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? handout 10b

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) handout 10b

Figure 1 handout 10b

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) handout 10b

Reference line for the shaft 30 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 handout 10b

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 handout 10b

Figure 2 IT grades are further associated with manufacturing processes handout 10b

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. handout 10b

Tolerance with respect to size group and IT group IT 1 IT 5 Basic size 0.0012 0.008 10-18 18-30 0.0015 0.009 handout 10b

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 handout 10b

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 handout 10b

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 handout 10b

Basic shaft system with indication of three types of fits Basic hole system with indication of three types of fits handout 10b

Complete set of FDs for hole and shaft Figure 3 Complete set of FDs for hole and shaft handout 10b

40 H7 Tolerance zone A combination of location of FD and IT grade Basic size 40 H7 Tolerance zone handout 10b

ISO Method to Determine Tolerance Hole - minimum hole size as basic diameter - denoted by Capital letter (say, H) Basic size Fundamental Deviation 40 H8 IT grade Tolerance zone handout 10b

- maximum shaft size as basic diameter - denoted by small letters (say, h) Basic size Fundamental Deviation 40 h7 IT grade Tolerance zone handout 10b

Figure 4. Preferred fit handout 10b

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. handout 10b

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 handout 10b

ISO Method to Determine Tolerance Procedure Basic size selection Determine the preferred fit International Grade Determine tolerances handout 10b

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 handout 10b

Step 3: Go to Table 1a, we will find that under the size 40, and column H8 H8 f7 Fit 40.039 39.975 0.089 (max clearance) 40.000 39.950 0.025 (min clearance) handout 10b

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 handout 10b

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More Examples Given: basic hole system locational transitional fit basic diameter =57 mm Figure 1 -> 60 Figure 4 -> H7/k6 Table 1: Hole Shaft 60.030 60.021 60.000 60.002 handout 10b

Hole Shaft 60.030 60.021 60.000 60.002 Tolerance Tolerance -0.021 60.030 60.021 60.000 60.002 Tolerance Tolerance -0.021 0.028 Max interference Max clearance 0.030 0.019 handout 10b

) ( ) ( Representation on the drawing 60.030 60H7 60.000 60.021 60.002 60k6 60.021 60.002 ( ) handout 10b