composite FCF question, please review and help

[AEROSPACE007]

Please see the attached example. Throughout many years of manufacturing round tight tolerance aerospace engine components I have never seen the call out in question. Please review and let me know your thoughts, internally we are debating the interpretation. My thoughts may be tainted due to previous discussions with the design engineer on this part. The lower call out going to datum C is where the confusion is coming in.

The drawing interpretation is to Y14.5-2004.

 

[pmarc]

If I am visualizing the geometry right - that is, 13 holes emanate radially from datum axis B and are located at certain distance from datum plane A (along datum axis B) - the callout in red is technically correct.

First of all, it is not composite FCF, but multiple single segment FCF (there are two Position symbols in each segment, not one for both). This is important, because in case of multiple single segment FCFs datum reference letters specified in second (lower) FCF do not have to be specified in the same order of precedence and with the same material condition modifiers as in upper FCF.

The upper dia. .010 FCF tells how 13 positional tolerance zones are located relative to datums A, B and C. They must be located at certain, most likely basic, distance from datum plane A (I do not see such dimension on this portion of drawing), must all cross datum axis B, and must be oriented to C through different basic angles. The lower FCF seems to say - tighten location of the pattern relative to A and its orientation to C within dia. .004, but keep location of the pattern relative to B at .010. Again, technically correct.

Unfortunately I do not really understand the picture shown on page 2. It shows dia. .010 tolerance zone (which looks OK), but I do not know why it shows .004 tolerance zone as distance between two parallel lines/planes. This is unclear to me, especially that the lower FCF also defines cylindrical tolerance zone (that should be seen as a circle on the picture). Am I missing something?

 

[AEROSPACE007]

Thanks pmarc for the reply. My apologies you are correct this is not a composite FCF, i incorrectly stated that in the post title.

I have attached another sketch to better explain the datum structure. Datum -A- is a face which controls the perpendicularity of these holes. Datum -B- is a Ø which locates the center-line on the part.

I understand the lower FCF establishes a Ø.004 tolerance zone. my interpretation is without datum -B- referenced in the lower FCF the Ø.004 tolerance zone can float inboard / out-board along the basic angular location for each hole. The holes must still meet the .010 requirement of the upper FCF, hence the two parallel lines establishing the Ø.004 tolerance zone.

Our current internal debate is whether or not the lower FCF requires you to pin the hole locations to the center line of datum -B-.

 

[pmarc]

Thanks for clarification, AEROSPACE007.

In this case here is how I am interpreting both callouts:
1. Upper FCF:
Defines pattern of 13 dia. .010 tolerance zones which axes are perfectly perpendicular to datum plane A, cross datum axis B and are oriented to C through different basic angles. I think you have no doubts about this one, right?

2. Lower FCF:
Now, that will be tricky. It defines pattern of 13 dia. .004 tolerance zones which axes are perfectly perpendicular to datum plane A (refinement of upper FCF), and ARE LOCATED from datum axis C. Why located? Notice that each of the tolerance zones within the pattern are at certain basic linear distance from C. The difficulty is that this basic linear distance is not given directly on the drawing, but has to be calculated based on polar coordinates shown on the print. If there was no upper FCF, there would be infinite number of tolerance zones locations that would satisfy requirement specified in this FCF (in other words, the pattern of 13 dia. .004 tolerance zones could freely rotate aroud datum axis C). But since the upper FCF defines pattern of 13 "fixed-in-space" dia. 010 tolerance zones, this free rotation of pattern of 13 dia. .004 tolerance zones around datum axis C is limited by upper FCF requirement.

I am not able to tell you exactly the shape of resultant tolerance zone for each hole - however I would not expect the zone to be limited by two parallel lines anywhere.

P.S.: I am really curious to hear the functional requirement hidden behind such two FCFs.