Bonus Tolerance Concept 2

[Jdho]

Hello all,

I've recently came across bonus tolerance with respect to positional tolerance during a quality inspections and wanted to verify my understanding of the concept. From my understanding when a positional tolerance is under maximum material condition it allows a "bonus tolerance" and this bonus tolerance is calculate using the upper limit - the lower limit of the hole diameter. So say for example if I have a positional tolerance of 6.0 mm on a hole with a diameter of 2.3 +/- .50 mm with respect to datum A, B (RFS), and C (RFS). This would translate into a 1.00 mm bonus tolerance on that positional tolerance of 6.0 mm, so my actual tolerance would be at 7.0 mm instead of 6.0 mm? Not entirely sure if I'm completely grasping this concept correctly, any input would be greatly appreciated. Thanks in advance friends!

Regards,

Jdho

 

[chez311]

Jdho,

Your bonus tolerance/available position tolerance changes depending on what size the holes comes in at, which if the specification is at MMC this would be when your hole is at LMC. For the situation you described, yes - when the hole is at 2.35mm the position tolerance would be 6 + 1 = 7mm. See Figures 2-12/13/15/16 for examples of these calculations for MMC/LMC. 2-14/17 deal with RFS - while this does not include the concept of bonus tolerance, they may be interesting for you to look at.

What I'm actually more interested in is what happens when your position tolerance is larger than the diameter of the hole, as you have described. I do not believe that this can be gauged despite the fact that MMC is called out as your virtual condition is negative (2.25 - 6 = -3.75) which would dictate a simulator of negative size. Is this a real application or just a theoretical question? Regardless I'm interested to hear others thoughts on this.

*Edit - the more I think about this, I do not think this would be a valid callout. I'm aware of course that a simulator of negative size is nonsensical. To me that means that it cannot be simulated by a pin fixed in size and position, as should be possible with an MMC control.

 

[Jdho]

Hi Chez311!

Thanks for the response. The situation is completely theoretical and I came up with it on the spot. Interesting, then it must mean that if my diameter measured were to be 2.30 it would mean I only have a bonus tolerance of .50 mm i.e. 6 + 0.50 = 6.5 mm? This is interesting, never came across this during school only when I got out into the "real" world did I find the opportunity to see this. IN any case, thanks for the input chez311!

Regards,

jdho

 

[Belanger]

Valid and nonsensical are two different things. Something can be nonsensical and still be valid per Y14.5.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[chez311]

Jdho,
Yes that is correct, your available tolerance zone would be as you say 6.5mm if the hole came in at 2.3mm. Glad to be of help! As I said theres plenty in the standard about this topic, I would recommend obtaining a copy and reading through it, especially if it is something you have to deal with regularly in your job (you mentioned inspection). I apologize by the way, I should stop assuming this but are you working to ASME or ISO standards? I was referencing ASME Y14.5-2009 (of which a new version 2018 has actually come out).


JP,
Any chance you could expound on that? I agree with you, I don't think theres anything in Y14.5 that prohibits it - I just am wondering about the implications. I don't think it could be gauged with a physical hard gauge for the reasons I mentioned, but could a CMM handle this? I guess it would technically just allow the axis of the hole to fall anywhere within that 6mm position, and this position tolerance zone would arbitrarily increase in size proportional to the amount of the hole's deviation away from MMC. I say arbitrarily because unlike your typical case (hole size > position tol) where a larger hole will allow it further deviation while not violating your VC (simulated by a pin of fixed size at VC) a larger hole for this case (hole size < position tol) while according to the rules allows bonus tolerance functionally does not allow further deviation in the same manner.

Maybe I shouldn't think too hard about it, especially since its just a theoretical question posed by OP, but it was intriguing.

 

[Belanger]

I knew as soon as I posted that that I should probably have thought of an example first. I just meant from a semantics viewpoint, something can be within the rules but not have any practical value.
One example might be a position tolerance applied to hole, but one that references that hole itself as a tertiary datum. Not blatantly illegal but perhaps meaningless/nonsensical?

Regarding how to check the OP's callout where position tol is greater than the size, I pretty much agree with you: A physical gage is not possible (I have to study what the new standard says about a negative virtual condition), but I presume a CMM can handle it because it's just doing math without a physical virtual condition check.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[chez311]

JP,

No problem - I knew what you meant! I was just wondering if while perhaps having little to no practical value, what the implications were. Thanks for the confirmation.

 

[Jdho]

Belanger Chez311,

You guys were great! Thank you for everything haha

Regards,

Johnny