ISO 1101 amendment 1

ISO 1101 is the principle ISO standard for geometrical tolerancing. It was last revised in 2004, just five years ago, but there are already a number of amendments queuing up to be incorporated into it.

The first of these, Amendment 1, is concerned with 3D annotation, and is going through the final stages of approval prior to publication. It is likely to come into effect some time in the next 12 months.

What is '3D annotation'?

It is established practice in most industries to use a 3D CAD model as an essential part of a component specification. The 3D model represents the 'nominal' geometry of the part, and as many manufacturing operations, and some inspection operations, can be driven directly from this 3D model, there is no longer a necessity to duplicate this data with dimensions on a 2D engineering drawing.

This has paved the way for the 'minimum content drawing' approach that is now widespread. The 2D drawing is used to convey information about datums, tolerances, surface finish and sometimes inspection requirements, but no longer needs to define the fundamental geometry of the part.

The next stage in the evolution of this process will be to apply the information about datums, tolerances etc directly to the 3D model, eliminating the requirement for a 2D drawing at all. This is what is meant by '3D annotation'.

Eliminating 2D drawings does not really become a practical proposition until everyone involved in the specification, manufacture and inspection of the component can work fully in 3D. For most of industry, that prospect is a distant one, so 2D drawings are going to be around for many year yet.

However, CAD software is increasingly providing the facilities to apply all specification requirements directly to the 3D model, and some manufacturing organisations are starting to work in this way.

The CAD software is currently a little ahead of the standards in this respect. While most 3D annotation requirements can be met quite happily by applying the 2D annotation elements in a 3D environment, there are some situations where this is not sufficient. Some specification elements depend on the positioning of a geometrical tolerance within a drawing view to convey part of the requirement. Working in 3D, we no longer have drawing views to position or orientate the geometrical tolerance, so other means have to be found to convey that information.

Amendment 1 provides some illustrations to indicate how a geomtrical tolerance requirement can be indicated in a 3D specification as well as in a 2D specification. More importantly, the amendment introduces some new annotation elements to enable all geometrical tolerance requirements to be fully specified in 3D. Symbols have been introduces to define 'intersection planes' and 'orientation planes', which can be used to define the location or orientation of a tolerance requirement relative to the 3D model.

The amendment also ties up one or two other loose ends. It will indicate how the 'all around' symbol can be used in 3D (although in my view there is no need to use it in 3D at all). There is also a new symbol to help make it clear when a derived feature, rather than a surface, is being used as a datum feature.

Finally, there will now be the facility to define offset tolerance zones. This last was something of a disappointment for those of us who argued in favour of adopting the American (Y14.5) symbology in the interests of simplicity, clarity and universal harmony. The ISO approach will be different from the Y14.5 approach, but at least we managed to avoid using the same symbol as the Americans', but with a different meaning (a very real possibility at one stage). Sometimes, these tiny, common-sense achievements rank as great victories when developing international standards.