Mechanical component tolerance

Summary of basic size, fundamental deviation and IT grades compared to minimum and maximum sizes of the shaft and hole.

Dimensional tolerance is related to, but different from fit in mechanical engineering, which is a

designed-in clearance or interference between two parts. Tolerances can be applied to any

dimension. The commonly used terms are:

▪ Basic size: the nominal diameter of the shaft (or bolt) and the hole. This is, in general,

the same for both components.

▪ Lower deviation: the difference between the basic size and the minimum possible

component size.

▪ Upper deviation: the difference between the basic size and the maximum possible

component size.

▪ Fundamental deviation: the minimum difference in size between a component and the

basic size. This is identical to the upper deviation for shafts and the lower deviation for holes.

If he fundamental deviation is greater than zero, the bolt will always be smaller than the basic

size and the hole will always be wider. Fundamental deviation is a form of allowance, rather

than tolerance.

▪ International Tolerance grade: this is a standardised measure of the maximum

difference in size between the component and the basic size (see below).

For example, if a shaft with a nominal diameter of 10 mm is to have a sliding fit within a hole,

the shaft might be specified with a tolerance range from 9.964 to 10 mm (i.e. a zero

fundamental deviation, but a lower deviation of 0.036 mm) and the hole might be specified

with a tolerance range from 10.04 mm to 10.076 mm (0.04 mm fundamental deviation and

0.076 mm upper deviation). This would provide a clearance fit of somewhere between

0.04 mm (largest shaft paired with the smallest hole, called the "maximum material

condition") and 0.112 mm (smallest shaft paired with the largest hole). In this case the size of

the tolerance range for both the shaft and hole is chosen to be the same (0.036 mm),

meaning that both components have the same International Tolerance grade but this need

not be the case in general.

When no other tolerances are provided, the machining industry uses the following standard

tolerances:[3][4]

1 decimal place (.x): ±0.2

2 decimal places (.0x): ±0.01

3 decimal places (.00x): ±0.005

4 decimal places (.000x): ±0.0005

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International Tolerance grades

Main article: IT Grade

When designing mechanical components, a system of standardized tolerances called

International Tolerance grades are often used. The standard (size) tolerances are divided

into two categories: hole and shaft. They are labelled with a letter (capitals for holes and

lowercase for shafts) and a number. For example: H7 (hole tapped hole or nut) and h7 (shaft

or bolt). H7/h6 is a very common standard tolerance which gives a rather tight fit, but not so

tight that you can't put the shaft in the hole, or turn the nut on the bolt, by hand. The

tolerances work in such a way that for a hole H7 means that the hole should be made slightly

larger than the base dimension (in this case for an ISO fit 10+0.015−0, meaning that it may

be up to 0.015 mm larger than the base dimension, and 0 mm smaller). The actual amount

bigger/smaller depends on the base dimension. For a shaft of the same size h6 would mean

10+0−0.009, which is the opposite of H7. This method of standard tolerances is also known

as Limits and Fits and can be found in ISO 286-1:2010 (Link to ISO catalog).

The table below summarises the International Tolerance (IT) grades and the general

applications of these grades:

  Measuring Tools Material

IT

Grade01 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

  Fits Large Manufacturing Tolerances

An analysis of fit by Statistical interference is also extremely useful: It indicates the frequency (or probability) of parts properly fitting together.