En Iso 14253-1

[Suyay Escarsega]

Thenewly released standard ISO 14253-1 for determining compliance ornon-compliance with a specification breaks with traditions in most industriesfor how these decisions are made by requiring that measurement uncertainty istaken into account. But is this an improvement or just an unnecessary extracomplication? This paper looks at how tolerances and decision rules interact andhow you may be able to expand your tolerances and save money while retaining thesame functionality, if you implement these new decision rules.

ISO 14253-1 contains decision rules that requires the tolerances to bereduced by the measuring uncertainty when measurements are made to proveconformance to a specification and expanded by the measuring uncertainty whenattempting to prove non-conformance to a specification.

While it is indisputable that these rules provide rigor to the process offormally proving conformance or non-conformance by requiring proof beyond areasonable doubt, there are other issues coming into play when these rules areapplied, such as:

We all know the legal phrase "prove beyond a reasonable doubt."What ISO 14253-1 does is to lay down a framework for how this principle can beintroduced in commercial transactions where workpieces or measuring equipmentare evaluated against a specification.

If we have a workpiece that is exactly on the limit of the tolerance and wehave no a priori knowledge of the composition of our uncertainty, we wouldexpect to have a 50% probability of finding the workpiece to be within thetolerance and have a 50% probability of finding it out of tolerance.

If the true value of the workpiece moves inside the tolerance, theprobability of measuring it to be outside the tolerance gets smaller and if thetrue value of the workpiece moves outside the tolerance, the probability offinding it to be inside the tolerance gets smaller.

But as long as we are no further away from the tolerance limit than ourmeasuring uncertainty, there is a chance that we may misclassify the workpiece.In a commercial transaction this is primarily a problem if the manufacturer of aworkpiece measures it to be inside the tolerance and the user of the workpiecesubsequently measures it to be outside the tolerance.

ISO 14253-1 states that in order to prove that a workpiece or measuringequipment is conforming to a tolerance, the manufacturer has to measure it to bewithin that tolerance by more than his measuring uncertainty.

On the other hand for the user to prove that a workpiece or measuringequipment is not conforming to a tolerance, he has to measure it to be outsidethat tolerance by more than his measuring uncertainty.

These are the rules of ISO 14253-1. While they are very simple, they are alsocontroversial, as they put definition to one of commerces gray areas that hasthe potential for a large economical impact.

The impact on metrology is more a question of accounting than it is aquestion of technology. Before this standard was available there was noscientifically substantiated guidance available, as to how one was to choose theright level of uncertainty for measuring a given tolerance. The only guidanceavailable was rules of thumb, such as a 4:1 or 10:1 relationship between thetolerance and the uncertainty.

It has always been the stigma of metrology that it is a cost without abenefit. The less resources you could spend on measurement and still meetarbitrary industry requirements, the better off you were. There was no way toshow how better metrology might lead to a better overall product or more costeffective production, so good metrology was cheap metrology.

Without a tool to account for the added value of improved metrology, therewas no way to evaluate the cost of metrology relative to the cost ofmanufacturing on an equal footing. While it might have been intuitively clearthat lower uncertainty was better, there was no way to put a value on betterand there was no economical incentive to choose a measuring process better thanthe absolute minimum prescribed by the rule of thumb prevalent in a particularindustry.

With the decision rules in ISO 14253-1 it is very easy to put a dollar figureon improved metrology. When the uncertainty goes from 20% of a tolerance to 10%of the tolerance, the amount left for manufacturing goes from 60% to 80% of thetolerance. Increasing the available tolerance by 33% generally allows for lowercost manufacturing. The reduction in manufacturing cost can then be compared tothe increase in measuring cost and an optimum can be found. This is the firsttime there is a tool available for applying logic to the business decision ofwhat the optimal measuring cost is, that does not come up with the result zero,which is intuitively known to be wrong.

It is true that if the decision rules of ISO 14253-1 are applied tospecifications that were developed using the traditional acceptance criteria,the results will be a de facto reduction of all tolerances. The real benefit ofISO 14253-1 is only realized, when tolerances are developed with these criteriain mind. In order to gain this benefit, it is important to stipulate incontracts referring to the standard, that the manufacturer has to proveconformance of all product shipped.

The tolerances can be expanded in most industries since there is normally anallowance for a certain amount of uncertainty built into the tolerances andspecifications. Especially in older designs, this allowance may be larger thanwhat is required with newer measuring equipment. Applying ISO 14253-1 will allowthe new zone of conformance to be larger than the original tolerance. This meanswe can save money by expanding the tolerances while still producing functioningparts.

Table 1 gives a number of scenarios from an implementation of ISO 14253-1. Inscenario 1 product is accepted or rejected strictly using the tolerance limitswithout regard to the measuring uncertainty. When ISO 14253-1 is implemented inscenario 2, the conformance zone is reduced by the measuring uncertainty at eachend, effectively reducing it by twice the uncertainty.

With ISO 14253-1 implemented, it is possible to go back and re-consider thetolerance. It may have been developed at a time when the uncertainty was higherthan it is today, or there may have been included a fudge-factor, since thedesigner did not know how the product was going to be measured. In scenario 3 itis assumed that one or both had taken place. Therefore the tolerance can beadjusted so the new conformance zone is larger than the original one.

Finally it is determined that an improvement in measuring uncertainty can beachieved for an attractive cost compared to the increase in the conformancezone. In scenario 4 this improvement is implemented.

A major expense in manufacturing is the resolution of disputes over suspectproduct. These disputes are very costly, because they may cause a productionline to shut down from part shortage; they may cause significant amounts to bespent on airfreight to keep production lines going; they may cause significantcost for re-measurement, maybe even by third parties, and they may requiresignificant meeting time for a large team of engineers and executives toresolve. A typical per incidence cost has been estimated to be $ 200k at a majorengine manufacturer2.

The implementation of ISO 14253-1 reduces product conformance disputesdramatically, when applied correctly. As stated above, it is important that theburden be put on the producer contractually to prove that the product he shipsconforms to the specification.

With this burden on the producer, the consumer should not receive any productthat is measured to be outside the conformance zone. If the producer hasestimated his measuring uncertainty correctly, no product should be received,that is outside the tolerance.

For the consumer subsequently to prove that the product is non-conforming, hehas to measure it to be outside the tolerance by more than his measuringuncertainty. This is of course impossible, if both parties have estimated theirrespective uncertainty correctly. While there is no guarantee that they have,ISO 14253-33 (currently in a draft stage) provides a resolution process for thatscenario, so such a conflict can be resolved in a scientifically sound manner.

One of the classic arguments for given relationships between the toleranceand the uncertainty is that they control the relationship between consumer risk(acceptance of bad parts) and supplier risk (rejection of good parts).Unfortunately, this is not the case.

In order to calculate these risks, it is necessary to make assumptions aboutthe distribution of the produced parts. A typical assumption is that the partsfollow a Normal distribution, centered in the tolerance and having a standarddeviation equal to 25% of the tolerance, such that 95 % of the produced partsare good.

As it turns out, the distribution that is assumed for the produced parts hasa much larger influence on the consumer and supplier risk, than the uncertaintyof the measurement. Therefore it is a very questionable practice to use thesecalculations as the basis for the choice of uncertainty ratio.

set of operations that establish, under specified conditions, therelationship between values of quantities indicated by a measuring instrument ormeasuring system, or values represented by a material measure or a referencematerial, and the corresponding values realized by standards. (VIM4 6.11)

There are two basic approaches to calibration. The first approach expressesthe result of the calibration as a value and an uncertainty. For example thecalibration of a 10 mm gage block may be expressed in the calibrationcertificate as a measured value, e.g. 10.0001 mm and a correspondinguncertainty, e.g. 0.05 μm.

This is a very clean format. The calibratedvalue can be used directly in measurements incorporating the gage block and theuncertainty can be used in the uncertainty budget for those measurements. Theonly considerations are whether the resulting uncertainty for this subsequentmeasurement is adequate for its purpose and whether the uncertainty of thecalibration is a major contributor to this uncertainty.

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