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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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  • But if I measure as a circle (thus projected in I vector defined by datum A and C) what is the point of using the C datum in the callout all together? Wouldn't it be the same to write A/B only? (I am not questioning your suggestion btw, just wanna learn how this works Slight smile ; Learning from this example/post is more important that the actual problem I am facing).

    Without datum C, the DRF is free to rotate about datum B. It can rotate until it is lined up with the hole where it will report 0 deviation from B, and only the deviation from datum A will be applied to the true position. You need to have datum C to lock the rotation about datum B down so that the deviation from datum B (normal to the midplane of datum C) is also taken into account.

    If you measure the hole as a circle without datum C, you will have 0 deviation from datum B. If you measure the hole as a cylinder, it will only report the deviation of the cylinder from datum B perpendicular to the hole cylinder. If there is any angular deviation between the cylinder and datum C, then this will still be an incorrect calculation for the true position. In every case, the deviation from datum A will be calculated correctly
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  • But if I measure as a circle (thus projected in I vector defined by datum A and C) what is the point of using the C datum in the callout all together? Wouldn't it be the same to write A/B only? (I am not questioning your suggestion btw, just wanna learn how this works Slight smile ; Learning from this example/post is more important that the actual problem I am facing).

    Without datum C, the DRF is free to rotate about datum B. It can rotate until it is lined up with the hole where it will report 0 deviation from B, and only the deviation from datum A will be applied to the true position. You need to have datum C to lock the rotation about datum B down so that the deviation from datum B (normal to the midplane of datum C) is also taken into account.

    If you measure the hole as a circle without datum C, you will have 0 deviation from datum B. If you measure the hole as a cylinder, it will only report the deviation of the cylinder from datum B perpendicular to the hole cylinder. If there is any angular deviation between the cylinder and datum C, then this will still be an incorrect calculation for the true position. In every case, the deviation from datum A will be calculated correctly
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