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0 @ MMC vs. 0 @ MMB
2

0 @ MMC vs. 0 @ MMB

0 @ MMC vs. 0 @ MMB

(OP)

RE: 0 @ MMC vs. 0 @ MMB

I'd say it's a violation, since the hole has zero tolerance, regardless of its size. The datum's shift just says that you might be able to jiggle the part around, but the hole's axis must still be perfectly aligned.

Interesting idea about the zones, however.

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

RE: 0 @ MMC vs. 0 @ MMB

weavedreamer,
May I know why you would want to apply a tolerancing scheme as shown on the picture on the right?

RE: 0 @ MMC vs. 0 @ MMB

(OP)
Per the explanation as I understand it, pmarc, the two dimensioning schemes should yield the same result.

On the left, Zone C would be the datum, regardless of feature size, while Zone A uses the 0.012 diameter bonus as the feature deviates from 20.045 toward 20.057.

Per the right illustration, Zone A would be the Datum with the 0.012 shift permitted as the feature deviates from 20.045 toward 20.057 while expecting Zone C to act, much as it would as the datum in the example on the left.

The obvious advantage is the more repeatable length avialable in Zone A to be used as Datum B.

RE: 0 @ MMC vs. 0 @ MMB

Unless leaving the MMC modifier was a typo (right hand figure) it's a bad callout.

Besides that, these will not yield the same results. Only on perfect parts will that happen. Don't be surprised to see a significant difference.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

RE: 0 @ MMC vs. 0 @ MMB

PH and Pmarc -- I do see why he/she is asking.
In the first case the datum is always true (based on an expanding gage) and the variation on the toleranced feature causes a "fudging" from the zero (called bonus). In the second case the datum might be "fudging" (called datum shift), while the other feature is always true. In either case there is still some variation between actual axis and actual axis.

It violates the standard, but I'm trying to verbalize the reasoning of the question.

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

RE: 0 @ MMC vs. 0 @ MMB

weavedreamer,

It seems to me that two schemes are exactly equivalent. In both cases, the surface in ZONE A must not violate a diameter 20.045 boundary concentric to the feature axis established from the surface in ZONE C.

If the two schemes are equivalent, then the "obvious advantage" you cite must not actually exist. I can't imagine any other possible advantages either. The major disadvantage of extreme strangeness remains.

I don't know whether it's a violation of the standard. However, using a tolerance of diameter 0.000000001 is certainly not, and would achieve practically the same result.

pylfrm

RE: 0 @ MMC vs. 0 @ MMB

The zones aren't exactly equivalent, but both versions are legitimate.

RE: 0 @ MMC vs. 0 @ MMB

(OP)

Quote (powerhound (Mechanical))


Unless leaving the MMC modifier was a typo (right hand figure) it's a bad callout.

Besides that, these will not yield the same results. Only on perfect parts will that happen. Don't be surprised to see a significant difference.
The MMC modifier was specified as MMB in conjunction with switching the Datum and Geometric callouts, to keep the bonus/shift with the same limit dimension.

Quote (Belanger (Automotive))


It violates the standard, but I'm trying to verbalize the reasoning of the question.
I did the two sketches, and did my best to lay out the virtual conditions to try to understand the reasoning behind the request.

Quote (pylfrm(Mechanical))

It seems to me that two schemes are exactly equivalent.
In both cases, the reasoning leads me to the same the same layout of the geometry.

I can't point to the specific "what" in the standard that it violates, unless I am looking at it, and just cannot articulate it.


Quote (3DDave (Aerospace))


The zones aren't exactly equivalent, but both versions are legitimate.
I'm coming up with exact equivalency. It is the legitimacy I am trying to ascertain.

Thanks, everyone, for the feedback.

RE: 0 @ MMC vs. 0 @ MMB

Your example is a form of the property => A*B = B*A. There's not much in the field to identify equivalencies like this.

Are you expecting a phrase or paragraph from a standard that says you can't do this? What proof is required?

RE: 0 @ MMC vs. 0 @ MMB

Perhaps the issue lies in how such a thing could be measured. Since there's no MMC modifier, you can't use a functional gage. You'd need a gage that is variable or something that outputs a number. So... how can any instrument verify a perfect part? (That's what the zero tolerance RFS means.) To what accuracy?

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

RE: 0 @ MMC vs. 0 @ MMB

Same gage as is variable to adapt to an RFS datum reference.

I think the point is the gage for either case is the same.

RE: 0 @ MMC vs. 0 @ MMB

(OP)

Quote (Belanger (Automotive))



Perhaps the issue lies in how such a thing could be measured. Since there's no MMC modifier, you can't use a functional gage. You'd need a gage that is variable or something that outputs a number. So... how can any instrument verify a perfect part? (That's what the zero tolerance RFS means.) To what accuracy?
I think you just helped to trigger why I didn't see the fallacy in this earlier. If two features were related back to Datum B at 0 RFS, the simultaneous gaging would ignore the shift produced by an MMB callout between the two features. The two features would be stipulated to be zero tolerance RFS to each other, where 0 at MMC would permit tolerance as the features moved toward their LMC counterparts.

Even though the shift acts like the bonus in this singular relationship, they are different ways of regarding the relationships between the features in question.

RE: 0 @ MMC vs. 0 @ MMB

Two (a little bit provoking winky smile) questions:

#1.

Quote (J-P)

Perhaps the issue lies in how such a thing could be measured. Since there's no MMC modifier, you can't use a functional gage. You'd need a gage that is variable or something that outputs a number. So... how can any instrument verify a perfect part? (That's what the zero tolerance RFS means.) To what accuracy?
Assuming a hard gage is not used, for an undoubtedly legitimate |pos|0(M)|B(M)| callout, if the size of Unrelated Actual Mating Envelope (UAME) of toleranced feature equals its MMC size, and the size of UAME of datum feature B equals its MMC/MMB size, the toleranced feature must be perfectly coaxial with datum axis B, right? So does it mean that such actual part condition is impossible to verify?

#2.

Quote (weavedreamer)

If two features were related back to Datum B at 0 RFS, the simultaneous gaging would ignore the shift produced by an MMB callout between the two features. The two features would be stipulated to be zero tolerance RFS to each other, where 0 at MMC would permit tolerance as the features moved toward their LMC counterparts.
What if the simultaneous requirement was overriden by use of SEP REQT note?

RE: 0 @ MMC vs. 0 @ MMB

Any zero position tolerance with 'bonus' on either feature or datum has the same problem. If the part runs up to the limit it has to be perfect. But then using any non-zero tolerance has exactly the same verification problem when the variation approaches the limit allowed.

Read up on how the NIST verifies gage blocks and accounts for the fact that light is not reflected from the metal surface, but a fraction of a wavelength inside the metal, leading to different values than from mechanical measurements. The closer one gets the more variables need attention.

RE: 0 @ MMC vs. 0 @ MMB

(OP)

Quote (pmarc (Mechanical))


#2. What if the simultaneous requirement was overriden by use of SEP REQT note?
If the standard were to adopt |pos|0|B(M)|, any and all simultaneous gauging between two or more of such instances would have to mandate the SEP REQT caveat.

RE: 0 @ MMC vs. 0 @ MMB

There are a lot of non-functional legitimate combinations that aren't proscribed by the standard. Dealing with them would make the standard very large indeed.

RE: 0 @ MMC vs. 0 @ MMB

So you guys are saying that the 0 tolerance RFS (with MMB) is legitimate, per Y14.5?

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

RE: 0 @ MMC vs. 0 @ MMB

Zero position tolerance at RFS to NOT per the standard. Logically it makes no sense as nothing can be perfect and must have tolerance to define how imperfect it can be. MMC must be used with zero position to gain tolerance from feature size change.

Regarding datum B at MMB: This is to the standard... but...I wonder, under what real-world design conditions would this dimension schema be used? I think its good for case-study but not much else. We have a "similar" condition with and assembly of a (rotating) turbine onto a shaft, where zone A and zone C are to be coaxial so the fit is uniform. We dimension zone B larger in diameter so it has clearance and is not part of the fit-up between the part and the shaft (at zones A and C). This is an RMB datum condition and not MMB, so shift it not allowed, but the application of MMB could be applied (as shown) depending on the function and fit-up requirements.

Certified Sr. GD&T Professional

RE: 0 @ MMC vs. 0 @ MMB

Quote (3DDave)

There are a lot of non-functional legitimate combinations that aren't proscribed by the standard. Dealing with them would make the standard very large indeed.

Quote (mkcski)

under what real-world design conditions would this dimension schema be used? I think its good for case-study but not much else.

These two quotes pretty much sum up this argument, in my opinion. It's an intriguing thought but what's the real world use of the right side drawing that isn't legally expressed another way?

I don't think this is a legitimate callout for the following reasons:

1. There is no support for a 0 @ RFS geometric tolerance in the standard.
2. There is a legitimately supported way to achieve the same results. No need to get overly creative about it.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

RE: 0 @ MMC vs. 0 @ MMB

mkcski - It's fine if you read the entire callout into words, to wit:

Does the axis of this feature align perfectly with the axis of the referenced datum feature that is allowed to shift within the tolerances allowed on that datum feature axis?

The exact same condition exists when a 0 position tolerance at MMC is verified when the feature is at MMC.

If the second callout is legal, then the first is also.

All that is necessary for a part to be accepted is for an inspector to find a case where the axes are aligned.

RE: 0 @ MMC vs. 0 @ MMB

While I agree with mkcski that 0 RFS with datum MMB is not legal, perhaps it could be thought of this way: The feature toleranced as 0 actually creates the datum, and the item identified as datum feature A will be the one either accepted or rejected.
In other words, the datum is supposed to be the "driving" factor, but in this case the datum is "driven," simply because the feature that is being tagged with position is stated as being perfect, while the datum feature is the one feeling the variation.
I wouldn't say that this is an extension of principles, but a misuse of the principles. (Makes for a good academic discussion, though.)

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

RE: 0 @ MMC vs. 0 @ MMB

JP - this is the same case for all RFS feature references to datums with MMC/MMB conditions. The Datum simulator will always be backdriven by the RFS simulator. It's just in some cases there is an additional flexibility when the RFS reference is not '0'.

Tolerance simulation software, such as VSA will have no trouble with this.

Too many are looking at their cookbooks without understanding the math.

Would it be much better if it was .000000000001 instead of 0?

RE: 0 @ MMC vs. 0 @ MMB

So I need a straight-up answer from you Dave smile
Is that right-hand picture acceptable per Y14.5, or not?


John-Paul Belanger

RE: 0 @ MMC vs. 0 @ MMB

Already said they were both legitimate. I was still considering at the time that there was some other difference but, after more consideration, I think they are identical.

Is there something in the standard or the interpretation that indicates otherwise?

RE: 0 @ MMC vs. 0 @ MMB

I'll say acceptable as well. Shame on me for being noncommittal earlier. This time I have some supporting evidence:

ASME Y14.5.1M-2004 provides two interpretations for positional tolerancing. Para. 5.2.1 defines the surface interpretation, and para. 5.2.2 defines the resolved geometry interpretation. Para. 5.1.1 states that the surface interpretation shall take precedence if the results are not equivalent.

For both interpretations, the tolerance specification is defined in such a way that a zero tolerance value applied on an RFS basis does not create a problem, and still allows conforming parts to exist.

Additionally, the actual value of position deviation is defined as the smallest tolerance value to which the feature conforms.

For any part that meets the requirements of OP's left-hand picture, can we agree that the actual value for the position tolerance in the right-hand picture is zero? If not, then what else might it be?

pylfrm

RE: 0 @ MMC vs. 0 @ MMB

It's true that nothing in the standard says that you cannot use zero RFS, but there's still something weird about it. However, I agree that the net effect is the same so I'll accept it at that.
pylfrm -- for your last question, I would say that you don't even need the qualifier that forms the first part of your sentence. This is because the actual value for the position tolerance in the RH picture is always zero for an acceptable part, simply because that's what the callout prescribes.

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

RE: 0 @ MMC vs. 0 @ MMB

(OP)
With a little more assistance, I was able to get clarified where the weirdness lay.

In Paragraph 7.3.4 is found "the positional tolerance allowed is totally dependent on the unrelated actual mating envelope size of the considered feature, as explained in para. 2.8.3." This is reiterated in the referenced paragraph 2.8.3 also.

When I earlier stated:
Even though the shift acts like the bonus in this singular relationship, they are different ways of regarding the relationships between the features in question.

The difference is: the positional tolerance comes from the unrelated actual mating envelope of the feature of size, not the the datum feature of size.

RE: 0 @ MMC vs. 0 @ MMB

I like your analysis of the difference in affect of size change between the feature vs the datum. Well expressed! The difference is "hard" to see when the datum and feature are both F.O.S. and have a coaxial relationship.

Certified Sr. GD&T Professional

RE: 0 @ MMC vs. 0 @ MMB

Weaverdreamer, I'm not sure I see that as being any different. Yes, the position tolerance is looking at the unrelated actual mating envelope, but the datum is also derived from the unrelated actual mating envelope.

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

RE: 0 @ MMC vs. 0 @ MMB

(OP)
Belanger,

As each dimension is supposed to have a tolerance, direct or indirect, the 0@MMC gets the tolerance zone from the unrelated actual mating envelope size of the feature of size.

Specifying it as 0@RFS, effectively leaves it without a tolerance zone. This is the violation of the standard.

Adding the datum feature of size in acted as a diversion.

RE: 0 @ MMC vs. 0 @ MMB

I mentioned in my first post that it was a violation, but subsequent comments swayed things to make me agree that they are effectively the same.
FYI, all of your initial pictures involve unrelated actual mating envelopes (UAME). The only difference is that MMC creates a constant-size UAME and RFS creates a variable-size UAME. But does that really make the two pictures different?

So allow me to rephrase your original post: An UAME has to be found for Zone A. And an UAME has to be found for Zone C.
The left-hand picture in your OP asks us to compare the axis of Zone A's constant-size UAME to the axis of Zone C's variable-size UAME.
Then, the right-hand picture asks us to compare the axis of Zone C's variable-size UAME with the axis of Zone A's constant-size UAME.
Notice how the words come out the same!

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

RE: 0 @ MMC vs. 0 @ MMB

I agree with John-Paul's conceptual statements. However....realistically is makes no sense to me to have the position tolerance on the Zone C feature in the right-hand sketch totally dependent on the size change of the Datum B. This just seems backwards and contrary to design practice using GD&T.

Additionally, the short length of the datum feature Zone C in the left-hand view compared to the feature length Zone A is questionable for creating a repeatable datum simulation.

Certified Sr. GD&T Professional

RE: 0 @ MMC vs. 0 @ MMB

The only requirement for RFS is that one orientation can be found that meets the requirements. In practice there is the potential for multiple, equally valid solutions.

RE: 0 @ MMC vs. 0 @ MMB

Quote (weavedreamer)

Specifying it as 0@RFS, effectively leaves it without a tolerance zone. This is the violation of the standard.

The tolerance zone does not disappear, it collapses to a line. If the feature axis is coincident with the "tolerance line", then the requirement is met. At least this is how it's described in Y14.5.1-1994.


Side note: In my previous post, I mistakenly wrote Y14.5.1-2004 (which does not exist). That should have been Y14.5.1-1994 (which was last reaffirmed in 2012).

pylfrm

RE: 0 @ MMC vs. 0 @ MMB

(OP)

Quote (pylfrm)

The tolerance zone does not disappear, it collapses to a line. If the feature axis is coincident with the "tolerance line", then the requirement is met. At least this is how it's described in Y14.5.1-1994.
My wording didn't convey this well. Rephrase as:

Specifying it as 0@RFS effectvely leaves the zone for the feature axis without a tolerance. This is the violation of the standard, per my ongoing struggle to clarify and articulate my understand of it here. The tolerance zone for the feature axis is supposed to come from its unrelated actual mating envelope (paragraph 7.3.4). Invoking MMB makes it appear to be solved via the related actual mating envelope.

In essence, this concurs with Mr. Belanger's first reply:

Quote ( )

I'd say it's a violation, since the hole has zero tolerance, regardless of its size. The datum's shift just says that you might be able to jiggle the part around, but the hole's axis must still be perfectly aligned.

RE: 0 @ MMC vs. 0 @ MMB

Quote:

Specifying it as 0@RFS effectively leaves the zone for the feature axis without a tolerance.
No -- it only leaves the zone without a tolerance with respect to the datum axis derived from a MMB datum feature.

In reality, the actual datum feature might deviate (most likely it will deviate) from the MMB. Thus, the zero tolerance is following a moving axis -- which really isn't a zero tolerance anymore.

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

RE: 0 @ MMC vs. 0 @ MMB

My head is spinning. In the real world, who would specify position like this when even those in-the-know cannot agree on an (the) interpretation. Please see Y4.5 2009, paragraph 1.4 (d)

Certified Sr. GD&T Professional

RE: 0 @ MMC vs. 0 @ MMB

This discussion is really interesting, but could we go back to the beginning of it for a moment?

My understanding of weavedreamer's second post is that the idea of changing datum feature from zone C to zone A was a result of concern about repeatability of datum axis B derived from such a short portion of the cylinder in zone C. So what if the repeatability problem is gone? In other words, perhaps in reality the cylinder in zone C works as a secondary datum feature, not primary? Any chance to learn more about part mating relationships with other components in assembly?

RE: 0 @ MMC vs. 0 @ MMB

(OP)

RE: 0 @ MMC vs. 0 @ MMB

weavedreamer,
Does the part in question mates with anything from the outside?

RE: 0 @ MMC vs. 0 @ MMB

(OP)
It is sandwiched top and bottom; capping off zone A, and riding the niche around the outside of zone C. If it helps, consider two horizontal plates, one with a hole it.

RE: 0 @ MMC vs. 0 @ MMB

Hmmm... Let me ask this way: what is the biggest player when it comes to orienting and locating this part in assembly? Two plates and the hole OR the plate with the hole only OR the piston OR the cover plate at the bottom?

RE: 0 @ MMC vs. 0 @ MMB

(OP)
The part would pilot on a large diameter concentric to the bore that the piston is in, sliding the zone c end into a hole until it rests on the mating side of the coverplate covering zone a.

The bigger issue is in the portion isolated and presented so far. It deals with the amount of rotation the piston can encounter. If you study the extended context, detail c[5], the piston has been rotated ccw. The challenge is keeping the smaller slip fit diameter concentric to the larger one.

RE: 0 @ MMC vs. 0 @ MMB

So why did not you consider controlling position of the bore in zone C relative to the bore axis in zone A from the very beginning?
If centering is your main concern, what is the reason behind applying position tolerance at MMC/MMB rather than at RFS/RMB?

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