Fig. 6-15, para. 6.4.4 Y14.5-2009 1

[Burunduk]

6.4.4:
"There may be applications where the full additional tolerance allowable may not meet the functional requirements.
In such cases, the amount of additional tolerance may be limited by stating a MAX following the MMC modifier. See Fig. 6-15."

1. dia. 0.1MAX shown in fig. 6-15 - is it the maximum additional ("bonus") tolerance (as I interpret the text quoted), or is it the maximum total (bonus + specified) tolerance of perpendicularity (as I think would be clearer and make more sense?). I am aware that in this example they are the same, but what if the tolerance at MMC was more than zero?

2. Is the concept applicable for position, or just orientation? Is there a good reason it is only in chapter 6, but not 7?

 

[tim_member]

Taking table 5-3 and b = D/2 it defines a tolerance zone for MMC axis interpretation as follows:

actual_value = D + size_MMC - size_UAME

I have some questions:

1. If I am getting the formula correctly, it may produce a negative actual value. This would happen if, for example, a hole axis was perfectly oriented and located (D=0) and the hole UAME size was greater than the MMC size. The very last sentence of the paragraph for surface interpretation in Y14.5.1M-1994 [para. 5.2.1 (c)] refers to footnote 1, which basically says that the actual value for a MMC or LMC callout may be negative in case of surface interpretation. If the negative actual value is also possible for axis interpretation, does anyone have any clue why there is no similar footnote in the paragraph for axis interpretation [para. 5.2.2 (c)]?

2. Let's imagine that there are two pieceparts, each containing a single hole for which the size specification is dia. 10.0-10.8 and the position tolerance specification is |POS|dia. 0.4(M)|A|B|C|.
- For the first piecepart, the actual hole is perfectly oriented and located (D=0) and its UAME=dia. 10.4. According to the above formula, the actual value would be -0.4 (0+10-10.4);
- For the second piecepart, the D value for the hole is 0.1 and its UAME = dia. 10.5. According to the above formula, the actual value would be also -0.4 (0.1+10-10.5), right? If yes, then doesn't it look strange at best.

3. Let's imagine another example of a piecepart containing a single hole for which the size specification is dia. 10.0-10.8 and the position tolerance specification is |POS|dia. 0.4(M)|A|B|C|. The D value for the hole is in this case 0.2 (which basically means that the axis of the hole somehow deviates from the true position) and the UAME size of the hole is 10.2. This will give the actual value equal to 0 (0.2+10-10.2), right? If yes, then what benefit can one get from reporting the actual value instead of just D=0.2 value? In other words, doesn't it look strange that the measurement of a hole produced with evident axis error with respect to its true position gives actual value of 0?

4. If the approach presented in the above formula is correct (EDIT), then could anyone explain why para. 5.2.2 (c) for Actual Value for axis interpretation doesn't simply say the same as para. 5.2.1 (c)?
"Actual value. The actual value of position deviation is the smallest value of t₀ to which the feature conforms."

The calculations of the actual value for the example in bullet 3 would then look as follows:

Based on the formula given in Table 5-3 for internal feature at MMC:
b = t₀/2 + (r_AM - r_MMC)
2b = t₀ + (d_AM - d_MMC)

since 2b = 0.2 (the diameter of the smallest tolerance zone containing the axis of the hole), then:

0.2 = t₀ + (d_AM - d_MMC)
t₀ = 0.2 - (d_AM - d_MMC)
t₀ = 0.2 - (10.2 - 10)
t₀ = 0

EDIT: D (uppercase) changed to d (lowercase) in the calculations at the bottom.

 

[chez311]

Tim_member,

1) I cannot say why a similar footnote was not included for the axis interpretation. The axis interpretation can be thought of as an approximation of the surface interpretation. When there is zero orientation/form error this approximation works well and they will be equal. Any deviation from this and the evaluated actual value for each will begin to deviate. All this is to say that if a negative actual value is possible with one, it should also be with the other - as under the aforementioned conditions they will be equal. We will see this below.

2) In short, no your answers do not look strange. The actual value is the same because the additional axis deviation (0->0.1) is offset by the increase in UAME size (10.4->10.5). Actual value for MMC captures the interrelationship of position and size deviation encoded in the definition of MMC, not just UAME axis deviation. Lets assume your examples include perfect orientation/form and therefore are an excellent approximation of the surface interpretation and will give us the same value.

Take the surface interpretation for MMC of an internal feature as I stated previously:
actual_value = size_MMC - size_RAME

In both cases - for two holes of perfect form/orientation one of size 10.4 whose axis is located at true position and another of size 10.5 offset from true position by 0.1 - the size_RAME is equal to 10.4 and will give the same -0.4 for actual value as you noted.

3) Again, no your answer does not look strange. Lets again assume perfect orientation/form, per the surface interpretation your size_RAME would be 10 (10.2 hole offset by 0.2) and therefore your actual value would also be 0.

In both your (2) and (3) the actual value does not correspond to UAME axis deviation alone (this would be RFS). A value of 0 does not mean the axis is perfectly positioned, and is really tied to the RAME deviation. For the surface interpretation this is an exact relationship (size_MMC - size_RAME). For the axis interpretation, this is an approximation.

what benefit can one get from reporting the actual value instead of just D=0.2 value?

I know this is something that quality departments struggle with. The actual value calculated for MMC may not provide a whole lot of information for process control, it only adheres to the geometric relationship provided in the standard. Other values may be necessary to gain the bigger picture (size, straightness, RFS position/orientation, or other fitting routines).

4) Your derivation is essentially the same as I showed, and comes up with the same result. The only difference being you stated the equivalent relationship 2b = D instead of b = D/2

t₀ = 0.2 - (d_AM - d_MMC)

This is the same as the equation I came up with (actual_value = D + size_MMC - size_UAME), just rearranged.

For the surface interpretation b is the radius of the smallest boundary (constrained in location/orientation to applicable datums) which contains the surface of the feature (b = size_RAME/2). For the axis interpretation b is the radius of the smallest boundary (constrained in location/orientation to applicable datums) which contains the axis of the feature (b = D/2).

 

[tim_member]

(1) and (4)

1) I cannot say why a similar footnote was not included for the axis interpretation. The axis interpretation can be thought of as an approximation of the surface interpretation. When there is zero orientation/form error this approximation works well and they will be equal. Any deviation from this and the evaluated actual value for each will begin to deviate. All this is to say that if a negative actual value is possible with one, it should also be with the other - as under the aforementioned conditions they will be equal. We will see this below.

4) Your derivation is essentially the same as I showed, and comes up with the same result. The only difference being you stated the equivalent relationship 2b = D instead of b = D/2

That's the whole problem here. The formulas for actual value (for axis interpretation) that you and I provided (I agree that mine is just a rearranged version of yours) were derived with the assumption that the actual values for the surface and axis interpretation of a MMC callout applied to a hole must be the same numbers for the same measured piecepart. The thing is that this is not required per the standard because the actual value for the axis interpretation is obtained in a different way than the for the surface intepretation. Again, the standard says:

For surface interpretation - para. 5.2.1 (c):
"Actual value. The actual value of position deviation is the smallest value of t₀ to which the feature conforms."

For axis interpretation - para. 5.2.2 (c):
"Actual value. The position deviation of a feature is the diameter of the smallest tolerance zone (smallest value of b) [not the smallest value of t₀ as in case of surface interpretation] which contains the center point or all points on the axis or center plane (within the extent of the feature) of the applicable actual envelope of the feature."

So the latter definition simply means that the actual value for axis interpretation is what you called D in your formula regardless if it is RFS, MMC, or LMC callout. Therefore, the formula is simply 'actual value = D' in all three cases.

What changes in the MMC and LMC cases compared to the RFS case for axis interpretation is the size of the tolerance zone that defines the conformance limit. See para. 5.2.2 (b).

And that is actually why there is no footnote in para. 5.2.2 (c) about possible negative actual value for axis interpretation. There simply can't be negative actual value because the smallest possible value is 0 (when the UAME axis is perfectly at its true position).

********

(2) and (3)
Knowing what the definition of actual value for axis interpretation says in para. 5.2.2 (c), I would say the values are not only strange but also incorrect.

I have no problem with the actual values for surface interpretation, though, as they match the actual value definition from para. 5.2.1 (c).

 

[pylfrm]

Sorry folks. Clearly I must retract the majority of my previous post. For some reason I thought that ASME Y14.5.1M-1994 para. 5.2.2(c) was identical to 5.2.1(c), but that's definitely not the case.

It seems crazy to me that it isn't.


pylfrm

 

[chez311]

pylfrm,

Now I don't understand what I thought I did. If actual value is the same for RFS/MMC/LMC (seems nonsensical to me but okay) and doesn't take into account size variation of the feature (???) then whats even the point of table 5-3? What is t₀ that we just calculated?

I'm sort of lost now. If he's following this, perhaps Evan could chime in too? The new draft of Y14.5.1 makes no change to this.

 

[pylfrm]

Now I don't understand what I thought I did. If actual value is the same for RFS/MMC/LMC (seems nonsensical to me but okay) and doesn't take into account size variation of the feature (???) then whats even the point of table 5-3? What is t₀ that we just calculated?

The point of Table 5-3 is to define the size of the tolerance zone, and it does that just fine. Our mistake was assuming that the actual value is the smallest value of t₀ to which the feature conforms, where t₀ is the tolerance value that would be in the feature control frame. For some mysterious reason that's not how the actual value is defined for the axis interpretation.


pylfrm

 

[chez311]

pylfrm,

I answered my own question about 5-3 after I asked it, I agree. I don't see why the actual value would be the smallest value of b instead of t₀ as calculated from the equation shown previously. In this way, this smallest value of t₀ to me still has meaning as a way to compare the axis and surface interpretations - if only by approximation (diverging values when deviating from perfect orientation/form). Why this would not be considered the actual value is confusing, but I guess thats the convention they chose.

 

[chez311]

tim_member,

Thank you for persevering in your assertions. It is frustrating because my initial interpretation for actual value provided an easily discernible link between surface and axis interpretations and fit the "approximation" paradigm. I still think this train of thought is still valid, the convention for actual value of the axis interpretation just doesn't reflect this. As such I continue to strive for a deeper understanding of the math standard.

To tim_member and all,

A few interesting notes on Y14.5-2018 and the emphasis on surface interpretation though. The new section 5.9.2 states in unambiguous language that the surface interpretation takes precedence for geometric tolerances applied at MMC and LMC, not conditional on any equivalence between the two interpretation. A side note - instead of not being mentioned as in 2009, it is explicitly stated that the surface interpretation is not valid for RFS.

5.9.2 Surface Method Default for Geometric Tolerances Modified at MMC or LMC
When there are form deviations on the feature, the deviation in terms of the feature axis or feature center plane may not be equivalent to the deviation of the surface limited by a VC boundary. See Figures 10-6 and 10-7. The surface method shall take precedence when the tolerances are applied at MMC or LMC. See para. 10.3.3.1(a). The surface method is not applicable when tolerances are applied RFS.

In 2018 the applicable section for MMC position has been changed to follow the verbiage in 5.9.2 above. As in 5.9.2 the extraneous word "extreme" in relation to form or orientation deviation has been removed. They have gone even further to state that "The axis method is shown in this Standard for visualization purposes."

10.3.3.1 Explanation of Positional Tolerance at MMC.
A positional tolerance applied at MMC may be explained in terms of the surface or the axis of the feature of size. In cases of form or orientation deviation of the feature of size, the tolerance requirements in terms of the axis method are not equivalent to the tolerance requirements in terms of the surface method. The surface method shall take precedence. See Figure 10-6 for an example of possible axis interpretation error due to form deviation. The axis method is shown in this Standard for visualization purposes. See ASME Y14.5.1M. In some instances, the additional tolerance may indirectly benefit features other than the one that departed from MMC.

There is an exception to this, stated in the footnote for 5.9.4.1 (MMC) and a similar footnote for 5.9.6.1 (LMC):

NOTE: When a geometric tolerance applied at MMC results in a negative VC (i.e., when a 2.0–2.5 internal feature of size has a position tolerance of 3.0, the VC is −1.0), the surface interpretation does not apply.

The section on orientation tolerance (9.3.5) does not reflect the changes noted above. Looks like a ball was dropped somewhere:

In certain cases of extreme form deviation (within limits of size) of the cylinder or width feature, the tolerance in terms of the feature axis or center plane may not be equivalent to the tolerance in terms of the surface. In such cases, the surface method shall take precedence as in Figure 10-6.

Additionally, to tie back into the OP (looks like we've gone full circle) the following detail about a "MAX" specification, in addition to being in section 9.3.4 for zero orientation tolerance at MMC essentially unchanged from 6.4.4 in Y14.5-2009, the below language as been added in the new sections for axis interpretation only 5.9.4.2 (MMC), 5.9.5.2 (zero tol at MMC), 5.9.6.2 (LMC), 5.9.7.2 (zero tol at LMC) - of course for 5.9.6.2/5.9.7.2 it reads "when the unrelated AME of the feature is at MMC" instead of at LMC. Perhaps this suggests that in addition to when the VC is negative, that a MMC/LMC "MAX" specification is an exception to when the surface interpretation takes precedence?

The total permissible variation in the specified geometric characteristic is maximum when the unrelated AME of the feature is at LMC, unless a maximum (“MAX”) is specified in the feature control frame.

 

[Burunduk]

Wow, surely a lot of catching up to do on the axis interpretation of position/orientation MMC. Could only look briefly at the discussion here so far...
But I did find a copy of Y14.5.1M-1994 and looked up the relevant Actual Value definition:

"(c) Actual value. The position deviation of a feature is the diameter of the smallest tolerance zone (smallest value of b) which contains the center point or all points on the axis or center plane (within the extent of the feature) of the applicable actual envelope of the feature."

So could it be that my assumption at the end of my post at 18 Aug 19 05:26 was correct after all?

Sorry again for not being able to read thoroughly the entire discussion here. I will try to do so later.

I think one of the concerns expressed here is that this definition is not size-dependent (?)
I would say that the allowed tolerance must be size-dependent, but the actual value is not size-dependent - it just the deviation from the true position of the feature that was produced. Whether it conforms to the specification or not, is where size dependency gets involved. Hope that makes sense. Not sure I can explain why it isn't the same for the surface interpretation.

 

[chez311]

So could it be that my assumption at the end of my post at 18 Aug 19 05:26 was correct after all?

It would seem so.

I would say that the allowed tolerance must be size-dependent, but the actual value is not size-dependent

Again, so it would seem from my latest understanding.

Not sure I can explain why it isn't the same for the surface interpretation.

I'm more confused about the converse - about why the actual value for the axis interpretation does not involve size (or more appropriately the approximation of size deviation, as form would get lumped in the difference between size_UAME and size_MMC) but I've mostly resigned myself to the fact that it is the convention that the committee has chosen per pylfrm's and tim_member's comments.

The surface interpretation for MMC/LMC doesn't really involve size directly but is affected by it as it utilizes the RAME. I don't see a way, or a need, to calculate an actual value in terms of the surface which is not, as you put it, "size-dependent". Do you?

 

[Burunduk]

I don't see a way, or a need, to calculate an actual value in terms of the surface which is not, as you put it, "size-dependent". Do you?

I haven't given enough thought to the formulas yet, and I actually would like to ignore them just for now and start with approaching the whole subject of tolerances that include a material condition modifier with just the basic concepts in mind; when I do so, the first thing I can tell is that as far as I'm concerned, "size dependency" should end at the determination of the amount of the allowed tolerance, and not be involved in anything else. There is not much sense in having both the allowed tolerance size and the actual value size-dependent, be it surface or axis interpretation. If you have taken size into account during the allowed tolerance determination, all that is left is to compare the actual deviation value to that tolerance. You don't have to reduce or adjust the deviation value in any way. Furthermore, an actual value should represent how much a feature as produced deviates from perfect location and/or orientation. Since size and location/orientation are absolutely different geometrical properties, I don't see how size should affect the measured deviation from perfect orientation/location. What size should do and it does, is being considered as a factor for determination of the amount of allowed tolerance for location/orientation, to account for possible consequences at assembly.

I should give more thought to the surface interpretation and the role of RAME (and therefore size) in the actual value calculation, but that is my very basic reasoning and my starting point to approaching this.

 

[pylfrm]

Burunduk,

I think the term "tolerance value" generally refers to the number in the feature control frame on the drawing, not to the size of the tolerance zone.

To me, the most sensible definition of "actual value" is the one that allows direct comparison with the tolerance value. It makes determination of conformance very simple: the tolerance is satisfied if and only if the actual value is less than or equal to the tolerance value. It also allows useful sorting, ranking, or comparison between multiple actual values. Do you really not see any value in this?

Furthermore, an actual value should represent how much a feature as produced deviates from perfect location and/or orientation.

If you want a value that represents how much the UAME axis deviates from perfect location or orientation, I'd say you should be asking for the actual value associated with an RFS tolerance. MMC and LMC tolerances generally don't control the UAME axis (due to the surface interpretation taking precedence), so it seems strange to expect their associated actual values to tell you anything about that.


pylfrm

 

[Burunduk]

I think the term "tolerance value" generally refers to the number in the feature control frame on the drawing, not to the size of the tolerance zone.

I agree with that.

To me, the most sensible definition of "actual value" is the one that allows direct comparison with the tolerance value.

I think I would prefer a definition that allows direct comparison to the total amount of permissible tolerance for a given as-produced feature, not just to the fixed value that is applicable at the special case of the feature produced at MMC size. The departure of "feature size" from MMC gives, to quote the standard: "additional tolerance" and that additional tolerance should be added to the "tolerance value", to get the total number that the "actual value" should be compared to.

MMC and LMC tolerances generally don't control the UAME axis (due to the surface interpretation taking precedence), so it seems strange to expect their associated actual values to tell you anything about that.

Note that I didn't mention the UAME axis, all I said is that it is location/orientation that is being controlled, not size.

From para. 2.8.3 Effect of Zero Tolerance at MMC:

"No tolerance of position or orientation is allowed if the feature is produced at its MMC limit of size; and in this case, it must be located at true position or be perfect in orientation, as applicable."

From para. 6.4.4 Application of Zero Tolerance at MMC:

"If the feature of size is at its MMC
limit of size, it must be perfect in orientation with respect to the datum. A tolerance can exist only as the feature of size departs from MMC. The allowable orientation tolerance is equal to the amount of such departure."

I believe those paragraphs are relevant to both axis and surface interpretations. If the terms "location" and "orientation" are not associated with the UAME axis/center plane in case of surface interpretation, then it should probably be simply said that they are associated with the surface ("surface location/orientation" for features of size - a valid concept?). I can accept that the axis/center plane can be uncontrolled by position/orientation FCFs with material condition modifier, but neither is the size controlled by them - so why should size be an additive or subtraction factor for the calculation of the actual value - the deviation from true position/perfect orientation? Again, I do see how it should be a factor for determining the allowed tolerance, but not for the actual value.

 

[tim_member]

tim_member,

Thank you for persevering in your assertions. It is frustrating because my initial interpretation for actual value provided an easily discernible link between surface and axis interpretations and fit the "approximation" paradigm. I still think this train of thought is still valid, the convention for actual value of the axis interpretation just doesn't reflect this. As such I continue to strive for a deeper understanding of the math standard.

You're welcome.

To me, the most sensible definition of "actual value" is the one that allows direct comparison with the tolerance value. It makes determination of conformance very simple: the tolerance is satisfied if and only if the actual value is less than or equal to the tolerance value. It also allows useful sorting, ranking, or comparison between multiple actual values. Do you really not see any value in this?

I would generally agree that "the most sensible definition of "actual value" is the one that allows direct comparison with the tolerance value", however I'm not sure it makes "determination of conformance very simple" and that "the tolerance is satisfied if and only if the actual value is less than or equal to the tolerance value" in case of axis interpretation of tolerances at MMC and LMC.

Let's this time take the example of a hole for which the size specification is dia. 10.0-10.8 and the position tolerance specification is |POS|dia. 0(M)|A|B|C|. Using the formula provided by chez311, "actual_value = D + size_MMC - size_UAME", just imagine that D = 0 and size_UAME = 9.9 - this basically means that the hole is perfectly located but doesn't conform to the size requirement. What will the actual value for position be in this case? It will be 0.1 = 0+10-9.9. Since 0.1 is greater than 0 specified in the position FCF, the feature position will have to be reported as non-conforming. But will this really mean that the there is something wrong with the position of the axis of the hole? Of course not.

The weirdness is all because the given formula actually treats the axis interpretation as the surface interpretation, and by doing so, the actual value must be dependent on the size of the feature, which works only if the UAME size of a feature is conforming.

 

[chez311]

But will this really mean that the there is something wrong with the position of the axis of the hole? Of course not.

To echo pylfrm - if the intent were to control a feature on the basis of axis deviation to be evaluated Regardless of the Feature Size then the control which embodies this namesake should be used. MMC position intimately relates the concepts of size and position, attempting to separate the two seems counterproductive to me.

Regardless - in the case of the example you provided, if we're going by the convention established by the standard the actual value would be 2b=D=0 and the tolerance zone to which it must conform would be calculated at 2b=0+(9.9-10)=-0.1 which I do not believe is valid - I'm not sure its explicitly stated but I would flag a tolerance zone with a negative diameter as invalid. Calculation of the minimum t₀ is just a restatement of this in other terms and as pylfrm pointed out can be compared to the FCF tolerance value. Additionally, the UAME of 9.9 in your example would mean that the surface would violate your VC of 10, so it doesn't matter if its due to size or how perfect the axis position is - this feature would fail the surface interpretation regardless of how the axis interpretation is calculated.

The weirdness is all because the given formula actually treats the axis interpretation as the surface interpretation, and by doing so, the actual value must be dependent on the size of the feature, which works only if the UAME size of a feature is conforming.

I would still consider the axis interpretation to be an approximation of the surface interpretation, because the tolerance zone to which it must conform is influenced by size - even if the reported actual value is D. If the convention of the standard were changed, why would it be so strange that the actual value for MMC would be affected by size? The interrelationship of size and position is coded in the definition for MMC, at least an actual value which reflects this allows direct comparison of axis and surface interpretations. Such a comparison is still possible with calculation of the minimum t₀ even if its not considered the actual value per the wording of the standard.

Noting the reasons I laid out above that in your example the feature would fail (not just due to size/UAME variation - it violates VC) lets modify your example to challenge your assertion that this "works only if the UAME size of a feature is conforming". If the feature has the same UAME=9.9 and located at true position so that D=0, but the specification now reads |POS|dia. 0.2(M)|A|B|C|. The tolerance zone is 2b=0.2+(9.9-10)=0.1 and the actual value is still 2b=D=0 (just for fun the minimum t₀=0+10-9.9=0.1) all of which shows that the feature passes the MMC position check (and also for the surface interpretation actual_value=10-9.9=0.1) despite being nonconforming for size (or form, or a combination of size and form – whatever creates the UAME of 9.9 it does not matter).

 

[chez311]

Note that I didn't mention the UAME axis, all I said is that it is location/orientation that is being controlled, not size.

In reality, in terms of the surface any variation of orientation/position/size/form as it relates to MMC position are inseparable - I do not see what is to be gained by attempting to parse them out, or really any meaningful way to do so in terms of the surface only. Not that these individual metrics can't be measured (here I’m talking about the axis), but it is ultimately their combination which influences MMC controls. What I mean by that is it does not matter how "perfect" the feature is by any other measurable metric - if even a small deviation of the surface violates your virtual condition the feature will fail, regardless of how that deviation arises. I believe your actual value for the surface interpretation should reflect this. Indeed, even if the convention was the same as the axis interpretation (smallest value of b) then it would still result in actual_value=b=size_RAME/2 which must conform to 2b=size_MMC-t₀ where t₀ is the stated tolerance in the FCF - either way the actual value is influenced by size. This is the case unless the very core definition of MMC is changed.

If the terms "location" and "orientation" are not associated with the UAME axis/center plane in case of surface interpretation, then it should probably be simply said that they are associated with the surface

If you take a look at my post (21 Aug 19 13:59) this is specifically addressed in Y14.5-2018. The surface interpretation for MMC unequivocally takes precedence (except in the noted instances) without the previous conditional "equivalence" to the axis interpretation and yet "The axis method is shown in this Standard for visualization purposes." The axis paradigm for some may be easier to grasp and concepts like position, orientation, size, and form deviation become nice, neat (mostly) mutually exclusive properties for the purposes of the figures and calculations shown in the standard. These concepts applied to the surface tend to get a bit fuzzy.

Position in terms of theoretically exact derived geometry such as an axis is a common concept – we can imagine the tilting and translation of this axis in space in relation to a fixed boundary. When instead we look at the concept of MMC position in terms of the surface, those lines blur. Any theoretical geometry derived from the surface can only serve as an approximation – ultimately it is only the relationship between the feature surface and the virtual condition which matters. The actual values can be thought of simply a measure of how close the surface is to violating this boundary (how much clearance to virtual condition/simulators) instead of measuring some discrete metric and the specification of position/orientation can be thought of as describing only how the boundaries/simulators should behave in reference to the DRF (position – fixed location/orientation, orientation – fixed orientation) instead of some expected measurement of exactly “how” the surface deviates.

This definition in terms of the surface is perhaps less satisfying or intuitive, but I think actually more streamlined and simple.

 

[Burunduk]

chez311, pylfrm, thanks to your responses in this thread I no longer think that the actual value for surface interpretation should be independent of size.

That being said, the more thought I give to the formula for actual value for a hole based on table 5-2, "actual_value = size_MMC - size_RAME", the less I like it.

If had to come up with my own way to calculate the actual value for a hole by the surface interpretation, the calculation I would suggest: actual_value = size_UAME - size_RAME.

The reasoning is simple: If there is no location, orientation or form deviation, size_UAME = size_RAME, so the feature utilizes zero tolerance, in other words, the deviation from true position/perfect orientation is zero.
If there is some deviation for a hole, the surface will get closer to the virtual condition boundary, and therefore the RAME diameter will be less than the UAME diameter by the amount of the deviation that occurred.

Example:
Specified hole diameter 10+-0.2
Perpendicularity requirement with reference to datum feature A: 0.1 at MMC.
Produced UAME size: 10.1
Produced RAME: 9.85.

First, what should the actual value be compared to? Everyone learns early at any GD&T training that departure of feature size from MMC gives additional tolerance that can be added to the specified tolerance value, to calculate the total allowed tolerance.
For this case departure from MMC is 10.1-9.8=0.3, so there is up to 0.1+0.3=0.4 orientation tolerance that can be utilized.
How much of it is used by the surface deviation of the feature from a boundary which is equal to its size and oriented perfectly? That is the "actual value" as I would prefer it to be defined: 10.1 (UAME dia.) - 9.85 (RAME dia.) = 0.25.
That is less than the allowed 0.4 for this UAME size, so the hole conforms to the specifications.

 

[tim_member]

Okay, I admit that the most recent example I made up reveals some issues with the axis interpretation as defined in the math standard, but that still doesn't mean that finding the actual value (that is, the value that would immediately and meaningfully tell by how much the feature axis deviates from its true position regardless if the size of the feature is at MMC, LMC or somewhere in between) is impossible. The conformance determination will be problematic / impossible, but this is something that someone trying to determine the conformance should always be aware of in cases where the UAME size is non-conforming.

The weirdness is all because the given formula actually treats the axis interpretation as the surface interpretation, and by doing so, the actual value must be dependent on the size of the feature, which works only if the UAME size of a feature is conforming.

Agreed, I shouldn't have added the striked through part.

 

[chez311]

If had to come up with my own way to calculate the actual value for a hole by the surface interpretation, the calculation I would suggest: actual_value = size_UAME - size_RAME

You have just described the inverse of the actual value for RFS surface interpretation per Y14.5-1994 (actual_value = size_RAME - size_UAME), which per Y14.5-2018 is explicitly not valid as I noted on (21 Aug 19 13:59).

I see no reason why the UAME would be involved in MMC surface interpretation calculation of actual value as it conflates axis deviation with surface deviation. To reiterate from my post (22 Aug 19 14:00):

"The actual values can be thought of simply a measure of how close the surface is to violating this boundary (how much clearance to virtual condition/simulators) instead of measuring some discrete metric and the specification of position/orientation can be thought of as describing only how the boundaries/simulators should behave in reference to the DRF (position – fixed location/orientation, orientation – fixed orientation) instead of some expected measurement of exactly “how” the surface deviates."

The current definition of MMC surface interpretation accomplishes this.

 

[chez311]

but that still doesn't mean that finding the actual value (that is, the value that would immediately and meaningfully tell by how much the feature axis deviates from its true position regardless if the size of the feature is at MMC, LMC or somewhere in between) is impossible

This is the current definition of all axis interpretation actual values for position (smallest value of b), as you pointed out previously - I'm not sure who said it is impossible? Maybe I'm missing what you're saying.

Are you referring to the fact that the diameter of the tolerance zone to which it must conform is negative? The actual value is certainly determinable as we showed - attempting to evaluate conformance to a tolerance zone of negative diameter should throw up some red flags though.