Learning Objectives - media.scc.losrios.edumedia.scc.losrios.edu/FitzpaK/books/Bk_AutoCAD_2008/Bk_AutoCAD_2008_Ch22.pdfPerspective Drawing The most realistic form of pictorial drawing

Learning Objectives

After completing this chapter, you will be able to do thefollowing:Describe the nature of isometric and oblique views.Set up an isometric grid.Construct isometric objects.Create isometric text styles.Demonstrate isometric and oblique dimensioning techniques.

Being able to visualize and draw three-dimensional shapes is a skill that everydrafter, designer, and engineer should possess. This is especially important in 3Dmodeling. However, there is a distinct difference between drawing a view that looksthree-dimensional and creating a true 3D model.

A 3D model can be rotated on the display screen and viewed from any angle.The computer calculates the points, lines, and surfaces of the objects in space. Forinformation on 3D modeling, see AutoCAD and Its Applications—Advanced. Thefocus of this chapter is creating views that look three-dimensional using some specialAutoCAD functions and two-dimensional coordinates and objects.

c Pictorial Drawing Overview )The word pictorial means "like a picture." It refers to any form of 2D drawing

that illustrates height, width, and depth. Several forms of pictorial drawings are usedin industry today. The least realistic is oblique. However, this is the simplest type.The most realistic, but also the most complex, is perspective. Isometric drawing fallsmidway between the two as far as realism and complexity are concerned.

Oblique DrawingsAn oblique drawing shows objects with one or more parallel faces having true

shape and size. A scale is selected for the orthographic, or front, faces. Then an anglefor the depth (receding axis) is chosen. The three types of oblique drawings are cavalier, cabinet, and general. See Figure 22-1. These vary in the scale of the receding axis.The receding axis is drawn at full scale for a cavalier view and at half scale for a cabinetview. The general oblique is normally drawn with a 3/4 sane for the receding axis.

pictorial: A drawingthat is similar to a

picture.

oblique drawing: Adrawing that showsobjects with one ormore parallel faceshaving true shapeand size.

723

axonometric

drawings:Drawings in which a3D object is rotatedfor display on a2D drawing sheetso that all three

dimensions can be

seen.

isometric

drawings:Drawings in whichthe three axes are

equally spacedat 120°.

dimetric: An

axonometric

drawing in whichtwo different scales

are used to measure

the three axes.

trimetric: An

axonometric

drawing in whichthree different

scales are used to

measure the three

axes.

foreshortening:Property of adrawing in whichobjects appearto recede in the

distance.

perspectivedrawing: The mostrealistic form of

pictorial drawing,in which recedingobjects meet at oneor more vanishingpoints on thehorizon.

vanishing point:The point at whichobjects seem toconverge in thedistance at an

imaginary point.

724

Figure 22-1.The scale of the

receding axis differsin the three types ofoblique drawings.

Cavalier

c—J/ d^Cabinet General

Axonometric DrawingsThe general term for drawings that display three dimensions on a two-dimen

sional surface such as a drawing sheet is axonometric draivings. The three types ofaxonometric drawings are isometric, dimetric, and trimetric.

Isometric draivings are more realistic than oblique drawings. The entire objectappears as if it is tilted toward the viewer. The word isometric means "equal measure."This equal measure refers to the angle between the three axes (120°) after the objecthas been tilted. The tilt angle is 35°16'. This is shown in Figure 22-2. The 120° anglecorresponds to an angle of 30° from horizontal. In the construction of isometric drawings, lines that are parallel in the orthogonal views must be parallel in the isometricview.

The most appealing aspect of isometric drawing is that all three axis lines canbe measured using the same scale. This saves time, while still producing a pleasingpictorial of the object.

Dimetric and trimetric drawings are closely related to isometric drawing. Theseforms of pictorial drawing differ from isometric in the scales used to measure thethree axes. Dimetric drawing uses two different scales and trimetric uses three scales.Using different scales is an attempt to create foreshortening. This means the lengths ofthe sides appear to recede. The relationship between isometric, dimetric, and trimetricdrawings is illustrated in Figure 22-3.

Perspective DrawingThe most realistic form of pictorial drawing is a perspective drawing. The eye

naturally sees objects in perspective. Look down a long hall and notice that the walland floor lines seem to converge in the distance at an imaginary point. That point iscalled the vanishing point. The most common types of perspective drawing are one-point and two-point perspectives. These forms of pictorial drawing are often used inarchitecture. They are also used in the automotive and aircraft industries. Examplesof one-point and two-point perspectives are shown in Figure 22-4. A perspective ofa true 3D model can be produced in AutoCAD using the 3DORBIT command. SeeAutoCAD and Its Applications—Advanced for complete coverage of 3DORB1T.

Figure 22-2.An object is tilted35°16' to achievean isometric

view having 120°between the three

axes. Notice the

highlighted face ineach view. Front Right

AutoCAD and Its Applications—Basics

Figure 22-3.Isometric, dimetric,and trimetric

drawings differ inthe scales used to

draw the three axes.

In the isometric

shown here, all threesides are drawn

at full scale, or 1.You can see how

the dimetric and

trimetric scales vary.

Figure 22-4.An example of one-point perspectiveand two-pointperspective.

Isometric

Vanishin

.point

One-Point

Perspective

Dimetric

Two-Point

Perspective

Isometric Drawing

Trimetric

)The most common method of pictorial drawing used in industry is isometric.

Isometric drawings provide a single view showing three sides that can be measuredusing the same scale. An isometric view has no perspective and may appear somewhatdistorted. Two of the isometric axes are drawn at 30° to horizontal, while the third isdrawn at 90°. See Figure 22-5.

The three axes shown in Figure 22-5 represent the width, height, and depthof the object. Lines that appear horizontal in an orthographic view are placed at a30° angle. Lines that are vertical in an orthographic view are placed vertically. Theselines are parallel to the axes. Any line parallel to an axis can be measured and is calledan isometric line. Lines that are not parallel to the axes are called nonisometric linesand cannot be measured. Note the two nonisometric lines in Figure 22-5.

Circles appear as ellipses in an isometric drawing. Circular features shown onisometric objects must be oriented properly or they appear distorted. The correctorientation of isometric circles on the three principal planes is shown in Figure 22-6.The small diameter (minor axis) of the ellipse must always align on the axis of thecircular feature. Notice that the centerline axes of the holes in Figure 22-6 are parallelto one of the isometric planes.

A basic rule to remember about isometric drawing is that lines that are parallelin an orthogonal view must be parallel in the isometric view. AutoCAD's ISOPLANEfeature makes that task, and the positioning of ellipses, easy.

Chapter 22 Basic Pictorial Drawings

isometric line: Anyline that is parallelto an axis in an

isometric drawing.

nonisometric

lines: Lines that are

not parallel to theaxes in an isometric

drawing.

725

726

Figure 22-5.Layout of theisometric axes.

Figure 22-6.Proper isometriccircle (ellipse)orientation on

isometric planes.The minor axis

always aligns withthe axis centerline.

Minor

Axis

Nonisometric

lines

MajorAxis

Axis centerline

DIf you are ever in doubt about the proper orientation of an ellipse inan isometric drawing, remember that the minor axis of the ellipsemust always be aligned on the centerline axis of the circular feature.This is shown clearly in Figure 22-6.

Settings for Isometric DrawingYou can quickly set your isometric variables in the Snap and Grid tab of the

Drafting Settings dialog box. See Figure 22-7. To access this dialog box, enter DS, SE,or DSETTINGS, or select Tools > Drafting Settings... from the pull-down menu. Thisdialog box can also be accessed by right-clicking the SNAP or GRID status bar buttonand then selecting Settings... from the shortcut menu.

To activate the isometric snap grid, pick the Isometric snap radio button in theSnap type area. Notice that the Snap X spacing and Grid X spacing text boxes are nowgrayed out. Since X spacing relates to horizontal measurements, it is not used in theisometric mode. You can only set the Yspacing for grid and snap in isometric. Besureto check the Snap On (F9) and Grid On (F7) check boxes if you want Snap and Gridmodes to be activated. Pick the OK button to display the grid dots on the screen in anisometric orientation, as shown in Figure 22-8. If the grid dots are not visible, turn thegrid on. You may also need to zoom in or out to display the grid.


1Drafting Settings

Snap and Grid |Pda, Tracking; Object Snap j| Dynamic Input]

Figure 22-7.The DraftingSettings dialog boxallows you to makesettings needed forisometric drawing.

• SnapOn(F9)

Snap spacing

SnagX spacing:

SnapYspacjng:

• Grid On(F7)

Gridspacing

GridX spacing:

GridY spacing

Major lineeveiy: 5

Grid behavior

0 Adaptive grid

I | Allow subdivision belowgridspacing

• Display grid beyond Limits

• Follow Dynamic UCS

0.36602540

0.5000 0.5000

•• Equal i =• -V spac ng

Polar spacing

Polar distance 0.0000

Snap type

© Giid snap

O Rectangular snap

ijisprneiiic snap

O PolaiSnapPick to activate

isometric snap grid

Figure 22-8.An example of anisometric grid setupin AutoCAD.

Grid dots align toisometric orientation

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Notice that the crosshairs appear angled. This aids you in drawing lines at theproper isometric angles. Try drawing a four-sided surface using the LINE command.Draw the surface so it appears to be the left side of a box in an isometric layout. SeeFigure 22-9. To draw nonparallel surfaces, you can change the angle of the crosshairsto make your task easier, as discussed in the next section.

To turn off the Isometric Snap mode, pick the Rectangular snap radio button inthe Snap type area. The Isometric Snap mode is turned off and you are returned to thedrawing area when you pick the OKbutton.

CYou can also set the Isometric Snap mode by typing SNAP or SN,selecting the Style option, and then typing I to select Isometric.

Chapter 22 Basic Pictorial Drawings 727

isoplanes: Thethree isometric

positions or planes.

728

Figure 22-9.Afour-sided objectdrawn with the LINE

command can be

used as the left side

of an isometric box.

Draw the left side

of an isometric box

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Changing the Isometric Crosshairs OrientationDrawing an isometric shape is possible without ever changing the angle of

the crosshairs. However, the drawing process is easier and quicker if the angle of thecrosshairs aligns with the isometric axes.

Whenever the isometric snap style is enabled, simply press the [F5] key or the[Ctrl]+[E] key combination to change the crosshairs immediately to the next isometricplane. AutoCAD refers to the isometricpositionsor planes as isoplanes. As you changeamong isoplanes, the current isoplane is displayed on the prompt line as a reference.The three crosshairs orientations and their angular values are shown in Figure 22-10.

Another method to toggle the crosshairs position is to use the ISOPLANE command.Enter ISOPLANE as follows.

Command: ISOPLANEJ

Current isoplane: currentEnter isometric plane setting [Left/Top/Right] <current>: J

Press [Enter] to toggle the crosshairs to the next position. The command line displaysthe new isoplane setting. You can toggle immediately to the next position by pressing[Enter] to repeat the ISOPLANE command and pressing [Enter] again. To specify theplane of orientation, type the first letter of that position. The ISOPLANE command canalso be used transparently.

The crosshairs are always in one of the isoplane positions when Isometric Snapmode is in effect. An exception occurs during a display or editing command when amultiple selection set method, such as a window, is used. In these cases, the crosshairs

Figure 22-10.You can toggleamong the threeisometric crosshairs

positions usingthe [F5] functionkey, the [Ctrl]+[E]key combination,or the ISOPLANE

command.Left

90° and 150c

Top30° and 150c

Right90° and 30c


change tothenormal vertical and horizontal positions. Atthecompletion ofthe displayor editing command, the crosshairs revert to their former isoplaneorientation.

CThe quickestway to change the isoplane is to press the [F5] functionkey or press the [Ctrl]+[E] key combination.

Exercise 22-1Complete the exercise on the Student CD.

Isometric EllipsesPlacing an isometric ellipse on an object is easy with AutoCAD because of the

Isocircle option of the ELLIPSE command. An ellipse is positioned automaticallyon the current isoplane. To use the ELLIPSE command, first make sure you are inIsometric Snap mode. Then, pick the Ellipse button from the 2D Draw control panel ofthe Dashboard or from the Draw toolbar, select Draw > Ellipse > Axis, End, or type ELor ELLIPSE. Once the ELLIPSE command is initiated, type I for the Isocircle option.Once you select the Isocircle option, pick the center point and then set the radius ordiameter. Note that the Isocircle option only appears when you are in Isometric Snapmode.

Always check the isoplane position before placing an ellipse (isocircle) on yourdrawing. You can dynamically view the three positions that an ellipse can take. Initiatethe ELLIPSE command, enter the Isocircleoption, pick a center point, and press [F5] totoggle the crosshairs orientation. See Figure 22-11. The ellipse rotates each time youtoggle the crosshairs.

The isometric ellipse (isocircle) is a true ellipse. If selected, grips are displayedat the center and four quadrant points. See Figure 22-12. However, do not use gripsto resize or otherwise adjust an isometric ellipse. As soon as you resize an isometricellipse in this manner, its angular value is changed and it is no longer isometric. Youcan use the center grip to move the ellipse. Also, if you rotate an isometric ellipsewhile Ortho mode is on, it will not appear in a proper isometric plane. You can rotatean isometric ellipse from one isometric plane to another, but you must enter a valueof 120°.

Figure 22-11.The orientation of an

isometric ellipse isdetermined by thecurrent isometric

plane.

Chapter 22 BasicPictorial Drawings

Screen crosshairs

ELLIPSE

2D Draw

> Ellipse

Ellipse

Draw

> Ellipse> Axis, End

LU

(/)CL

LU

729

</) I ELLIPSE0-1 EL* 1—i E535fflB

LU2D Draw

> Ellipse Arc

o

Ellipse Arc

EDraw

> Ellipse> Arc

730

Figure 22-12.An isometric ellipsehas grips at its fourquadrant points andcenter.

.a

n

•

--.-a'

JPrior to drawing isometric ellipses, it is good practice to place a markerat the ellipse center point. A good technique is to draw a point at thecenter using an easily visible point style. This is especially useful ifthe ellipse does not fall on grid or snap points.


Constructing Isometric ArcsThe ELLIPSE command can also be used to draw an isometric arc of any included

angle. To construct an isometric arc, use the Arc option of the ELLIPSE commandwhile in isometric mode. Toaccess the Arc option, pick the Ellipse Arc button from the2D Draw control panel of the Dashboard or from the Draw toolbar, type EL or ELLIPSEfollowed by Afor the Arc option, or select Draw > Ellipse > Arc. Once the Arc option isinitiated, the following prompts appear:

Specify axis endpoint of elliptical arc or [Center/lsocircle]: USpecifycenter of isocircle: (pick the center of the arc)Specify radius of isocircle or [Diameter]: (pick the radius ortype a value and press [Enter])Specifystart angle or [Parameter]: (pick a start angle or type a value andpress [Enter])Specify end angle or [Parameter/Included angle]: (pick an endangle or typean

included angle value and press [Enter])Command:

A common application of isometric arcs is drawing fillets and rounds. Once around is created in isometric, the edge (corner) of the object sits back from its original,unfilleted position.See Figure 22-13A. You can draw the completeobject first and thentrim away the excess after locating the fillets. Youcan also draw the isometric arcs andthen the connecting lines. Either way, the center point of the ellipse is a critical featureand should be located first. The left-hand arc in Figure 22-13A was drawn first andcopied to the back position. Use Ortho mode to help quickly draw 90° arcs.

The next step is to move the original edge to its new position, which is tangent tothe isometric arcs. You can do this by snapping the endpoint of the line to the quadrant point of the arc. See Figure 22-13B. Notice the grips on the line and on the arc.The endpoint of the line is snapped to the quadrant grip on the arc. The final step isto trim away the excess lines and arc segment. The completed feature is shown inFigure 22-13C.


Figure 22-13.Fillets and rounds can be drawn with the Arc option of the ELLIPSE command. A broken lineis used to represent an edge that is viewed straight on.

Rounded edges, when viewed straight on, cannot be shown as complete-edgelines that extend to the ends of the object. Instead,a good technique to use is a brokenline in the original location of the edge. This is clearly shown on the right-hand edgein Figure 22-13C.


Creating Isometric Text StylesText placed in an isometric drawing should appear to be parallel to one of the

isometric planes. Text should align with the plane to which it applies. Text may belocated on the object or positioned away from it as a note. Drafters and artists occasionally neglect this aspect of pictorial drawing and it shows on the final product.

Properly placing text on an isometric drawing involves creating new text styles.Figure 22-14 illustrates possible orientations of text on an isometric drawing. Theseexamples were created using only two text styles. The text styles have an obliquingangle of either 30° or -30°. The labels in Figure 22-14 refer to the chart below. Theangle in the figure indicates the rotation angle entered when using one of the textcommands. For example, ISO-2 90 means that the ISO-2 style was used and the textwas rotated 90°. This technique can be applied to any font.

Name Font Obliquing Angle

ISO-1

vlSO-2

Romans

Romans

30°

-30c


Chapter 22 BasicPictorial Drawings 731

732

Figure 22-14.Isometric text

applications. Thetext shown here

indicates which

style and angle wereused.

c Isometric Dimensioning )An important aspect of dimensioning in isometric is to place dimension lines,

text, and arrowheads on the proper plane. Remember these guidelines:/ Extension lines should always extend in the plane being dimensioned./ The heel of the arrowhead should always be parallel to the extension line./ Strokes of the text that would normally be vertical should always be parallel

with the extension lines or dimension lines.

These techniques are shown on the dimensioned isometric part in Figure 22-15.AutoCAD does not automatically dimension isometric objects. You must first createisometric arrowheads and text styles. Then, manually draw the dimension lines andtext as they should appear in each of the three isometric planes. This is time-consumingwhen compared to dimensioning normal 2D drawings.

You have already learned how to create isometric text styles. These can be set upin an isometric template drawing if you draw isometrics often. Examples of arrows forthe three isometric planes are shown in Figure 22-16.

Figure 22-15.A dimensioned

isometric part.Note the text

and arrowhead

orientation in

relation to the

extension lines.


Figure 22-16.Examples ofarrowheads in

each of the three

isometric planes.

S id?

Isometric Arrowheads

You can draw isometric arrowheads and fill them in with a solid hatch pattern,or you can use the SOLID command to create a filled arrowhead. A variable-widthpolyline cannot be used because the heel of the arrowhead will not be parallel to theextension lines.

Every arrowhead does not need to be drawn individually. First, draw two isometricaxes, as shown in Figure 22-17A. Then, draw one arrowhead like the one shown inFigure 22-17B. Use the MIRROR command to create additional arrows. As you createnew arrows, move them to their proper plane.

You can save each arrowhead as a block in your isometric template or prototype.Use block names that are easy to remember. Blocks are discussed in Chapter 23.

Oblique DimensioningAutoCAD has a way to dimension isometric and oblique lines semiautomati-

cally. First, draw the dimensions using any of the linear dimensioning commands.The object in Figure 22-18Awas dimensioned using the DIMALIGNED and DIMLINEARcommands. Then, use the Oblique option of the DIMEDIT command to rotate theextension lines. See Figure 22-18B.

To access the Oblique option, type DED or DIMEDIT and then type O for Oblique.You can also select Dimension > Oblique. When prompted, select the dimension andenter the obliquing angle.

This technique creates suitable dimensions for an isometric drawing and isquicker than the method previously discussed. However, this method does not rotatethe arrows to align the arrowhead heels with the extension lines. It also does not drawthe dimension text aligned in the plane of the dimension. Therefore, this method doesnot produce technically correct dimensions.

Figure 22-17.Creating isometricarrowheads.

A—Draw the two

isometric axes

for arrowhead

placement. B—Drawthe first arrowhead

on one of the axis

lines. Then mirror

the arrowhead to

create the others.


DIMEDIT

DED

•wrarn

H

QLU

Dimension

> Oblique

733

734

Figure 22-18.Using the Oblique option of the DIMEDIT command, you can create semiautomatic isometricdimensions by editing existing dimensions.

[ Dimension Obliquing Angle

1 30°

2 -30°

3 30°

4 -30°

C" 5 30° j

Chapter TestAnswer thefollowing questions. Write your answers on a separate sheet ofpaperorcomplete the electronic chapter test on the Student CD.

1. The simplest form of pictorial drawing is .2. How does isometric drawing differ from oblique drawing?3. How do dimetric and trimetric drawings differ from isometric drawings?4. The most realistic form of pictorial drawing is .5. Which command allows you to access the Drafting Settings dialog box?6. What must be set in the Drafting Settings dialog box to turn on Isometric Snap

mode and set a snap spacing of .2?7. What function does the ISOPLANE command perform?8. Name the command and option used to draw an isometric circle.9. What factor determines the orientation of an isometric ellipse?

10. Where are grips located on a circle drawn in isometric?11. Can grips be used to resize an isometric circle correctly? Explain your answer.12. How are isometric arcs drawn?

13. Which text style setting allows you to create text that can be used on an isometricdrawing?

14. What technique does AutoCAD provide for dimensioning isometric objects?


Drawing ProblemsCreate an isometric template drawing. Items that shoidd be set in the template include gridspacing, snap spacing, ortho setting, and text size. Save the template as isoproto.dwt. Use thetemplate to construct the isometric drawings in Problems 1-10. Save the drawing problems asP22-(problem number).

i. 2.

3. 4.

5. 6.

7. o.


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735

736

9. 10.

For Problems 11-14, create isometric drawings using the views shown. Measure the drawingsto obtain the dimensions.

11.

12.

13.


14.

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15. Construct a set of isometric arrowheads to use when dimensioning isometricdrawings. Load your isometric template drawing. Create arrowheads for each ofthe three isometric planes. Name them with the first letter indicating the plane:T for top, L for left, and R for right. Also, number them clockwise from the top.See the example below for the right isometric plane. Do not include the labels inyour drawing. Save the template again when finished.

16. Create a set of isometric text styles like those shown in Figure 22-14. Load yourtemplate drawing and make a complete set in one font. Make additional sets inother fonts if you wish. Enter a text height of 0 so that you can specify the heightwhen placing the text. Save the template when finished.

17. Begin a new drawing using your isometric template. Select one of the followingproblems from this chapter and dimension it: Problem 5, 7, 8, or 9. When addingdimensions, be sure to use the proper arrowhead and text style for the plane onwhich you are working. Save the drawing as P22-17.

Cliapter 22 Basic Pictorial Drawings 737

738

18. Create an isometric drawing of the switch plate shown below. Select a view thatbest displays the features of the object. Do not include dimensions. Save thedrawing as P22-18.

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Documents

Learning Objectives - media.scc.losrios.edumedia.scc.losrios.edu/FitzpaK/books/Bk_AutoCAD_2008/Bk_AutoCAD_2008_Ch22.pdfPerspective Drawing The most realistic form of pictorial drawing