In our recent series, we’ve devoted a blog to explaining and understanding each of the key terms used in measurement technology (terms like true value, accuracy, precision, etc.) In this final instalment, we’ll investigate the term “tolerance”.
Unlikely a way as it sounds to start a blog about a key aspect of measurement systems, the Glastonbury Festival is playing in the background as I write and as coincidence would have it, the Manic Street Preachers are on stage playing their best-known song: “If you tolerate this, then your children will be next”. (If you want to, you can check out the performance here). The lyrics provide one way of looking at tolerance (although for our purposes probably not the right one. Not exactly, anyway).
Still, the song title does underline a point we’ve made throughout this series. Almost all the terms we use in measurement technology are also used in everyday life with proximate, though not identical meanings. So, for those working in the field, remember that there’s a need for caution. Careless language has consequences.
On to tolerance, then. In measurement technology it doesn’t mean putting up with other people’s opinions or behaviors or even risking your children’s futures with a smile plastered across your face as something unacceptable happens. In contrast, it means:
What is the size of the deviation from an expected value, and is that degree of deviation acceptable?
Nothing is perfect but tolerance is the answer
We already know from previous blogs that no measurement is exactly error free or perfect. That being the case, a measurement must simply be “good enough”. What does that mean? We can think of “good enough” as what is represented by the term tolerance. Something that’s good enough can be said to be “within tolerance”.
For an example, let’s refer back to the idea of any two parts of a mechanical system which, after they’ve been manufactured, have to fit together; for instance, a hub and an axle (though it could be any other two components in any machine).
In manufacturing, the diameter of each of those parts is specified. Let’s say a specification of 1.00mm is required. In measurement technology this would be stated as 1.00mm ±0.01. That means that the actual diameter (the True Value) of the manufactured part will be acceptable between 0.99 and 1.01mm. Put another way, manufactured (and then measured) within that range the two components will fit together as required. So, another way of looking at tolerance is as the acceptable margin for error.
Tolerance in practice
How do we get to tolerance when we’re manufacturing something? How we do we achieve an outcome that is within tolerance?
Clearly, you must measure the part being manufactured with a measurement system that’s accurate enough to determine whether the part’s specified diameter falls within the required tolerance range. Let’s use an example to explain how the process works.
Say that you wanted to measure the diameter of a drawing die and were using a Conoptica CU11 measurement system to do so. If the expected (specified) diameter is 1.00 m, the appropriate objective to use in measuring would be what’s called 1X, which has an accuracy of ±0.0012 mm.
If you measured a die and it proved to be 1.0243 mm, then it would be outside the tolerance and could not be used. If you then measured a second die and found it to be 0.9901 mm, it would be within the tolerance and usable. You need, and use, a measurement system to determine whether tolerance is met.
But what about a more marginal outcome? Suppose a die measured 0.99 mm. Would that be in tolerance, or outside it? In this case, you would have a situation where the Conoptica System’s own accuracy tells us that in fact the die may be between 0.9888 mm and 0.9912 mm (remember, no measurement is perfect, even those yielded by measurement systems). In this case, the die could be outside the tolerance required due to the accuracy of the measurement system rather than the die itself and should therefore not be used.
When you’re manufacturing parts that need to fit together, you also have to take into account that they different components will have different tolerances. Using the example of a hub and axle (which is related to construction, rather than the measurement system itself), the hub will have a narrower tolerance and the axle a larger tolerance, necessary so they can be joined. Tolerance in such cases is not absolute.
To summarize, we can think of tolerance in measurement as the amount of variation that is allowable within a measurement. You can see it as an amount of variation allowable that will not have a significant impact on the specified outcome. Only if the amount of variance goes beyond the tolerance will the inaccuracy present you with a problem. Therefore, determining what amount of tolerance is acceptable without having a negative outcome is a key factor in manufacturing processes.
You should also bear in mind that tolerance will change when using different measurement systems, as well as between varying fields utilizing measurement and, indeed, in different physical conditions. For instance, the tolerance required in construction will be slightly different from that necessary in engineering. Knowing the tolerance of a measurement tool will enable you to account for all possible variance statistically.
It’s also worth noting that many engineers tend to confuse tolerance with another term, accuracy, even though they are not the same thing. Keep in mind that tolerance is the acceptable distance from an objects true value; thus, you choose the acceptable tolerance of an object or part; the measurement instrument decides its accuracy. This means that while you can demand the degree of tolerance from a manufacturer, when doing so accuracy must be taken into account.
Lastly, a reminder: knowing about, understanding, and using the correct terms is always important and particularly so in the world of measurement. At Conoptica, we manufacture equipment for measuring very small tools and parts with high accuracy and repeatability, so for us this statement is obvious. We’re hoping that after reading these pages you will have come to the same conclusion.
Conoptica is a leading source of high accuracy measurement systems and offers a variety of solutions to meet the requirements described in this blog series. The company enables accurate measurement, so that you can track consistency and take appropriate actions – avoiding excessive quality deterioration and enabling better management of your stocks.
Conoptica’s measurement systems will give an average measurement expected to be within ±1.2 µm (for the 1X objective) from the True Value. That expectation is based on reference to the PTB measurements, which have a much smaller accuracy value though even those will have an error compared to the true value.
Conoptica is the market leader for measurement equipment in the wire & cable industry and has been providing high tech camera-based measurement solutions since 1993. We make sure that the metal working industry has access to key quantitative data about their products and tools.