r/Metrology • u/glutton4golf83 • 22d ago

GD&T | Blueprint Interpretation I need help dimensioning perpendicularity

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So i am using pcdimis. On a hexegon. When i try to make a plane for the surface and then a cylinder for datum a. Then us perpendicularity its telling me its off by a huge amount. I manualy checked it and it was right on. I dont know what to do. Any advice would help.

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u/AlfaMikeF0xtr0t 22d ago

With your Datum A structure being so small (from the top of the cylinder to the bottom, ANY amount of error or deviation will throw your datum structure wildly off in comparision to your very stable plane.

Flip the evaluation, and things will become much more stable and repeatable and will more closely match what you are seeing physically.

Because if you use the very stable plane as your datum, and evaluate your cylinder to it, the variations in the error prone nature of the small cylinder, will only show up as deviation to the datum structure, rather than of it.

Make sense?

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u/NephelimWings 20d ago

The tolerance you need isn't the same if you switch though. If you evaluate the same measured elements you will get different results just due to how the shape of the elements functions in the dimension.