协调计量中的一致性，可追溯性和测量不确定性

客户 - 供应商分歧对产品规范的符合性通常是昂贵的贡献。在确定与几何尺寸和公差（GD＆T）规格确定符合时，近年来识别测量不确定性。本文将重点关注这一领域的一些重要发展。

GD＆T要求需要评估零件特征特征的复杂3D相互关系。为了满足中等大小零件的这一挑战，坐标测量机（CMM）是一种流行的工具。强大的相关数据分析软件允许将零件表面上的原始采样点转换为可报告的GD＆T参数，例如发动机块中的汽缸的并行性。可以在CMM报告的价值观中放置多少置信度？

美国国家标准与技术研究院y (NIST) documents make the importance of measurement uncertainty clear: "A measurement result is complete only when accompanied by a quantitative statement of its uncertainty." Ordinarily, uncertainty is expressed as a range of values within which, at a specified level of confidence, the true value of the quantity measured is believed to lie. In CMM-based measurements, a task-specific uncertainty for each and every GD&T parameter is necessary. Required are statements like, "The uncertainty of the diameter of this particular 75.00-mm diameter hole (produced under specific manufacturing conditions) is ± 0.05 mm at 95 percent confidence (when determined with this particular measurement system, using this particular measuring protocol, under this particular set of environmental conditions). As suggested by the parenthetic phrases in the last sentence, many factors will contribute to task-specific measurement uncertainties. The same power and versatility that make the CMM attractive as a measuring device also make assessing these measurement uncertainties a formidable task. First, consider two important roles that measurement uncertainty plays in CMM applications. The first is in measurement traceability; the second is in conformance decisions.

The ISO defines traceability as "the property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties." The prominent role of uncertainty in completing the traceability chain is evident. Simply having a CMM calibrated does not make its measurement results traceable. If CMM-derived GD&T measurements are traceable, defensible task-specific uncertainty evaluations should be included in a report (see Figure 1).

最近的标准（ISO 14253-1和ASME B89.7.3.1）进一步强调了不确定性评估的重要性，该标准为决策规则制定制定了指导，以管理商务条款的接受或拒绝。图2表明，如果决定规则要求通过测量不确定性降低规范区以确定可接受值的区域，则有明显的经济处罚以更大的不确定性。

许多因素可以在基于CMM的GD＆T参数的测量中有助于不确定性。这些影响的数量和他们的互动的复杂性已经被专家认可，因为在对CMMS的测量不确定性的测量不确定的方法中，他们的应用是对CMMS的不确定性的应用。ISO 15530系列提供了几种替代方法。

When considering versatility, economy, robustness and comprehensive character, the winner is computer simulation. Here, the part measurement is simulated repeatedly under various conditions. The ranges of factor variability such as CMM geometry errors, sensor errors, environmental conditions, and so on are employed in a mathematical model that allows their influences to be reflected in the resulting GD&T parameter ranges—precisely consonant with the measurement uncertainty concept.

史蒂文菲利普斯博士等NIST研究人员认为，他的同事构想了称为“通过约束模拟”（SBC）的统计方法。该方法识别出通常情况下，CMM用户不会有关于错误的完整和详细信息，例如，在他的CMM中。实际上，他可能只有CMM制造商的指示以及给定机器模型的最大允许误差。

CMM性能数据，如ASME B89.4.1指定的数据可以用作基于SBC的软件的输入，以及关于测量系统（例如，传感器性能测试数据），环境（例如机器的热条件）的其他信息部件的部分）和测量协议（例如，表面采样策略）。根据这些数据，为所需的GD和T参数生成不确定性估计。已经进行了SBC技术的商业实施，使CMM用户访问评估他最终报告的测量数据的不确定性。

Because the software evaluates measurement uncertainties by simulation, it is possible to compare the behavior of different CMMs (operating under otherwise identical conditions) to assess their relative abilities to measure any given part, or set of parts, with requisite precision. This is useful when choosing a new CMM for purchase or the right machine for a given job. The user can even see how small his measurement uncertainties might be if he had a perfect CMM. From this, one can recognize how much uncertainty is contributed by the other factors mentioned above. Similar experiments based on a perfect sensor, for example, can identify the weak link in a measuring system, allowing the user to efficiently focus resources on improving measurement precision. Finally, simulation software of this sort is an effective means of training CMM operators and programmers in good measurement practice without tying up valuable CMM time in the process.

- Metrosage LLC.