Datum example registration#
Land registration user guides and resources.If those dowels are designed to "expand" functionally as an interference fit such as roll-pins specify them RFS if not MMC.Defining ‘where’ - through maps, surveys and titles. designate them as a 2X pattern secondary datum feature (constraining the remaining three degrees-of-freedom simultaneously) rather than making one secondary and the other tertiary. if a pair of solid dowels are have identical clearance allowances. My advice is to select the datum features characterize the function directly "if possible" to minimize the need for alternate stack predictions that may fail to define function. then the design specifications will fail to predict the function. If your design does not account for it via "trustworthy" alternate stack paths. the ultimate variation that is inherent in assembly and function is subject to that variation. I agree with your apprehension in selecting "features-of-size" as datum features considering that they complicate the measurement and predictive design processes. Sometimes, you have to do it, but usually, there is a better way. If I do it, the feature of size usually is accurate enough that I do not care about MMC. I avoid using features of size as datums. RE: MMC on datums drawoh (Mechanical) 26 Jan 09 13:57 This is a whole other discussion - perhaps another time. So should a nonconformance on a datum feature mean that all controls referencing it get labeled as a fail? I would say that the actual values of any controls referencing the nonconforming datum feature are "suspect".Īnother part of the question relates to whether you look at the considered feature(s) and datum feature(s) as separate entities, or as an overall system. If the datum holes fail a tolerance that is there for cosmetic reasons (such as cylindricity or perpendicularity), this may have no effect on the position tolerance on the considered feature.
Datum example full#
If they are undersize, the considered feature control isn't given its full "liberty" as Paul mentioned. If they are oversize, this is more forgiving. One example would be the size of the holes in the pattern that is referenced as a datum feature at MMC.
The problem is that a nonconformance in a datum feature might result in a more forgiving condition that helps the considered feature tolerance to pass.
It may be useful to know the relationship between the considered feature pattern and the datum feature(s). I wouldn't necessarily stop there though. I agree with drawoh that if the datum feature fails its size tolerance then the part fails.
Any combination is legal and one could dream up applications for each. As drawoh said, a hole pattern can have a position tolerance at MMC and be referenced as a datum feature at MMC (or RFS).
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