Hc A p T E R 5

mechanical drafting for completelydescribing an object are multiviewdrawings as shown in Fig. 5-1-1.

Pictorial projections, such asaxonometric, obliq ue, and perspectiveprojection, are useful for illustrative pur-poses and are frequently employed ininstallation and maintenance drawingsand design sketches.

As a result of new drawing techniquesand equipment, pictorial drawings arebecoming a popular form of commu-nication, especially with people nottrained to read engineering drawings.Practically all drawings of do-i t-yourselfprojects for the general public or ofassembly-line instructions for non-technical personnel are done in pictorialform.

UNIT 5-1Theory of ShapeDescription

Chapter 4 illustrated many simple partsthat required only one view to com-pletely describe them. However, inindustry, the majority of parts that haveto be drawn are more complicated thanthe ones previously described. Morethan one view of the object is required toshow all the construction features.Except for complex objects of irregularshape, it is seldom necessary to drawmore than three views.

Pictorial (three-dimensional) draw-ings of objects are sometimes used, butthe majority of drawings used in

ORTHOGRAPHIC PROJECTION

Fig. 5.1.1 Types of projection used in draftin~.

66 BASIC DRAWING

OBLIQUE

PICTORIAL DRAWINGS

PERSPECTIVE

SHAPE DESCRIPTION BYVIEWSWhen looking at objects, we normallysee them as three-dimensional, havingwidth, depth, and height, or length, width,and height. The choice of terms useddepends on the shape and proportions ofthe object.

Spherical shapes, such as a basket-ball, are described as having a certaindiameter (one term).

Cylindrical shapes, such as a baseballbat, have diameter and length. However, ahockey puck would have diameter andthickness (two terms).

Objects which are not spherical orcylindrical require three terms todescribe their overall shape. The termsused for a car would probably be length,width, and height; for a filing cabinet,width, height, and depth; for a sheet ofdrawing paper, length, width, and thick-ness. These terms are used interchange-ably according to the proportions of theobject being described, and the position itis in when being viewed. For example, atelephone pole lying on the groundwould be described as having diameterand length, but when placed in a verticalposition, its dimensions would be diam-eter and heigh t.

In general, distances from left toright are referred to as width or length,distances from front to back as depth orwidth, and vertical distances (exceptwhen very small in proportion to theothers) as height. On drawings, the mul-tidimensional shape is represented by a

-------- - --

view or views on the flat surface of thedrawing paper.

Many mechanical parts do not have adefinite "front" or "side" or "top," as doobjects, such as refrigerators, desks, orhouses, and their shapes vary from thesimple to the complex. Decisions have tobe made on how many views, and whichviews, will be drawn. Following are somebasic guidelines.

1. Draw those views that are necessaryto fully explain the shape.

2. The front view is usually the "key"view; it shows the width or length ofthe object and gives the most infor-mation about its shape. When thelongest dimension is drawn in a hori-zontal position, the object will seembalanced.

3. Choose those views that will showmost of the detailed features of theobject as "visible," thus avoiding theextensive use of "hidden" featurelines.

Surface TermsWhen describing the shape of an object,reference is often made to the types ofsurfaces found on the object relative tothe three principal viewing planes-horizontal plane, vertical plane, profileplane. These surfaces can be identifiedas follows:

Parallel flat surfaces that are parallel tothe three principal viewingplanes;

Hidden surfaces that are hidden in oneor more reference planes;

Inclined flat surfaces that are inclined inone plane and parallel to theother two planes;

Oblique flat surfaces that are inclined inall three reference planes;

Circular surfaces that have diameter orradius.

PICTORIAL VIEWSPictorial drawings represent the shapewithjust one view. See Fig. 5- l-2A. How-

ever, the majority of parts manufacturedin industry are too complicated in shapeand detail to be described successfullyby a pictorial view.

ORTHOGRAPHICPROJECTIONThe drafter must represent the partwhich appears as three-dimensional(width, height, depth) to the eye on theflat plane of the drawing paper. Differentviews of the object-front, side, and topviews-are systematically arranged onthe drawing paper to convey the neces-sary information to the reader (Figs.5-l.2B and 5-1-3). Features are projectedfrom one view to another. This type ofdrawing is called an orthographic projec-tion. The word orthographic is derivedfrom two Greek words: orthos, meaningstraight, correct, at right angles to; andgraphikos, meaning to write or describeby drawing lines.

An orthographic view is what youwould see looking directly at one side or"face" of the object. See Fig. 5-1-2(C).When looking directly at the front face,you would see width and h.eight. (twodimensions) but not the thIrd dImen-sion, depth. Each orthographic viewgives two of the three major dimensions.

(A) PICTORIAL DRAWING (iSOMETRIC)

Orthographic SystemsTwo systems of orthographic projection,known as first- and third-angle projec-tion, are used (Fig. 5-1-4). Third-angleprojection is used in the United Stat:s,Canada, and many other countrIesthroughout the world. First-angle pro-jection, which will be described in detailin Unit 5-8, is used mainly in manyEuropean and Asiatic countries. Asworld trade has brought about theexchange of engineering drawings aswell as the end products, drafters arenow called upon to communicate in, aswell as understand, both types oforthographic projection.

Fig. 5.1-3 Systematic arrangement of views.

VIEW

rHEIGHT

~

WIDTH--- •...••~1

FRONT VIEW

I.

(B) ORTHOGRAPHIC PROJECTIONDRAWING (THIRD ANGLE)

(C) ORTHOGRAPHIC VIEW

Fig. 5-1-2 A simple object shown in pictorialand orthographic projection.

THICK SOLID LINE USED TO

INDICATE VISIBLE OBJECT LINES

THEORY OF SHAPE DESCRIPTION 67

VERTICAL PLANEPROFILE PLANE

Fig. 5.1-4 The three planes used in orthographicprojection.

Fig. 5.1.5 ISO projection symbol.

-$- E31,--_T_I_T_LE_B_LO_C_K_~

Fig. 5-1-' Relationship of object with viewingplanes in third-angle projection.

Fig. 5-1-6 Locating ISO projection symbol ondrawing paper.

RIGHT

,L-Z7TOP

FRONTLEFT

VIEWEDQ,,_FROM .RIGHTSIDE

-0-VIEWED FROM TOP

VIEWED FROMLEFTSIDE~

UNFOLDING GLASS BOX TO GIVETHIRD ANGLE LAYOUT OF VIEWS

rnTOP VIEW FRONT

G ES 8 ~LEFT.SIDE FRONT VIEW RIGHT-SIDE REAR VIEW

VIEW VIEW

IT]BOTTOM VIEW

VIEW OF OBJECTPROJECTED ONTOWALLS OF GLASS BOX

VIEWING THE OBJECT FROM ALL SIX SIDES

-0

OBJECT ENCLOSED IN GLASS BOXFig. 5.1-8 Systematic arrangement of views.

68 BASIC DRAWING

BORDERLINES "...

PLANE 2

OR

VERTICALDRAWINGSPACE

HORIZONTALDRAWING SPACE

PLANE I

1. Decide on the views to be drawn andthe scale to be used, e.g., 1:1 or 1:2.

2. Make a sketch of the space requiredfor each of the views to be drawnshowing these views in their correc;location. (A simple rectangle for eachview will be adequate, Fig. 5-2-IB.)

3. Put on the overall drawing sizes foreach view. (These sizes are shown asW, D, and H.)

4. Decide upon the space to be leftbetween views. (These spaces shouldbe sufficient for the parallel dimen-sion lines to be placed between views.For most drawing projects, 1.50 in. issufficient.)

5. Total these dimensions to get theoverall horizontal distance (A) andoverall vertical distance (B).

6. Select the drawing sheet to bestaccommodate the overall size of thedrawing with suitable open spacearound the views.

7. Measure the "drawing space"remaining after all border lines, titlestrip or title block, etc., are in place(Fig.5-2-1C).

8. Take one-half of the differencebetween distance A and the horizon-

SPACING THE VIEWS

UNIT 5-2Arrangement ofViews

ITait'r'"o'"v". EliDEVIEW

+-------+t=-~1.50~Dl

(A) DECIDING THE VIEWS TO BE DRAWNAND THE SCALE TO BE USED

It is important for clarity and goodappearance that the views be well bal-anced on the drawing paper, whether thedrawing shows one view two viewsthree views, or more. The'drafter mus;anti.cipate ~h~ approximate spacereqUIred. This IS determined from thesize of the object to be drawn, thenumber of views, the scale used, and thespace between views. Ample spaceshould be provided between views topermit placement of dimensions on thedrawing without crowding. Spaceshould also be alotted so that notes canbe added without crowding. However,space between views should not beexcessive.Figure 5-2-1 shows how to balance the

views for a three-view drawing. For adrawing with two or more views, followthese guidelines:

nlLd

ISO Projection SymbolWith two types of projection being usedon engineering drawings, a method ofidentifying the type of projection is nec-essary. The I nternational StandardsOrganization, known as ISO, has rec-ommended that the symbol shown inFig. 5-1-5 be shown on all drawings andlocated preferably in the lower right-hand corner of the drawing, adjacent tothe title block (Fig. 5-1-6).Drawings made in the United States

are understood to be shown in third-angle projection if the ISO symbol isnot used.

ASSIGNMENTS

Fig. 5.1.9 Six views required to show a die.

Third-Angle ProjectionIn third-angle projection, the object ispositioned in the third-angle quadrant,as shown in Fig. 5-1-7. The person view-ing the object does so from six differentpositions, namely, from the top, front,right side, left side, rear, and bottom.The views or pictures seen from thesepositions are then recorded or drawn onthe plane located between the viewerand the object. These six viewing planesare then rotated or positioned so thatthey lie in a single plane, as shown inFig. 5-1-8. Rarely are all six views used.An exception would be the drawing of adie (one ofa pair of dice). See Fig. 5-1-9.Only the views which are necessary tofully describe the object are drawn. Sim-ple objects, such as a gasket, can bedescribed sufficiently by one view alone.However, in mechanical drafting two- orthree-view drawings of objects are morecommon, the rear, bottom, and one ofthe two side views being rarely used.

DDDDD..• •

See Assignments I and 2 for Unit 5-1 onpage 78.

lC) ESTABLISHING LOCATION OFlB) CALCULATING DISTANCES A AND B PLANES I AND 2

Fig. 5.2.1 Balancing the drawing on the drawing paper or monitor.

THEORY OF SHAPE DESCRIPTION 69

CAD

the side view intersect the miter line,project horizontal lines to the left.

5. Project vertical lines up from thefront view and complete the top view.

AFTERBEFORE

The working area on the CRT monitormust be established prior to selecting thepaper size for the completed drawing.

Construction lines are menu optionsused in the preparation of multiviewdrawings on the CRT monitor (Fig.5-2-3).

3. Construct the miter line at 45° to thehorizon.

4. Where the horizontal projection linesof the top view intersect the miterline, drop vertical projection lines.

5. Project horizontal lines to the right ofthe front view and complete the sidevIew.

Using a Miter Line toConstruct the Top View1. Given the front and side views, pro-

ject vertical lines up from the sideview.

2. Establish how far away from the frontview the top view is to be drawn (dis-tance D).

3. Construct the miter line at 45° to thehorizon.

4. Where the vertical projection lines of

1. Given the top and front views, pro-ject lines to the right of the top view.

2. Establish how far from the front viewthe side view is to be drawn (distanceD).

tal "drawing space" to establishPlane I.

9. Take one-half of the difference be-tween distance B and the vertical"drawing space" to establish Plane 2.

The use of a miter line provides a fastand accurate method of constructing thethird view once two views are estab-lished (Fig. 5-2-2).

USE OF A MITER LINE

Using a Miter Line toConstruct the Right Side View

REFERENCE LINES ASSIST MUL TIVIEWDRAWING CONSTRUCTION. THIS ISESPECIALLY USEFUL WHEN A GRIDPATTERN CANNOT BE USED. THELINES ARE FOR REFERENCE PUR-POSES ONLY AND WILL NOT APPEARON THE FINISHED PLOT.

Fig. 5-2-3 CAD construction line command.

ASSIGNMENTS

+

ID

8STEP I STEP I

See Assignments 3 through 6 for Unit5-2 on pages 78 and 79.

UNIT 5-3All SurfacesParallel and AllEdges and LinesVisible

nr==~9 "STEP 2

(A) ESTABLISHING WIDTH LINES ONSIDE VIEW

Fig. 5-2-2 Usc of a miter line.

STEP 2

(B) ESTABLISHING WIDTH LINESON TOP VIEW

To fully appreciate the shape and detailof views drawn in third-angleorthographic projection, the units forthis chapter have been designed accord-ing to the types of surfaces generallyfound on objects. These surfaces can bedivided into flat surfaces parallel to theviewing planes with and without hiddenfeatures; nat surfaces which appearinclined in one plane and parallel to theother two principal reference planes(called inclined surfaces); l1at surfaceswhich are inclined in all three referenceplanes (called oblique surfaces); and sur-faces which have diameters or radii.These drawings are so designed that

70 BASIC DRA WING

HIDDEN EDGE LINE

All CAD systems have the option tocreate different line styles. On large sys-tems, these options are found on the au x-

CAD

(lines, holes, etc.), cannot be seen whenviewed from outside the piece. Thesehidden edges are shown with hidden linesand are normally required on the draw-ing to show the true shape of the object.

Hidden lines consist of short, evenlyspaced dashes. They should be omittedwhen not required to preserve the clarityof the drawing. The length of dashesmay vary slightly in relation to the size ofthe drawing.

Lines depicting hidden features andphantom details should always beginand end with a dash in contact with theline at which they start and end, exceptwhen such a dash would form a con-tinuation of a visible detail line. Dashesshould join at corners. Arcs should startwith dashes at the tangent points (Fig.5-4-2). Figure 5-4-3 shows additionalexamples of objects requiring hiddenlines.

HIDDEN EDGELINES SHOWNIN FRONT VIEW

SPACEFig. 5-4-1 Hidden lines.

I

1

UNIT 5-4Hidden Surfacesand Edges

Most objects drawn in engineeringoffices are more complicated than theones shown in Fig. 5-4-1. Many features

ASSIGNMENTSSee Assignments 7 and 8 for Unit 5-3 onpages 79 and 80.

only the top, front, and right side viewsare required.All Surfaces Parallel to the ViewingPlanes and All Edges and LinesVisible When a surface is parallel tothe viewing planes, that surface willshow as a surface on one view and a lineon the other views. The lengths of theselines are the same as the lines shown onthe surface view. Figure 5-3-1 showsexamples.

B FAo;~ [] FR~@ OJ FR~~

BcCJ egCJ GEaA B C

D E F

NOTE: ARROWS INDICATE DIRECTION OF SIGHT WHEN LOOKING AT THE FRONT VIEW.

Fig. 5.3.1 Illustrations of objects drawn in third-angle orthographic projection.

THEORY OF SHAPE DESCRIPTION 71

IA) GATE (B) INK BOTTLE STAND IC) CAP

2 3

Fig. 5-4.2 Application of hiddcn lincs.

"- )--I

4

I

" I /''Yo''

5

I-;:;J./

6

-~- 'J1

-~ ~' __ LI

789

[Q ~ ~EtB ~EErJI I I II I

FRONT FRONT FRONT

D [JJ ~~ tEB~I I I I I I __ -.J

A B C

Fig. 5-4-3 Illustration of objccts ha\'in~ hiddcn fcaturcs.

iliary menu. On smaller systems, theline style selection is made directly fromthe tablet menu. Any style ofline may bedrawn by following the line commandsexplained in Unit 4-2.

ASSIGNMENTS

not seen in its true shape in the top,front, or side view. I t is, however, seen intwo views as a distorted surface. On thethird view it appears as a line.The true length of surfaces A and B in

Fig. 5-5-l is seen in the front view only.In the top and side views, only the

width of surfaces A and B appears in ilstrue size. The length of lhese surfaces isforeshortened. Figure 5-5-2 shows addi-tional examples.Where an inclined surface has impor-

lalH features that must be shown clearlyand withoul distortion, an allxiliar..y

NOTE: THE TRUE SHAPE OF SURFACES AAND B DO NOT APPEAR ON THE TOP ORSIDE VIEWS.

See Assignments 9 through 12 for Unit5-4 on pages 80 and 81.

UNIT 5-5Inclined Surfaces

If the surfaces of an object lie in either ahorizontal or a vertical position, the sur-faces appear in their true shapes in oneof the three views, and these surfacesappear as a line in the other two views.When a surface is inclined or sloped in

only one direction, then that surface is

72 BASIC DRA WING

Fig. 5.5-1 Sloping surfaccs.

A B

F

Fig. 5-5-2 Illustrations of objects having sloping surfaces. NOTE: ARROW INDICATES DIRECTION OF FRONT VIEW

or helper view must be used. This type ofview will be discussed in detail inChap. 15.

ASSIGNMENTSSee Assignments 13 through 16 for Unit5-5 on page 83.

UNIT 5-6Circular Features

Typical parts with circular features areillustrated in Fig. 5-6-1. Note that thecircular feature appears circular in oneview only and that no line is used toshow where a curved surface joins a flat

surface. Hidden circles, like hidden flatsurfaces, are represented on drawings bya hidden line.The in tersection of unfinished sur-

faces, such as found on cast parts, thatare rounded or filleted at the point oftheoretical intersection, may be indi-cated conventionally by a lin~. See Unit8-18.

D

~ /

W'II I 1T:l

E

C

F

Fig. 5-6-1 Illustrations of objccts having circular features. NOTE: ARROWS INDICATE DIRECTION OF FRONT VIEW

THEORY OF SHAPE DESCRIPTION 73

Center LinesA center line is drawn as a thin, brokenline of long and short dashes, spacedalternately. Such lines may be used tolocate center points, axes of cylindricalparts, and axes of symmetry, as shown inFig. 5-6-2. Solid center lines are oftenused when the circular features aresmall. Center lines should project for ashort distance beyond the outline of thepart or feature to which they refer. Theymust be extended for use as extensionlines for dimensioning purposes, but inthis case the extended portion is notbroken.

all three views bu t never in its trueshape. This is referred to as an obliquesllI:faee (Fig. 5-7-1). Since the oblique sur-face is not perpendicular to the viewingplanes, it cannot be parallel to them andconsequently appears foreshortened. Ifa true view is required for this surface,two auxiliary views-a primary and asecondary view-need to be drawn.This is discussed in detail under Second-ary Auxiliary Views in Unit 15-4. Figure5-7-2 shows additional examples ofol~jects having oblique surl:lces.

ASSIGNMENTSSee Assignmen ts 22 and 23 for U ni t 5-7on page 91.

UNIT 5-8

First-AngleOrthographicProjection

As mentioned previously, first-angleorthographic projection is used in manycountries throughout the world. Todaywith global marketing and the inter-change of drawings with different coun-tries, drafters are called upon to prepareand interpret drawings in both first- andthird-angle projection. In first-angleprojection, all the views are projectedonto the planes located behind theobjects rather than onto the planes lyingbetween the objects and the viewer, as inthird-angle projection. This is shown inFig. 5-8-2. The unfolding and position-ing of the views in one plane are shownin Fig. 5-8-3. Note that the views are onopposite sides of the front view with theexception of the rear view. A comparisonbetween the views of first- and third-angle projections is shown in Figs. 5-8-1and 5-8-4. Remember that the views areidentical in shape and detail, and onlytheir location in reference to the frontview has changed.Fig. 5.7.1 Oblique surface is not its true shape

in any of the three views.

E-=aUSE TWO SHORT DASHES ATTHE POINT OF INTERSECTION

Fig. 5.6-2 Center line applications.

LENTER LINE SHOULDNOT BE BROKEN WHENIT EXTENDS BEYONDTHE CIRCULAR FEATURE

NOTE: AT TIME OF WRITING, CAD SYSTEMSDO NOT CONFORM TO THIS STANDARD.

Fig. 5-7.2 Examples of objects having oblique surfaces.

See Assignments 17 through 21 for Unit5-6 on page 87 and 88.

ASSIGNMENTS

When a surface is sloped so that it is notperpendicular to any of the three view-ing planes, it will appear as a surface in

On views showing the circular fea-tures, the point of intersection of the twocenter lines is shown by the two inter-secting short dashes.

UNIT 5-7Oblique Surfaces

74 BASIC DRAWING

N

R

v

o

s

w

p

T

x

Q

u

y

13

~

14 15 16OJ II IIEd ,I-- - -- -" J ,

[b~ [Jc;j Ow C8bE17 18 /9 20I' " [IE [d II

"I'" , I ,I

IIII I' I -- - IIwD ~BJ h3H~ DafTI

21 22 23 24[]Jj IT] OIJI ,I II I I I I ,

DJB wrn Wu ~BFig. 5.5.L Matching test.

THEORY OF SHAPE DESCRIPTION 87

A

E

J

B

F

K

c

G

L

o

H

M

E=J'~'d=J'~ ·g~ bS~ J;jdJ wu£J[;J '~ 'EJ ' E? ·~cdJ cQrr!J L;3dJ wcl1E? ' ~ "~ "[;J "

.wrrB b1dJ b=J~ ~cdJFig. 5-4-J Match pictorial drawings A through M with orthographic drawings.

THEORY OF SHAPE DESCRIPTION 83