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Stackup RFS-MMC combo, simple part, same results?

Stackup RFS-MMC combo, simple part, same results?

2
(OP)
I’ve studied a bit the stackup calculations presented on eng-tips, especially the ones where a “simple” OD-ID relationship is shown (a.k.a. ID = datum feature and OD = controlled feature)
For the feature control frames that have only RFS and MMC/MMB modifiers, am I safe to conclude that the X max and X min calculation RESULTS are the same regardless of how the modifiers/ or lack thereof are shown?

In other words:
position RFS to datum RFS
position MMC to datum RFS
position MMC to datum MMC/MMB
position RFS to datum MMC/MMB
will give the same results for the maximum / minimum wall calculations ?

Is this true? Yes, no, maybe ?

At least look like that might be the case unless I am missing something.

For reference one relevant thread, but extended to my questions above
http://www.eng-tips.com/viewthread.cfm?qid=426288



RE: Stackup RFS-MMC combo, simple part, same results?

aniiben,

Not sure what exactly makes you think that all these four different combinations, that is:
1. position RFS to datum RFS
2. position MMC to datum RFS
3. position MMC to datum MMC/MMB
4. position RFS to datum MMC/MMB
always give the same results for min and max wall thickness calculations, but I don't think this is true statement.

RE: Stackup RFS-MMC combo, simple part, same results?

(OP)
I used your sketch (for reference) from the thread above and change the callouts


ID: 20 ± 0.5 — datum feature A
OD: 50 ±1.3,
position Ø3 -case 1, 2, 3 or 4
1. position RFS to datum RFS
2. position MMC to datum RFS
3. position MMC to datum MMC/MMB
4. position RFS to datum MMC/MMB

I got 10.8 for minimum and 17.4 for maximum not matter what!

Which case do you think DOES NOT fit the pattern?

RE: Stackup RFS-MMC combo, simple part, same results?

2
I stand corrected. In all 4 cases MIN=10.8 and MAX=17.4.

Thank you.

RE: Stackup RFS-MMC combo, simple part, same results?

Sure seems very counterintuitive. Would you expected that?! But nice to know.

RE: Stackup RFS-MMC combo, simple part, same results?

2

Quote (greenimi)

Sure seems very counter-intuitive

On the other hand, think of the idea of bonus tolerance - as hole is getting larger, it can be moved further away from theoretical position for exactly the same amount, so bonus tolerance does not affect wall thickness.
Hence no difference between MMC (bonus tolerance) and RFS (no bonus tolerance)

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

RE: Stackup RFS-MMC combo, simple part, same results?

I would like to ask a couple of questions related with this thread:
Looks like when RFS and MMC modifiers are invoked there would be no difference between the callouts, hence min. / max. results are the same.
What about when LMC/ LMB modifier is used in combination with RFS or MMC/MMB?
I would expect:
5. position MMC to datum LMC/LMB AND
6. position RFS to datum LMC/LMB
to get the same min./max. values

Also

7. position LMC to datum MMC/MMB
8. position LMC to datum RFS (12.1 / 18.7)
to be the same

However, I find that was not the case. Either, I am doing (conceptually)something wrong or what I expected is not realistically true and the analogy does not work (for some reasons that I am not aware). I was able to get “verifiable (on this forum)” results only for case#8 == values shown==

RE: Stackup RFS-MMC combo, simple part, same results?

2
(OP)
Let me try to provide some values. I am using VC/RC calculations. These results might not be the correct ones. Would be nice for someone with more experience and knowledge than I, to confirm or invalidate these values, if possible.
Thank you
ANI


5. position MMC to datum LMC/LMB (x min=10.8, X Max.= 17.4)

6. position RFS to datum LMC/LMB (x min=12.1, X Max.= 18.4)



7. position LMC to datum MMC/MMB (x min=12.1, X Max.= 18.7)

8. position LMC to datum RFS (x min=12.1, X Max.= 18.7)

RE: Stackup RFS-MMC combo, simple part, same results?

Quote (aniiben)


5. position MMC to datum LMC/LMB (x min=10.8, X Max.= 17.4)
6. position RFS to datum LMC/LMB (x min=12.1, X Max.= 18.4)


7. position LMC to datum MMC/MMB (x min=12.1, X Max.= 18.7)
8. position LMC to datum RFS (x min=12.1, X Max.= 18.7)


Interesting thread.
I am not able to get the same numbers.
Could somone, please, confirm that shown values are the correct ones. I am trying to learn from this kind of stacks.
Thank you

RE: Stackup RFS-MMC combo, simple part, same results?

Nobody would like to do some match calculations and confirm the values above?
I did it, but unfortulately, due to my lack of knowledge/abilities I got different results.


5. position MMC to datum LMC/LMB (x min=10.8, X Max.= 17.4) (x min=12.1, X Max.= 15.9)

6. position RFS to datum LMC/LMB (x min=12.1, X Max.= 18.4) (x min=12.1, X Max.= 17.4)



7. position LMC to datum MMC/MMB (x min=12.1, X Max.= 18.7) (x min=12.1, X Max.= 20.0)

8. position LMC to datum RFS (x min=12.1, X Max.= 18.7) --ONLY THIS IS THE SAME---shame of me:(


Who is right? What are the coorect values?

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