basics of sketching

7/29/2019 Basics of Sketching

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UNIT 7

59

PICTORIAL SKETCHING

Pictorial sketching is widely used in industry becausethis type of sketching is easy to read and under-stand, Figure 71. It is also a quick and easy meansof communicating technical ideas. Isometric sketch-ing, one of several types of pictorial drawing, is the

most frequently used. With the use of pictorial gridsheets and ellipse templates, pictorial drawings canbe sketched quickly and accurately.

Viewing Direction

The pictorial sketch may be drawn so the part is viewedfrom above (birds eye view), or from below (worms eyeview), Figure 72. The part features you wish to shownormally govern the viewing direction selected.

ISOMETRIC SKETCHING

All isometric sketches are started by constructingthe isometric axes, which includes a vertical line forheight and isometric lines to the left and right, at an

angle of 30 from the horizon, for width and depth.The three faces seen in the isometric view are thesame faces that would be seen in the normal ortho-graphic views: top, front, and side, as shown inFigure 73(A). Figure 73(B) shows the selection of

NOSE LANDING GEAR

PAYLOA

D

18MBA

Y60FT

FORWARD ENGINES

SPACE SHUTTLE ORBITER

SIDE HATCHUMBILICAL PANEL

LAUNCHUMBILICAL PANEL

ELEVONS

RUDDER/SPEEDBRAKE

STARTRACKERPANEL

ORBITALPROPULSION

AFT REACTIONCONTROL ENGINES

BODY FLAP

MAIN ENGINES (3)

MAIN LANDING GEAR

LENGTH: 122 FT

WINGSPAN: 78 FT

WEIGHT: 150,000 LBS

HEIGHT: 57 FT

FIGURE 71 Application of a pictorial sketch.

(A) BIRDS EYE VIEW

(B) WORMS EYE VIEW

FIGURE 72 Viewing direction.


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60 Interpreting Engineering Drawings

120

120

30 30

120

(A) ORTHOGRAPHIC VIEWS (B) ISOMETRIC AXIS (C) ISOMETRIC SKETCH

A

A A

AFRONT

FRONT

TOP

TOP

SIDE

SIDE

30

A

FIGURE 73 Isometric axes and projection

.25

.38

.38

.38

.50

.50

.38

.38

.25

.38

EXAMPLE A EXAMPLE B

.38

.38

.25

.25

FIGURE 74 Construction of nonisometric lines.

the front corner A and the construction of theisometric axes. Figure 73(C) shows the completedisometric view. All lines are drawn to their true

length, measured along the isometric axes, and hid-den lines are usually omitted.

Isometric Grid Sheets

This type of isometric sketching paper has evenlyspaced lines running in three directions. Two sets oflines are sloped in the direction of the isometricaxes. The third set of lines is vertical and passesthrough the intersection of the sloping lines, as inFigure 72. The most commonly used grids are the

inch, which is further subdivided into either 4 or 10equal grids, and the centimeter, which is furthersubdivided into 10 equal grids of 1 mm. No units ofmeasure are shown on these sheets; therefore thespaces could represent any convenient unit of size.

Inclined Surfaces

Many objects have inclined surfaces that are repre-sented by sloping lines in orthographic views. In iso-metric drawings, sloping surfaces appear asnonisometric lines. To create them, their endpoints,which are found on the ends of isometric lines, are

joined with a straight line. Figure 74 shows how toconstruct nonisometric lines.

Circles and Arcs

A circle on the three faces of an object drawn inisometric has the shape of an ellipse, as shown inFigure 75. Practically all circles and arcs shownon isometric sketches are made with the use of anisometric ellipse template. The template shown in

Figure 75 combines ellipses, scales, and angles.

Markings on the ellipse coincide with the centerlines of the holes, speeding up the drawing of cir-cles and arcs.

Basic Steps to Followfor Isometric Sketching

To save time and to make a more accurate andneater-looking sketch, use an isometric ellipse tem-plate for drawing arcs and circles and a straightedgefor drawing long lines.


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UNIT 7 61

CIRCLES TOUCH SQUARESAT MID-POINT OF EACH SIDE

POSITION OF ISOMETRIC ELLIPSE

TEMPLATE FOR DRAWING CIRCLES

AND ARCS ON THE TOP VIEW



AND ARCS ON THE SIDE VIEW



AND ARCS ON THE FRONT VIEW

TOP PLANE

FRONT PLANESIDE PLANE

FIGURE 75 Using the isometric ellipse template for drawing circles and arcs.

A commonly used technique for sketching is tosketch a boxhavingthemaximumheight, width, and

depth of the object, and then the parts of the box,which are not part of the object, are removed, leav-ing the parts that form the total object, Figure 76.

Step 1. Build a Frame. The frame (or box) is theoverall size of the part to be drawn. It is drawn withconstruction lines.

Step 2. Block in the Overall Sizes for Each De-tail. These subblocks or frames enclose each detail.They are drawn with construction lines.

Step 3. Add the Details. Lightly sketch the shapesof the details using construction lines. For circles,

draw squares equal to the size of the diameter. Alsosketch in the lines to represent the center lines ofthe circle.

Step 4. Darken the Lines. Using a soft lead pencil,darken in the visible object lines.

OBLIQUE SKETCHING

This method of pictorial drawing is based on theprocedure of placing the object with one face par-allel to the frontal plane and placing the other two

faces on oblique (or receding) planes, to left or right,top or bottom, at any convenient angle. The three

axes of projection are vertical, horizontal, and re-ceding. Figure 77 illustrates a cube drawn in typi-cal positions with the receding axes at 60, 45, and30. This form of projection has the advantage ofshowing one face of the object without distortion.The face with the greatest irregularity of outline orcontour, the face with the greatest number of circu-lar features, or the face with the longest dimensionfaces the front, Figure 78.

Two types of oblique projection are used exten-sively. In cavalier oblique, all lines are made to theirtrue length, measured on the axes of the projection.

In cabinet oblique, the lines on the receding axis areshortened by one-half their true length to compen-sate for distortion and to approximate more closelywhat the human eye would see. For this reason, andbecause of the simplicity of projection, cabinetoblique is a commonly used form of pictorial repre-sentation, especially when circles and arcs are to bedrawn. Figure 79 shows a comparison of cavalierand cabinet oblique. Note that hidden lines areomitted unless required for clarity. Most of thedrawing techniques for isometric projection apply tooblique projection.


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(B) BASIC SKETCHING STEPS

STEP 1

BUILD THE FRAME

STEP 2

BLOCK IN THE DETAILS

STEP 3

ADD THE DETAILS

STEP 4

DARKEN THE LINES

(A) THE PART

FIGURE 76 Basic steps to follow for isometric sketching.

30 45 4560 60 30

FIGURE 77 Typical positions of receding axes for oblique projection.

Oblique Grid Sheets

This type of sketching paper is similar to the two-dimensional sketching paper except that 45 lines,which pass through the intersecting horizontaland vertical lines, are added in either one or both

directions. The most commonly used grids are theinch, which is subdivided into smaller evenly spacedgrids, and the centimeter. As there are no unitsof measurements shown on these sheets, thespaces can represent any convenient unit of length,Figure 710.


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UNIT 7 63

NOT ACCEPTABLE

NOT ACCEPTABLE

ACCEPTABLE

ACCEPTABLE

ACCEPTABLE

PART B

PART A

FIGURE 78 Two general rules for oblique projection.

Inclined Surfaces

Angles that are parallel to the picture plane aredrawn as their true size. Other angles can be laid offby locating the ends of the inclined line.

A part with notched corners is shown in Figure711(A). An oblique drawing with the angles paral-lel to the picture plane is shown in Figure 711(B).In Figure 711(C) the angles are parallel to the pro-file plane. In each case, the angle is laid off by mea-surement parallel to the oblique axes, as shown bythe construction lines. Because the part, in eachcase, is drawn in cabinet oblique, the receding linesare shortened by one-half their true length.

Circles and Arcs

Whenever possible, the face of the object havingcircles or arcs should be selected as the front face,so that such circles or arcs can be easily drawn intheir true shape, Figure 712.

When circles or arcs must be drawn on one of theoblique faces, the following method is recommended.With reference to Figure 712(B):

Block off an oblique square with center linesequal to the diameter of the circle required.Blocking in the circle first also helps to getthe proper size and shape of the ellipse. If anellipse template is available, select an ellipsethat fits within the square and touches thesides of the square at its midpoints. Usingthick, dark lines (object lines), draw the

oblique circle (ellipse), Figure 712(C). If an ellipse template is not available, lightly

sketch an ellipse within this square with thecircumference of the ellipse making contactwith the square at its midpoints, Figure712(B).

Using object lines, darken the oblique circle,Figure 712(C).

Basic Steps to Follow for Oblique

Sketching (Figure 713)

Step 1. Build a Frame. The frame or box is theoverall size of the part to be drawn. It is drawn withlight, thin lines.

Step 2. Block in the Overall Size of Each Detail.These subblocks or frames enclose each detail. Forcircles, draw squares equal to the diameter size.

L

CAVALIER PROJECTION CABINET PROJECTION

LL2

FIGURE 79 Types of oblique projection.


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(A)

THE PART

(B) (C)

PART SHOWN IN CABINET OBLIQUE PROJECTION

(D)

AC

C

C

C

F F

FF

D D

D

D

EE

E

EB

A AA

B BB

FIGURE 711 Drawing inclined surfaces.

(A) ADDING OBLIQUE SQUARES

AND CENTER LINES WHERE

CIRCLES ARE REQUIRED

(C) COMPLETING THE OBLIQUE

CIRCLES

(B) LIGHTLY SKETCHING IN THE

SIZE AND LOCATION OF

OBLIQUE CIRCLES

OBLIQUE CIRCLES PASSTHROUGH THESE LINE

INTERSECTIONS

D FOR CAVALIER

FOR CABINET

D

D2

FIGURE 712 Sketching oblique circles.

Also sketch the center lines. They are drawn usinglight, thin lines.

Step 3. Add the Details. Lightly sketch the shapeof the details in each of their frames. These detailsare drawn using light, thin lines. If an oblique circle(ellipse) template is available, the arcs and circles aredrawn using thick, dark (visible object) lines.

Step 4. Darken the Lines. Use a soft lead pencil todarken the lines.

REFERENCES

ASME Y14.4M-1989 (R1999) Pictorial DrawingFIGURE 710 Oblique sketching paper.


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UNIT 7 65

INTERNET RESOURCES

Animated Worksheets. For information onisometric and perspective drawings, see:http://www.animatedworksheets.co.uk.

(B) BASIC SKETCHING STEPS

STEP 1

BUILD THE FRAME

STEP 2

BLOCK IN THE DETAILS

STEP 3

ADD THE DETAILS

STEP 4

DARKEN THE LINES

(A) THE PART

FIGURE 713 Basic steps to follow for oblique sketching.


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PICTORIAL SKETCHING OF PARTS HAVING FLAT

SURFACES USING DECIMAL-INCH DIMENSIONING

ASSIGNMENT:

STIRRUP

BIRDS EYE VIEW

ISOMETRIC LAYOUT OBLIQUE LAYOUT

WORMS EYE VIEW


BRACE

ON AN ISOMETRIC OR OBLIQUE GRID SHEET SKETCH

A PICTORIAL DRAWING OF ONE OF THE PARTS SHOWN.

DO NOT DIMENSION. ONE SQUARE ON THE GRAPH

PAPER REPRESENTS .50 IN.

A-23

5.00

.50 .50

1.50

.50

6.50

.50

2.50

6.00 4.00

.50

7.50

1.50

1.501.50

.50

.50

5.00


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UNIT 7 67

PICTORIAL SKETCHING OF PARTS HAVING FLAT

SURFACES USING METRIC DIMENSIONING

ASSIGNMENT:

RATCHET

TABLET

BIRDS EYE VIEW


WORMS EYE VIEW





PAPER REPRESENTS 10 mm.

A-24M

20

20

20

20

15 25

35

75

35

140

35

60

90 60

20

25

15

15

814040

20

30

30

20

1030

10 15

60

METRIC

DIMENSIONS IN MILLIMETERS


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ASSIGNMENT:




PAPER REPRESENTS .25 IN.

A-25

ROD SPACER

SWIVEL HANGER

2.50

1.00

2.50

.50

.50

.50

4X1.25

4X.75

R1.25

1.75

2.75

1.50

R1.00

R.75

R.75

1.00

2.00

.50

.50

1.00

PICTORIAL SKETCHING OF PARTS HAVING CIRCULAR

FEATURES USING DECIMAL-INCH DIMENSIONING

BIRDS EYE VIEW


WORMS EYE VIEW


1.00

.50

1.50


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UNIT 7 69

PICTORIAL SKETCHING OF PARTS HAVING

CIRCULAR FEATURES USING METRIC DIMENSIONING

ASSIGNMENT:

BEARING

BIRDS EYE VIEW


WORMS EYE VIEW





PAPER REPRESENTS 10 mm.

A-26M

110

60

70 40

100

METRIC

DIMENSIONS IN MILLIMETERS

BEARING SUPPORT

200

2X 20

R30

100

100

80

20

140

20

basics of sketching

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