Editing Engineering tolerance (section)

== Mechanical component tolerance ==
[[File:Mechanical Tolerance Definitions.svg|thumb|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 [[engineering fit|fit]] in mechanical engineering, which is a ''designed-in'' clearance or interference between two parts. Tolerances are assigned to parts for manufacturing purposes, as boundaries for acceptable build. No machine can hold dimensions precisely to the nominal value, so there must be acceptable degrees of variation. If a part is manufactured, but has dimensions that are out of tolerance, it is not a usable part according to the design intent. 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 minimum possible component size and the basic size.
; Upper deviation: The difference between the maximum possible component size and the basic 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.<ref>{{cite book |last1= C. Brown|first1= Walter|last2= K. Brown|first2= Ryan |date= 2011|title=Print Reading for Industry, 10th edition |publisher= The Goodheart-Wilcox Company, Inc.|page= 37|isbn=978-1-63126-051-3}}</ref> If the fundamental deviation is greater than zero, the bolt will always be smaller than the basic size and he hole will always be wider. Fundamental deviation is a form of [[allowance (engineering)|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{{nbsp}}[[millimeter|mm]] is to have a sliding fit within a hole, the shaft might be specified with a tolerance range from 9.964 to 10&nbsp;mm (i.e., a zero fundamental deviation, but a lower deviation of 0.036&nbsp;mm) and the hole might be specified with a tolerance range from 10.04&nbsp;mm to 10.076&nbsp;mm (0.04&nbsp;mm fundamental deviation and 0.076&nbsp;mm upper deviation). This would provide a clearance fit of somewhere between 0.04&nbsp;mm (largest shaft paired with the smallest hole, called the ''Maximum Material Condition'' - MMC) and 0.112&nbsp;mm (smallest shaft paired with the largest hole, ''Least Material Condition'' - LMC). In this case the size of the tolerance range for both the shaft and hole is chosen to be the same (0.036&nbsp;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|machining industry]] uses the following '''standard tolerances''':<ref>2, 3 and 4 decimal places quoted from page 29 of "Machine Tool Practices", 6th edition, by R.R.;&nbsp;Kibbe, J.E.;&nbsp;Neely, R.O.;&nbsp;Meyer & W.T.;&nbsp;White, {{ISBN|0-13-270232-0}}, 2nd printing, copyright 1999, 1995, 1991, 1987, 1982 and 1979 by Prentice Hall.<br>(All four places, including the single decimal place, are common knowledge in the field, although a reference for the single place could not be found.)</ref><ref>According to Chris McCauley, Editor-In-Chief of Industrial Press' [[Machinery's Handbook]]: '''Standard Tolerance''' "…''does not appear to originate with any of the recent editions (24-28) of [[Machinery's Handbook]], although those tolerances may have been mentioned somewhere in one of the many old editions of the Handbook.''" (4/24/2009 8:47 AM)</ref>

{| border="0"
|-
| style="padding-left:2em;" | 1 decimal place  ||    (.x): || ±0.2"
|-
| style="padding-left:2em;" | 2 decimal places ||   (.0x): || ±0.01"
|-
| style="padding-left:2em;" | 3 decimal places ||  (.00x): || ±0.005"
|-
| style="padding-left:2em;" | 4 decimal places || (.000x): || ±0.0005"
|}
[[File:Limits and Fits.jpg|thumb|Limits and fits establish in 1980, not corresponding to the current ISO tolerances]]

===International Tolerance grades===
{{main|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, [[tap and die#Machine tapping|tapped hole]], or [[nut (hardware)|nut]]) and h7 (shaft or bolt). H7/h6 is a very common standard tolerance which gives a tight fit. 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&nbsp;mm larger than the base dimension, and 0&nbsp;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 means the shaft may be as small as 0.009&nbsp;mm smaller than the base dimension and 0&nbsp;mm larger. This method of standard tolerances is also known as Limits and Fits and can be found in [http://www.iso.org/iso/iso_catalogue/catalogue_ics/catalogue_detail_ics.htm?csnumber=45975&ICS1=17&ICS2=40&ICS3=10 ISO 286-1:2010 (Link to ISO catalog)].

The table below summarises the International Tolerance (IT) grades and the general applications of these grades:

{| class="wikitable" style="text-align:center; margin: 1em auto 1em auto"
|width=70|&nbsp;
|colspan=9|Measuring Tools
|colspan=7|Material
|colspan=5|&nbsp;
|-
!IT Grade
!width=25|01
!width=25|0
!width=25|1
!width=25|2
!width=25|3
!width=25|4
!width=25|5
!width=25|6
!width=25|7
!width=25|8
!width=25|9
!width=25|10
!width=25|11
!width=25|12
!width=25|13
!width=25|14
!width=25|15
!width=25|16
|-
|colspan=7|&nbsp;
|colspan=7|Fits
|colspan=5|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.