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Screw threadsGeneral representationThe methods shown in Figure 1.42 are recommendedfor right-hand or left-hand representation of screwthreads. The diameter (0 DIA) of a thread is thenominal size of the thread, for example for a 12 mmthread (M12, see p. 28), DIA = 12 mm.

(a) external thread: side and end view

Threads on assembly and special threadsFigure 1.43(a) illustrates the method of representingtwo threads in assembly. Figure 1.43(b) shows theassembly of two members by a stud mounted in oneof them. Special threads are usually represented bya scrap sectional view illustrating the form of thethread, as shown in Figure 1.43(c).

(b) external thread: section

DRILLPOINT120'

1--4--'CLEARANCE

(e) internal thread: outside view (d) internal thread: sectional view (e) internal thread: end viewFig. 1.42 Methods of representing screw threads

STUD

(b) assembly 01 a sludin a blind hole

NUTSPRING WASHER

MEMBER A

MEMBER B

Fig. 1.43 Methods of representing assembled and special threads

(a) threads in assembly

(e) square thread

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Dimensioning full and runout threadsWhen full and runout threads have to bedistinguished, the methods of dimensioning shown inFigure 1.44 are recommended. Where there is nodesign requirement, the runout threads need not bedimensioned.Dimensioning metric threads in holesFigure 1.45 (below) shows various methods used todimension threaded holes. The diameter of thethread is always preceded by the capital letter M,which indicates metric threads.The coarse thread series is dimensioned simplyby the letter M followed by a numeral, for exampleM12. However, fine threads should show the pitch of

20 'jI:

the thread as well, for example M12 x 1.25. Theterm 6H contained in the thread dimensions of Figure1.45(b), (c) and (d) refers to the grade of toleranceto be used in the manufacture of these threadedholes. This tolerance combined with a similartolerance on the mating screw provides a certain 'fit'when the screw is assembled into the threaded hole.The system used for threaded 'fits' is the same asthat used for plain shaft and hole 'fits' described onpage 54.If it is not important, the runout threads need notbe dimensioned. However, in blind holes it is oftenimportant to have fully formed threads for a certaindepth, and dimensioning must be provided to controlthis.

20 MIN

[(a) dimensioning length of full threadII[ I----t - I + - - ~ -(b) dimensioning to end of full thread

25 MIN

I, --+-[ t ~.18 MAX(e) dimensioning length of full thread and runout

Fig. 1.44 Methods of dimensioning threaded membersM12 20 MIN LGFULL THO

(a)

M18 x 1.5- 6H22 MIN LGFULL THO

(b) M18Fig. 1.45 Methods of dimensioning threads in holes28

(e)

oN N

M20x l . S - 6H20 MIN LG FULLTHO 26 MAX INCLRUNQUT

(d)

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The Australian metric thread profileFigure 1.46 shows the profile of the internal andexternal metric threads, which are suitable for singlepoint screw cutting at maximum material conditionaccording to AS 1721 :1985. If the rounded projectionis not used, a flat should be ground on theappropriate tool to the values of Wn and W, and thecorners will round off as the tool wears.

INTERNAL THREAD/

Detail A

Fig. 1.46 Profile of the Australian metric thread

The ISO metric threadFigure 1.47 shows the profile of the ISO metricthread, together with proportions of the variousdefined parts of the thread.

p = pitch of threadH = 0.866 PHS=0.108p

3H"8 = 0.325 PH4 = 0.217 P5H"8 = 0.541 P

MAJORDIAMETER

IPITCHDIAMETER

I MINORDIAMETERI

In practice the crestmay be rouhded

Detail A

Internal thread(Detail A)h, = 0.577 PR, = 0.072 PW,(max) = 0.072 PW,(min) = 0

p

Fig. 1.47 Basic profife and proportions of the ISO metric thread

p4

External thread(Detail B)h. = 0.604 PR,=0.125PW,(max) = 0.177 PW.(min) = 0.144 P

:I:"'''''

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//4.54

3.5ISO metric c o a r s e ~ /I

ISO metric fine~ / 32.5 E..// '". , /

/ ' " / ' "/ '" / --/ ._ -

/r .V/

V

.

'0..!2 -5'01.75 .s:.5 c::

1 2510.80.70.5OA

o 1 2 3 4 5 6 7 B 9 10 12 14 16 1B 20 22 24 27 30 33 36 39 o42Diamete, (mm)

Fig. 1.48 Graphical comparison of metric threads

Graphical comparison of metric thread seriesISO metric threads are of two kinds: coarse and finethread. A graphical comparison of these two seriesis shown in Figure 1.48.Tapping size and clearance holes fo rISO metric threadsTapping sizes and clearance holes for metric threadsare shown in Table 1.9 (page 32). In this tablecolumn 1 represents first and second choices ofthread diameters. The sizes listed under secondchoice should be used only when it is not possibleto use sizes in the first choice column.The pitches listed in column 2 are compared onthe graph in Figure 1.48. These pitches, togetherwith the corresponding first and second choicediameters of column 1, are those combinations whichhave been recommended by the ISO as a selected'coarse' and 'fine' series for screws, bolts, nuts andother threaded fasteners commonly used in mostgeneral engineering applications. Column 3 is thetapping size for the coarse and fine series. Thesevalues represent approximately 83 per cent full depthof thread, and can be calculated simply by theformula:

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tapping drill size = outside diameter - pitch3.3 = 4 - 0.7

Sometimes the drill size has to be rounded off tothe next largest stock drill size; this can be obtainedfrom Table 1.10 (page 33).Column 4 of Table 1.9 gives tapping sizes forcoarse threads in mild steel only; these will giveapproximately 71 per cent of the full depth of thread.In most general engineering applications this depthof thread is sufficient and desirable for the followingreasons:1. Tapping 83 per cent depth of threadnecessitates about three times more powerthan tapping 71 per cent depth of thread.2. The possibility of tap breakage is greater asthe depth of thread increases.3. The 83 per cent depth of thread hasapproximately 5 per cent more strength thanthe 71 per cent depth of thread.4. The amount of metal removed from a 71 percent depth of thread is much less than thatremoved for 83 per cent depth of thread.There are cases when a deeper thread isnecessary, for example on machines and insituations where movement in the mating threads isto be kept to a minimum.Column 5 of Table 1.9 gives three classes ofclearance holes recommended for the various sizes

of metric threads.