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We have been asked many times what the difference between "measuring a part" and "verifying a part" is and when to use each.
There may be information that R&D or Engineering wants from the process you are using. In that case, to give them data you would need to measure the parts in order to provide continuous data that gives the most information about the process capabilities. Other times, you just want to know if the features on the part fall in the range of tolerance you are given. This can be accomplished through gaging, often referred to as go/no-go gaging. You can use a functional gage to help ensure the part will assemble and all criteria for the features the gage checks are within the tolerance zones allowed by the design.
Sometimes a gage may not be practical, especially when smaller numbers of parts are produced. In those cases, you may use a CMM, height gage, micrometers, etc. to measure the part and make sure that those measurements are within the acceptable range. Note that functional gaging (fixed limit gage) also requires features to be defined at MMC concept in order to define a constant size boundary for the gage.
How do you know when to verify and when to measure? This is more difficult to answer. If you are gathering data, you must measure. If you want a higher confidence level that the assembly will go together and perform its intended function you may wish to gage the feature. Measuring only checks a limited number of surface points, while gaging checks the entire surface. The risk of passing bad parts versus the cost and work in ensuring your method is accurate should be taken into consideration. Remember that gages have tolerance and there are various gaging tolerancing policies that a company may choose for their product. Typical recommendations are using absolute tolerancing policy for GO gages and the practical absolute tolerancing policy for functional gages.
If you need help with inspecting Geometric Tolerances, let us know. Because remember, here at Tec-Ease, GD&T Rules!