1. ISO Method to Determine Tolerance
A practical problem statement:
Basic hole system
Running of accurate machines
Basic diameter, say 39.00
Question: to determine the tolerance?
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2. Basic size (preferred)
Dimensions are initially determined by designers from a point of view of functions.
From the view point of function, the length of a bar may be like 39.6
From a point of view of manufacturing, 39.6 is not a convenient figure, and therefore needs to be rounded up (say, 40) (see figure 1)
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3. Figure 1
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4. Output from the system: fit and tolerance.
Design a system (ISO standard) which is a set of tables or charts to determine the tolerance.
Input to the system: basic size (decided by the designer), machine running condition, manufacturing facility available (option).
Output from the system: fit and tolerance.
Constraint to the system: machine running condition, assembly requirement, manufacturing facility available (option)
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5. Reference line for the shaft30
Fundamental deviation block. Its location is measured with reference to the basic size
(b) 30
Tolerance: composed of location of the fundamental deviation block and thickness of the block
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6. Fundamental deviation (FD)
Deviation closest to the basic size or the location of the FD block
International Tolerance Grade (IT)
FD can vary in an infinite number of possible numbers. To restrict FD a finite number of possibilities, we group FDs into 16 as follows (IT1, IT2, …, IT16, the number means grade:
Small Deviation
large Deviation
IT0, IT1, IT2, IT16
large tolerance is given to large grade
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7. Figure 2 IT grades are further associated with manufacturing processes
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8. Basic size group
With the same idea of IT grade, we group basic sizes into groups. The tolerance is the same to any dimension in the groups.
Large basic size gets large tolerance.
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9. Tolerance with respect to size group and IT group
IT 1 IT 5
Basic size
10-18
18-30
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10. FD (block in the following diagram) is located with respect to basic size (in total there are 27 FDs)
Different location is given a name (letter)
Different location of FD finally comes up with tolerance
50.005
50.030 H G J h k 50
49.05
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11. H: a special location of FD, and this location makes the minimum diameter of the hole is the basic size of the hole, so the tolerance of the hole implies the basic hole system in the mind of designer
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12. Shaft
h: a special location of FD, and this location makes the maximum diameter of the shaft is the basic size of the shaft, so the tolerance of the shaft implies the basic shaft system in the mind of designer
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13. Basic shaft system with indication of three types of fits
Basic hole system with indication of three types of fits
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14. Complete set of FDs for hole and shaft
Figure 3
Complete set of FDs for hole and shaft
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15. 40 H7 Tolerance zone A combination of location of FD and IT grade
Basic size
40 H7
Tolerance zone
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16. ISO Method to Determine Tolerance
Hole
- minimum hole size as basic diameter
- denoted by Capital letter (say, H)
Basic size Fundamental Deviation
40 H IT grade
Tolerance zone
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17. - maximum shaft size as basic diameter
- denoted by small letters (say, h)
Basic size Fundamental Deviation
40 h IT grade
Tolerance zone
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18. Figure 4. Preferred fit
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19. ISO Method to Determine Tolerance
Preferred fit:
Product Function determines Fit. For instance, two parts need to have relative motion, so we require therefore clearance fit.
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20. Thickness of the FD block Location of the FD block
Tolerance
Thickness of the FD block
Location of the FD block
Basic size
IT grade (required accuracy)
Basic hole and basic shaft
Quality of machine running
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21. ISO Method to Determine Tolerance
Procedure
Basic size selection
Determine the preferred fit
International Grade
Determine tolerances
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22. Procedure to determine tolerance
Re-visit the example:
Basic hole system
Running of accurate machines
Basic diameter, say 39
Step 1: go to Figure 1, the closed size to 39 is 40.
Step 2: Go to Figure 4, H8/f7
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23. Step 3: Go to Table 1a, we will find that under the size 40, and column H8
H8 f7 Fit
(max clearance)
(min clearance)
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24. Summary of the procedure
The following figures and table are used:
Figure 1: Get a preferred size as well as IT grade
Figure 4: Get a preferred fit
Table 1a: Get tolerance
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26. More Examples Given: basic hole system locational transitional fit
basic diameter =57 mm
Figure 1 -> 60
Figure 4 -> H7/k6
Table 1: Hole Shaft
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27. Hole Shaft 60.030 60.021 60.000 60.002 Tolerance Tolerance -0.021
Tolerance
Tolerance
-0.021
0.028
Max interference Max clearance
0.030
0.019
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28. ) ( ) ( Representation on the drawing 60.030 60H7 60.000 60.021 60.002
60k6
60.021
60.002 ( )
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