1. TOLERANCES - Introduction Nearly impossible to make the part to the exact dimension by any means of manufacturing approach  tolerances of the dimension.
- Dimension 30 (mm) won’t be made exactly as 30 (mm)
- It may be made as (mm) or (mm).
- maximum may be (mm) 30
(a)
(b)
Fig. 1
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2. Introduction (a) 30.01 (shaft) (b) 30.005 (hole)
- situations for assembly of (a) and (b)?
(a) (shaft)
(b) (hole)
(a) and (b) are impossible to be assembled without any special treatment.
(a) (shaft)
(b) (hole)
(a) and (b) are assembled with a possibility of poor function of the system (see Figure 2)
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3. Introduction
Figure 2 . L’ L
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4. Introduction In summary, designers need to specify tolerances for
(a) Parts manufacturing: interchangeable
(b) System functions: satisfactory with low cost
Specify a tighter tolerance only if necessary, because the tighter the tolerance, the higher the cost.
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5. Objectives of the lecture:
Introduction
Objectives of the lecture:
To learn principles behind the international standard (e.g., ISO) for determining tolerances.
To learn the procedure to determine tolerances based on the standard.
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6. Tolerance is always a positive number !
Basic Concept
Definition of tolerance:
Tolerance is the total amount a specific dimension is permitted to vary, which is the difference between the maximum and the minimum dimensions.
Tolerance is always a positive number !
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7. Basic Concept Three types of fits (a) 1.247 - 1.248 shaft
(b) hole Clearance fit
(a) shaft
(b) hole Interference fit
(a) shaft
(b) hole Transition fit
Tolerance refers to a single part, while fit refers to a connection of two parts.
he tolerances of the two connected parts contribute to a fit between the two connected parts.
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8. Standard
Suppose tolerance range is 0.1 and the nominal dimension is 30.
(1) max: 30.10, min: 30.00; (2) max: 30.00, min: 29.90; (3) max: 30.05, min: 29.95; (4) max: 30.15, min: 30.05, …… 30
Min 30
Max 30 30 30
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9. Standard
Basic idea:
To give restriction on the possibility of the dimension and then its tolerance
Nominal dimension is a positive integer with the last letter 0 or 5.
Limit (max or min) dimension is the same as the nominal dimension. For instance, the low limit (min) dimension for a hole is 30, which is the nominal dimension.
Tolerance can only be a certain groups, the same as the nominal dimension with 0 or 5 as the last letter.
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10. Basic hole versus basic shaft concept:
making the nominal dimension as one of the dimension limit (max or min)
Tolerance is developed along this direction 30
(b) 30
(a)
Tolerance is developed along this direction
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11. Purpose: take a hole as a reference to determine the shaft limit.
Basic Hole System
Basic size = Nominal size
Purpose: take a hole as a reference to determine the shaft limit.
The minimal hole size as the basic size
Reason: in some applications, the hole can be made more precise (Reamers, Broachers, Gages), while the machining of the shaft varies.
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12. Basic Shaft System Different fits with the same shaft
Purpose & reason: in some applications, the shaft could be better made as a reference to determine the hole limit.
Different fits with the same shaft
Take the shaft as a reference
The maximal shaft size as the basic size
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13. Example Basic size =0.5 0.502 0.498 0.500 0.495 Basic hole system
Basic size =0.5
0.505
0.502
Basic shaft system
0.500
0.499
Basic hole system
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14. Basic size (preferred)
Dimensions are initially determined by designers from a point of view of function.
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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15. Figure 1
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16. To determine the tolerance
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17. Reference line for the shaft30
Fundamental deviation block. Its location is measured with reference to the basic size
(b) 30
Tolerance: determined by location of the fundamental deviation block and thickness of the block
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18. 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 to a finite number of possibilities, we group FDs into 16 groups or grades as follows (IT1, IT2, …, IT16):
Small Deviation
large Deviation
IT0, IT1, IT2, IT16
large tolerance is given to large grade
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19. Figure 2 IT grades are further associated with manufacturing processes
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20. Basic size group
Group basic sizes into groups. The tolerance is the same to all dimensions in the groups.
Large basic size gets large tolerance.
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21. Tolerance with respect to size group and IT group
IT 1 IT 5
Basic size
10-18
18-30
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22. 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)
50.005
Hole
50.030 H G J h k 50
49.05
Shaft
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23. H: a special location of FD, and this location makes the minimum diameter of the hole is the basic size of the hole (basic hole system)
Hole
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24. Shaft
h: a special location of FD, and this location makes the maximum diameter of the shaft is the basic size of the shaft (basic shaft system)
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25. Basic hole system with the indication of three types of fits
Basic shaft system with the indication of three types of fits
Hole
Shaft
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26. Complete set of FDs for hole and shaft
Figure 3
Complete set of FDs for hole and shaft
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27. 40 H7 Tolerance zone Basic size Tolerance zone = FD
Location of FD IT grade
Basic size
40 H7
Tolerance zone
Tolerance zone = FD
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28. ISO Method to Determine Tolerance
Hole
minimum hole size as basic diameter
denoted by capital letter (say, H)
Basic size Location of Fundamental Deviation
40 H IT grade
Tolerance zone
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29. Shaft maximum shaft size as basic diameter
denoted by small letters (say, h)
Basic size location of Fundamental Deviation
40 h IT grade
Tolerance zone
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30. Either basic hole (H) or basic shaft (h)
Figure 4. Preferred fit
Either basic hole (H) or basic shaft (h)
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31. The tolerance for a part (A) is also constrained by the fit of the part with the other part (B).
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32. 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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33. 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)
Design and manufacturing
Basic hole and basic shaft
Fit
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34. Determine the preferred fit International Grade Determine tolerances
Procedure for determining the tolerance
Basic size selection
Determine the preferred fit
International Grade
Determine tolerances
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35. Running of accurate machines Basic diameter, say 39
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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36. 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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37. 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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41. 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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42. 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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43. ) ( ) ( Representation on the drawing 60.030 60H7 60.000 60.021 60.002
60k6
60.021
60.002 ( )
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44. Summary (expected to know):
Tolerance is about one part but it has effect on the fit of two parts.
It is the fit that makes sense for the quality of machine running.
Tolerance is determined by the constraints of (a) machine running condition, (2) manufacturing technology, (c) design to meet the function.
Standard is to assist in determination of the tolerance fast and to facilitate the part exchange.
Standard consists of several tables and charts.
Procedure: (a) decide basic hole or basic shat, (b) decide the basic dimension, (c) decide the fit (which contains the IT grade), (d) find the tolerance with the basic size and tolerance zone code (e.g., H7).
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