Datum precedence for two single segment location tolerance

[erik3000]

I know that on a composite feature control frame, the feature relating (second row) tolerance must have the same datum precedence as the pattern locating (first row) tolerance. Is this the case also for two separate single segment controls applied to a feature?

e.g., could I have an FCF locating a hole to A, B and C, and then a separate FCF under it locating the hole tighter to C only? Does it matter which one is on top and which is on the bottom?

 

[chez311]

erik3000,

No - there is no requirement of preserving datum precedence between each FCF/segment for multiple single segment. You can hold different segments to different DRF's - though this might cause some issues/complaint at your QC department having to set up completely different DRF's when measuring and gauging. Each segment is verified separately - as long as the feature satisfies both tolerance zones it will result in a passing part.

I don't think there is any requirement as to which is top/bottom except from a comprehension perspective (ie: might get alot of questions and confusion if you flip them in such a way as may be counter-intuitive, say [C] on the top frame above [A|B|C]) - the segments could technically be placed on two different leader lines attached to the same feature and still have the same result.

Note that obviously, none of this applies to composite tolerance - it is a similar, but different, animal.

**Edits in italics, I realized my wording could be misconstrued so I tried to be a little more precise.

 

[fsincox]

See ASME Y14.5-2009 for just such an example: Fig 7-55
Frank