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True position Question

constadin
Hello guys,

Sorry for the dump question (and draft paint image) but I am a bit confused:




I have a TP callout for the hole (center of image) on a part like that. Datum A is the Upper plane (basic dimension value not important), Datum B is the vertical Ø through the part as displayed in dashed lines and Datums C as per picture. The callout is A/B/C and I was wondering since C is controlling the rotation in this datum structure, it would be wrong to control this hole as a circle right ? It would need to be a cylinder...
Thanks in advance for your help Slight smile
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  • In ISO world :
    A is the top plane using minimax algo (tangent plane on the free side of material that minimizes the max deviation)
    B is a perfect cylinder perp to A, max inscribed (If you want to check if PC-DMIS construct it as well, you can project all the points of the cylinder on the plane, then construct a max inscribed circle with them).
    By definition (in this case), C must be perp to A (and // to B, that's why the datum system is not correct IMO), so you can construct a line at the theo distance by intersecting the mid plane and a plane // to A. C becomes a line, and block the rotation... Just what I would do... But I would begin by calling the designer !!!!!!!
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  • In ISO world :
    A is the top plane using minimax algo (tangent plane on the free side of material that minimizes the max deviation)
    B is a perfect cylinder perp to A, max inscribed (If you want to check if PC-DMIS construct it as well, you can project all the points of the cylinder on the plane, then construct a max inscribed circle with them).
    By definition (in this case), C must be perp to A (and // to B, that's why the datum system is not correct IMO), so you can construct a line at the theo distance by intersecting the mid plane and a plane // to A. C becomes a line, and block the rotation... Just what I would do... But I would begin by calling the designer !!!!!!!
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