ISO tolerance ranges

Tolerance bands need to be defined which can be related to functional performance and manufacturing processes. The ISO has published tolerance ranges to help designers. Examples of these tolerance ranges are shown in Figure 5.4. This table is only a selection from the full table given in ISO 286-2:1988. The full range goes up to IT18 and 3m nominal size. The tolerance ranges are defined by 'IT' ranges as shown in the diagram from IT1 to IT11. The range given in the ISO standard is significantly more complicated than the extract in Figure 5.4. It should be noted that the range increases as the IT number gets larger and the range increases as the nominal size increases. The latter is fairly logical in that one would expect the tolerance range to be larger as the diameter increases because the precision that can be achieved must be relative. The ranges were not chosen out of the blue but empirically derived and based on the fact that the relationship between manufacturing errors and basic size can be approximated by a parabolic function.

The trace from a flat surface shown in Figure 4.11 has shown the maximum deviation over the 10mm length to be 4,2um. The nominal size was 22mm. If this surface was to be inspected with respect to the tolerance grades in Figure 5.4, the 22mm nominal size would fall within the row 18 to 30mm. Along this row, the 4,2um corresponds to IT4 since, if the tolerance on a drawing was given by IT1 to IT3, the surface would fail inspection whereas if the drawing

I Nomit Over

ml size Up to & incl

IT1

1T2

IT3

ISC IT4

Tolera IT5

ice rant IT6

jes in u IT7

icrons IT8

IT9

IT10

IT11

-

3

0,8

1,2

2

3

4

6

10

14

25

40

60

3

6

1

1,5

2,5

4

5

8

12

18

30

48

75

6

10

1

1,5

2,5

4

6

9

15

22

36

58

90

10

18

1,2

2

3

5

8

11

18

27

43

70

110

18

30

1,5

2,5

4

6

9

13

21

33

52

84

130

30

50

1,5

2,5

4

7

11

16

25

39

62

100

160

50

80

2

3

5

8

13

19

30

46

74

120

190

80

120

2,5

4

6

10

15

22

35

54

87

140

220

120

180

3,5

5

8

12

18

25

40

63

100

160

250

180

250

4,5

7

10

14

20

29

46

72

115

185

290

Figure 5.4 Standard ISO tolerance ranges adapted from ISO 286-2:1988

Figure 5.4 Standard ISO tolerance ranges adapted from ISO 286-2:1988

specified IT4 or above, it would pass the inspection. Similarly, with respect to the gun-drilled hole out-of-roundness deviation in Figure 5.2, the bars on the graph show that with a sharp drill, the out-of-roundness is 4,5um whereas when the drill is worn the out-of-roundness is 9,lum. These values beg the question as to what IT class this gun-drilling hole belongs to. The quick answer is that it depends on drill wear. With reference to Figure 5.4, the appropriate row is 6 to 10mm (i.e. the third row). The 4,5um out-of-roundness corresponds to class IT5 whereas the 9, lum out-of-roundness corresponds to class IT7. If the tolerance class IT4 is to be met by gun-drilling then a drill can only be used for a short proportion of its life. If, on the other hand, class IT7 is acceptable, this can be achieved throughout the life of the drill.

Figure 5.5 shows the IT tolerance ranges for various situations. These are the ranges for measuring tools, for common manufacturing processes, for limits and fits and for the production of materials. It is perhaps of no surprise that the range produced by common manufacturing processes is almost the same as the range of limits and fits from which designers can select functional performance tolerances.

Figure 5.6 is a table that is essentially an expansion of the manufacturing processes range in Figure 5.5. This table shows the range of tolerances achieved by the most common manufacturing processes. High-precision processes like lapping can achieve tolerance IT4 whereas, at the other end, roughing processes like shaping are only IT11. The range within any one process represents the variabilities caused by such things as wear, feed and speed, etc.

Figure 5.5 ISO tolerance ranges for various situations

Figure 5.5 ISO tolerance ranges for various situations

(Process

Lapping

Cylindrical grinding_

Honing

Diamond turning

Surface grinding_

Broaching_

Electro-Discharge M/cing Deep hole drilling

Reaming_

Boring_

i Turning_

Miliing_

Drilling_

Planing_

Shaping

Sintering_

Precision casling_

General casting

Figure 5.6 ISO tolerance ranges for a variety of manufacturing processes

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Responses

  • julio
    How tolerance range increase with length?
    8 years ago

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