Screw threads and conventional representations

Screw threads

The most common application of the helix is in a screw thread which follows the path of the helix. Screw threads may be either left or right hand and these are shown pictorially in Fig. 15.1. Notice the slope of the thread and the position of the index finger on each hand. The left hand thread is used for special applications and the right hand thread is the one normally used on nuts and bolts. The thread illustrated has a vee-section.

Pitch n

Miwi

Left hand thread Fig. 15.1

Pitch

Right hand thread

The following terms are associated with screw threads:

The thread pitch is the distance between corresponding points on adjacent threads. Measurements must be taken parallel to the thread axis.

The major diameter or outside diameter is the diameter over the crests of the thread, measured at right angles to the thread axis.

The crest is the most prominent part of the thread, internal or external.

The root lies at the bottom of the groove between two adjacent threads.

The flank of the thread is the straight side of the thread between the crest and root.

The minor diameter, root diameter or core diameter is the smallest diameter of the thread measured at right angles to the thread axis.

The effective diameter is measured at right angles to the thread axis and is the diameter on which the width of the spaces is equal to the width of the threads.

The lead of a thread is the axial movement of the screw in one revolution.

The terms are illustrated in Fig. 15.2.

Pitch 2

Crest

Pitch 2

Crest

Drawing Metric Coarse Threads
Fig. 15.2 External form to illustrate thread terms

ISO metric threads

Figure 15.3 shows the ISO metric thread form for a nut (internal) and for a bolt (external). In the case of the nut, the root is rounded in practice. For the mating bolt, the crest of the thread may be rounded within the maximum outline, as shown, and the root radiused to the given dimension. Both male and female threads are subject to manufacturing tolerances and for complete information, reference should be made to BS 3643-1.

BS 3643-2 defines two series of diameters with graded pitches for general use in nuts, bolts and screwed fittings, one series with coarse and the other with fine pitches. The extract given in Table 15.1 from the standard gives thread sizes from 1.6 to 24 mm diameter. Note that first, second and third choices of basic diameters are quoted, to limit the number of sizes within each range.

On a drawing, a thread will be designated by the letter M followed by the size of the nominal diameter and the pitch required, e.g. M10 x 1

If a thread is dimensioned without reference to the pitch, e.g. M16, then it is assumed that the coarse-series thread is required.

Table 15.1

1 2 3

4

5

6

7 8

9

10

11

12

13

14

15

16

Basic major diameters

Coarse series with graded pitches

Choice

Fine

series

with constant pitches

1st 2nd 3rd

6

4

3 2

1.5

1.25

1

0.75

0.5

0.35

0.25

0.2

1.6

0.35

-

-

- -

-

-

-

-

-

-

-

0.2

1.8

0.35

-

-

- -

-

-

-

-

-

-

-

0.2

2

0.4

-

-

- -

-

-

-

-

-

-

0.25

-

2.2

0.45

-

-

- -

-

-

-

-

-

-

0.25

-

2.5

0.45

-

-

- -

-

-

-

-

-

0.35

-

-

3

0.5

-

-

- -

-

-

-

-

-

0.35

-

-

3.5

0.6

-

-

- -

-

-

-

-

-

0.35

-

-

4 - -

0.7

-

-

- -

-

-

-

-

0.5

-

-

-

4.5

0.75

-

-

- -

-

-

-

-

0.5

-

-

-

5

8.8

-

-

- -

-

-

-

-

0.5

-

-

-

5.5

-

-

-

- -

-

-

-

-

0.5

-

-

-

6

1

-

-

- -

-

-

-

0.75

-

-

-

-

7

1

-

-

- -

-

-

-

0.75

-

-

-

-

8--

1.25

-

-

- -

-

-

1

0.75

-

-

-

-

9

1.25

-

-

- -

-

-

1

0.75

-

-

-

-

10 - -

1.5

-

-

- -

-

1.25

1

0.75

-

-

-

-

- - 11

1.5

-

-

- -

-

-

1

0.75

-

-

-

-

12 - -

1.75

-

-

- -

1.5

1.25

1

-

-

-

-

-

- 14 -

2

-

-

- -

1.5

1.25*

1

-

-

-

-

-

15

-

-

-

- -

1.5

-

1

-

-

-

-

-

16

2

-

-

- -

1.5

-

1

-

-

-

-

-

17

-

-

-

- -

1.5

-

1

-

-

-

-

-

18

2.5

-

-

2

1.5

-

1

-

-

-

-

-

20

2.5

-

-

2

1.5

-

1

-

-

-

-

-

22

2.5

-

-

2

1.5

-

1

-

-

-

-

-

24

3

-

-

2

1.5

-

1

-

-

-

-

-

Note. For preference, choose the diameters given in Column 1. If these are not suitable, choose from Column 2, or finally from Column 3. *The pitch of 1.25 mm for 14 mm diameter is to be used only for sparking plugs.

Engineering Representation Screw

Fig. 15.3 ISO metric thread

Fig. 15.3 ISO metric thread

5 H = 0.54127P, where P is the pitch of the thread 8

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    What is the necessity of conventional representation of screw threads?
    3 years ago
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    What is the necessity of conventional representation of screw thread?
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