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Simultaneous requirement
5

Simultaneous requirement

Simultaneous requirement

(OP)
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

 

RE: Simultaneous requirement

Design,

I agree with your statement as I understand it.  Have you made commento to Tec Ease and if so, ressults?

RE: Simultaneous requirement

(OP)
Ringster,

At this time, I was just looking for input/feedback as to what I may be missing or if others agree.

No comment at this point to Tec Ease. I have never contacted a forum member directly outside of thread posts.
As far as my experience goes, this is the place to review standard concepts and opinion; however if there is some protocol that I should be aware of then please advise.



 

DesignBiz

 

RE: Simultaneous requirement

It can't be wrong!  They are certified! wink

"Good to know you got shoes to wear when you find the floor." - Robert Hunter
 

RE: Simultaneous requirement

DesignBiz:

The Tec-Ease example shown does not comply with 5.3.6.1 & 5.3.6.2 on pages 92 & 93. The reference datum structure must be identical and not having one feature control frame only referencing datum A while other feature control frames referencing more than datum A. These are not simultaneous requirements under the 94 standard.

The ASME Y14.5M-94 standard supersedes interpretation examples as shown here by Tec-Ease.  

Dave D.
www.qmsi.ca

RE: Simultaneous requirement

The Tec-Ease tip looks correct to me.  There are no B and C datums in the lower drawing, but all of the features (and patterns) referenced to A are to be treated as a single pattern (as described in 5.3.6.2).  This is a good example of the rule of simultaneous requirements.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Simultaneous requirement

The second drawing in the Tec-Ease example does reflect simultaneous requirements but not the top drawing shown.

Maybe it is the verbiage that is confusing rather than the examples.  

Dave D.
www.qmsi.ca

RE: Simultaneous requirement

(OP)
I disagree Axym.

The link claims that the same hard gage could be used for both upper and lower drawings. No one can tell that without horizontal and vertical dimensions orienting and locating the features. If the center hole is over positive x and up positive y from the center of the outside profile; there are no dimension to indicate that or if they are centered. Just what are the tolerance of postion callouts in the lower drawing relative to regarding horizontal and vertical datums; or even to each other in the absence of datum references?

The simultaneous requirement does not state that what looks like a number of features sharing the same center, precludes us from providing dimensions for location and/or orientation.


The upper and lower dwgs could not use the same gage based on the simultaneous requirement in the standard.  

DesignBiz

 

RE: Simultaneous requirement

Dave,

You're right, I think the intent of the verbiage is that only the lower drawing illustrates simultaneous requirements.

DesignBiz,

The first part of your objection relates to the "implied basic zero" issue that has been recently discussed in other threads.  There is nothing on the drawing that explicitly indicates that the center hole is nominally 75 mm from the left hand side of the part.  It's implied, because there are no dimensions showing otherwise.  This is admittedly a gray area in the '94 standard.  But it has nothing to do with simultaneous requirements.

The tolerance of position callouts define a "single pattern" of tolerance zones that are relative to each other.  This extends the idea of a pattern to include groups of dissimilar features.  All of the tolerance zones (and therefore the gage elements) have a basic relationship to each other.  The relationship between any two features in the pattern (such as one of the holes relative to one of the edge features) is controlled indirectly.

This is hard to explain in words.  A diagram is needed here!

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Simultaneous requirement

2
DesignBiz,

The rule for simultaneous requirements makes all feature controls that have identical datum reference designations "one pattern". Since all the features in the pattern must be evaluated simultaneously or relative to one instantantaneous-common reference, their relationship to one another is not subject to any rotational or translational degrees of freedom. So the point that Don Day is making is true. All six degrees of freedom are always  constrained among pattern features... the simultaneous requirements rule extends that constraint to feature controls that have identical datum references that are modified identically as well.

I believe that the intent rule in the beginning (82 and prior) was to insure that datum shift resulting from "datum features of size" could not be applied independently to feature controls that had identical datum references...   however when the condition was described as "one composite pattern" it constrained all six degrees of freedom rather than just those that the DRF is capable of controlling. What is even more incredible is that simultaneous requirements apply to feature controls that have datum features that are not "features of size" in other words RFS... the reason... "a rocking primary datum surface".
If two separate callouts both referenced  to |A|B|C| "RFS" were verified independently and the primary datum because of its form deviation allowed it to rock... then the verifications could be subject to two unique |A|B|C| "RFS" datum references... hence the simultaneous  requirement.

The only way to unlock this rule is with a "sep req't" annotation or a control in the lower segment of a composite composite FCF.

Paul

RE: Simultaneous requirement

(OP)
Axym,

Even if I were to buy in on the "implied basic zero" concept, the 1st and 2nd dwgs do not have the same DRF for the FCFs which along with the same material conditions required by the standard's definition of simultaneous requirement.

In this case it is obvious that the datums B and C are missing in the lower drawing's FCFs. This is in direct opposition to what the standard states.

As in the start of the thread and the last sentence in my post I acknowledge that the lower drawing has all features requiring conformance to the simultaneous requirement rule. My position is that the rule does not lend itself to the 1st and 2nd dwgs. They obviously do not have the same info in the FCF's.

Paul,
I do not disagree with you regarding the rule and orientation. Actually I do agree with your statemet,

"The rule for simultaneous requirements makes all feature controls that have identical datum reference designations "one pattern". "

The upper and lower drawings do not meet this requirement.

If the upper and lower drawings could fit the same simultaneous requirement then it would mean that the FCF's do not have to be identical.

DesignBiz

 

RE: Simultaneous requirement

OK, I think we've found the root cause of the problem.

The upper and lower drawings in the Tec-Ease tip are intended to be two completely independent examples.

The upper drawing shows a "conventional" approach with three datum features and other features referenced to them.

The lower drawing shows a different approach, with all of the features having identical datum references (to A) and therefore simultaneous requirements.

Nothing on the lower drawing is simultaneous with anything on the upper drawing, because the two drawings are not intended to exist together.  It's one or the other.

The fact that Dingy2 and DesignBiz both interpreted the Tec-Ease tip that way indicates that it could have been worded more clearly.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Simultaneous requirement

Evan:

I agree with your statement above about being different drawings. Although both might use the same gauge, one must use it in a different manner.

In the first example, we must qualify each datum individually and we certainly would not check any further if, as an example, datum B failed to meet the requirement.

In the 2nd example, we do check simultaneously since all are considered one pattern.  

Dave D.
www.qmsi.ca

RE: Simultaneous requirement

(OP)
Axym, ding2

agreed...

Yes, the wording can give someone a different impression other than the central intent.

agreed with ding2's comments on the gaging...

My thinking regarding the standard has been stimulated again. I had really never considered the "implied basic zero" including all  features of a part. I had for a couple of features, but not the entire part.
To me, that example is  a brain teaser.

DesignBiz

 

RE: Simultaneous requirement

OK, just so I'm clear here before weighing in (Evan & Paul, nice explanations), everyone is ok that in the second example having only a single datum reference (Datum A) is adequate to fully constrain the wokpiece and all features as a single pattern?  If so, consider a slight extension to that thought... if you have even a single feature on the workpiece which has SEP REQT on it, can you still get by with only a single datum reference?

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: Simultaneous requirement

That feature would be unconstrained for location relative to the composite pattern but it would be constrained for orientation relative to A and if it was the 4X pattern of holes they would still be constrained as its own pattern.

Without a location constraint the hole or hole pattern are free to wander to the edges or even breech the edges to the point where the measured size could be regarded non-conforming. It wouldn't be pretty.

Consider this however... If the 2.0+/-0.1 material thickness had a position callout on it |POS|0(M)| and the datum identifier "A" and its flatness note was removed as well as the the "A" from all the other callouts... Is it still "one composite pattern" without any datum reference?

Paul   

RE: Simultaneous requirement

(OP)
Okay, maybe I should be more reluctant but I'm game.

MechNorth,
No; no more single pattern, I dont believe so. The "Sep Req" would exclude that feature from the pattern. You would need another datum to locate it relative to the other features.

Paul,
I dont believe it is a fully defined positional control without a datum reference. I dont believe that removal of the flatness has much to do with removal of the datum reference nor the "0" at MMC callout, other than it would be flat within .2 . In the standard in 5.2.1.3 states that, it is "neccessary" to identfy features on a part to establish datums for dimensions locating true positions.

And now you've done it. After review of this paragraph (5.2.1.3) I am now not so sure about the "implied basic zero" that Axym brought up. Zero form what? what datum(s) is the zero from/to? I can accept the concept but now I want a datum to reference where the zero dim comes from!

DesignBiz

 

RE: Simultaneous requirement

DesignBiz,

The "implied basic zero" that I mentioned doesn't relate to datums.  It's relates to basic dimensions.  In the drawings in the Tec-Ease tip, the center hole looks like it is exactly in the middle of the hole pattern and there are no dimensions showing otherwise.  So it is implied that the center hole is nominally supposed to be exactly in the middle.  

The term "implied basic zero" is a bit misleading in this case.  It applies better to things like features that look coaxial on the drawing, and therefore implied to be nominally coaxial.  If a basic dimension were shown between the axis of one and the axis of the other, it would be zero.

It's the same kind of thing as the implied basic 90 degree angle.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Simultaneous requirement

(OP)
Axym,

1. That is my point. Actually coaxially is maybe a more easily viewed example. Before GDT and even in the 1973 standard when it was actually legal to attach a Datum symbol to the centerline itself, came the argument as to which of let's say 3 diameters is the Datum that the other 2 are located from. It was an argument long before GDT was widely accepted that features represented with a shared centerline meant that the centerline belonged to all of them. When identifying a datum in a FCF, it does identify which one. Even without the actual "zero" dimension by identifying  the datums in a FCF, it gives me insight into part function; and how to setup for inspection.

2. How do you reconcile?

"In the standard in 5.2.1.3 states that, it is "neccessary" to identfy features on a part to establish datums for dimensions locating true positions."

One of the principles of a tolerance of position control is that it is measured by orienting a part the same way each time it is inspected. What if a number of parts are made by 3 different suppliers and inspected by 3 different suppliers. What says that all must use hard gage tooling to inspect the part? If another method is employed then how is the part garuanteed to be positioned the same way each time one is inspected in the absence of identified datums?
  

DesignBiz

 

RE: Simultaneous requirement

Design,

When and where was it allowed to 'routinely' identify a CL as a datum?

I was unaware of that one.

RE: Simultaneous requirement

DesignBiz,

I second ringster's question.

RE: Simultaneous requirement

2
Great discussion!
Zero basic was implied in the 1994 standard.  See Figs. 4.26.5.45.18, 5.23 and others. The Standard shows both ways.  With or without "half" dimensions.  Since they are basic, it doesn't matter.
Even the 1982 Standard has examples.  Old figures that have been carried over from the old ± days show half dimensions. New figures, in general, do not.
In the 2009 revision of the Standard, to clear this up
"1.4 FUNDAMENTAL RULES,(k) A zero basic dimension applies where axes, center
planes, or surfaces are shown coincident on a drawing,
and geometric tolerances establish the relationship
among the features." Also, zero basic is illustrated in Figs. 4.29 thru 4.33, 4.39, 7.4, 7-18 and others.
As far as the Tec-Ease, Inc. Tip is concerned, simultaneous requirements is only mentioned with regards to the lower drawing.
If you do not believe the two drawings are equivalent in that Tip, please describe how the gages would differ if you were to gage these parts.  Given that the parts are pretty thin, a comparator and an overlay chart would allow inspection of either part and the charts would be identical.  Granted, this approach would not check the perpendicularity to the datum plane established by datum feature A.  The control is there, mainly, to tie the features together through "simultaneous requirements".

RE: Simultaneous requirement

(OP)


1973 Standard allowed for the datum symbol to be attached to the centerline.

DesignBiz

 

RE: Simultaneous requirement

Design,

I do not have a copy of the 1973, and do not recall that one.  I am originally from Missouri, any way to post a scan or it.  

And there are exceptions to all rules. There is one example in the 1982 that has ident attached.  It is an exception.

RE: Simultaneous requirement

(OP)
In an earlier post I have commented that Axym brought up the "implied basic zero" which I hadnt considered in terms of all the features of a part. Dingy2 points out that a single gage is not sufficient for inspection of all the controls called out in the 1st drawing.

If both of these drawings are the same, then how is the "datum shift" for B and C accounted for in the lower dwg?

DesignBiz

 

RE: Simultaneous requirement

Datum shift occurs between the datum feature and the simulator.  In both cases the gage elements for B and C are made at their virtual conditions (maximum material boundaries per 2009).  This will allow the part to "shift" on the Ø29.8 pin and inside the 100.7 width in order to allow the other features to fit their virtual conditions (maximum material boundaries per 2009)defined in the gage "simultaneously".

RE: Simultaneous requirement

(OP)
I suppose as interesting as the 2009 standard appears to be, we will need to stay focused on the standard referred to in the post. If not noted before I believe that this post is based on the 1994 standard.

That aside the lower drawing controlled by the simultaneous requirement supposition would be checked with a gage built at the virtual sizes of the features involved. There is no reference to allowable datum shift in the lower drawing as the upper drawing does allow for, that I can see. The gage locations are fixed by an implied basic zero dimensions to each other, only to deviate relative to the difference from the gage's virtual size pins and the actual mating size.

Whereas the 1994 standard para. 5.3.2.2 states, as the referenced datums at MMC depart from MMC the feature located referencing those datums can have the pattern axis displaced by one half of the difference between the datum's actual mating size and its MMC size. This "bonus shift" is not accounted for in the lower drawing that does not reference datum B nor C at MMC.  I read this as the feature locations have more allowable movement which reference the datums at MMC rather than what the lower drawing allows. For this reason the drawings cannot have the same controls as depicted. Or tell me where the "bonus shift" is for the lower (2nd drawing)? I contend that it is not there.
 

DesignBiz

 

RE: Simultaneous requirement

Question,

Has anyone ever considered the implementation of GD and T with the the problems of the Auto Manufacturer's?

Too much or not enough?

RE: Simultaneous requirement

If DesignBiz or anyone else thinks the drawings do not have the same meaning, please explain how the gage would differ.  The gage shown does, in fact, automatically provide the datum shift.  If datum feture B is produced at its LMC size of 30.2 it could "shift" a total of 0.4 on the Ø29.8 gage.  If datum feature C was produced at its LMC of 99.5 it could "shift" a total of 1.2.  Since B limits the vertical and horizontal shift to a max of 0.4, most of the allowable shift of datum feature C would be in rotation. This identical shift or float on the Ø30 hole and 100 width is allowed by the lower drawing.

RE: Simultaneous requirement

Does 'green side up' play any part in this problem.

Looks like a good case for the 'good old datum feature identification' to me.

 

RE: Simultaneous requirement

(OP)
ProfDon,
Your statements continually rely solely on the difference of the holes' virtual size and an actual mating size as the total locational allowance. In other words the hole size limits  control the amount of possible deviation from location. This is true in the lower drawing.

Have you read 1994 standard para. 5.3.2.2 pg 85?  This is an additional allowance, for example the 4 hole pattern's relative location to datum B in this case. This distinctly states that beyond the difference between the 4 holes' actual mating size and the virtual size for loction tolerance "to each other", there is a possible bonus shift  "of the pattern's axis" to datum B.

If for instance datum B actual mating size is  30.2 but is shifted off "the shared implied zero" by 0.2 then the 4 hole pattern's ( all 4 virtual gage pins as a pattern) can shift by an additional 0.2.

 30.2 Actual size mating size (LMC in this case)
-29.8 Virtual size (MMC in this case)
------------------------
 0.4  difference
 0.4 / 2 = 0.2 max allowable bonus shift for the "pattern of 4 holes".

This is relative to allowable movement of the "pattern" of 4 gage pins at the virtual hole size.
Each of the 4 holes have an allowable location displacement of 0.2 dia. up to 0.4 dia. "to each other".
The "bonus shift" is in the 1st drawing and is NOT in the lower drawing. The upper drawing allows the "pattern of holes axis" to potentially float 0.2 off of the virtual locations of the referenced datums at MMC.

The lower simultaneous requirement example can be verified with a single gage, whereas the upper drawing referencing datums at MMC could use the same gage initially, however would require further steps to inspect.

I have been know to be "slightly mistaken" before; however as of this point I am not being convinced that is the case here. poke
 

DesignBiz

 

RE: Simultaneous requirement

Well, as an active participant of Y14 standards since 1988 and a member of the Y14.5 committee since 1995 assigned to the datums section, yes, I have read para. 5.3.2.2 pg 85 numerous times.  In all fairness, this was a confusing area of the Standard for a lot of folks.  We have spent a lot of time on the 2009 revision in an effort to make this more understandable.  This is one of the reasons the maximum material boundary terminology has been introduced.  I joined this thread to help clear things up but apparently have not done a very good job of it.  
The Tip is correct.  The gage would be identical for the two drawings. With regards to the upper drawing, the paragraph to look at is "2.11.3 Datum Features at Virtual Condition.  A virtual condition exists for a datum feature of size where its axis or center plane is controlled by a geometric tolerance. In such cases, the datum feature applies at its virtual condition even though it is referenced in a feature control frame at MMC or LMC."  This is what accounts for "datum shift".  The gage is made at the virtual condition of B and C.  As the actual mating envelopes of the datum features depart from the virtual condition you get the datum shift which might permit the other features to meet their specifications.  This gage is verifying the position tolerances on the datum features as well as the other position controls. No other verification of the geometric tolerances, other than the flatness tolerance, is needed.
I regret not being able to explain it better. wink     

RE: Simultaneous requirement

DesignBiz,

Both examples would use the same go-gage for acceptance of the variable position tolerances and...
Both require additional 2-point gages to check LMC sizes , go plug gages to check MMC sizes of all features except the 30+/-0.2 diameter (its MMC size is verified in the go position gage), the material thickness, and the flatness.
 
BTW the scenario that I proposed, (all "features-of-size" |POS|0(M)| with no datum ref.), actually does create one composite pattern and therefore fully constrains the features relative to one another. The gauge for it would look the same but it would have to be capped so that it limited max thickness to MMC 2.1 just like the other two slab thicknesses are limited to their MMC.

Paul

RE: Simultaneous requirement

Oops I meant:
(2+/-0.1 |POS|0(M)| and all other position callouts with no datum ref.)
not (all "features-of-size" |POS|0(M)| with no datum ref.).

If they all changed to 0@MMC the go-position MMC sizes would increase by 0.2 but we could eliminate the mmc size go-plugs and slab thickness gages.

Paul

RE: Simultaneous requirement

I may be confused, not the first time for sure.  I have always thought the standards were to stand alone without being supplemented by a newer or older version.  

Anyone?

If true, how can we apply 2009 as an aid in the interpretation of 88 or 94?
 

RE: Simultaneous requirement

Ringster is right that the standards stand alone.  However, the standards do evolve.  As each concept is introduced, the users find applications that were not anticipated by the developers of the Standard.  GD&T is a language.  And like any language, it is determined by usage.  Otherwise we would all be speaking as they did in Shakespeare's time.  Composite tolerancing is a lot easier to understand in the 2009 revision than it was in previous revisions.  One of the purposes of a revision to any standard is to clarify previous revisions.  It is called continuous improvement.  Has this thread been settled or are there still issues?   

RE: Simultaneous requirement

Don,

Your last post included:  "One of the purposes of a revision to any standard is to clarify previous revisions." And additionally that the Standards stand alone.  This sounds like doublespeak

RE: Simultaneous requirement

Reality check time, guys.  Standards, text books, reference materials, and even the Bible are revised and updated periodically to reflect changes in societal needs and technological evolution.  In each case, there is little value in revisiting older versions except for historical perspective.  The greater problem is that people, for whatever reason, do not upgrade their knowledge and practices to reflect the latest offerings of standards, texts and references.  There is also due consideration needed for the source of these learned materials.  They are generated by well-intentioned people, participating on public committees, who document the current status and near-term evolution and cutting-edge applications and knowledge.  Whenever a group of humans attempts to reach agreement on something substantial, there is invariably negotiation and trade-offs.  Each party has personal and corporate biases that they hopefully try to subordinate to the greater good.  It's very easy to poke holes in a standard because it doesn't immediately reflect your preferences.  The problem all too often in the application of any body of knowledge is that the users' knowledge is based primarily or solely on their personal exposure and application rather than to the body of knowledge as a whole.

Would you reasonably expect that a Mechanic's guide for a Model-T should be inclusive of all technologies in a modern Ford?  The Model-T was great for its day, but its day has long past, and I wouldn't want to try driving cross-country in one today.  You?

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: Simultaneous requirement

Regarding Ringsters comment on "doublespeak":  It is unreasonable to believe that when a concept is introduced, all possible applications and extensions of that concept can be addressed.  When composite tolerancing was first introduced, no thought was given to what it would mean to have additional datum references in a lower segment.  When it was seen that folks were using multiple datum references in a lower segment, it was time to standardize the meaning.  The same is true of a 3 segment composite, application of LMC to datum references, embedding tolerances in a solid model and the list goes on. Language is determined by usage.  The committee is there to standardize that usage.
Each revision gives us more tools to say what we mean.  It doesn't, nor will it ever cover every situation.  Today, it covers most situations.  As the Tip points out, as the standard grows there will be some overlap and some things can be said in more than one way, especially on simple parts.

RE: Simultaneous requirement



To the best of my knowledge, a goodly portion of Space Station hardware was designed in accordance with 1988 Std.
Supposing that replacement parts are required at a later date.  Would not those parts require the knowledge of the 1988 Std. in order to properly create the hardware.

Or is one to suppose that the drawings all be revised to update to the 1994 Standard?  Now 2009?

This represents some of my concerns, hope I have made it clear.

RE: Simultaneous requirement

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.

RE: Simultaneous requirement

(OP)
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

 

RE: Simultaneous requirement

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!".  

RE: Simultaneous requirement

Quote:

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   

RE: Simultaneous requirement

(OP)
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

 

RE: Simultaneous requirement

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

RE: Simultaneous requirement

(OP)
I do like your "shift happens" comments  or dazed

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

 

RE: Simultaneous requirement

DesignBiz,

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"

Paul

RE: Simultaneous requirement

(OP)
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"....    dazed

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

 

RE: Simultaneous requirement

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.

RE: Simultaneous requirement

DesignBiz,

Quote:

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!   

Quote:

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


     

RE: Simultaneous requirement

Don

Quote:

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

Quote:

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  

RE: Simultaneous requirement

(OP)
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

 

RE: Simultaneous requirement

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.
 

RE: Simultaneous requirement

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

RE: Simultaneous requirement

(OP)
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

 

RE: Simultaneous requirement

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

DesignBiz

 

RE: Simultaneous requirement

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.   

RE: Simultaneous requirement

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.   

RE: Simultaneous requirement

(OP)
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

 

RE: Simultaneous requirement

'94 also states in 4.3.2 "The datum feature symbol identifies physical features and shall not be applied to center lines, center planes, or axes except as defined in paras. 4.6.6 and 4.6.7."
4.6.6 refers to equalizing datums and 4.6.7 refers to those established from complex or irregular surfaces.

"Good to know you got shoes to wear when you find the floor." - Robert Hunter
 

RE: Simultaneous requirement

DesignBiz,

To illustrate my concerns over any attemts to label the centerline of a part as a datum feature, let's look at the Fig. 5-18 in the 94 standard.  Imagine a datum identifier B attached to the vertical CL and C attached to the horizontal.  I know of and dealt with a similar arrangement on a aircraft part years ago.  It made no sense then nor would it now.  The drawing at that time had been in existance for some time and NO ONE  previously apparently had the capability to see the flaws in the application. (in that case there were actully more holes in the plate that were on the same cl)

I can verify the MO thing.

RE: Simultaneous requirement

(OP)
Ringster,

I am in complete agreement with you.

I only was trying to make a point regarding the 2nd drawing where the center hole, profile of the plate, and the pattern of the plate share the same centerlines without any reference to an explicit datum in the simultaneous requirement example. This to me makes things very unclear as the former datum symnbol practice was also confusing.

Like I said before, I will trust you on the MO thing.

DesignBiz

 

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