GD&T composite true position assistance

ndfightingirish1971

New Member
Joined
May 14, 2013
Messages
6
Good day!

I am currently dealing with a problem that I cannot seem to find any information for. Here is a screen grab of the feature control frame. I realize that the FCF may look odd to some GD&T users, however, I do not wish to debate the accuracy of the FCF to ASME standards. I only need help with the composite portion of the FCF. This is not my engineering drawing and the customer is unwilling to change the FCF.

fcf.jpg


I have a spreadsheet that uses the following formula to attain the true position of 0.0200 to A, B, & C: =2*SQRT((A7*A7)+(B7*B7)+(C7*C7))

A7 is the deviation in the X direction, B7 is the deviation in the Y direction, & C7 is the deviation in the Z direction. I have another series of formulas that establish MMC bonus tolerance and a formula that uses an IF statement for "PASS" or "FAIL". The formulas all seem to work accurately as I have confirmed the numbers using an online calculator, as well as, my CMM software.

Here is where I need help. I am trying to write a series of formulas using PLTZF (Pattern Locating Tolerance Zone Framework) and FRTZF (Feature-Relating Tolerance Zone Framework) that will determine if the pattern of holes is accurate and I have no idea where to begin. I cannot find any mathematical formulas for hole pattern true position. As a general rule, I measure products on the CMM but in this case, the supplier is sending me raw data from manual measurements and they need help reporting the data. Any assistance would be greatly appreciated!!!

Thanks!
 

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KRice

Well-known Member
Joined
Dec 9, 2003
Messages
842
Office Version
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The 0.0200 is the diameter of the tolerance zones in which the axes of the holes must lie. Those tolerance zones are allowed to expand by the same amount as the diameters of the holes deviate from MMC. I'm not familiar with a reason why the root sum of squares computation would be needed. I'm assuming your three datums are mutually orthogonal, datum A represents the surface into which the holes are bored, and the drawing specifies hole locations in terms of distances from datums B and C...are these assumptions correct?
 

ndfightingirish1971

New Member
Joined
May 14, 2013
Messages
6
The 0.0200 is the diameter of the tolerance zones in which the axes of the holes must lie. Those tolerance zones are allowed to expand by the same amount as the diameters of the holes deviate from MMC. I'm not familiar with a reason why the root sum of squares computation would be needed. I'm assuming your three datums are mutually orthogonal, datum A represents the surface into which the holes are bored, and the drawing specifies hole locations in terms of distances from datums B and C...are these assumptions correct?

Sorry for the delayed response. I thought I would get an email letting me know someone replied. I had given up hope.

That is correct, however, datum A is not perfectly flat and I cannot assume that the hole is bored exactly perpendicular to the surface. This is not my normal method of extracting the information I need but it is the customer engineering intent. I would like to post the Excel file I created for critique but I'm not really sure how to do that with the unbelievable IT restrictions we have here. I uploaded a screen grab of the single hole tab so you can see the minimal difference it makes using all 3 datums in the calculations. On a personal level, I call BS on the calculations but when you are tied to the restrictions of the customer, sometimes you just have to bite your tongue and agree.

The sole purpose for this post was to get some feedback on the 0.0130 dimension. I have researched this at length and to no avail. I can measure a group of holes using PCDMIS for CMM software and it does the calculations for me. In this situation, however, the data was collected manually so I am trying to figure out the math involved in hole pattern tolerances. From what I can gather, it is going to take some exceptionally long and complicated formulas. Thanks for the reply.
 

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