BS 308 and BS 8888
Six Myths
about BS 8888
1
BS 8888 is just the new name for BS
308.about BS 8888
No. BS 8888 is an entirely new standard, and is quite different in nature and content to BS 308.
2 The only difference between BS 8888 and BS 308 is that now we have to use a comma instead of a full stop as the decimal marker.
No. Although BS 8888 does specify the comma as the decimal marker (as this is a requirement of the ISO system), this is just one of a number of presentational changes introduced by the new standard.
More important are the fundamental changes brought in with the system of Geometrical Product Specification.
3 BS 308 was fine as it was, there was no need for a new standard.
No. The developments in the ISO system over the last decade have been driven by technical deficiencies in the traditional methods of specifying component geometry.
If BS 308 had been retained, the technical changes that have been introduced in the ISO system would also have been introduced into BS 308, and the problem of two parallel sets of standards serving the same purpose would remain.
4 BS 8888 is just more bureaucracy being imposed from Brussels.
No. BS 8888 does provide a gateway to the ISO system, but the ISO system is nothing to do with the EU. ISO is a global federation of national standards organisations representing over 150 nations.
BSI believes that the adoption of the ISO system has the potential to bring considerable benefits to industry.
5 To comply with BS 8888, you have to introduce new procedures for document management, system security, handling of digital data etc, as well as changes to engineering drawing practice.
Not necessarily. If your engineering drawings are ‘drawn in accordance with BS 8888’, this claim only refers to the drawing itself.
If you wish to comply with the requirements of BS 8888 for document management etc, that is a separate issue.
6 If we change to BS 8888, we will have to go through all our drawing archives to bring them all up to date with the new standard.
Wrong. An explicit requirement of BS 8888 is that drawings are interpreted according to the standards that were in force at their ‘acceptance date’. If a drawing was released on 15th June 1957, correct to the version of BS 308 current on 15th June 1957, it should be interpreted according to the appropriate version of BS 308 and it does not have to be up-dated.
The BS 308 Story
In 1927, the British Standards Institute (BSI) introduced a national standard for engineering drawing known as BS 308. This standard was developed and expanded over the next 73 odd years, until it was finally withdrawn in 2000.
While BSI maintained and developed BS 308 over the years, as a key member of the International Organization for Standardization (ISO), BSI were playing an important part in the development of international (ISO) standards for technical specifications in parallel with this. Many of these international standards were subsequently incorporated as British Standards. By 2000, a full set of British standards for engineering drawing included around 30 ISO standards as well as all three parts of BS 308.
The maintenance of two parallel sets of standards, which duplicated each other in many ways, was hard to justify. It led to confusion among users about which set of standards they should be working to, and about the differences between them. In the second half of 2000, BS 308 was withdrawn, and BSI took the logical and inevitable step of adopting ISO standards in full for engineering documentation. Due to the high level of compatibility between the two sets of standards, this was a relatively smooth transition, but there have been some changes to working practices.
About BS 8888
In order to make the transition from British to ISO standards as painless as possible, BSI produced a new standard to provide a kind of gateway or interface between the user and the ISO system.
In 1927, the British Standards Institute (BSI) introduced a national standard for engineering drawing known as BS 308. This standard was developed and expanded over the next 73 odd years, until it was finally withdrawn in 2000.
While BSI maintained and developed BS 308 over the years, as a key member of the International Organization for Standardization (ISO), BSI were playing an important part in the development of international (ISO) standards for technical specifications in parallel with this. Many of these international standards were subsequently incorporated as British Standards. By 2000, a full set of British standards for engineering drawing included around 30 ISO standards as well as all three parts of BS 308.
The maintenance of two parallel sets of standards, which duplicated each other in many ways, was hard to justify. It led to confusion among users about which set of standards they should be working to, and about the differences between them. In the second half of 2000, BS 308 was withdrawn, and BSI took the logical and inevitable step of adopting ISO standards in full for engineering documentation. Due to the high level of compatibility between the two sets of standards, this was a relatively smooth transition, but there have been some changes to working practices.
About BS 8888
In order to make the transition from British to ISO standards as painless as possible, BSI produced a new standard to provide a kind of gateway or interface between the user and the ISO system.
This new standard performed three fundamental tasks:
- firstly, it provided a unifying identity for all those ISO standards which relate to Technical Product Specification;
- secondly, it provided an index showing which ISO standards dealt with which aspect of technical product specification (TPS);
- finally, it provided BSI with a platform for providing additional explanation and commentary where this was felt to be useful
BSI currently plan to publish revised versions of this standard approximately every two years, and the next version is due in October 2010. Altogether BS 8888 refers 'normatively' to over 130 ISO and EN ISO standards (i.e. these must be complied with if documents claim to comply with BS 8888), and informatively to around another 30 (i.e. for information but not mandatory).
BS 8888 - What does it mean in practice?
There are a number of changes and developments to working practices, many of which would have been introduced through a new version of BS 308 if it had not been withdrawn. Principle changes can be summarized as follows:-:
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The standards cover more than just engineering drawing, they cover all aspects of technical product specification, including, for instance, the use of 3D CAD models to define component geometry.
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Requirements for drawing borders and title blocks have been formalised and changed in some areas
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The comma is to be used as a decimal marker instead of the full stop.
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Size tolerances should be interpreted in accordance with ISO 8015 (i.e. according to the Principle of Independency)*. This differs to their interpretation under BS 308, or the American ASME Y14.5 standard.
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The use of dimensions, size tolerances, datums, geometrical tolerances, edge and surface specifications have been formalised and systematised under the heading of Geometrical Product Specification (GPS). This approach is the driving philosophy behind BS 8888 and the current generation of ISO standards, and industry will potentially benefit enormously from its adoption.
A number of 'fundamental principles' which were previously listed in the front of the document, and which were largely incomprehensible to readers, have been simplified, relocated, and in some cases removed altogether.
The rules governing the use of implied dimensions have been spelt out. Additional references to ISO standards have been added to reflect the publication of new ISO standards in some areas, and to correct omissions in others.
The most important change in the 2008 revision is that there is no longer a requirement to indicate whether specifications have been toleranced in accordance with the Principle of Independency or the Principle of Dependency. BS 8888 now simply follows the ISO practice of working to the Principle of Independency as defined in ISO 8015.
The fact that there is no longer an option to choose to work to the Principle of Dependency has not led to any technical deficiency in the system, as the Envelope Requirement may still be invoked when required. The 'Dependency' option was also in contradiction to the fundamental purpose of BS8888, which was to act as a gateway into the ISO system. The benefits of the new approach are:
- BS 8888 is now fully consistent with the ISO system.
- industry is no longer being required to
choose between two different tolerancing principles, when neither is
well understood
- there is no longer a risk of two different 'dialects' of BS 8888
developing
- drawings are now to be marked 'TOLERANCING ISO 8015', which is
an internationally recognized means of indicating which standards
govern their interpretation
*The Principle
of Dependency and the Principle of Independency describe the two fundamental ways
in which size tolerances can be interpreted. It is important to know
which principle you are working to from both a design and an inspection point of view.
Under the Principle of Dependency, the envelope requirement is applied by default to all features-of-size. Under the Principle of Independency, it has to be specified.
Both approaches were acceptable under previous revisions of BS 8888, as they were under BS 308.
The Principle of Dependency was the default under BS 308, early revisions of BS 8888, and remains the default for the American ASME Y14.5 standard. However, the rules governing its operation have never been fully defined in any BS or ISO standard, so there were always potentially some difficulties with it.
The Principle of Independency is defined in ISO 8015, which also defines the envelope requirement. ISO 8015 is now normatively referenced from BS 8888. A consequence of this is that all drawings compliant with BS 8888 must now also state 'TOLERANCING ISO 8015', which is an internationally recognized indication.
ISO 8015 is due to be replaced by a new standard, ISO 14405, at some point. This is likely to happen in 2010. Watch this space - we will post further details about ISO 14405 when they become available.
Under the Principle of Dependency, the envelope requirement is applied by default to all features-of-size. Under the Principle of Independency, it has to be specified.
Both approaches were acceptable under previous revisions of BS 8888, as they were under BS 308.
The Principle of Dependency was the default under BS 308, early revisions of BS 8888, and remains the default for the American ASME Y14.5 standard. However, the rules governing its operation have never been fully defined in any BS or ISO standard, so there were always potentially some difficulties with it.
The Principle of Independency is defined in ISO 8015, which also defines the envelope requirement. ISO 8015 is now normatively referenced from BS 8888. A consequence of this is that all drawings compliant with BS 8888 must now also state 'TOLERANCING ISO 8015', which is an internationally recognized indication.
ISO 8015 is due to be replaced by a new standard, ISO 14405, at some point. This is likely to happen in 2010. Watch this space - we will post further details about ISO 14405 when they become available.
BS 8888 formats:
BS 8888:2008 can be purchased in paper format, either on its own or in 'kits' (where it is supplied with sub-sets of the referenced ISO standards), or as a CD-ROM.
The CD ROM includes all the ISO standards that are referenced from within BS 8888 in PDF format. Each reference within BS 8888 is hyper-linked to the appropriate ISO standard to ease navigation through the system.
BSI have also been working on the development of an 'on-line' version of the standard. When available, this will work in a similar way to the CD ROM version, with links to all the referenced ISO standards, but also with additional guidance and explanation. This web-based version has been delayed, and is now likely to appear later in 2009.
All engineering drawing and geometrical tolerancing training offered by Iain Macleod Associates takes full account of the changes to BS 8888 (and is now fully updated to take account of changes in BS 8888:2008), and conforms with current ISO standards.
BS 8888:2008 can be purchased in paper format, either on its own or in 'kits' (where it is supplied with sub-sets of the referenced ISO standards), or as a CD-ROM.
The CD ROM includes all the ISO standards that are referenced from within BS 8888 in PDF format. Each reference within BS 8888 is hyper-linked to the appropriate ISO standard to ease navigation through the system.
BSI have also been working on the development of an 'on-line' version of the standard. When available, this will work in a similar way to the CD ROM version, with links to all the referenced ISO standards, but also with additional guidance and explanation. This web-based version has been delayed, and is now likely to appear later in 2009.
All engineering drawing and geometrical tolerancing training offered by Iain Macleod Associates takes full account of the changes to BS 8888 (and is now fully updated to take account of changes in BS 8888:2008), and conforms with current ISO standards.