Tolerancing from Standards? Use Tolcap for a fast route to the baseline.

6th September 2017

There are a lot of standards out there to guide you when you put dimensions and tolerances on drawings. Do you have to know them all? Which ones are relevant and useful? How easy are they to apply and how far do they get you? Is there a quicker way to get to the right tolerance?

Standards for tolerances may be divided into three types: those that tell you how to construct a set of dimensions and tolerances appropriately, those that tell you the tolerance you need to specify in order to achieve a particular fit or function, and those that specify tolerances that are recommended for particular materials and processes.

In the first set we have standards explaining how to set out geometric tolerances and what they mean. There include BS 8888, ISO 2768 and ASME Y14.5-2009 (from the American Society of Mechanical Engineers). Then standards such as ISO 14405-1 treat Geometrical Product Specifications. This is about how dimensions on drawings must show the proper relationships and alignment of features. BS EN ISO 2692 relates to both the above and addresses maximum material requirement.

In the second group there is ANSI B4.1, Limits & Fits - what is necessary on shafts and holes for running, sliding or interference fits. ANSI B4.2 is the metric version ISO 286 appears to be the ISO version and has a part 2 that treats Geometrical Product Specifications, i.e. misalignments in tolerancing holes & shafts. For geometric tolerances there is ISO 286 (which replaces ISO 1829). BS EN 20286 is an equivalent British Standard.

The third group includes such standards as DIN 16901, now replaced by DIN 16742, for plastics, particularly injection mouldings, DIN 1688-4 for pressure die castings and ISO 8062 for iron castings (sand through to investment castings), and ISO 2768 for machining and sheet metal processes.

These are just examples: there are many standards out there in each of these categories. So which are really worth deep study and understanding? Which are useful references? Are there alternatives to having a shelf full (or these days a pdf folder) of standards?

It is clear we need to know the content and workings of the first group. Drawings are the vehicle of communication between supplier and customer, the language of engineering - and as with a language, you can get by, but if you don’t know the grammar and syntax you may occasionally be misunderstood! So we should know these standards - and much of their content will have been built into the training we needed to work as designers.

When it comes to the second group, the tables tell you what you need to achieve. The existence of such standards means these limits are basically achievable of course, but can you keep within them consistently with the material you are using and the geometry of the part you are making using the manufacturing process you have assumed - and costed for? Can you get that diameter sufficiently accurately with a drill, or do you need to drill then ream? Here Tolcap can help you answer this basic question - and tells you the process capability to expect. You want to know this anyway, and you need to if there is a tolerance stack-up involved.

Standards in the the third group set out to advise you what is achievable, some of them qualifying this with tables for precision engineering rather than general requirements. But what does that mean? Look up the standard by all means, but use Tolcap to get a numeric estimate of the process capability, so you have an idea whether the tolerance is readily achievable or whether you are pushing into special control, which must attract more costs in manufacture.
The component parts of most products require a range of manufacturing processes, and Tolcap covers the majority of those commonly used and quite a few of the more esoteric ones. The Pareto principle or 80/20 rule generally applies, and designers frequently find themselves designing with one or two processes again and again. If you are always designing plastic injection mouldings, for example, it obviously pays you to study the appropriate standards - but Tolcap provides a cross-check on your findings in a particular case, and we note it takes account of some factors the standards ignore. But what about an unfamiliar process you need? Is it worth struggling through a standard - if you can find one, of course? Even if you can, perhaps a better strategy is to find a supplier with the expertise, get an idea of the fundamentals of the process** and use Tolcap to start your tolerancing and get discussions going on a data driven basis.

In summary, Tolcap does provide you with a process capability estimate when you take a tolerance requirement from standards, and can give you a process capability estimate for a tolerance derived from standards for particular materials and processes or indeed where you are not familiar with the standard or there is no standard at all.

Written by:

Richard Batchelor MA, MBA, CEng, FIEE

Richard Batchelor MA, MBA, CEng, FIEE

Richard is a founding member of the Capra Technology team.

** See for example K.G. Swift and J.D.Booker ‘Process Selection from Design to Manufacture’ [Butterworth Heinemann 2003] ISBN 0 7506 5437 6