Cucchi pointed out that print tolerances are not concerned with what metrology method one uses to meet them but only with the requirements of the part. Print tolerance refers to the amount of dimension deviation allowable on a part as defined by the customer’s blueprints or specifications. A student manipulates a CMM controller to move a touch probe at the Center for Precision Metrology at The University of North Carolina at Charlotte. Its resolution wouldn’t really reflect its accuracy. You’d trust a hardware store yardstick to measure and cut a fence post, but not to check a precision aerospace or medical component-and that’s so even if the yardstick had hashmarks to 1 µm apart. Resolution is simply how fine the measuring instrument is set to read out-whether to tenths, hundreds, thousands or whatever.” It defines the limits of the errors made when the instrument is used in normal operating conditions. “In industrial instrumentation, accuracy is the measurement tolerance of the instrument. Starrett Co., Athol, Mass., warned not to confuse accuracy with resolution. For example, Tim Cucchi, product manager, precision hand tools at the L.S. If you’re a novice, first make sure you understand some basic terms. So, what’s a manufacturer to do? Accuracy and Print Tolerance They are an important resource-but, as we’ll see, they alone can’t tell you what method is best for a particular task. The American Society of Mechanical Engineers (ASME), New York, and the International Organization for Standardization (ISO), Geneva, Switzerland, for example, offer standards to communicate accuracy levels for various metrology methods. To find the most accurate way to measure your part, you need to understand the uncertainty inherent in any given measurement system.įortunately, there are national and international organizations that develop standards to help codify measurement uncertainty levels.
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So, what’s the best method for making sure the part is within spec? The question is not only how to measure the part-a CMM touch probe, or a laser scanner?-but also how to evaluate measurement quality.
It’s an old challenge: You’re a manufacturer whose customer needs you to assure that the part you’ve contracted to make for them will be held to specified tolerances. The video-based Starrett AV350 CNC vision system features 12:1 zoom optics. A look at the evolving meaning of measurement accuracy, and how published accuracy standards can help you evaluate metrology devices-but only to a point.