Simultaneous requirement 5

[DesignBiz]

In a previous thread regarding “profile and a datum”.

http://www.eng-tips.com/viewthread.cfm?qid=241508

A dwg was posted in support of the datums callouts. However as I looked at the dwgs from this post more closely I saw at the bottom a dwg with a different dimensioning scheme.

http://www.tec-ease.com/tips/dec-07.htm

The link for the drawings claims the lower dwg is the same effect as the upper drawing dimension scheme, based on the simultaneous requirement. (pg 92 para 5.3.6.1 and 5.2.6.2 for the ASME Y14. 5M standard)

In the 1st (upper) dwg, a large hole in the center of a square part with a pattern of 4 holes located around a center hole is dimensioned to datums A (back surf of part); B (center hole); and C (height of part). Simple enough, however the bottom drawing shows the same part with the same callouts minus the B and C datums and claims this 2nd dimensioning scheme is the same as the 1st dimensioning scheme based on the “simultaneous requirement” rule.

I disagree. When datums B and C are taken away there is no datum to orient and locate the features to each other. The 2nd drawing only controls mutual perpendicularity to datum A, and the dimensional relationship to the pattern of 4 holes to each other. There is no longer any horizontal or vertical relationship to the features of the lower dwg as in the upper dwg.
I don’t believe this is a valid interpretation of the standards “simultaneous requirement” rule comparing these 2 drawings.

The lower drawing indeed shows a “simultaneous requirement” for all of it's feature callouts, however it is not comparable to the 1st or upper drawing.

Agree? Or tell me how am I incorrect.







DesignBiz

 

[ProfDon]

Not to worry, Ringster. People often say that if a part is controlled by ± tolerances the mating part should be also. Just because one part is poorly defined doesn't mean that poorly definning the mating part will make it OK. The newer revisions allow us to better define our parts. If anything, we will have a better shot at having them work right the first time, even if the matng parts were toleranced to a previous Standard. NASA has had problems with components not fitting or working once they get to the Space Station (toilets, etc.) because they do not always use GD&T (public knowledge). They have been receiving GD&T training lately and this will improve the odds of success. But, this sounds like a new thread.

 

[DesignBiz]

Thanks to all for your contributions. There is one reason I spend time on this site and it is to “learn” more about the application of the standard(s)and how designers and engineers interpret it. I have been in the business for many years and have seen evolutions of Y14.5, yet in the course of creating and modifying components and assemblies that make up products in the real world, there is the practical side of applying standards.

ProfDon, I appreciate your efforts to respond to this question regarding the “bonus shift” however I am eyes wide-opened when you present your credentials as committee member and follow that up by stating the difficulty in explaining what the committee has written in the standard. WOW!!! How could any management endorse a design standard that most design engineers couldn’t drive a common meaning in light of the authors’ admittance that the work is not so understandable? This is beginning to sound like something Congress wrote. That is not good, not with today’s congress!

I am into perseverance though. I am quite confident in my ability to understand words even if I am not on a committee. BTW North, “For God so loved the world, He did not send a committee!” (That has never been revised)

Okay, back to the subject that seems to be disappearing from the dialog.

Paul, you say that omitting the reference to any datum is a valid control callout. However you have not reconciled the requirement for tolerance of position callouts to have datum references. “In the standard para. 5.2.1.3 states that; it is "necessary" to identify features on a part to establish datums for dimensions locating true positions.” What do you do with this? (This particular example is “implied zero” dimension, never the less it is a dimension of location which requires a datum)

ProfDon, North,
The standard on the drawing regards what is legally required to conform to, neither past, nor future versions.
No, a Model T manual would probably not work with a modern day vehicle, for the most part, however if I have a Model T to work on then it is in the mechanic’s and the owner’s best interest, for the mechanic to use a Model T repair manual. This example is relevant to the 1994 standard I believe, not 1966, not 1973, not 1982, not 2009.

ProfDon,
You state; “The gage is made at the virtual condition of B and C.”
The 1994 standard states; (para. 5.3.2.2 pg 85) “Since the axis of the datum feature’s actual mating envelope MUST serve as the origin of measurements for the pattern of features, the features are therefore viewed as if, as a group, had been displaced relative to the axis of the datum feature’s actual mating envelope”.

1.3.24 Size, Actual = the general term for the size of a produced feature. This term includes the actual mating size and the actual local sizes.

1.3.26 Size, Actual Mating = the dimensional value of the actual mating envelope.

In my above post I used the formulas from the standard. Just to claim that I don’t understand, without pointing out the specific errors I may have made, and also not to provide a corrected solution does nothing to prove your point in my view. If I posted errors then please point them out.

Context, context, context !!! The words; “let’s eat honey”. If I am with my wife and am talking about dinner, then it would be reasonable to presume I am talking about eating dinner. If I am walking thru the woods and discover a beehive then it is reasonable to presume that I want to eat bee’s honey.

What are we talking about here? The claim is that |position |dia. 0.2 MMC| A| B MMC| C MMC| = |position| dia. 0.2 MMC| A| ???? Sure doesn’t seem logical. Really? That regardless of a “bonus shift allowance”, all gages of this kind use the virtual gage pin center as the origin for measurements? There may have been some confusion on the meaning, however the 1994 standard states what it states, and it states that the measurement is taken from the center of the actual mating envelope. It does not state it is measured from the virtual gage pin center.

I learn by challenging where I perceive potential errors even if an acknowledged authority says it. The true authority is in the meaning of the standard, not just whatever a credentialed person claims. I respect the credentials of the certified and committee members, however their teachings must be in line with the words of the document to qualify as valid to me.

I may not be getting the correct interpretation, however I have not as of yet been convinced in the alleged errors of my ways. I do appreciate the inputs of all. Hopefully in the interest of debating this subject I have not personally offended anyone, it is not my intent.


DesignBiz

 

[ProfDon]

First, DesignBiz, in addition to my standards work and the Prof next to my name I own and operate a small manufacturing business producing pneumatic clutch/brake units. So unlike many engineers, not only am I responsible for the design, I also work with suppliers and am responsible for solving production problems which have been greatly reduced thanks to GD&T.
Now that we are acquainted, everything I have said is consistent with para.5.3.2.2. In 5.3.2.2 there is a Note which states: "NOTE: If a functional gage is used to check the part, this shift of the axis of the datum feature is automatically accommodated." This is consistent with the Tip and what I have been saying. In your calculation you divide by 2 to get the shift in one direction whereas I stated the total shift. It is the same answer. From the upper drawing it is datum shift. For the lower drawing it is slop in the gage. In both cases, "shift happens!".

 

[PaulJackson]

Paul, you say that omitting the reference to any datum is a valid control callout. However you have not reconciled the requirement for tolerance of position callouts to have datum references. "In the standard para. 5.2.1.3 states that; it is "necessary" to identify features on a part to establish datums for dimensions locating true positions." What do you do with this?

DesignBiz, No datum features are being established in my example... only a single composite pattern.
paul

 

[DesignBiz]

Paul,

Please point me to where the standard states that it is okay to omitt all reference to datums in tolerance of position FCF.



DesignBiz

 

[PaulJackson]

Sorry I should have said... No datums are being established therefore no datum features need be identified.

The title of the paragraph 5.2.3.1 is "Identifying Features to Establish Datums"

Paul

 

[DesignBiz]

I do like your "shift happens" comments or %-)

You do realize that your statment (the note) starts with "IF a functional gage...".

No one seems to be willing or can reconcile how the main text (and now the note) makes a distinction for taking measurements from the gage's virtual pin as the origin (which is a fixed location); and the main body of text which states the origin as the "actual mating envelope" (which would not be a fixed location).

It has been my understanding as I have been taught that a functional gage as the lower drawing calls for would be used as a first measurement for the upper drawing callouts; however if the upper drawing part fails the 1st gage, then it is not necessarily a failed part at that point. Compensation for the "bonus shift" should be allowed and inspected for.

No one is debating whether the same gage can be used for both callouts at this point, at least not me.

I do take exception that; "Since the design intent didnt change and both drawings have the same meaning,...." statement.





DesignBiz

 

[PaulJackson]

DesignBiz,

I would cite peragraph 5.1(b) "location of features [such as in (a) above] [red]as a group[/red], from datum features, such as a plane and cylindrical surfaces"

Paul

 

[DesignBiz]

Paul,

I take it that it is your position to omitt the 1st sentence of that para. in 5.2.1.3;

"It is "necessary" to identify features on a part to establish datums for dimensions locating true positions.

and from...

"No datums are being established therefore no datum features need be identified."

that... if one simply doesn't establish datums then they simply are not necessary?

and it is okay to ignore the "from datum features"...

I would cite peragraph 5.1(b) "location of features [such as in (a) above] as a group, from datum features, such as a plane and cylindrical surfaces";

Interesting, however I don't see that interpretation and neither does our in-house consultant for GD&T.

ProfDon,
I am curious to ask you or anyone else that has taken the certification exam if there is a question(s) regarding this subject? and if so, what is considered to be the correct answer?

I might be willing to give them the answer that is being looked for and set aside my personal views in order to pass the exam.

I may be willing to sit down on the "outside" while I stand up on the "inside". ....just maybe in that circumstance, "discretion would be the better part of valor".... %-)

seriously,

I just can't find it in my comprehension skills (or lack there of) to agree with your efforts to see the bonus shift incompassed in deviation from the virtual size, vs an additional bonus based on the "actual mating size".

DesignBiz

 

[ProfDon]

In the 1994 standard Position requires a datum reference except in the lower segment of a composite position tolerance. In the 2009 revision: "7.6.2.3 Coaxial Features Without Datum References. A coaxial relationship may be controlled by specifying a positional tolerance without datum references, as shown in Fig. 7-59. This method allows specific
control of feature-to-feature coaxiality." The place this usually makes sense is if the coaxial features are used to establish a primary datum axis. The meaning of a position tolerance on coaxial features has been illustrated since the 1982 Standard where it occurs in the second segment of a composite position tolerance so the concept is not new, however, it has been extended to coaxial features serving as a datum feature.

 

[PaulJackson]

DesignBiz,

if one simply doesn't establish datums then they simply are not necessary?

If none are needed to control the feature location or orientation relationships to one another as in patterns then none are required, such as in 5.1(c) coaxiality of features.
If datums features are required to locate and orient those patterns relative to other features (not part of the pattern) then they must be identified to establish datums.

See figure 5-51, Its lower segment composite control would have an equivalent meaning if the control was two single segments rather than a composite because it specifies no datum features to control the orientation of the zones as they are controlled in figure 5-52.

Before composite tolerances were redefined and the rule for simultaneous requirements was amplified to encompass alike feature controls at RFS in 94, we had designs for valve bore sections that were two single segment controls like figure 5-51. The upper controlled location and orientation of the bores to the structure and the lower without any datum reference controlled coaxiality of the bores. Needless to say we had to change all of these dual single segment feature controls on new designs to composite controls just so that all of the lower segment controls, those controlling coaxiality without a datum reference, did not become "one composite pattern" and therefore have each separate "coaxial go plug" rigidly linked to every other "coaxial go plug" just because they all had identical datum references... none!

it is okay to ignore the "from datum features"...?

The conditions "location of features [such as in (a) above] as a group" and "from datum features" and "such as a plane and cylindrical surfaces" are isolated by a commas, meaning that each in this case is a separate condition. If datum features were required to control the location of features as a group there would be no comma!

paul

 

[PaulJackson]

Don

Quoted from DesignBiz,
Please point me to where the standard states that

Quoted from ProfDon
In the 1994 standard Position requires a datum reference

What restricts a simultaneous requirement consisting of features of size with position controls and identical datum references (none) from being considered "one composite pattern" which would constrain all six degrees-of-freedom among the pattern feature tolerance zones and consequently locate and orient them to one another.

Paul

 

[DesignBiz]

Paul,
5.1 General (tolerances of location)
1st sentence is the subject of the paragraph (principles of tolerances of location); 2nd sentence lists principles and a “list of relationships” as to what they control. (a) thru (d )
There is no way this is a “list of conditions”; rather a” list of principles and the relationships that they control”.
Each paragraph (a) thru (d ) names a control and what it is relative to.

(a) Control: “center distance”; in relationship to; “distance between features” and gives list of those features
(b) Control: “location of features as a group” and refers to a list of these features in (a); in relationship to; “location of feature as a group from datum features” and example of a couple datum features
(c) Control: “coaxiality”; in relationship to; “features that share the same axis”
(d) Control: “Concentricity or symmetry”; in relationship to; “center distances of correspondingly-located feature elements equally disposed about a datum axis or plane”

The below is link to, “the use of comma’s”

http://owl.english.purdue.edu/handouts/grammar/g_comma.html

DesignBiz

 

[ProfDon]

Paul, the default is stated in 4.5.12 of the 1994 standard. Where two or more features or patterns of features are located
by basic dimensions related to common datum features referenced in the same order of precedence and at the same material condition, as applicable, they are considered a composite pattern with the geometric tolerances applied simultaneously as illustrated by Fig. 4-26, You override this by stating SEP REQT next to any feature control frames you want to exempt. The key is that there must be a datum reference.

 

[PaulJackson]

OK OK I'm convinced. I'll abandon my myopically reasoned path that "one composite pattern" can be established via simultaneous requirements from Features of Size controlled by position callouts without datum references.

Paul

 

[DesignBiz]

ProfDon,

I am genuinely excited to tell you that I had a break through in my understanding regarding the "bonus shift" included in a gage.
I can now see that you are 100% correct in supporting the Tec-Ease posting. Powerhound has provided a claim that really makes a person think regarding a pattern as a feature. After much thought and digging deeper into the standard I can see where I was too focused on the wording and terms of the standard, while not focusing enough on how a physical part would fit to the gage. I am not so glad that I was mistaken; rather I am very glad to have learned a valuable lesson about patterns.
My mental block was coming from the wording of the standard with the lack of more detailed information as how the gage includes the "bonus shift". I believe I can describe how the happens with a fairly simple sketch to demonstate the wording of the standard when applied to a gage.

Thanks to all, whether or not we agreed your posts help me to learn more.


DesignBiz

 

[DesignBiz]

Paul,
I hope on our last exchange I didnt offend you. Your posts have made me think alot. I apprectiate them.

DesignBiz

 

[ringster]

ProfDon,

My compliments to you for your example in your tips showing Separate Requirements. It makes perfectly good sense to me and is 'contradictory' to the one in the Standard.

The example is 2 keyways on a shaft.

 

[ProfDon]

Hi Ringster
Thanks. But, it is just another example of simultaneous and separate requirements. When I heard someone say simultaneous requirements only applies when datums are modified at MMC, I took that example to the committee. As a result, the 2009 revision has a prettier version of that figure. So, I do not believe it contradicts anything in the 1994 standard. I have uploaded a page from my Update text which is similar to what ended up in the 2009 revision but I have shown a couple of applications. Perhaps this will help folks better understand simultneous vs. separate requirements.

 

[DesignBiz]

Ringster,
In attempt to produce the basis for my claim on an earlier post that ANSI y14.5 1973 allowed for the datum feature symbol to be placed on a centerline, in order to accommodate the people of Missouri (show me state?) (and I really don’t have documentation verifying you are a Missourian… I’ll trust you).

The 1973 standard in section 5.3.5 para. “Feature Control Symbol Placement” (a) states methods of placement;
(a) “Adding the symbol to a note or dimension pertaining to the feature”

I understand that this does say feature control placement and references “dimension”, which could be argued that this does not specify “datum symbol”; and the fact it could be argued that a centerline is not a dimension (and I would agree with that thinking). However, right or wrong, this is one basis that people argued back then, that it is legal to place the datum symbol on a centerline. I and others I have talked with recall this was a common practice at the time, even if their interpretation did not agree with the practice. I didn't agree with it at the time, thought it was confusing. That was the point of my post.

AMSE Y14.5m (maybe 1982 also, I don’t know) worded 3.3.2 (section for “Datum Feature Symbol”) words para (a) as follows;
…..The datum feature symbol is applied to the concerned feature surface outline, extension line, dimension line or feature control frame as follows:
(b) Placed on an extension of the dimension line of a feature of size when the datum is the axis or center plane…..


DesignBiz