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Tolerance & Fits (Part 1)

A Vital Part of the Metric System

In a previous blog post, we wrote at length about the benefits of metric fasteners – and the use of the metric system, in general. The importance of tolerance systems was mentioned, but not expanded upon. The ISO tolerance system is, in essence, the key to global interchangeability.

The key to understanding the metric system as it relates to fasteners is the understanding of its tolerance system. The inch/pound system tolerance is done by minimums and maximums that are determined by design.

The ISO fits and tolerance system is based on a number/letter system that relates to the ISO tolerance chart. This chart is applied globally and allows the designed parts to be interchangeable.

The letters indicate the location of the tolerance and the numbers indicate the tolerance range. The bigger the number, the bigger the spread between the maximum and minimum tolerance. Capital letters show internal dimensions (holes, sockets key, etc.) while lower case letters refer to external dimensions (shaft diameters, pin diameters, width across the flats, etc.)

Each letter has its exact location related to a so-called zero line. The zero line refers to the nominal dimensions such as nominal diameter and nominal length.

In our next blog post, we’ll cover fits as it relates to tolerances, so be sure to check back next week! Until then, if you’re interested in engaging further about tolerance systems, please do not hesitate to reach out! Contact us at ProvenProductivity@bossard.com to get the conversation started.


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November 20, 2015

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