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?

 

[chez311]

Burunduk,

1) I would interpret it as maximum total allowable tolerance. Noted that it only appears in the section for O@MMC but I don't see why it couldn't be applied to a nonzero tolerance at MMC.

2) I'm honestly not sure. Something perhaps about the tolerance zone of a position tolerance being location constrained? Maybe the committee saw less reasoning why a design would require such behavior for position vs. orientation.

What I do see as inherently problematic is that for MMC, the surface interpretation takes precedence. The figure shows an axis interpretation of a "nice" feature - and leads us to believe it is as simple as shown. While the concept of bonus tolerance with axis interpretation and a "nice" feature with zero form error and a constant size is a convenient concept to help envision the variation allowed by MMC and accompanying variation in size. In reality, as long as the feature falls within its size tolerance and doesn't violate its virtual condition then it will pass.

I'm not sure that placing a limit on variation allowed per the axis interpretation jives with a control for which the surface interpretation takes precedence. Perhaps something like a variable virtual condition could be utilized? This seems strange and complex, certainly more complex than the figure would lead us to believe.

 

[Burunduk]

chez311,

I'd also like to interpret it as total allowable tolerance, but in that case, shouldn't the wording in para. 6.4.4 have been: "In such cases, the amount of additional total allowable tolerance may be limited by stating a MAX following the MMC modifier. See Fig. 6-15."?

Maybe the committee saw less reasoning why a design would require such behavior for position vs. orientation.

Recently I considered using this concept with position to limit the resultant condition of a hole, to preserve some minimum wall thickness around the hole. It wasn't desirable to tighten the hole size tolerance for this purpose, in this particular case.

I'm not sure that placing a limit on variation allowed per the axis interpretation jives with a control for which the surface interpretation takes precedence. Perhaps something like a variable virtual condition could be utilized?

I think that they decided that the surface interpretation takes precedence to always allow the use of functional gages, which is the main advantage of MMC modifier. A functional gage depicted in a figure could easily be used for visualization of a virtual condition along with the surface which is not allowed to violate it, but I'm not sure how can the maximum allowable orientation (dia 0.1 tolerance zone in this case) could be visualized with the surface interpretation. What would be your suggestion?

 

[chez311]

shouldn't the wording in para. 6.4.4 have been: "In such cases, the amount of additional total allowable tolerance may be limited by stating a MAX following the MMC modifier

I'm okay with the wording in the standard. I don't see anything wrong with the sentence as written - I don't have a good succinct reason, as it would require tearing down the sentence semantics and grammar, but it makes logical sense to me. That in addition to the fact that the table for 6-15 shows "diameter tolerance zone allowed" with no column for "bonus tolerance allowed".

Regardless - such confusion could be fixed by avoiding the practice altogether.

Recently I considered using this concept with position to limit the resultant condition of a hole, to preserve some minimum wall thickness around the hole. It wasn't desirable to tighten the hole size tolerance for this purpose, in this particular case.

If your desire is to limit wall thickness at LMC, a control specified at LMC can be added in an additional segment/FCF. It may be strange to some, but MMC and LMC can exist together on a single feature - there is no prohibition for it, and I would say its more easily inspected and interpreted than an MMC MAX control per 6.4.4, as well as better captures functional intent.

but I'm not sure how can the maximum allowable orientation (dia 0.1 tolerance zone in this case) could be visualized with the surface interpretation. What would be your suggestion?

Perhaps someone could come up with a way to evaluate a virtual condition which varies along its length depending on the measured size of each segment/cross-section of the feature. This would exist in the virtual world through CMM software, if this is even a valid concept. The other option would be to establish a fixed virtual condition like normal and limit the feature's UAME variation to the MAX value - this combination seems unwieldy at best, and I'm not sure it would fully accomplish the result you're looking for (limiting wall thickness).

I think overall this is a poorly thought out concept.

 

[Burunduk]

The other option would be to establish a fixed virtual condition like normal and limit the feature's UAME variation to the MAX value - this combination seems unwieldy at best, and I'm not sure it would fully accomplish the result you're looking for (limiting wall thickness).

When talking about the virtual condition we should probably be discussing position, not perpendicularity as the 0.1 MAX tolerance zone in fig 6-15, right?

 

[Burunduk]

I am also wondering how MMC and LMC can work together. Might need some help grasping it.
Let's suppose position 0 at MMC and position 0 at LMC applied on the same feature. The first says the entire size tolerance is available for position if the UAME size = LMC. The latter says 0 position at LMC. Not sure what to make out of it.

 

[chez311]

When talking about the virtual condition we should probably be discussing position, not perpendicularity as the 0.1 MAX tolerance zone in fig 6-15, right?

I don't follow. Both position and perpendicularity at MMC create a virtual condition, I was not specifically talking about one or the other.

Let's suppose position 0 at MMC and position 0 at LMC applied on the same feature. The first says the entire size tolerance is available for position if the UAME size = LMC. The latter says 0 position at LMC.

MMC does not say that. Let's presume your 0 at MMC and 0 at LMC is applied to a hole (internal feature). Each segment must be satisfied individually just like any other multiple segment callout, they do not contradict each other. All that MMC dictates is a virtual condition of MMC size minus position tolerance which cannot be violated and the feature must be within its size tolerance. LMC likewise dictates a virtual condition of LMC size plus position tolerance which cannot be violated, of course the feature must be within its size tolerance. In this case because your position tolerance at MMC and LMC is 0, you have two boundaries of LMC and MMC size (LMC position VC and MMC position VC respectively) fixed at true position. As long as the feature does not violate either of these boundaries and falls within the specified size tolerance it passes.

If you want to visualize the feature's behavior, when it is at MMC size it is allowed 0 position error. The amount of position error allowed increases as the size increases towards LMC. This reaches a maximum when the feature's size is directly in the middle of the specified size tolerance. At this point the amount of position error allowed begins to decrease, and when the feature is at LMC size the amount of position error allowed is once again 0.

 

[greenimi]

In this case because your position tolerance at MMC and LMC is 0, you have two boundaries of LMC and MMC size (LMC position VC and MMC position VC respectively) fixed at true position.

The amount of position error allowed increases as the size increases towards LMC. This reaches a maximum when the feature's size is directly in the middle of the specified size tolerance. At this point the amount of position error allowed begins to decrease, and when the feature is at LMC size the amount of position error allowed is once again 0.

Is this behavior same as profile applied to the hole?
Maybe not.

But perfect form at MMC and perfect form the LMC is required when profile is applied (and the nominal size is basic). Am I correct?

 

[chez311]

Greenimi,

Yes, perfect form would be required at both LMC and MMC. In this example with zero position at both LMC and MMC and analogous profile control would create identical boundaries and have similar if not identical Behavior between those boundaries. The feature would of course have to be specified at a basic diameter and any controls requiring a regular feature of size would have to be modified or eliminated, for example a perpendicularity refinement.

I do not think the same could be said for non-zero position at either LMC, MMC, or both especially when the feature is part of a pattern. It could possibly be approximated with profile, but not nearly identical.

I would say in that case a combined MMC and LMC position control would be far more elegant and concise. Especially if the intent was to avoid use of the MMC MAX specification.

 

[Burunduk]

I don't follow. Both position and perpendicularity at MMC create a virtual condition, I was not specifically talking about one or the other.

Now I understand that you were speaking generally. I had in mind position and perpendicularity as a refinement, as this is what shown in fig. 6-15.
Taking into account both, the virtual condition boundary diameter would be 49.7 - actually not affected by the perpendicularity refinement. Am I right?

Still not following you about the varying (local?) Virtual condition boundary to replace the axis representation of dia. 0.1 tolerance zone. Regarding the second option, not sure why you suggested limiting the UAME variation (Feature size?) to the MAX value.

Thank you for the explanation about the combination of MMC and LMC on the same feature. I now realize that they don't contradict each other. I now understand that they create 2 limiting boundaries - one inside the material, the other outside the material, and each control is only able to give "bonus" at its side, limited by the mid-tolerance diameter. Correct?

 

[chez311]

Taking into account both, the virtual condition boundary diameter would be 49.7 - actually not affected by the perpendicularity refinement. Am I right?

Each control would create a separate virtual condition, none of which may be violated. On fig 6-15 the position control creates a VC of 49.7 fixed in location/orientation to |A|B|. The perpendicularity control creates a VC of 50 fixed in orientation only to |A|.

Still not following you about the varying (local?) Virtual condition boundary to replace the axis representation of dia. 0.1 tolerance zone.

I suggested it only as a hypothetical as a way to evaluate a MMC MAX callout fully under the surface interpretation. I'm not sure if its a valid concept as I don't really like the idea of a varying virtual condition. Someone else could possibly chime in on this.

Regarding the second option, not sure why you suggested limiting the UAME variation (Feature size?) to the MAX value.

The UAME is affected by size variation but the two are not analogous, but that is what is utilized with the axis (resolved geometry) interpretation - the UAME is what defines the feature axis.

 

[chez311]

Thank you for the explanation about the combination of MMC and LMC on the same feature. I now realize that they don't contradict each other. I now understand that they create 2 limiting boundaries - one inside the material, the other outside the material, and each control is only able to give "bonus" at its side, limited by the mid-tolerance diameter. Correct?

Glad to help. You generally have the correct idea, but like I said the concept of "bonus" is just a convenient way to visualize variation allowed by an MMC or LMC callout. In fact, the word "bonus" does not even appear in Y14.5 at all. At its purest form, MMC/LMC controls specify a boundary of fixed size*, constrained to applicable datum features, which may not be violated. The feature is still subject to the specified size tolerance and rule#1 (if applicable).


*Which is why the concept of a "variable virtual condition" doesn't sit well with me. It was just a hypothetical suggestion.

 

[chez311]

As a side note - perhaps an MMC MAX specification forces a fully axis interpretation (instead of something combined or fully surface interpretation which I suggested before)? A formula for axis/resolved geometry interpretation of MMC and LMC is provided in Y14.5.1-1994 and was present in the new 20XX draft as well (yet to be seen if it will be present in the final release). This is not explicitly stated, and conversely Y14.5 specifically notes that the surface interpretation takes precedence for an MMC control with no exceptions.

I'm interested to hear others thoughts on this.

 

[Burunduk]

chez311, thank you for reminding me that each feature control frame that includes a material condition modifier has its own virtual condition. I somehow got used to using this term when referring to the absolute worst case for assembly (in terms of parts not fitting together).

Attempting the surface interpretation for the MMC MAX concept - perhaps the resultant condition boundary should also be looked at? I think that the RC boundary diameter for perpendicularity in fig. 6-15 is 50.26 (instead of 50.32 that should have been without the MAX specification, but the complicated thing here is that it is not enough to say that the VC and RC are the only boundaries that limit the surface at tilting of the hole. Starting from dia. 50.1 and up to 5.16 there is a UAME size-dependent boundary (of a diameter that equals to UAME size + 0.1) that limits the surface for any given hole within that size range.

That is probably why the axis interpretation is a much simpler visualization method for this concept.

Hope I'm not utterly wrong here about anything.

 

[pylfrm]

The requirements described in ASME Y14.5-2009 Fig. 6-15 can also be expressed as follows:

⌀50.00-50.16
[box]⌖[/box][box]⌀0.3Ⓜ[/box][box]A[/box][box]B[/box]
[box]⟂[/box][box]⌀0Ⓜ[/box][box]A[/box]
[box]⟂[/box][box]⌀0.1[/box][box]A[/box]

The standard should not have introduced an obscure new method to achieve the same result in slightly less space.

A surface interpretation for the RFS tolerance is described in ASME Y14.5.1M-1994, but not Y14.5-2009.


Burunduk,

A resultant condition boundary doesn't limit anything. It's just a consequence of the actual tolerance requirements.


pylfrm

 

[chez311]

pylfrm,

Thank you, I agree if the desire is to limit the UAME deviation as was one of my suggestions then your explicit method would be far better than trying to infer such through a single MMC MAX specification per Y14.5 fig 6-15. My other suggestion of a "variable virtual condition" for a purely surface interpretation as I said did not really sit well with me, but I was trying to brainstorm exactly what was meant by the control in the standard.

Unfortunately this figure and associated text remains almost unchanged in the 2018 version (paragraph 9.3.4 and 9-15) with no additional clarification.

If for some reason it is desired to limit UAME deviation by an RFS control then I would go with the method you described. If on the other hand the desire is to limit wall thickness as suggested by OP in one of their subsequent posts - would you agree that a combined MMC and LMC control would be viable instead?

A surface interpretation for the RFS tolerance is described in ASME Y14.5.1M-1994, but not Y14.5-2009.

I know we have discussed this in the past, and I still don't care for this interpretation though I at least have a better understanding of it thanks to you. What application do you see for it here though? It initially seemed to me that an axis interpretation of MMC would better capture the intended meaning of the MMC MAX specification.

 

[Burunduk]

The requirements described in ASME Y14.5-2009 Fig. 6-15 can also be expressed as follows:

⌀50.00-50.16
⌖⌀0.3ⓂAB
⟂⌀0ⓂA
⟂⌀0.1A

Interesting! Thanks pylfrm for this insight.

It should be noted that saying that the two schemes are equivalent is essentially associating the dia 0.1 MAX restriction with an RFS control. That means that there is no reason to look for a surface interpretation, the axis interpretation shown in the figure represents the control just fine.

 

[tim_member]

To nit-pick a bit, as long as '94 or '09 version of Y14.5 governs the drawing, the combination of position and two perpendicularities, as suggested by pylfrm, isn't technically identical with what is shown in Fig. 6-15. The perpendicularity tolerances are not subject to simultaneous requirement rule, therefore it is possible, although rather unlikely, that they could be verified relative to different candidate datums A.

The problem doesn't exist in Y14.5-2018 where a single solution is the default condition for establishment of a datum from a datum feature with irregularities.

 

[chez311]

the combination of position and two perpendicularities, as suggested by pylfrm, isn't technically identical with what is shown in Fig. 6-15. The perpendicularity tolerances are not subject to simultaneous requirement rule

Simultaneous requirements only applies to position and profile. The difference you describe is by virtue of it being two separate perpendicularity controls instead of a single unclear and ambiguous control.

 

[Burunduk]

chez311,
A note related to the surface interpretation taking precedence for MMC controls: the text at the bottom of the "means this" section of fig. 6-15 ends with the statement: "The feature axis must be within the specified tolerance of location".
Given that the position tolerance is specified MMC I would expect the something like: "No portion of the surface may violate the boundary of..."
Thoughts?