BB

Fig. 37 Two ways of setting out 'successive sections'.

over the orientation of the section lines -hatching - if confusion is to be avoided. First, note that the cotters are not sectioned, and hence are shown projecting in front of the section planes. Then, note the more or less flat alignment of the hatching on the horizontal surface of the section. A very odd optical illusion results from reversing the run of the lines in the two planes. Using slightly bolder lines on the horizontal surface can enhance the effect. Finally, you will see that a different type of hatching has been used for the bearing brasses. This is now regarded as old-fashioned, and it certainly takes a bit longer to draw. However, the object of the exercise is clarity of presentation and the alternative - to use hatching at closer pitch - would take almost as long.

Representing materials in section

This was an almost universal practice when I was taught engineering drawing, more years ago than I care to remember. It is used today only in a very limited way (chiefly on assembly drawings) as material specifications are very tight now and should always be laid down in detail in a schedule. However, we do have to use 'old' drawings occasionally, and there are times (as we have just seen) where an indication is helpful. Fig. 39 shows both the old and the current recommended conventions. Current practice is stated against each example first, current permitted and often-useful practice is shown in brackets (ferrous alloys) and the old or historical practice in fcrtrait Jiff tal [Lead, *jnc]

ALU H ET AUS .(Femkovs METM.) ccayr iron)

L&teeJ

(COPPER Aloma) Cbrass.qh)

fcrtrait Jiff tal [Lead, *jnc]

ALU H ET AUS .(Femkovs METM.) ccayr iron)

(COPPER Aloma) Cbrass.qh)

Rock

"thick THN

EiUtS Etc VNSftATtcM

COMCÄCTE

EARTH . ALSO D»tD MITM FLAT SOAFACJI ToSHoW 'FU3CX2 LEVCU"

Rock

TlMBKR

Fig. 39 Conventional section lining to denote various materials. Those in ( ) are permitted, those in [ ] are not encouraged but may be found on old drawings.

square brackets [cast iron]. So, plain hatching nowadays shows all metals. Perhaps 50 years ago this would have been used only for cast iron and the steels, with alternating plain and dotted lines for copper alloys. But my father would have used plain lines only for cast iron, double lines for steel and wrought iron, plain-and-dotted for brass and gunmetal and other combinations for bronze, zinc, copper and so on. On pictorial drawings the use of contrasting types of line can be useful, but they should be avoided on detail drawings unless they are essential. One small point: the convention for 'earth' is sometimes drawn with a straight horizontal line at the top on general arrangement drawings, to indicate floor level.

Section 4

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