Perpendicularity And MMC 2

[Alan Lowbands]

Hi,
I was hoping someone might be able to help.

I have just received a report and can't understand how the perpendicularity tolerance was calculated using MMC.
There are two diameter measurements reported, one is stated 'mated' and used for the perpendicularity tolerance at MMC.
The other diameter measurement is different.

Does anyone know which of the two measurements are correct and how the 'mated' measurement is achieved ?
I was wondering if it's software generated but unsure.

I have attached a screen grab.

Any help would be really appreciated

best regards
Alan

 

[Jacob Cheverie]

Alan,

My initial assumption would be that both measurements are "correct", with the non-mating diameter representing the UAME (unrelated actual mating envelope) and the mating diameter representing the RAME (related actual mating envelope) as measured while constrained to primary datum C.

 

[Alan Lowbands]

Thanks Jacob,

I'll have a read up.

I was a little confused as to how this was calculated.
I'm assuming that the perpendicularity tolerance is linear not angular as the tolerance is taken from the Ø90 'mating' result.
How is this done without a depth value to calculate it. (can't think of another way to describe it)

does that make any sense :/

thanks again
alan

 

[Alan Lowbands]

sorry Jacob,

That was me being thick
think I've got my head around it now

best regards
Alan

 

[chez311]

What standard are you using, is it ASME Y14.5? My calculation below is based on that.

Actually I would have to disagree, I believe that the mating diameter is actually the UAME. I'm not exactly sure, but perhaps the other diameter refers to the maximum measured diameter at any one section, or perhaps the UAMME.

I say this because of the calculation for perpendicularity. The tolerance Zone calculation for surfaceaxis interpretation* at MMC would be b = t_0/2 + (size_UAME/2 - size_MMC/2) which lines up with the calculation shown (radial, multiply by 2 for the diametrical zone).

Actual value and tolerance zone measurements of any geometrical control is never measured in angular units. Only if a direct plus minus angular tolerance is applied to a feature would we want the results be angular to determine conformance. We can measure angular deviation, but again it would not guarantee conformance to a geometric control (position, perpendicularity, etc..).

*where t_0 is the tolerance specified in the FCF.

*Edit2: I should have reviewed it first - just realized that the calculation for the size of the orientation tolerance zone at MMC is not the same as position (obviously the tolerance zone would not be location constrained for an orientation tolerance but the size of the tolerance zone would be the same). This seems backwards to me, as the tolerance zone calculation should be the same it would just be actual value that changes. I'll have to look into this further, the result may be essentially the same but the path to get there is certainly different.

 

[greenimi]

Agree with Chez311,
The calculations seems to be wrong.
90.170-90.000 (MMC) = .170 (and this is the bonus)
Measured perpendicularity is .147 and you are at .170 .

Agreed that we should used UAME and not RAME as the reports looks like it is using

 

[Alan Lowbands]

thanks for taking the time to reply guys.

the only info I have is for the drawing standards ISO-GPS-8888

don't know if that helps

 

[greenimi]

If ISO, I think that the "bonus concept" does not exist , but that does not mean the calculations are correct.
MMVC boundary shall not be violated in ISO and I think the same conceptual calculations (for "bonus") should be used. I will stand corrected if the ISO experts proves me wrong.

 

[Jacob Cheverie]

chez311,

t_0/2 + size_UAME/2 - size_MMC/2 = 0/2 + 90.075/2 - 90/2 = .075/2 = .0375 ==> This is what you get using the equation that you have provided and assuming that the mating diameter is the UAME. I'm not sure how .0375 connects to anything else on the report.

This is an internal cylinder. The UAME will almost always be larger than the RAME if I am not mistaken. Also, the RAME will determine how much bonus tolerance is granted as it is constrained to the DRF. That is the reasoning behind my claim.

It appears that the measured value of Perpendicularity came out to .147, but only .075 bonus tolerance was granted from the RAME size, so the characteristic is out of tolerance by .072 (.147 - .075).

 

[chez311]

greenimi,

I was going off the assumption that the calculations were correct and the "mating" diameter was indeed the UAME diameter - its not unusual to refer to the UAME as simply the mating or actual mating diameter (I know we're talking about ISO here apparently but Y14.5.1 simply refers to the UAME size as r_AM "actual mating size" and the location constrained RAME size as r_TP "true position mating size", there is no notation provided for the non-location constrained RAME).

If the 90.075 mating diameter is indeed the RAME as you suggest and 90.170 is the UAME then I agree the calculations are incorrect, the feature would be within tolerance. OP would have to go back to their metrology team and confirm as the feature shown as nonconforming would actually pass.

 

[greenimi]

This is an internal cylinder. The UAME will almost always be larger than the RAME if I am not mistaken

Jacob,

Question #1: How do you know it is an internal cylinder?
Question #2: What RAME has to do with the bonus for perpendicularity (considering bonus concept valid in ISO)?

 

[Jacob Cheverie]

greenimi,

1. The size tolerance is 90 to 90.22. Measuring 90.075 yielded .075 bonus which implies 90.075 - MMC = .075 which implies MMC = 90. The feature is at MMC at it's smallest size, which sounds like an internal feature to me.

2. I was assuming Y14.5, I'm not proficient to ISO so consider my statements involving bonus invalid.

 

[chez311]

Jacob,

As I said the calculation I show is radial, multiply by two to get the diametrical tolerance zone. 0.0375 x 2 = 0.075 which is the tolerance zone to which the perpendicularity tolerance is compared against in the report provided by OP (0.147 deviation vs 0.075 tolerance zone = FAIL).

As greenimi noted, the RAME would not be utilized for bonus tolerance calculation in Y14.5 (I assume it is not either in ISO).

I would concur that judging by the tolerances involved (bonus is calculated from 90 which would imply 90 is the MMC size) the feature would seem to be internal. I also am in agreement that typically the RAME is smaller than the UAME for an internal feature (I would say it should be always equal to or less than the UAME), if 90.075 was the UAME I was unsure what the 90.170 referred to - I could only guess. If 90.170 is instead the UAME then the calculations are incorrect.

 

[Jacob Cheverie]

chez311,

I missed that, sorry. I'll always have a lot to learn, what you say makes more sense. It's hard to guess what the report contains, possibly a functional maximum inscribed evaluation on the "mating" dimension and a least squares evaluation on the 90.170 dimension. There would have to be quite a bit of form error on the feature to explain the discrepancy.

Further, I wonder why you have [Ø0(M)|C] Perpendicularity and [Ø0.500|C] Position.

 

[Alan Lowbands]

Once again thanks for looking at this.

It is the report for a hole and I have asked what standard they have inspected it too (waiting a reply).
From what I understand ISO-8888 does use bonus tolerances.

Jacob, could you explain further what you mean by 'Further, I wonder why you have [Ø0(M)|C] Perpendicularity and [Ø0.500|C] Position.'
Is this not usual ?

big learning curve for me so any help is gratefully accepted.

regards
Alan

 

[chez311]

Alan,

I would also request more information about what the reported diameters are (does "mating diameter" = RAME or UAME? What is the other diameter just reported "diameter"?).

FYI I think you're actually referring to BS-8888 which is a British Standard developed by the BSI Group for product specification which utilizes the ISO GPS family of standards.

 

[drawoh]

Alan Lowbands,

Given that you have allowed 0 perpendicularity error at MMC/MMB, you are relying on the bonus tolerance. 0.147 appears to represent a linear error, at least as per ASME Y14.5. You are 0.075 from MMC/MMB, so your 0.147 perpendicularity error is a fail.

--
JHG

 

[Jacob Cheverie]

Alan,

The Positional tolerance and the Perpendicularity tolerance don't really make sense together if datum C is a plane with normal vector nominally along the axis of the hole. I assumed it were, but it may be another feature. What is datum feature C?

 

[greenimi]

You are 0.075 from MMC/MMB, so your 0.147 perpendicularity error is a fail.

That is the problem. The MMC bonus was calculated incorrectly meaning this 0.075 ("bonus") is not correct calculated.
In my opinion the bonus is .170 and the perpendicularity should have pass its orientation requirements.

 

[drawoh]

greenimi,

I had problems with that one too. This is a hole. MMC/MMB is 90mm.

--
JHG