Why ISO Standards Should Be Preferred For Metric Fasteners?

The adoption of ISO Fastener Standards by suppliers and end users will end 40 years of fastener industry confusion. The adoption of ISO standards is good engineering practice, it simplifies inventory, it enhances assembly uniformity, and it makes export products more attractive to consumers outside the USA. 
 
THE “BIG THREE” STIMULATED THE ADOPTION OF THE METRIC SYSTEM IN THE US. 
 
The United states started moving into the use of metric fasteners in a significant way in the early 1970s when the “Big Three” (FORD, GM, and Chrysler) made a commitment to use the metric system for all new vehicle designs. Since the automotive industry then, and still today, use more threaded fasteners than any other industry, the major fastener suppliers in the USA started getting involved in the production of metric fasteners. This auto industry commitment to metric design was adopted so that one car design could be produced all over the world instead of having one design for North America and another design for all markets outside North America. The huge international equipment manufacturers like Caterpillar and John Deere soon followed suit. 
 
Until that time relatively low volumes of metric fasteners were used in the United States. They were mostly used in maintenance applications for the maintenance of imported manufacturing equipment from Europe, with the majority coming from Germany. The German standards system is referred to as the DIN (Deutsches Institut fur Normung e. V.) system and most of the imported fasteners used to support the import equipment were made in Europe to the DIN fastener standards. 
 
The Big Three decision to adopt the metric system of measurement in design impacted all industrialized countries in the world. The car manufacturers wanted to be able to source products anywhere in the world and have the components be compatible regardless of where the parts were made, purchased, or assembled. This decision drove the need for the creation and use of common world standards for all components including fasteners. 
 
THE ISO FASTENER COMMITTEE WAS FORMED TO COMMONIZE METRIC FASTENER SYSTEMS. 
 
At that time several European and Asian countries had their own designs of metric fasteners as defined by their country standards. To commonize these designs the ISO TC2 Fastener Committee was formed. The predominant metric fastener standard at the time was DIN so it became the foundational metric fastener standard system from which the eventual ISO fastener standards evolved. 
The US joined the ISO efforts, but at the same time tried to mold a unique metric standard called the Optimum Metric Fastener System (OMFS). This work began in the Industrial Fasteners (IFI) Technical Committees and was later transitioned into the fastener committees in the American Society of Mechanical Engineers (ASME) B18 Committee and the American Society for Testing, Materials (ASTM) F16 Committee and the SAE Fastener Committee. 
 
The concept behind the OMFS was to narrow the choices that were available in the other metric fastener systems in ways such as limiting the thread series to only course threads. OMFS included efforts to introduce an external spline-head design to replace hex heads on bolts and a new gaging system for threads in an effort to improve thread quality. 
 
The work on the OMFS concept within the ASME, ASTM, and SAE committees resulted in an entire series of USA created metric fastener standards. Eventually most of the unique ideas in the OMFS program faded away and the resultant standards were interchangeable with, but not exactly like, the evolved ISO fastener standards. In retrospect, the USA effort just added another metric fastener standard option to metric fastener technology, increasing the complexity instead of achieving its goal of simplifying it. The OMFS concept was noble but its objectives never materialized.
 
In the meantime more and more countries joined the ISO fastener standards efforts and a true worldwide fastener standards system was created. The growing adoption of ISO fastener standards has resulted in most industrialized countries withdrawing their country-specific standards and formally adopting the ISO standards as their metric fastener system. 
 
GERMANY WITHDRAWS DIN STANDARDS IN 2001 TO ADOPT ISO FASTENER STANDARDS. 
 
The biggest endorsement of the ISO fastener standards was Germany’s official withdrawal of their DIN fastener standards in 2001 which is documented in DIN 918, Supplement 3. A table showing the withdrawn DIN standards and the ISO standards that should now be used instead is included in this article. 
 
International trade is increasing every day. USA exports are critical for controlling the USA balance of trade and reducing unemployment. USA firms should be adopting ISO fastener standards to assure what is produced in the USA is acceptable to purchasers outside the USA. 
The good news is that with very few exceptions, fasteners made to the ISO standards, the withdrawn DIN standards, and the USA metric standards are interchangeable. The most significant differences are in the AF (across the flats) sizes on M10, M12, and M14 bolts and nuts. The DIN standards specify a one millimeter larger hex size than do the ISO and ASME standards. Fortunately, all designs have the same strength requirements and capabilities. The only practical difference is that installers will have to use different driver socket sizes depending on which metric standard the bolts or nuts are used in manufacturing. 
 
A concerted effort is being made in the ASME B18 and ASTM F16 fastener committees to start systematically withdrawing their metric fastener standards. Users will be directed to the comparable ISO metric standard. 
 
US METRIC FASTENER SUPPLIERS AND END USERS SHOULD TRANSITION INTO THE USE OF ISO STANDARDS. 
 
It is understood today that a lot of the stocked imported metric fasteners are made to the DIN standards. The good news is that many of the product requirements are identical in the DIN and the ISO standards. Few of the differences adversely affect fit or function. Importers should start working with their suppliers to transition from following the DIN standards to following the ISO requirements. For many years Bossard has shown the references in their catalog for the comparable DIN, ISO, and ASME standards. All stocking metric fastener suppliers are encouraged to consider doing the same if they are not already doing so.
It is also understood that many end users are still referring to DIN fastener standards because they are unaware of DIN withdrawing their standards in preference for the ISO standards. Suppliers are encouraged to suggest to end users to adopt ISO fastener standards for all new designs and to transition into ISO standards away from the DIN standards where possible for current usage. 
There is no law or rule that states that users cannot continue to use a withdrawn standard forever if they wish to do so. They should, however realize that of the DIN, ASME, and ISO metric fastener standards, only the ISO will be technically maintained and updated in the future. 
 
ISO IS THE WORLDWIDE-ACCEPTED STANDARD. 
 
It is not good engineering practice to have more than one standard for any given product. Multiple metric fastener standards cause unnecessary confusion and adversely affect product uniformity. US industry has been in a state of confusion since the 1970s with its use of metric fasteners, because users have been given three standards to choose from for essentially the same products. It is time for all companies in the supply chain of metric fasteners to start an orderly transition towards the uniform adoption of fasteners made to the ISO fastener standards. To effectively compete in worldwide commerce, US product manufacturers need to utilize the worldwide-accepted ISO fastener standards
 

Date : 23.11.2017