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

 

[PaulJackson]

DesignBiz,

I second ringster's question.

 

[ProfDon]

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".

 

[DesignBiz]

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

DesignBiz

 

[ringster]

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.

 

[DesignBiz]

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

 

[ProfDon]

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".

 

[DesignBiz]

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

 

[ringster]

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?

 

[ProfDon]

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.

 

[ringster]

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

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

 

[DesignBiz]

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.


DesignBiz

 

[ProfDon]

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.

 

[PaulJackson]

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

 

[PaulJackson]

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

 

[ringster]

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?

 

[ProfDon]

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?

 

[ringster]

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

 

[MechNorth]

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[ProfDon]

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.

 

[ringster]

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.