Cpk on Position w/ MMC on Location and Size

Proud Liberal

Quite Involved in Discussions
#1
I have several bolt hole pattern FCF's with MMC on both the tolerance and feature size with the added wrinkle of no rotational alignment (datums only on a plane and a center of the bolt hole pattern).

1. How should the lack of fixed alignment system be handled w/respect to capability?

2. On some prints, the FCF is indicated with the number of holes (ie: x12) and others do not indicate. Should these differences be handled differently.

Because I can identify each hole separately, I'm tempted to overthink this issue. The practical side of me says to only look at the hole location with the worst position. But that isn't always the same hole. Can someone save me from myself?

To demonstrate how easy it is for me to overthink the problem, I've attached a file showing what was done to evaluate the 6 pc layout samples.
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Bev D

Heretical Statistician
Staff member
Super Moderator
#2
Ask yourself what the purpose of 'determining' or understanding capability really is. This should keep you from over thinking it and designing a study - adn it's calculations - that will help YOU improve your process...
 

Paul F. Jackson

Quite Involved in Discussions
#3
1. How should the lack of fixed alignment system be handled w/respect to capability?
A mobile alignment system (if it mirrors functional liberties for translation and rotation) will reflect those of the mating assembly conditions at their most restrictive limits. Your inspection should be handled by addressing those functional liberties for translation and rotation if you don’t want to reject conforming product or require greater accuracy and precision w/respect to capability.
2. On some prints, the FCF is indicated with the number of holes (ie: x12) and others do not indicate. Should these differences be handled differently?
No. Not if the specification ascribes to the rules governing simultaneous requirements as applied in ASME Y14.5.
Because I can identify each hole separately, I'm tempted to overthink this issue. The practical side of me says to only look at the hole location with the worst position. But that isn't always the same hole. Can someone save me from myself?
There are a couple of ways to check the pattern and address those functional tolerance liberties by allowing the secondary datum feature translate within its residual size from MMC and rotating about it to minimize the pattern deviation according to each member’s variable position tolerance with respect size.
• You can make an attribute “GO” gage with the datum feature plug/hole sized to its MMC limit and the feature holes/plugs sized to their virtual conditions.
• Or you can analyze the pattern visually (or analytically) by iterating translation and rotation of the pattern’s basic coordinates within graphically amplified boundaries respective of size liberties just like the “GO” gage. See PatternFittingSeasonLee.xls attached
To demonstrate how easy it is for me to overthink the problem, I've attached a file showing what was done to evaluate the 6 pc layout samples.
Take your overthink a little further and tabulate the sample coordinates figuring X, Y, Ø Position deviation, and Size. Then plot both Ø position deviation and the size on the same histogram with MMC size corresponding with MMC position and extending beyond to LMC size corresponding with LMC position.
If you use the same iterative calculations to rotate and slightly translate the basic coordinates again in the table will reveal how rotation and slight translation relative to datum feature size reduces position error collectively on the histogram. The intersection of the upper tail of the position with the lower tail of the size reflects the probability of position deviation and the encroachment of the size distribution on its USL reveals what potential mean size shift is possible to optimize the process to reduce the probability of a defect for the dependent variables simultaneously. See MMC_Histogram.pdf PAGE 8 attached
Of course if you look for PpkMMCXY.xls in the forum files you will see some of these techniques blended.
Paul
https://elsmar.com/Forums/attachment.php?attachmentid=13377&d=1287239602
View attachment PatternFittingSeasonLee.xls

View attachment MMC_Histogram.pdf
 
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