Best Fit Bore Pattern

[[To...]]

I have a part with 4 large diameters, known as Datum E. Each diameter has 4 tapped holes around it. For the position of the 4 large diameters, I am going to use Best Fit Bore Pattern to C (plane on top) B (OD). I will activate "view tolerance with MMC/LMC". I will activate full translation and rotation for best fit. I will activate MMC for the pattern and for Datum B. I think this part is easy.

For the tapped holes, I was thinking a best fit bore pattern of the 4 tapped holes to each large diameter but I was wondering if I shouldn't use best fit bore pattern of the 16 tapped holes to the best fit bore pattern created for the 4 diameters.

Which way do you think is right?

Screenshot 2020-11-03 073327.jpg

[[De...]]

If there is a different detail/component that attaches to each of the four locations, I would recommend doing 4 patterns of 4 holes. If there is a single detail/component that attaches to all 4 locations at the same time, I would recommend including all 16 in the pattern. Not knowing more about the part, or it's mating parts, this would be my suggestion.

[[Ri...]]

I agree with Dean.

I think you are taking the literal understanding of this drawing, but I don't think that is what they mean.

Would need to see more of the drawing, and possibly have a discussion with the customer, but I would say that there are (4) Datum Es, and that there is a pattern of (4) bolt holes that are evaluated to the corresponding Datum E for that pattern.

In addition, I don't know what the Datums are, but I think each Datum E appears to be constrained except for maybe (1) degree of freedom (possibly rotation), and the (4) bolt holes are fully constrained, so for those there is no allowable translation/rotation.

[[ch...]]

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I think your job is fit for macro programming, l don't have this like job otherwise I could have experiment on macro programming.

[[Ja...]]

You must position all 16 holes to [C|E(M)|B(M)] due to simultaneous requirements. They share the same datum reference frame and must be evaluated so.

[[Gr...]]

We have a part we manufacture that presents the same question.
It has 4 pads on it with a thru hole referenced as a datum. The thru hole has a pattern of 4 tapped holes around it.
The customer told us each pad should have its own True Position call out.
Basically, in our instance, 4 separate datum D

[[To...]]

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That is what I was thinking but my guess is that this part is hold 4 individual components. I feel like they may not have considered the "simultaneous requirement". The cool thing is I can show the results both ways. Customer is a vendor making the part for another vendor so contacting end customer might not be so easy.

Anyway, this is for a demo and customer doesn't have a cmm..... yet.

Thank you to everyone for feebback.

[[Ja...]]

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The simultaneous evaluation will be more a more conservative evaluation anyway, so it may be better to be safe than sorry. If the specifications are not meant to be evaluated simultaneously, then the drawing should include "SEP REQT" or something along those lines.

[[Cl...]]

I have never used this option and know little about it. If I understand correctly, by ticking translation
and rotation, Tom is allowing the pattern of hole to rotate about Z and translate about X &Y?

[[Jo...]]

I have a similar situation. So as I understand it. If all 16x were tied to Datum E you would effectively have rotation possible about Datum E in the 4 holes located about its center. You would only get translation for the other 12 since they are not centered about Datum E.

If you had 4 Datum "E" you could get rotation on each location

[[To...]]

Demo was a success.

As in my original post, I did a best fit bore pattern of the 4 Datum E diameters to C (top plane) and B (OD with MMB) I set Best Fit to "View tolerance WITH MMC/LMC of references" and allowed translation and rotation. Datum B was oversize so no translation occurred but there was .0095° of rotation.

Then, I did a best fit bore pattern of all 16 tapped holes to the Best Fit alignment created in the previous paragraph. No rotation or translation.

Even though I didn't need to do any "fitting", I used Best Fit Bore Pattern instead of 16 individual true positions is because of the plotting capability found only in the Best Fit Bore Pattern option. Being able to show where all 16 holes were was an impressive detail.

[[To...]]

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Yes. Because there is no tertiary datum, the pattern is allowed to rotate about the B diameter. Since B was also called out with the MMB symbol, that means datum shift was allowed so translations were active.

[[Jo...]]

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I'm presenting this as a question: Simultaneous 16x when using ONE datum is not the same as using Simultaneous of 4 sets of 4. If the other 3 Datums are not in nominal position, your center point of each set could change ?

[[To...]]

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I get what you're saying. As it was, the position for all 4 datum E diameters to C and B came in under .0015" and the position of the 16 holes to the alignment came in under .002", so in this case, we may have gotten away with it.

In this particular application, I found out that each datum E and it's surrounding 4 holes were for mounting an individual part. So, the first part of my measurement of the 4 datum E diameters was ok, but tying all 16 holes to the alignment created from the 4 datum E diameters was not necessary.

[[Ri...]]

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I don't think you would want to allow translation for the best-fit. If you have the View Tolerance applied, you can select MMB from inside of the characteristic - that will give you the allowable datum translation. Applying translation inside of the best-fit is allowing the pattern to shift away from the Datum, and is no longer constrained to Datum B.

[[To...]]

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Are you sure? On this particular part, Datum B was oversize because they finish it later. During the best fit, there was no translation whatsoever. I assumed to this was because of the oversized condition. I will have to do some testing.

[[Ri...]]

I was always under the impression that allowing translation frees the pattern from those particular degrees of freedom - it disregards the datum that is controlling that particular degree of freedom. The same for allowing rotation.