0 @ MMC vs. 0 @ MMB
0 @ MMC vs. 0 @ MMB
(OP)

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RE: 0 @ MMC vs. 0 @ MMB
Interesting idea about the zones, however.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: 0 @ MMC vs. 0 @ MMB
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
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
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
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
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
RE: 0 @ MMC vs. 0 @ MMB
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.
I did the two sketches, and did my best to lay out the virtual conditions to try to understand the reasoning behind the request.
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.
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
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
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: 0 @ MMC vs. 0 @ MMB
I think the point is the gage for either case is the same.
RE: 0 @ MMC vs. 0 @ MMB
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
#1.
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.
What if the simultaneous requirement was overriden by use of SEP REQT note?
RE: 0 @ MMC vs. 0 @ MMB
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
RE: 0 @ MMC vs. 0 @ MMB
RE: 0 @ MMC vs. 0 @ MMB
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: 0 @ MMC vs. 0 @ MMB
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
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
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
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
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
Is that right-hand picture acceptable per Y14.5, or not?
John-Paul Belanger
RE: 0 @ MMC vs. 0 @ MMB
Is there something in the standard or the interpretation that indicates otherwise?
RE: 0 @ MMC vs. 0 @ MMB
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
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
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:
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
Certified Sr. GD&T Professional
RE: 0 @ MMC vs. 0 @ MMB
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: 0 @ MMC vs. 0 @ MMB
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
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
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
RE: 0 @ MMC vs. 0 @ MMB
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
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:
RE: 0 @ MMC vs. 0 @ MMB
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
Certified Sr. GD&T Professional
RE: 0 @ MMC vs. 0 @ MMB
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
RE: 0 @ MMC vs. 0 @ MMB
Does the part in question mates with anything from the outside?
RE: 0 @ MMC vs. 0 @ MMB
RE: 0 @ MMC vs. 0 @ MMB
RE: 0 @ MMC vs. 0 @ MMB
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
If centering is your main concern, what is the reason behind applying position tolerance at MMC/MMB rather than at RFS/RMB?