Creating datum from rounded slot

Hello, in the beginning i want to emphasize that i'm a new use of pc-dmis software (4 months) and worked with the other software (ArcoCad) for 3 years.
Now I'm stuck with one thing in XactMeasure GD&T. I got part that has datums like in the drawing.The problem is that I'm not sure if I created datum C in the right way. I simply measured auto-feature round slot and then defined it as a datum C. Now, when I create a dimension with XactMeasure, PC-Dmis creates an FCF alignment that I see on CAD as yellow arrows. But when I think about it and talk with my colleagues, I started to think that it should look like the red arrows. And now, if I create a centerline from the slot to create datum C from it, I will not be able to use the MMC in this datum.

I checked the forum topics, but still have not find a solution:
https://www.pcdmisforum.com/forum/pc...und-slot/page2
https://www.pcdmisforum.com/forum/pc...h-length-bonus

PS. I am not the best in English because I do not use it very often, but I hope that my post is understandable, if not i will try to explain in further.

Attached Files

marcc over 5 years ago

What you have is a typical 4-way locator (B) and 2-way locator (width of the slot -C-). You should have basic dimension given between centers of B and C, measure B and slot, use centers to make offset alignment based on that basic dimension. I think diameter and M symbol associated with slot length are incorrect because you will be only checking location in one axis. You need to check not only position of the center but also position of 2 most outside points on both ends along long axis of the slot - that tells you if the slot is oriented within limit or skewed too much. After you have your offset alignment, create center line from 2 long sides of the slot, intersect with both opposite radiuses of the slot, create generic circles at those points with diameter equal to actual width of the slot and report location using only X axis. If this is good you can check profile - again create generic circle in the center of the slot with diameter equal to slot's width so you can use C from the callout.
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Mike Ruff over 5 years ago

I think I would agree with PC-DMIS, since this is more inline with how a mating part would align to the datums. Your results will be much more consistent because the line from centroid of B to the centroid of C is much longer than the centerline of C is. If you clock to C only, and the angle is just slightly off, this error will by projected out and will be worse the further away you get from the datums.
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