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| Question on bonus tolerance when datum is oversized |  | ||
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| Posted by: brosas ® 07/24/2008, 11:43:46 Author Profile eMail author Edit |
I have a situation, where on a drawing, the engineer calls for true position to be calculated for hole A. Hole A has a tolerance of 17.4 + 0.2. In the true positioning control box located under the hole tolerance, there is a positioning call out for 1.4 @ MMC, while referencing Datums: A, D, and E@ MMC. Now I understand that to get the bonus tolerance we must first do ACTUAL SIZE - MMC= BONUS TOLERANCE, which in this cases is 17.41-17.2= 0.21. From my understanding 0.21 is my bonus tolerance.
Now I also know that since I have Datum E @ MMC I must factor in the additional bonus of the datum E hole. This is where the confusion is, because if the Datum E hole was in tolerance I know that to get the bonus tolerance I would use ACTUAL SIZE - MMC= BONUS TOLERANCE. But in this case, Datum E hole actual measurement is 21.19, and hole tolerance is 19.0 + 0.3, so this means that my hole, which is also my datum is Oversized. Now to get my additional bonus tolerance from Datum E do I still use ACTUAL SIZE - MMC= BONUS TOLERANCE( 21.19 - 18.7 = 2.46), or should I use the LMC - MMC = BONUS TOLERANCE( 19.3 - 18.7 = 0.6) , or should I completely disregard the reference to Datum E @ MMC because the hole is out of tolerance????? So what do I do with a Datum @ MMC reference, when the Datum is oversized???????????????????????? Please any information would be helpful. Modified by brosas at Thu, Jul 24, 2008, 11:53:39 |
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| : Question on bonus tolerance when datum is oversized -- brosas | Post Reply | Top of thread | Forum |
| Posted by: Kelly Bramble ® 07/24/2008, 13:22:21 Author Profile eMail author Edit |
If you decide that the Datum hole feature being oversized does not effect the performance of the part, then the most correct way to determine the bonus tolerance from datum E is ACTUAL DATUM E SIZE - MMC= BONUS TOLERANCE. I'm not aware of any rules in ASME or ISO that declare LMC is a bonus tolerance limit when a feature is out of tolerance. |
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| : : Question on bonus tolerance when datum is oversized -- Kelly Bramble | Post Reply | Top of thread | Forum |
| Posted by: brosas ® 07/24/2008, 14:33:59 Author Profile eMail author Edit |
Just to clarify, in this situation, i would take 0.21 + 2.46 = 2.66 as my total bonus tolerance of hole A in ref. to datums A,D,and E@MMC? |
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| Posted by: traingdt ® 07/25/2008, 10:29:56 Author Profile eMail author Edit |
This is a good discussion! Kelly, wouldn't you agree that we need to be careful about saying that Hole A's bonus tolerance grows to be 2.66? Because datum E is tertiary, that datum's contribution to the bonus would not be diametrical, but only felt in one direction. And I would agree that if E is oversized, the part should be rejected outright, or if you can live with a larger datum E, then the spec should be changed. But at this point, you're looking at a part that doesn't meet the print... |
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| Posted by: Kelly Bramble ® 07/25/2008, 11:44:05 Author Profile eMail author Edit |
I think that there is insufficient information to understand exactly what is going on. So far, we have only been told that the tertiary datum is too big not offset or out-of-position to far. If the tertiary datum (feature of size) is related to the primary and secondary at MMC, then an internal boundary (VC) will be maintained regardless of being manufactured oversized. Assuming that somebody did the math and designed the part and mating part with interchangeability in mind, the part could assemble. Additionally, if the tertiary datum is controlled with RFS and is manufactured very oversized, but within the positional tolerance, I doubt interference could occur. All of this assumes that the mating parts are allowed some wiggle room during assembly and interference does not occur due to external features or other limitations. |
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| Posted by: Kelly Bramble ® 07/24/2008, 17:42:41 Author Profile eMail author Edit |
Yes... Lots of tolerance, what does the feature do? Also, if the datum is allowed to be built out of size tolerance, why is it specified 19.0 + 0.3 if 21.19 works? |
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