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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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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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Figure 1

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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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Tolerance: composed of location of the fundamental deviation block and thickness of the block

30

30

(a)

(b)

Reference line for the shaft

Fundamental deviation block. Its location is measured with reference to the basic size

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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:

IT0, IT1, IT2, .... IT16

Small Deviation large Deviation

large tolerance is given to large grade

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Figure 2 IT grades are further associated with manufacturing processes

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Basic size group

1. With the same idea of IT grade, we group basic sizes

into groups. The tolerance is the same to any

dimension in the groups.

2. Large basic size gets large tolerance.

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IT 1 IT 5Basic size

0.0012 0.00810-18

18-30 0.0015 0.009

Tolerance with respect to size group and IT group

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50

50.03050.005

HG

J

49.05

hk

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

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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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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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Basic shaft system with indication of three types of fits

Basic hole system with indication of three types of fits

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Figure 3

Complete set of FDs for hole and shaft

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A combination of location of FD and IT grade

Basic size

40 H7

Tolerance zone

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Hole - minimum hole size as basic diameter

- denoted by Capital letter (say, H)

Basic size Fundamental Deviation

40 H8 IT grade

Tolerance zone

ISO Method to Determine Tolerance

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Shaft

- maximum shaft size as basic diameter

- denoted by small letters (say, h)

Basic size Fundamental Deviation

40 h7 IT grade

Tolerance zone

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Figure 4. Preferred fit

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Preferred fit:

Product Function determines Fit. For instance,

two parts need to have relative motion, so we

require therefore clearance fit.

ISO Method to Determine Tolerance

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Tolerance

Location of the FD block Thickness of the FD block

Basic size IT grade (required accuracy)

Basic hole and basic shaft Quality of machine running

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Procedure

1. Basic size selection

2. Determine the preferred fit

3. International Grade

4. Determine tolerances

ISO Method to Determine Tolerance

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Procedure to determine tolerance

Re-visit the example: Basic hole system Running of accurate machines Basic diameter, say 39

Step 2: Go to Figure 4, H8/f7

Step 1: go to Figure 1, the closed size to 39 is 40.

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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)

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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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More Examples

Given: basic hole systemlocational transitional fitbasic diameter =57 mm

Figure 1 -> 60Figure 4 -> H7/k6Table 1: Hole Shaft

60.030 60.02160.000 60.002

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Hole Shaft

60.030 60.021

60.000 60.002

-0.021 0.028

Max interference Max clearance

0.030 0.019

Tol

eran

ce

Tol

eran

ce

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Representation on the drawing

60H760.03060.000( )

60.02160.002

60k660.02160.002( )