vellum is
replaced with a software file. This section of the chapter covers the basic practices of
creating 2-D CAD-based drawings. Most 2-D CAD software uses similar features, commands, and geometry creation methods, although the precise terminology may vary.
22.03.01 Basic Setup Commands
FIGURE 22.20. CAD drawings require the use of a computer and software to create the graphics.
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The basic setup commands are used to prepare a drawing by specifying various user
preferences for parameters such as the overall size of the drawing area, units of measurement, sizes of fonts, formats of numbers, and special shortcuts and aids used to
speed up the process of creating drawings. Many companies have common settings for
these commands to make it easy for different people to work on the same drawing.
Following are some common setup commands and their functions:
Angle Control—helps draw lines at specific angular orientations by forcing, or
snapping, any line drawn near specified angular intervals to align to those intervals.
This command setting aids in the drawing of angled lines by eliminating the need to
enter the values of certain angles. Angle control, sometimes called polar tracking, is also
useful for drawing horizontal and vertical lines. An example of a dialog box used to
specify angular increments for polar tracking is shown in Figure 22.21.
Layer—manages how objects or entities are grouped for convenience within the
drawing. Layers also control color, line type, and line weights and control
whether the objects on the layer appear on a drawing when it is plotted. It is
common, for example, for an annotation to be kept on a separate layer from the
geometry of the object. The annotation layer can then be temporarily turned off
to reduce the complexity of the drawing when only the geometry needs to be
viewed. An example of a dialog box used to control layer properties is shown in
Figure 22.22.
Limits—defines the overall size of the drawing area, which is often determined by
the size of the sheet when the drawing is printed at its full size.
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FIGURE 22.21. A dialog box used to set up angular control in the user interface of
a 2-D CAD software product.
Object Snap—helps specify the exact locations of selection points by forcing any
selection point near certain types of geometry locations to become coincident with
those locations. This command setting aids in the accurate placement of entities or
finishing points. Typical selection point locations include the following:
■
■
■
■
■
■
■
■
■
Center: locates to the centers of circles and arcs
Midpoint: locates to the midpoint of lines and arcs
Endpoint: locates the endpoints of lines and arcs
Perpendicular: after the first endpoint of a line has been specified, locates the
second endpoint to draw the line perpendicular to another line
Intersection: locates the nearest intersection of two objects
Apparent Intersection: locates the intersection of two entities that would meet if
those entities were extended
Parallel: after the first endpoint of a line has been specified, locates the second
endpoint to draw the line parallel to another line
Quadrant: locates the 0°, 90°, 180°, and 270° positions on a circle
Tangent: locates the tangent positions of a line, an arc, or a circle with another
arc or circle
Examples of various symbols used to notify the user that an object snap point has
been specified are shown in Figure 22.23.
FIGURE 22.22. A dialog box used to set up layer control in the user interface of a
2-D CAD software product.
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FIGURE 22.23. The endpoint of
the line being created can snap to
any endpoint (a), perpendicular
point (b), or midpoint (c) of
another entity. Different symbols
are used to indicate the particular
snap geometry engaged.
First point established
Endpoint
Second point snaps
to existing entity
Perpendicular
point
(a)
Midpoint
(b)
(c)
Extension
Midpoint
Tangent
Center
Intersection
Node
Perpendicular
Quadrant
Parallel
Indicates active snap
Precision—specifies the decimal place accuracy of dimensions when they are added
to a drawing. This setup command does not affect precision of the location or the
size of drawn entities.
Text Format—specifies the font and size of the letters and numbers used to annotate
the drawing.
Units—defines the units of measurement in the drawing. For example, decimal
inches are commonly used in manufacturing drawings, whereas fractional inches
are commonly found in construction and architectural drawings. An example of a
dialog box used to specify units is shown in Figure 22.24.
22.03.02 Basic Creation Commands
The basic entity creation commands, listed and detailed next, are found on most
2-D CAD programs. The procedures for their use are slightly different for each software
tool, but the overall utilization concept is the same. Typically, these commands can be
initiated by entering a key word or a letter from the keyboard or by selecting a shortcut
icon on the screen menu of the program.
Arc—creates sections of circles.
Chamfer—replaces the intersection of two entities with a line segment of a specified
length and orientation.
FIGURE 22.24. Part of a dialog box used to specify the type and precision of units to be used
on the drawing.
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Endpoint
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Circle—creates a circle.
Dimension—creates the wide variety of dimensions that are used to specify the exact
sizes and locations of points, lines, and arcs. The common types of dimensions that
can be created are as follows:
Angular—specifies the angle between two lines.
Aligned—specifies the shortest distance between two points regardless of angular
orientation.
Baseline—specifies sizes and locations from a common extension line.
Leader—creates annotations that are directed to a specific feature.
Linear—specifies sizes and locations in horizontal and vertical directions.
Ordinate—specifies sizes and locations from a newly defined coordinate system.
Radial/Diameter—specifies the diameters of circles and the radii of arcs.
Fillet (or Round)—replaces the intersection of two entities with an arc of a specified radius.
Hatch—fills the area inside a closed loop with a specified pattern, color, or shade.
Line—creates straight lines at any orientation. Examples of lines created by keyboard
entries on the drawing screen are shown in Figure 22.25. Note that decimal inch
units are used in one case and fractional inch units are used in the other case.
Point—creates a point at a specified location.
Text—creates letters, numbers, and symbols.
Value entered
from keyboard
Location of
pointing device
2.3520
15 deg
(a)
Value entered
from keyboard
First point established
1'- 4 1/2"
17 deg
(b)
FIGURE 22.25. The creation of lines with polar coordinates entered from the keyboard.
Decimal units (engineering) are used in (a). Fractional units (architectural) are used in (b).
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22.03.03 Basic Editing Commands
The basic editing commands are used to modify entities that have already been created. As with the creation commands, the procedures for using editing commands are
slightly different for each software tool, but the overall utilization concept is the same.
Typically, these commands can be initiated by entering a key word or a letter from the
keyboard or by selecting a shortcut icon on the screen menu of the program.
Copy—is similar to the Move command except that this tool creates new objects at
the second location and keeps the original objects in the original location.
Divide—places points at equally spaced intervals along a selected object.
Erase—removes the selected objects from the drawing.
Extend—lengthens an object until it intersects a selected boundary object.
Move—relocates the selected entities from one location in a drawing to another
location within the same drawing.
Offset—creates a new object that is placed at a constant prespecified distance from
the original object. This command is commonly used to create parallel lines and
concentric circles.
Rotate—changes the orientation angle of selected objects about a selected rotation center.
Scale—changes the overall size of selected objects. A new scale number above the
value of 1.00 will increase the size of the objects. A new scale number below 1.00
will decrease the size of the objects.
Trim—removes that part of a selected object that extends beyond its intersection
with a selected boundary object.
22.03.04 Making a Simple 2-D CAD Drawing
The use of the setup, creation, and editing commands to make a simple 2-D CAD drawing is now demonstrated through the creation of a drawing of the gasket shown in
Figure 22.26, which was created earlier using manual techniques. First, the setup commands are specified, as shown next.
Angle control—specify a snap to every 90°.
Layer—specify a single layer.
Limits—specify a letter-sized (8.5" × 11") drawing limit with landscape orientation
(i.e., a size A drawing).
Object snap—snap to endpoints, midpoints, and intersections.
Precision—specify accuracy to two decimal places.
Text format—specify a drafting font that is 0.25 inch in height.
Units—specify decimal inches.
If this setup will often be used for future drawings, it would be wise to save the drawing
as a template. Template files can then be used to create drawings with the desired setup command specifications already in place so the specifications do not need tobe entered again.
FIGURE 22.26. The 2-D CAD
drawing of the gasket begins with
a horizontal line. The placement
and length are not important as
long as the line is horizontal and
longer than 4 inches.
4.7521 >0 deg
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Pattern (or array)—copies selected objects and creates a circular group of them
around a selected point or creates a rectangular group of them extending from a
selected point.
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Create a horizontal line using the Line command as shown in Figure 22.26. The line
should be at least 4 inches long and snap to the horizontal position. Next, create a vertical
line from the right end of the horizontal line as shown in Figure 22.27. The vertical line
should be at least 2 inches long.
Specifying an offset value of 4 inches, use the Offset command to create a copy
of the vertical line to the left of the original vertical line as shown in Figure 22.28.
Use the Offset command again, this time specifying an offset value of 2 inches, to
create a copy of the horizontal line above the original horizontal line as shown in
Figure 22.29.
Use the Fillet command to round the corners of the enclosed rectangle as shown in
Figure 22.30. The radius of the fillet should be specified as 0.5 inch. Select the edges of
the rectangle that are to be trimmed to create the fillet. Do this for all four corners of the
enclosed rectangle.
FIGURE 22.27. A vertical line is
added at one endpoint. The
length is not important as long
as the line is vertical and longer
than 2 inches.
3.2502 >90 deg
FIGURE 22.28. The horizontal
line is offset by 2 inches in the
direction shown.
OFFSET, 2 inches
FIGURE 22.29. The vertical line
is offset by 4 inches in the
direction shown.
OFFSET, 4 inches
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(a)
0.5-inch radius
FIGURE 22.30. The corners are filleted by specifying the entities to be modified (a) and
the radius (b).
The center of the 1-inch circle is located using construction lines, which will later
be erased. Using the Line command, create a (vertical) line connecting the midpoints of
horizontal lines. Then use the Line command to create a (horizontal) line connecting
the midpoints of the vertical lines. This construction is shown in Figure 22.31.
Use the Circle command to draw a 1-inch diameter circle as shown in Figure 22.32.
Snap the center of the circle to the intersection of the two lines just drawn. Specify the
diameter of the circle as 1 inch (or the radius as 0.5 inch). After the circle is created,
the two lines used to locate its center are erased.
The dimensions are added as shown in Figure 22.33. Use the Linear Dimension
command, selecting the two vertical lines, to label the width of the gasket. Use the Linear
Dimension command again, selecting the two horizontal lines, to label the height of the
gasket. Use the Diameter Dimension command, selecting the circle, to label the size of
the center hole. Use the Radius Dimension command, selecting one of the corner fillets,
to label the radius of the corners. If a Smart Dimension command is available, it can be
used to label all of the previous features.
FIGURE 22.31. The center of the circle is located with construction lines drawn from the
midpoints of the sides.
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(b)
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1-inch diameter
FIGURE 22.32. The center of the circle snaps to the intersection of the construction lines.
The diameter is specified. The construction lines can then be erased.
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4.00
2.00
2.00
1.00
R 0.5 TYP
Ø 1.00
FIGURE 22.33. The dimensions are added. Adding dimensions to circles and radii usually
inserts their centerlines.
22.03.05 Manually Drawn versus 2-D CAD Drawn
The current trend is toward increased usage and dependence on 2-D CAD-based tools
for creating and editing all types and styles of technical drawings. Manually created
drawings have the advantage of requiring simpler and less expensive tools than 2-D
CAD-created drawings. Manual drawing tools are also simpler and easier to use. Twodimensional CAD-created drawings require the use of a computer and the appropriate
software. Mastering the use of the software typically requires longer and more sophisticated training than mastering the use of manual drawing instruments. However, once
mastered, 2-D CAD-based drawings have many advantages over manually created
drawings. Two-dimensional CAD-created drawings typically can be produced more
quickly than manually created drawings. Manually created drawings are prone to inaccuracies caused by errors in tool placement, measurement, and imperfect drawing technique. Two-dimensional CAD-created drawings are easier to edit compared to erasing
entities on a manually created drawing, which is a time-consuming process. Finally,
because 2-D CAD-created drawings can be saved as computer files, they require less
storage space and can be reproduced and transmitted quickly and easily.
22.04 Making a Typical 2-D CAD Drawing
A more sophisticated drawing is shown in Figure 22.34, which shows the front and
right-side views of a flange. The right-side view is presented as a half section. The steps
in the creation of a 2-D CAD-based drawing of this flange are detailed to demonstrate
how additional 2-D CAD settings and commands can be used.
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2X R .63
FIGURE 22.34. The front and
right-side views of a flange. The
right-side view is presented as a
half section.
.38
2X Ø .50
Ø 1.56
2X .06
1.49
2X .25
2.99
R 1.13
Ø 1.25
Layer—specify five layers.
Limits—specify a letter-sized (8.5" × 11") drawing limit with landscape orientation
(i.e., a size A drawing).
Object snap—snap to endpoints, midpoints, intersections, and tangents.
Precision—specify accuracy to two decimal places.
Text format—specify a drafting font that is 0.25 inch in height.
Units—specify decimal inches.
The five layers shown in the example dialog box in Figure 22.35 are used to organize different aspects of the drawing. Entities used to create the dimensions are grouped
into a layer named “0,” the centerlines in a layer named “center,” the hidden lines in a
layer named “hidden,” the outline of the object in a layer called “object,” and the section lines in a layer called “section.” The advantage of using five separate layers is that
different line styles and colors may be assigned to each layer, as in the case of the flange
drawing. For example, an entity created on the center layer will appear as a sequence of
short and long dashes, whereas an entity created on the object layer will appear unbroken. An entity on the hidden layer will appear with dashed lines. In addition, the object
lines and section lines should be slightly thicker than the default line type. The color
and line style of every entity in a layer can be changed, if needed, in a single operation by
changing the default settings in that layer.
Once the setup is complete, the front and right-side views can be created. The center layer is activated, and the Line command is used to draw intersecting horizontal and
vertical lines as shown in Figure 22.36. These lines automatically appear as centerlines
because they have been drawn on the center layer. These lines will be used to locate the
large hole on the front view. Next, use the Offset command to create a copy of the horizontal line 1.49 inches above the original line; then use Offset again to create a copy
1.49 inches below the original line. The intersection of the vertical line with the two
new horizontal lines will be used to locate the centers of the two smaller holes on the
front view.
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First, the setup commands are specified as shown below. This setup is the same as
that used in the previous example except for the additional layers and object snaps. If
the setup used in the previous example was saved as a template, that template could be
recalled, modified, and reused for this example.
Angle control—specify a snap to every 90°.
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FIGURE 22.35. An example of a
dialog box used to specify the
names, the line types, the colors, and other information
about the layers on the drawing.
FIGURE 22.36. Start the front
view of the flange by creating
intersecting horizontal and vertical centerlines on the center
layer. Offset the horizontal
lines to locate the centers of the
two holes.
Vertical line
Offset by 1.49 inches
Horizontal line
Offset by 1.49 inches
In creating the outline of the part, the object layer is activated. The holes are created with the Circle command. When this command is selected, the center of the circle being drawn should snap to the intersections of the lines. A 1.25-inch diameter
circle is created with its center at the middle intersection of the lines. A 0.5-inch
diameter circle is created at each of the upper- and lower-intersections of the lines.
In addition, a 1.56-inch circle, representing the outer surface of the cylindrical part
of the flange, and a 2.25-inch diameter circle, representing the large round at the
base of the flange, are created with their centers at the middle intersection of the
lines. Also, 1.25-inch diameter circles, representing the smaller round at the base of
the flange, are created at the upper- and lower-intersections of the lines. These circles, which appear unbroken because they are located on the object layer, are shown
in Figure 22.37.
Using the Line command, the straight edges of the base are created with tangent
lines, which are drawn between each of the circle edges as shown in Figure 22.38. The
endpoints of each line will snap to the nearest tangent point of each circle. The Trim
command is used to remove the unneeded arc segments to complete the front view,
which is shown in Figure 22.39.
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FIGURE 22.37. After the switch
is made to the object layer,
circles are drawn to represent
the holes, outer diameter, and
rounds on the flange.
0.5-inch diameter
1.25-inch diameter
1.25-inch diameter
2.25-inch diameter
1.56-inch diameter
Tangent
Tangent
FIGURE 22.39. The outer
circles are trimmed to the
tangent lines to create the
outer edges of the part.
Tangent
Tangent
Trimmed
Trimmed
In creating the right-side view, new horizontal centerlines, created in the center
layer, are attached to the ends of the existing horizontal centerlines and extended
into the right-side view. These new centerlines will be used to help locate the flange
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FIGURE 22.38. Tangent lines
are drawn to the outer circles to
create the outer edges.
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holes in the right-side view. The object layer is then activated, and a vertical line is
created to represent the back surface (shown as an edge) of the base on the flange.
The front surface of the base can be located by using the Offset command to create a
copy of the back edge (the vertical line just created) at a distance of 0.3 inch to the left
of the back edge. The front surface of the flange can be located by using the Offset
command to create a copy of the back edge at a distance of 1.75 inches to the left of
the back edge. Key points can then be projected from the front view to the right-side
view to locate other key locations on that view. On some software, this projection
may require construction of temporary geometry (that will be erased later). On
Figure 22.40, for example, a horizontal construction line is created through the intersection of the vertical centerline and the outer cylindrical surface on the front view.
This construction line can be used to locate the outer cylindrical surface on the rightside view. The limits of the base on the flange and the limits of the holes can be
located with this method. After the needed lines are trimmed, the right-side view will
appear as shown in Figure 22.41.
The right-side view is to be presented as a half section. Therefore, the internal features of the flange will be shown with section lines on half of the view, and the other
half will be shown with hidden lines. The hidden lines are created by selecting the
interior lines on the lower half of the right-side view, as shown in Figure 22.42, and
reassigning them to the hidden layer. This action will convert these solid lines into
dashed lines.
Section lines are created on the upper half of the right-side view by using the
Hatch command. By default, this command usually fills a selected closed area with
uniformly spaced lines at a 45º angle. The pattern can be easily edited to adjust the
line spacing, density, or pattern. The areas that have been hypothetically cut should
be selected on the upper half of the right-side view of the flange. The result is shown
in Figure 22.43.
Now the cutting plane line and arrow are fabricated. If the 2-D CAD software being
used does not have a tool for creating a cutting plane line, the line can be fabricated
with a leader line and a standard line. Adjust the line properties to produce a thick
dashed line as shown in Figure 22.44.
After the dimension layer is activated, dimensions can be placed on both views as
shown in Figure 22.45. Arcs that are less than 180º are typically labeled as radii instead
of diameters.
0.375-inch offset
FIGURE 22.40. The right-side
view is drawn by using horizontal lines to project key points
from the front view.
1.625-inch offset
Back
Top
Edges located by projection
from the front view
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FIGURE 22.41. The outline of
the right-side view is completed.
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FIGURE 22.42. The internal
features are added. Lines become
dashed when they are reassigned
to the hidden layer. Lines that are
to be centerlines should be
assigned to the center layer.
Assigned to hidden layer
FIGURE 22.43. The external
edges are trimmed on the section
view, and section lines are added
to the areas that have been cut.
Section lines are added
to the closed-loop areas
formed by areas that have
been cut
Trimmed
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Cutting plane line
constructed from
dashed line and leader
FIGURE 22.44. The cutting
plane line can be constructed
using a combination of a continuous line, a dashed line, and a
leader.
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22-29
2X R .63
FIGURE 22.45. Dimensions and
other annotations are added on
the dimension layer.
.38
2X Ø .50
Ø 1.56
2X .06
1.49
2X .25
2.99
R 1.13
Ø 1.25
22.05 Creating Auxiliary Views Using 2-D CAD
Auxiliary views are commonly used to show the true shape of an inclined surface on an
object. The object shown in Figure 22.46, for example, is presented in front, right-side,
and auxiliary views. The creation of the auxiliary view requires the creation of lines that
are parallel or perpendicular to the inclined edge in the front view. When such a construction is presented, two popular methods can be used to create lines with the necessary orientation in the auxiliary view. One method entails using the Offset, Parallel,
and Perpendicular tools; the other method focuses on changing the orientation of the
xy coordinate system. Both work well.
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22.05.01 Using Offset, Parallel, and Perpendicular
The Offset command can be used to copy the inclined edge and place the copy at the
desired distance away from the original view, as shown in Figure 22.47, to create a projected edge. The Offset command can be used again to copy the inclined edge and place it
at the correct distance to represent the object’s depth, as shown in Figure 22.48. The true
shape of the inclined surface in the auxiliary view is completed by connecting the endpoints of the projected edges. Additional edges that may be needed to complete the object
in the auxiliary view are either parallel or perpendicular to the projected edge. Lines that
are parallel to the projected edge can be created using the Line command with the parallel object snap option and specifying the projected edge as the parallel reference. Lines
that are perpendicular to the projected edge can be created using the Line command with
the perpendicular object snap option and specifying the projected edge as the parallel
reference.
Offset
FIGURE 22.47. To construct the auxiliary view, offset the edge of the inclined surface.
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FIGURE 22.46. The front, side, and auxiliary views of an object with an inclined surface and
a hole.
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Offset the Depth
Depth
FIGURE 22.48. Locate the back of the object on the auxiliary view by offsetting the edge by
the depth of the object as seen in the side view.
22.05.02 Rotating the Coordinate System
In the default xy coordinate system on most 2-D CAD software, the positive x-axis
points to the right (horizontal) and the positive y-axis points up (vertical). The coordinate system can be reoriented so that the x-axis or y-axis points in any desired direction.
In the construction of an auxiliary view, the goal is to create lines that are either parallel or perpendicular to the inclined edge in an existing view. For this purpose, it is useful to reorient the coordinate axis such that the x-axis points in the same direction as
the inclined edge, which automatically forces the y-axis to point in a direction perpendicular to this edge. This reorientation is shown in Figure 22.49.
With the coordinate system in this orientation, any line that is created as “horizontal”
will be parallel to the inclined edge. Any line that is created as “vertical” will be perpendicular to the inclined edge. Creating the auxiliary view, as shown in Figure 22.50 and
Figure 22.51, is simplified because the inclination angles need not be measured or
calculated.
Y
X
Y
X
(a)
(b)
FIGURE 22.49. Realignment of the original drawing coordinate system (a) to be parallel and
perpendicular to the inclined edge (b).
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"Vertical" line
FIGURE 22.50. In the realigned
coordinate system, “vertical”
lines will automatically be
created perpendicular to the
inclined edge (a) and “horizontal”
lines will automatically be created
parallel to the inclined edge (b).
Y
Y
"Horizontal" line
X
X
(a)
(b)
Offset depth
Y
X
"Vertical" line
22.05.03 Projecting Features from Auxiliary Views
Auxiliary views are most often used to show inclined surfaces or features on inclined
surfaces in their true shape and size. Consequently, features on inclined planes are
most easily created in the auxiliary view because their distortion need not be considered. However, such features, when projected onto one of the standard views, do
become distorted; and this distortion should be presented. Consider the hole in the
object shown in Figure 22.52.
In the right-side view, the hole becomes elliptical. Four points, which are the endpoints of the major and minor axes of the ellipse, are needed to define its shape.
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FIGURE 22.51. The limits of
the inclined plane in the
auxiliary view are drawn with
“vertical” lines and offset
“horizontal” lines.
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FIGURE 22.52. The hole, seen in its true shape in the auxiliary view, should be projected into
the side view.
The vertical location of the ellipse can be found by projecting the center location
from the front view. The horizontal location of the ellipse can be found by measuring
its location from the front edge of the inclined surface as seen in the auxiliary view. The
minor axes can be found by projecting the limits of the circle from the auxiliary view to
the front view and then into the right-side view, as shown in Figure 22.53.
The length of the major axis of the ellipse is the same as the diameter of the hole.
Thus, with the length of the major and minor axes defined and the location of the center known, an ellipse can be drawn using the Ellipse command found in most 2-D CAD
software, as shown in Figure 22.54.
Center
location
Center
location
FIGURE 22.53. The location of the center and limits of the minor axis are projected from the
auxiliary view to the inclined surface in the front view, then into the side view.
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FIGURE 22.54. The ellipse
drawn in the side view, with the
major axis equal to the diameter
of the circle.
Circle
diameter
Circle
diameter
Following are some hints, options, and settings that will make 2-D CAD faster and
more accurate, with fewer resultant errors. Depending on the specific software you are
using, some of these settings may not be available.
■ When drawing horizontal and vertical lines, activate the Angular Control option
and set it to align to these positions. Unless this option is properly set and active,
horizontal and vertical lines will not be exactly horizontal or vertical, no matter
how good they look.
■ When entities need to be connected, activate the Object Snap option and set it to
snap to endpoints. Unless this option is properly set and active, different entities
will not be connected, no matter how good they look.
■ When starting a new drawing, adjust the size of the viewing screen (using the
Zoom command) so that the drawing limits fill the entire screen. This preparation
will help you draw your first entities to approximately the correct size by allowing
the size of the drawing area to be used as a visual reference.
■ Placing the endpoints of lines in their correct relative positions is usually quicker
and easier to do when using construction lines, offsets, and trims than when
determining the Cartesian coordinates of their endpoints and entering these
coordinates from a keyboard.
■ If single-letter keyboard shortcuts are available for activating commands, memorize
them. These are usually much faster to use than selecting an icon from the screen.
■ The buttons on a pointing device such as a mouse can usually be customized to
speed up command entries. For example, the right button can usually be customized to act as the Enter key, which is usually used to complete commands.
■
■
A sharp corner between two intersecting entities can usually be created using the
Fillet command with a zero radius.
Tangent snaps are sometimes difficult to place when the circle quadrant and
intersection snaps also are active. If you have difficulty making an entity tangent
to a circle or an arc, activate the tangent snap only.
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22.06 Useful 2-D CAD Tricks
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■
■
■
■
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CAUTION
22-35
Hatch patterns require enclosed boundaries for the design to be generated. If an
“open boundary” error occurs, closely examine corners and tangents to see if there
is a gap. It is sometimes easier to redraw the boundary than to find a gap.
For complex drawings, place the object views, dimensions, and other annotations
on different layers. Dimensions and annotations can then be easily hidden, if
needed, to clarify the views of the object.
For assembly drawing, place each part on a different layer. Different parts can then
be easily hidden, if needed, to clarify the views of the remaining parts.
Make the default line colors different for each layer. This way, it becomes easy
to see whether you are using the intended layer.
Inexperienced engineers, designers, and drafters can unwittingly introduce errors into
their drawings with poor drawing techniques. The following sections, a compilation of
the most common beginner errors, discuss how to avoid or fix them.
Nonconnecting Entities
One of the most common errors made in 2-D CAD is the nonconnection of entities. The
example in Figure 22.55 shows lines that appear to be connected. However, a magnification
of the figure shows that the connection is not made. This error is typically discovered when
the wrong dimension appears when dimensions are added to the drawing or when an
operation requiring a closed boundary, such as section lines being added to an area, is used.
This error usually results when the Object Snap option has not been activated. To avoid this
problem, make sure the Object Snap option is active whenever entities are to be connected.
NO!
Yes
FIGURE 22.55. Entities that appear connected may, under magnification, not actually
be connected.
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Overlapping Entities
Similar to the problem of nonconnected entities is the problem of overlapping entities.
The example in Figure 22.56 shows lines that appear to be connected at their endpoints.
However, a magnification of the figure shows that the lines overlap. This error is typically
discovered when the wrong dimension appears when dimensions are added to the
drawing. This error usually results when the Object Snap option has not been activated. To
avoid this problem, make sure the Object Snap option is active whenever entities are
to be connected.
Not Really Tangent
The example in Figure 22.57 shows a line that is apparently tangent to an arc. However,
a closer look shows that the line is not attached to the arc at its tangent point, but
rather at one of the quadrant points. This error usually occurs when the Object Snap
option is active and is set to snap to the quadrant point and tangent points. When this
is the case, the attaching entity attaches to the closest acceptable option, which may
not be the intended point. To avoid this error, whenever multiple acceptable snap
possibilities exist, make sure you activate only the Snap option you want.
In an attempt to fit and print a drawing on a particular sized sheet, inexperienced 2-D CAD
users sometimes decrease (or increase) the size of all of the entities on the drawing
using a Scale command. This operation does change the size of the entities; it also
changes the dimensions attached to those entities, yielding undesired dimension
values as shown in Figure 22.58. To avoid this error, create all entities on a drawing to
their correct size and print on the desired sheet size by adjusting the scale of the
printing, not the scale of the drawing.
NO!
Yes
FIGURE 22.56. Entities that appear connected may, under magnification, actually overlap.
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Improper Scaling
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NO!
Yes
FIGURE 22.57. Lines are sometimes inadvertently attached to quadrant points when tangent
points are desired.
1.00
.50
NO!
1.00
Yes
FIGURE 22.58. When the size of a drawing is changed, scaling the object changes its
dimensions. Dimensions can be maintained by changing the size of the sheet.
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Attaching Dimensions to the Wrong Entity
The example shown in Figure 22.59 shows a drawing that, at first glance, is correctly
dimensioned. However, one dimension seems odd because its value is not a nice round
number like the other dimensions. Upon closer inspection, it can be seen that this
dimension accidentally specified the distance to the end of the extension line of
another dimension instead of the distance to the edge of the object. This type of error
is difficult to spot; to a fabricator, the dimension looks correct, but it will result in the
part being produced with an incorrect size. To avoid this error, when adding dimensions to a drawing, make sure you select the correct points on the object, not points on
extension lines or another annotation.
Overreliance on Grids
2.00
1.06
NO!
Yes
FIGURE 22.59. Dimensions are sometimes inadvertently attached to other dimensions
instead of to the object.
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In the 1980s, rectangular arrays of points, called grids, were commonly used to promote
accuracy in a drawing. All drawing entities (such as the endpoints of lines and arcs and
the centers of circles) would be connected to dots on the grid in some way, as shown in
Figure 22.60. The user counted the dots on the screen or set up the pointer to jump
from dot to dot to make counting the distance easier. If the desired location did not
match the grid spacing, the grid spacing could be changed. Today, however, the counting
of dots and adjusting of grid spacing is considerably less efficient than using the
creation and editing commands described earlier. Today grids are still used, but only
for a small percentage of simple objects that have feature sizes and locations that
correspond precisely to the spacing of a grid. Grids are also useful for creating cell
outlines for text tables and drawing borders. Otherwise, grids unnecessarily limit the
possible sizes and locations of features on an object.
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FIGURE 22.60. Grids are handy for creating data and information tables, but they can
unnecessarily limit creativity and optimal sizes for a design.
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Grid Accidentally Turned On
When a grid option is available, it can be inadvertently activated even if the grid is not
visible. When this happens, entities can snap to the grid points in the same manner
they snap to objects. Errors occur when a user, in an attempt to snap to an entity, accidentally snaps to a point on the grid. To avoid these errors, make sure the grid is inactive if it is not needed.
22.07 Chapter Summary
This chapter presented the basics of 2-D drawing. It was not intended to be a comprehensive tutor on this topic. Instead, only the basic information needed to understand
the process of creating a 2-D drawing was presented. The instruments used for manual
drawing creation were shown and described, with a quick example of how they are used.
Different types of geometry require the use of different instruments. These instruments must be used properly if accuracy and precision are to be maintained. Manual
drawing creation is prone to errors in precision because their creation is dependent on
the skill of the person making the drawing. The use of CAD improves the efficiency
with which drawings can be created, and also dramatically improves their accuracy and
precision because these are much less dependent on the skill of the operator. However,
the basic CAD commands and their proper use must be mastered. In this chapter, the
basic commands found in most 2-D CAD software were described, with examples of
how they are typically used. Most CAD software includes many more commands in
addition to these basic commands. Mastery of specific software comes only with its use
and practice.
22.08 glossary of key terms
2-D CAD: Two-dimensional computer aided design.
Computer software used for the creation of twodimensional art and text in engineering drawings.
creation commands: User input in 2-D CAD software
that creates annotations and entities used on a drawing.
dimensions: Annotations that indicate the size, location, or orientation of the features on an object; they are
used for fabrication of the object. The process of adding
notation to indicate the size, location, or orientation of
features on an object.
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22.08 glossary of key terms (continued)
drafting: The process of creating an engineering drawing using manual techniques and tools.
drafting instruments: Tools such as triangles, pencils,
T squares, erasers, and compasses used to aid users in
creating drawings.
drawings: Collections of images and annotations containing sufficient information for places and things to be
re-created. The process of creating an engineering drawing.
editing commands: User input in 2-D CAD software
that edits annotations and entities used on a drawing.
setup commands: User input in 2-D CAD software
that helps create the overall process and interface
environment.
snapping: Forcing the location of a point to particular
location on a grid or an entity as that point approaches
the location.
22.09 questions for review
7. What are some difference between manual drafting
and 2-D CAD?
8. In the creation of drawings, what are some advantages of drawing them manually compared to using
2-D CAD software?
9. In the creation of drawings, what are some advantages of drawing them manually compared to using
2-D CAD software?
10. What is the purpose of the Object Snap command in
2-D CAD?
11. How are layers used in 2-D CAD?
22 Chapter CXXXX 40757 Page 40 03/18/08 MD
1. What is the purpose of different lead hardnesses in
the creation of manual drawings?
2. How can 30-60 and 45 triangles be used to create
angles other than 30°, 45°, 60°, and 90°?
3. How is the circle template used to create rounded
edges?
4. When should a circle instead of a compass be used to
create a circle?
5. What is the purpose of the eraser shield?
6. What are some similarities between manual drafting
and 2-D CAD?
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22-41
22.10 problems
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1. Reproduce the geometric patterns using either manual drawing instruments or 2-D CAD as directed by your
instructor.
(a)
(b)
(c)
(d)
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22.10 problems (continued)
(f )
50
R10
1.75
35
2X R.50
R15
45º
4X Ø.50
2X Ø.38
1.50
45º
24
15º
14
17
R.25
8
R.50
R.50
5
R10
R14
1.38
R.75
44
60
65
(g)
.87
R.75
R.75
1.00
1.88
2.25
(h)
.50
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(e)
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chapter 22 Two-Dimensional Drawing
22.10 problems (continued)
.62
4.12
R3.00
2X R.55
.375
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.63
.65
3.24
2X R1.18
.64
.76
.375
R2.25
.58
2X R.60
(i)
(j)
(k)
22-43
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22-44 sectionfive Advanced Topics in Engineering Graphics
22.10 problems (continued)
2. Draw the front, top, and right-side views of the following objects along with their dimensions. Use either manual
drawing instruments or 2-D CAD as directed by your instructor.
(b)
(c)
(d)
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(a)
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22.10 problems (continued)
(e)
(f )
(g)
(h)
22-45
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22-46 sectionfive Advanced Topics in Engineering Graphics
22.10 problems (continued)
(k)
(j)
(l)
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(i)
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22.10 problems (continued)
(m)
22-47
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