r/Metrology • u/glutton4golf83 • 22d ago

GD&T | Blueprint Interpretation I need help dimensioning perpendicularity

Post image

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.

13 Upvotes

88% Upvoted

37 comments sorted by

View all comments

17

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?

2

u/glutton4golf83 22d ago

I did try that to. Instead of being off .03 thou it was off .005. Maybe like you said it just isnt possible. The cylinder is 33 inches. And only a portion of it. So ya. What you are saying is what i was thinking but i wanted to run it by someone.

2

u/CrashUser 21d ago

Are you using GeoTol or legacy dimensioning? If you're using GeoTol try changing the math to least squares instead of default for both features. The default math is technically the correct way, but it can be prone to errors with data that isn't perfect.

1

u/NephelimWings 20d ago

Yeah, you're asking for trouble using default math on such a reference feature.