Incomplete or profile general note applicable

[Kedu]

What is your opinion about this drawing?
Are datum feature B and datum feature C fully defined or are incomplete? Otherwise stated, is profile general note (profile|3|A|B|C|) applicable to "B" and "C" datum features?

 

[chez311]

Looks exactly like Fig 11-21 from Y14.5-2018.

 

[greenimi]

I see this note encouraging a lot of laziness. Instead of calling out secondary and tertiary datum features with their relationship to the primary and/or form control on primary, someone decided to use the general note as a cover. I don’t like it.
Same discussion found on linkedin about UOS note.

I know I don't make any friends of saying that, but that's my personal opinion.

Also, the OP figure is not THE SAME , it is not EXACTLY as the standard's figure: datum C is the middle or center plane of the width versus the left side of the part. Sorry, chez311, I had to nit-pick on your posting...

 

[chez311]

greenimi,

I feel similarly, however it isn't too uncommon to see such schemes on casted/molded parts especially with datum targets such as is found on the following figure 11-22. I'd much rather see either each datum feature controlled and qualified wrt higher precedence datum features, and in this respect the datum targets scheme makes more intuitive sense because they don't actually have to be "qualified". Its definitely strange though to see A held wrt |A|B|C|, but there was (mostly) consensus previously on how to treat such schemes and now its codified in the standard.

I agree it is not the same, I was going to amend my post after the fact as I noticed the difference in C being width-shaped however I didn't initially think the ramifications were that substantial. After reading your comment I realize I should have considered it more carefully - it doesn't seem to be to affect OP's question about "completeness" as I don't think its any less complete than 11-21. That said, I think that it changes the amount of tolerance zone available for datum feature C - unlike A and B where the tolerance zone is effectively halved. Since the profile tolerance is now located wrt the midplane of C instead of either surface the entire tolerance zone is potentially available for C.

 

[Belanger]

Also note that the given figure from the 2018 standard has an error: It says that "Shaded areas show unusable portion of the tolerance zone." But then they shade the entire zone.

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

 

[fsincox]

J-P,
Thank you, I was just looking at that and saying to myself something is wrong with that.

Chez,
Interesting observation on the centerplane derived datum, I think I would agree, meanwhile under the 2009-2018 standard can datum "B" be modified "MMB" to capture the whole part?
Frank

 

[chez311]

fsincox,

As JP pointed out that figure is definitely strange - see his other post on that (

https://www.eng-tips.com/viewthread.cfm?qid=465737)

it changes from fully shaded to not shaded at all depending on the level of zoom on the pdf version.

As far as setting B@MMB the effects would depend on what version you were using. In Y14.5-2009 datum features modified at MMB which do not fully "capture" the part/feature like fig 4-31(c) are required to make contact with the simulator. In Y14.5-2018 the same type of features are no longer required to make contact with the simulator see fig 7-36 as long as "one or more extremities shall remain between the MMB and the LMB". Since in both of these cases the simulator is fixed at MMB presumably the entire tolerance zone exists on one side of the simulator (entire tolerance zone available) instead of equally disposed on either side as the default in the RMB case (half the tolerance zone available), it surely doesn't say that anywhere explicitly even in 2018 but I think thats not too big of a leap to make. That said, with the contact required in 2009 the amount of the tolerance zone used depends on variation of the surface and in 2018 I'm having a hard time envisioning the consequences of a datum feature used to establish its own tolerance zone and is allowed to migrate away from itself as long as "one or more extremities shall remain between the MMB and the LMB". For the latter case I guess the entire surface is still required to be within the LMB/MMB instead of just the extremities.

Anyhow - I'm not sure the addition of the MMB modifier is a good solution to the problem of not being able to utilize the full tolerance zone. The standard presents an alternate case (and indeed, has been known as a solution some time before written into the standard) of utilizing a unilateral tolerance zone with the entire tolerance zone biased inside the material of the feature. This is stated in Y14.5-2018 para 11.4.3.1(b). Unfortunately as pointed out in the thread I referenced above, no figures are presented to show this alternate solution in the new standard.

 

[axym]

Hi All,

Another nuance with the OP drawing is that datum feature symbol C is in line with the 200 mm basic width dimension. Here's a question: does this mean that datum feature C includes both surfaces (not just the RHS), and datum C is then the centerplane? * I think I hear a can of worms opening * ;^)

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[chez311]

Evan,

I've seen this discussed elsewhere, but I'm not sure I understand the reasoning behind why this is controversial. Y14.5-2009 fig 3-4 (h) and figs 4-13/4-14 clearly show this type of notation to refer to a FOS datum feature and datum center plane. Is it an issue in combination with the basic dimension? Y14.5-2009 fig 4-33 specifically shows this notation in combination with a basic dimension to refer to a center plane (or rather center planes - 3 to be exact).

 

[Belanger]

chez311 -- I suspect the issue is that datum C in the OP graphic is not derived from a FOS.
That's because a FOS is defined as something that is "directly toleranced." The basic dimension makes it, well, not directly toleranced. That might bring up some internal wording issues in paragraph 4.17 (2009) when compared to paragraph 13.3.32.
I vaguely remember this issue being discussed earlier on this forum.

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

 

[Belanger]

As a tangent to my previous post -- in years past I also recall a debate regarding whether a general tolerance (title block tolerance) counts as "directly toleranced."
In 2018 they added a new paragraph (3.27) to explicitly answer that question (in the affirmative).

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

 

[chez311]

JP,

I thought so too a while back, until a colleague pointed out to me that fig 4-33 is labeled "Irregular and Regular Features of Size Datum Features". There are only three datum features in this figure, and all three are identically notated like OP's with basic dimensions* (assumption is that A is irregular FOS and B/C are regular FOS). I agree this opens up a looser interpretation as to what is meant by "directly toleranced" however the alternative is that this figure is labeled wrong and indeed there are actually no FOS (regular or irregular) datum features on this figure. Its worth noting that this figure remains in the 2018 version largely unchanged* (now fig 7-40) with the same title.

*Edit - I just noticed that C is not designated as basic, however no tolerances are applied and A and B are certainly basic. In 2018 the 80 width for C is also changed to basic. I'm willing to bet this was an error in 2009 not a purposeful omission.

 

[axym]

chez311,

Sorry, I didn't see your Apr 17 post that discussed the issue of C being a width and not just one surface. I agree with what you said - the profile tolerance zone is not halved for the C surfaces.

The issue I'm having is that a "width" datum feature is defined using a basic dimension. I would have thought that it needs to be a directly toleranced dimension - I wonder what others think of this?

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[chez311]

Evan,

You don't think the combination of basic dimensions and width-shaped datum features shown in fig 4-33 is sufficient to allow such definition?

 

[axym]

chez311,

I suppose I have an issue with Fig. 4-33 in 2009. Width B has a basic dimension, and width C has a directly toleranced dimension. I was never really comfortable with that. In Fig. 7-40 in 2018, width C now has a basic dimension. So Y14.5's intent is that the same type of datum feature (and datum) is created whether the dimension is directly toleranced or basic. In fact, there doesn't need to be a dimension at all. In the model-based view in Fig. 7-40, there are no width dimensions shown - just dimension lines! That just seems wrong to me. A datum feature in a model-based view is partly defined by a dimension line, for a dimension that is not present? Yuck. I wish that they had found a different way to encode that, such as explicitly indicating the two surfaces that are included in datum feature B.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

chez -- I thought of that too (the idea that it's an irregular FOS), but if you go back and look at the definition of an irregular FOS, even that sub-definition includes the idea of "directly toleranced." (para. 3.35.1 in 2018 or para. 1.2.32.2 in 2009)


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

 

[chez311]

So Y14.5's intent is that the same type of datum feature (and datum) is created whether the dimension is directly toleranced or basic.

Do you believe that a width shaped datum feature should be established or behave differently depending on whether it is associated with a directly toleranced(+/-) or basic dimension? If so, what would that look like? It seems to me that it is perfectly reasonable to want to establish a width shaped datum feature (and associated datum center plane) in conjunction with a basic dimension/profile tolerance and I can't think of a reason I would want the inherent behavior to be different in the way the datum feature is established (I'm of course not talking about conformance of the feature - that would be different for a directly toleranced vs basic dimension).

I won't debate the point about MBD, I don't know enough about it and it seems like a rats nest of its own.

 

[Kedu]

The discussion kind of drifted away from my original intent, but I guess my initial inquiry has been answered.

Chez311, Evan and all,

Regarding the new topic developed based on the original post, I would like to ask:
Why do you think the title of fig 7-40/ 2018 has been kept unchanged versus 2009 (just 80 was changed to basic and some profile FCF’s within 1.5|A|B(M)|)?

Which one of the datum features shown is regular Feature of Size?
“A”? Hmm, I don’t believe.
“B”? and “C”? Are they REGULAR feature of size (meaning rule#1 is applicable to them)?

2018: fig 7-40 has its title:
Figure 7-40 Irregular and Regular Features of Size as Datum Features

Going a little back to my original intent: the qualification of the datum features and implicit the usage of the Unless Otherwise Specified note -UOS (or lack thereof), (and I posted the same question on another forum), I would like to ask:

How do you read this “L” shape drawing attached?

Are the datum features B and C subject to the default profile note? Yes, No, maybe?

In other words, what qualifies/ what is the definition of datum features “B” and “C”? Are they incomplete?
(It is not the first time when I am posting this “L” shape drawing, but for different reason/ different topic discussed)

 

[Belanger]

In your latest graphic, to determine if datum features B and C appeal to the general profile note, we have to assess whether they qualify as regular FOS (thus subjecting them to Rule #1, which would impose a form control upon them). According to paragraph 3.35.2, a regular FOS must have "opposed surfaces." I would say that your datums B and C are not directly opposed, so they are not covered by Rule #1 and thus they get the general profile tolerance.

A similar idea applies to Fig. 7-40 in 2018: to determine if datums B or C are derived from features of size, we have to look at ASME's definition of FOS as given in paragraphs 3.35.1 and 3.35.2, which state that both regular and irregular FOS have to be directly toleranced.
So that leads us to ask what they mean by "directly toleranced," and that's given in paragraph 5.2.

So I'll pose to everyone: According to 5.2(c), are the datum features in Fig. 7-40 directly toleranced? In reading that, I'm now leaning toward the affirmative.

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

 

[fsincox]

J-P,
What am I missing here, in my 2018 version, para 5.2 only goes up to (c)??
The same figure is in 2009 (fig 4-13)
I can't help but think as they extend the limit on what are "features of size" (1982 did NOT say "OPPOSED") and move toward MBD the centerplane derived from basic dimensions has to be in the mix.

Frank