composite true position and 3d best fit

[[Ha...]]

Does anyone have a solid way to check composite true positions? I have been using 3d best fit alignments and we cannot agree on which answer is correct. The way I check is by measuring all the holes in the pattern, whether it is 2 or 10 and reference to the first callout (ex ABC). Then, create the 3d best fit by selecting all the holes in the pattern, and reference back to the first alignment (ABC). Next, create the true position with the 3d best fit and call out the holes to the new alignment. Any ideas on the correct way to call out these dimensions? Any help would be appreciated.
Thank You

[[Te...]]

What is the call out for the composite tolerance.

6 x Ø 12.0 ± 0.1
Postion to ABC
Position to A

Is it that kind of callout?

[[Ha...]]

Yes exactly. However, this situation is just 2 diameters.

[[Te...]]

I think all you have to do is 2 positions both of a pattern.

Position 1 to Bore Pattern
Primary A
Secondary B
Tertiary C
NO ROTATION and NO TRANSLATION

Position 2 to bore pattern
Primary A
TRANSLATION (both axis) ON and ROTATION ON

Page 53 of the cookbook.

That should do it

REMEMBER All True Positions and Profiles should be broken out into there datum structure DO NOT use alignment of feature to get the ISO 5459 to calculate correctly.

[[Ha...]]

Thank you Ted,

I called out the true position exactly as you described and got the same exact results. However, I'm having trouble selling the results to our engineers. I'm under the impression calling the dimension out this way allows the second part of the true position to "float". Nonetheless, if you check the part manually, the hole to hole appears to be on the dead high. Also, if you make the first hole the datum, align back to the second diameter, then recall the distance between the features, the distance will also be at the dead high. It seems like the way the cook book method almost splits the difference in half, while the other method makes one hole "perfect" and puts all the deviation on the other diameter. Any thoughts?

[[Aa...]]

Haha! The composite FCF doesn't say that one hole has to be in tolerance while the other is dead nuts. It only says they both have to be in tolerance. So, for regular circular pattern of holes, for example, each hole could be moved out by as much as half of the diametral position tolerance. The second method you described (using the first hole as the datum and aligning off the second hole) throws away part of the tolerance (in your case, half of it), and is definitely not an acceptable way to determine conformance.

The bottom line is that for the FRTZF (the lower tier) no translational degrees of freedom may be constrained. You are allowed to translate the DRF as needed to achieve acceptable results. (And if no datums are referenced in the FRTZF, you can also rotate it as needed to achieve acceptable results.)

[[To...]]

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Something is fishy in Denmark (Sorry to forum members from Denmark)
I am having a hard time with this one.

[[To...]]

I really think the reason you're getting the same result on both positions is that you may have left the translate and rotation check boxes checked on the position to ABC.

One benefit of using Best Fit Bore Pattern is the ability to plot a graph. Open the position characteristic. Click on the graph icon at upper right. If you get "unable to calculate", you may need to click the "Calculate Now" button. In the plot, you may have to play around with magnification and/or scale to get it to look real good, but you should see the center points of both holes equally spaced away from the nominal.

See example below. (3 pages)

In Metric
In Inches