Morling K. Geometric and Engineering Drawing

Подождите немного. Документ загружается.

Geometric and Engineering Drawing234

11. A cylinder, made of transparent material, 88 mm O/D, 50 mm I/D and 126 mm long, has

its axis parallel to the VP. Two helical lines marked on its curved surface – one on the

outside and the other on the inside – have a common pitch of 63 mm.

Draw the elevation of the cylinder, showing both helices starting from the same radial

line and completing two turns.

Associated Examining Board

12. Draw a longitudinal elevation, accurately projected, showing two turns of a helical spring.

The spring is of 100 mm outside diameter, the pitch of the coils is 62 mm and the spring

material is of 10 mm diameter.

Cambridge Local Examinations

140 mm

φ100 mm

Figure 5

6. A cylinder is 48 mm diameter and a piece of string is equal in length to the circumfer-

ence. One end of the string is attached to a point on the cylinder.

(a) Draw the path of the free end of the string when it is wound round the cylinder in a

plane perpendicular to the axis of the cylinder.

(b) In block letters, name the curve produced.

string) construct a tangent to the circle representing the cylinder.

Southern Universities ’ Joint Board

7. A circle 50 mm diameter rests on a horizontal line. Construct the involute to this circle,

making the last point on the curve 2 π mm from the point at which the circle makes con-

tact with the horizontal line.

Cambridge Local Examinations

8. P, O and Q are three points in that order on a straight line so that PO  34 mm and OQ 

21 mm. O is the pole of an Archimedean spiral. Q is the nearest point on the curve and P

another point on the first convolution of the curve. Draw the Archimedean spiral showing

two convolutions.

Southern Universities ’ Joint Board

9. Draw two convolutions of an Archimedean spiral such that in two revolutions the radius

increases from 18 to 76 mm.

Oxford Local Examinations

10. A piece of cotton is wrapped around the cylinder shown in Fig. 5 . The cotton starts at C

and after one turn passes through D, forming a helix. The start of the helix is shown in

the figure. Construct the helix, showing hidden detail.

Middlesex Regional Examining Board

Geometric and Engineering Drawing. DOI:

10.1016/B978-0-08-096768-4.00016-4

Freehand Sketching

The ability to sketch neatly and accurately is one of the most useful attributes that a

draughtsperson can have. Freehand drawing is done on many occasions: to explain

a piece of design quickly to a colleague; to develop a design (see Fig. 16.10 ); and

even to draw a map showing someone how to get from one place to another.

Technical sketching is a disciplined form of art. Objects must be drawn exactly

as they are seen, not as one would like to see them. Neat, accurate sketches are only

achieved after plenty of practice, but there are some guiding rules.

Most engineering components have outlines composed of straight lines, circles

and circular arcs: if you can sketch these accurately, you are halfway towards produc-

ing good sketches. You may find the method illustrated in Fig. 16.1 helpful. When

drawing straight lines, as on the left, rest the weight of your hand on the backs of

your fingers. When drawing curved lines, as on the right, rest the weight on that part

of your hand between the knuckle of your little finger and your wrist. This provides a

pivot about which to swing your pencil. Always keep your hand on the inside of the

curve, even if it means moving the paper around.

Straight lines

(Rest weight of hand

on backs of fingers)

Curved lines

(Pivot hand about wrist or

knuckle of little finger)

Figure 16.1

Geometric and Engineering Drawing236

Pictorial Sketching

Freehand pictorial sketching looks very much like isometric drawing. Circles appear

as ellipses and lines are drawn at approximately 30 ° . Circles have been sketched

onto an isometric cube in Fig. 16.2 . You can see how these same ellipses appear on

sketches of a round bar material.

Circles sketched on

an isometric cube

Application to

freehand sketching

of cylindrical objects

Figure 16.2

When sketching, you may find it an advantage to draw a faint ‘ box ’ first and draw

in the ellipses afterwards. With practice you should find that you can draw quite a

good ellipse if you mark out its centre lines and the major and minor axes.

Although drawing is a continuous process, the work can be divided into three

basic stages.

Stage 1: Construction

This should be done with a hard pencil (6H), used lightly, and the strokes with the

pencil should be rapid. Slow movements produce wavy, uncertain lines. Since these

constructed lines are very faint, errors can easily be erased.

Stage 2: Lining in

Carefully line in with a soft pencil (HB), following the construction lines drawn in

stage 1.

The completion of stage 2 should give a drawing that shows all the details and

you may decide, particularly in an examination, not to proceed to stage 3.

Freehand Sketching 237

Stage 2

(Line in and detail)

Stage 1

(Construction)

Stage 3

(Shade)

Figure 16.3

Stage 3: Shading

Shading brings a drawing to life. It is not necessary on most sketches, and in some

cases it may tend to hide details that need to be seen. If the drawings are to be dis-

played, however, some shading should certainly be done.

Shading is done with a soft pencil (HB). It is very easy to overshade, so be care-

ful. For the smooth merging of shading, the dry tip of a finger can be gently rubbed

over the area.

Figures 16.3 – 16.7 are examples of freehand pictorial sketching.

Geometric and Engineering Drawing238

Stage 1

(Construction)

Stage 2

(Line in and detail)

Stage 3

(Shade)

Figure 16.4

Stage 1

(Construction)

Stage 2

(Line in and detail)

Stage 3

(Shade)

Figure 16.5

Freehand Sketching 239

Stage 1

(Construction)

Stage 2

(Line in and detail)

Stage 3

(Shade)

Figure 16.6

Geometric and Engineering Drawing240

Sketching in Orthographic Projection

More detail can be seen on an orthographic drawing than on an isometric, mainly

because more than one view is drawn. For this reason it is often advantageous for a

draughtsperson to make an orthographic sketch.

The views should be drawn in the conventional orthographic positions, i.e. in third

angle projection, a FE, a plan above the FE, and an EE to the left or right of the FE;

in first angle projection, a FE, a plan below the FE and an EE to the left or right of

the FE. These views should be linked together with projection lines.

Figures 16.8 and 16.9 show pictorial and orthographic sketches of two engineer-

ing components.

Circles are difficult to draw freehand but you can use your hand as a compass.

Hold your pencil upright and, using your little finger as a compass ‘ point ’ , revolve

the paper keeping your hand quite still.

Stage 1

(Construction)

Stage 2

(Line in and detail)

Stage 3

(Shade)

Figure 16.7

Freehand Sketching 241

Third angle projection

Section X–X

Figure 16.9

Section X–X

First angle projection

Figure 16.8

Geometric and Engineering Drawing242

Serrated jaws

Stage 1

Pictorial sketch

(Formulates ideas)

Spring

Wing nut

and washe

Stage 3

(Detail design

of components)

R15

Third angle projection

Diamond

serration

90° C’S’K’, φ7.5

Vert. to face

Dimensions in mm

Drill φ10

Drill φ5

R50

R73

R52

12 CTS

121

Stage 2

Orthographic sketch

(More details can be shown)

Spring

(2 off)

Recess to

take sq. hd

bolt (Prevent

turning)

Rivet to

retain spring

Tight fit

Recess for

head of rivet when

jaws fully closed

6 mm bolt

Large clearance

to allow jaws to close

Figure 16.10 Sketching and design.

Freehand Sketching 243

Figure 16.10 shows how a draughtsperson might use sketching to aid a piece of

design. They wish to design a small hand vice.

First they make a freehand pictorial sketch so that they can see what the vice will

look like. They also make a few notes about some details of the vice.

They then make an orthographic sketch. This shows much more detail and they

make some more notes.

This is quite a simple vice and they may now feel that they are ready to make the

detailed drawings. If it was more complicated they might make a few more sketches

showing even more detail. Details of one of the legs of the vice are shown.