Tapered wedge as a datum feature

[gabimo]

If a tapered wedge IS defined with ± angle and direct toleranced dimension on one end, can its middle plane be used as a datum feature?
How to generate the centerplane / middle plane as a datum from the non-parallel but planar adjacent surfaces?

Details: A part has its left and right sides tapered and has its primary datum the feature that connect these two sides.
Secondary datum is intended to be the middle / center plane of the tapered sides.
There are three holes on the part (normal/perpendicular to the primary) that should be centered (within some tolerance) to the secondary (middle plane).
Is the middle plane created by this tapered side walls a valid datum?

 

[greenimi]

chez311,

Thank you for your comprehensive replay.
We have (for reference) some important discussions in one place due to your work (search) to put them together. Thank you again.
Now, I have to stick with my original answer to the OP’s question. I do not think my solution is self-referencing (my solution is below for reference).

“datum feature B: profile (or perpendicularity/ angularity) to A primary and B-C secondary
datum feature C: profile (or perpendicularity/ angularity) to A primary and B-C secondary”


I might agree that is not well documented, but I do not see why cannot be used in the OP’s case. What is your proposal?

“1) all basic angles/dimensions. 2x profile tolerance applied to B and C separately in reference to A. referenced as [B-C] (similar to fig 4-22 but the datum features are at an angle to each other instead of parallel)”

Why your proposal #1 is not similar with mine (especially when you are mentioning figure 4-22 from 2009 standard)

Also, I do need a little bit of clarification from your quote

“B and C controlled separately with B-C instead of both B and C controlled together with B-C”

What do you mean?


Anyone else, think proposal in red is not kosher? Just curious what I am doing wrong?

“datum feature B: profile (or perpendicularity/ angularity) to A primary and B-C secondary
datum feature C: profile (or perpendicularity/ angularity) to A primary and B-C secondary”

 

[chez311]

greenimi,

I do not think my solution is self-referencing (my solution is below for reference)

“datum feature B: profile (or perpendicularity/ angularity) to A primary and B-C secondary
datum feature C: profile (or perpendicularity/ angularity) to A primary and B-C secondary”

I'm going to have to disagree - if you read through some of the threads I referenced your proposal is similar to the ones under discussion as self-referencing. See my attached presentation for some explanations on the different versions.

Also in regards to the term self-referencing or what qualifies as such I will stick to my original statement:

it seems that I may differ from some in my definition of "self-referencing", I would consider ANY feature which is controlled with a DRF which includes that feature to be self-referencing - not just ones that result in an invalid combination (ie: position of A relative to A). Maybe more precise terminology is needed to differentiate the two?

In the file:
#1 = my proposal (NOT self-referencing)
#2 = a variation of self-referencing similar to those discussed in the referenced threads, modified for this example
#3 = your proposal (I claim to be self-referencing)

The more I look at the how the datum features in your proposal (#3) interact* to form the DRF and control the datum features themselves, the more I am unsure if it is even valid at all. The interactions seems to be more complex than the #2 example, and at the very least this makes it even more difficult to inspect/interpret. If you can show exactly how this would work please feel free, I think its a bit more complicated than it seems on the surface.

To reiterate from my previous post - while a solution may be viable similar to the #2 where the tolerance zone is halved** this is not a well known concept and unless truly necessary to define the part properly should probably be avoided if at all possible (both my #2 and #3). Defining a feature in this way is almost guaranteed to confuse and/or mislead just about everyone who comes into contact with it and will likely be interpreted differently each time.

*Hopefully the pseudo flow chart/lines make sense. I tried to visualize them as best as possible to show how I mean self referencing.

**See Evan's post from 7 Mar 16 18:02 in

https://www.eng-tips.com/viewthread.cfm?qid=404621

for a nice graphic showing why it is halved.

 

[Kedu]

Well.... I am not fluent in GD&T, but what is the difference between option 2 and 3 in chez311 cases above?

2x profile versus two independent profiles shown separately....for me looks similar...same mathematical scheme

 

[greenimi]

Nice graphics. Excellent work.

And yes, option #3 as you said is what I am proposing. Still not sure why it is not valid.

The only thing I would not call it composite but I would call it "multiple datum feature" (primary or secondary, as applicable. (I guess in my proposed case secondary). See fig 3-25/2009

So, you are proposing option #1 and you say it is NOT self referencing. Okay. I agree that is not, but the question for you is: what is establishing the mutual relationship between datum feature B and C in your option #1? 5.000 basic and 40degrees basic are location and orienting the tolerance zones, but not the mutual relationship between B and C. Am I correct? I might missed it entirelly, but I am asking.

 

[chez311]

greenimi,

Thank you - I hope the graphics were helpful in communicating my intent.

I didn't say your #3 was necessarily invalid, only that it might be. I'm just not sure - it might be as Kedu suggested and the same mathematical result as #2, someone with a bit more expertise than I might be able to say one way or another. That being said, even if the result is the same, as I stated previously I would avoid #2/#3 unless truly required as I think you will find this halving concept is not well known and in practice you will probably get wildly varying interpretations. See the below attached summaries from thread (

https://www.eng-tips.com/viewthread.cfm?qid=405622)

- one was supplied by JP and the other from Alex Krulikowski's book (supplied by SeasonLee).

https://files.engineering.com/getfi...&file=Profile_Datum_Rule-Alex_Krulikowski.jpg

https://files.engineering.com/getfi...b6-abe6-ac582c9532ad&file=ProfileOnDatums.pdf

I have seen the term utilized many places as a "composite datum feature" (ie: [B-C] is composite of B and C) though I do see the standard does refer to it as a "multiple datum feature" per Fig 3-25. I admit "composite" may confuse some with a composite tolerance, however I think "multiple datum feature" seems even less descriptive and more ambiguous to me. Again the terminology available fails us somewhat but we are definitely talking about the same thing.

what is establishing the mutual relationship between datum feature B and C in your option #1? 5.000 basic and 40degrees basic are location and orienting the tolerance zones, but not the mutual relationship between B and C.

All the basic dimensions required to fully define the tapered feature and its location/orientation to A are included. I was going with the conventions provided by gabimoang (OP) in his 28 Sep 18 17:13 post, but for sure it is fully defined. Just to be clear, are you suggesting that for some reason the 2x profile tolerance only succeeds in locating/orienting the 2x surfaces to A but not to each other? If you take a look at Fig 4-22 the same type of 2x profile tolerance maintains the same "mutual relationship" between the two surfaces in basic location and orientation to each other - I would say this also applies to my example #1 in the same way.

 

[greenimi]

I would say the difference between figure 4-22/ 2009 and the OP case (and your proposed solution #1) from the relationship between datum features and their mutual relationship is the following:

- In fig 4-22 from the standard A-B are multiple datum primary and they don’t have to be oriented and located to their higher order precedence (again because are primary datums)

- In proposed solution #1, datum features B-C is secondary and (I guess) should have somehow a relationship to its primary “A” datum feature. And “B” and “C” relationship to “A” is angularity. Probably you would say their (“B” and “C”) mutual relationship is ensured thru simultaneous requirements? Am I correct?

Anyone else thinks my proposed solution #3, does not hold any water? If yes, why not?

 

[chez311]

greenimi,

I am aware of the difference, but I dont think that [B-C] referenced as secondary vs. primary changes the conversation much, I was utilizing 4-22 as an example of how to establish a compound (or "multiple ") datum feature utilizing a patterned profile tolerance. I don't see why the concept wouldn't be just as valid with a secondary datum - datum precedence will just dictate what DOF each datum feature constrains. In this case [B-C] constrains only the 2x translational DOF in x and y.

Probably you would say their (“B” and “C”) mutual relationship is ensured thru simultaneous requirements?

Simultaneous requirements is not needed* - as in 4-22 the 2x notation means they are part of a pattern and are held in common orientation/location to the referenced datum(s) as such.

Edit: Simultaneous requirements is not needed, but could be utilized if I had 2 separate callouts. In this case I do not, and the 2x pattern notation takes care of that.

 

[pylfrm]

In the file:
#1 = my proposal (NOT self-referencing)
#2 = a variation of self-referencing similar to those discussed in the referenced threads, modified for this example
#3 = your proposal (I claim to be self-referencing)

I agree that #1 is perfectly reasonable. Although exactly equivalent, my preference would be to attach a datum feature symbol (let's say D) to the profile FCF, and then reference |A|D| instead of |A|B-C| for the position tolerance. Assuming datum features B and C are never referenced individually, this seems cleaner to me.

For #2, I disagree that the tolerance zone is halved (or more properly, that only half of it is usable). Some fraction of the tolerance may be unusable due to being on the wrong side of the datum feature simulator, but that fraction could be anything from 0 to 1. The difference is that in this case the datum feature simulator for B-C is not fixed coincident to the true profile.

I think #3 is equivalent to #2.

pylfrm

 

[chez311]

pylfrm,

Glad to see you think my first proposal is viable! I was mainly following the convention in 4-22, but I agree it could possibly be slightly cleaner to reference them together as a single datum feature D since they are part of the same pattern. Definitely something to keep in mind.

For #2, I disagree that the tolerance zone is halved (or more properly, that only half of it is usable). Some fraction of the tolerance may be unusable due to being on the wrong side of the datum feature simulator, but that fraction could be anything from 0 to 1. The difference is that in this case the datum feature simulator for B-C is not fixed coincident to the true profile.

Could you expand on this a bit? Are you saying this because its referenced as a secondary datum feature, because it is a tapered feature, or both? Or for some other reason? If it was referenced as primary would that mean the tolerance zone is effectively halved then?

In regards to #3 - when I attempted to draw the relationships out graphically (my poor attempt at visually explaining it with "flow" lines) it seemed more complicated, but that may have been misleading. You and Kedu (and I assume greenimi feels the same way as well) may be correct that they are essentially equivalent, I could have been a bit confused that for some reason with a separate FCF controlling B and C separately but still to [B-C] would impart another layer of complication. I see now that is likely not be the case.

 

[3DDave]

I have not seen mentioned that most of the undesirable effects of self-referencing surfaces controlled by a profile (of surface, just for the pedants) tolerance are eliminated by making the profile (of surface) tolerance zone one-sided, which most clearly represents the situation if the item is used as a keystone or dovetail arrangement. Obviously the allowable material removal would be to the inside volume of the part with the ideal part perfectly conforming to the geometry as controlled by the basic dimensions.

 

[greenimi]

chez311,

Looks like pylfrm thinks that my option #3 is not self-referencing....And in addition that your option #2 is same as mine (#3). Go figure...... Looks like it is a matter of preference as both are the same....maybe just drafting difference.
Good to know.

 

[chez311]

3DDave,
I had seen that discussed on one of the threads I referenced, I just failed to mention it. Thanks for pointing that out. Nonetheless, as I said before regardless of equally/unequally disposed I would prefer to avoid any self-referencing datum features. Especially since often being discussed is a self-referencing datum feature wrapped in an all around profile tolerance where other features (anywhere else in the "all around" profile besides the datum features) are also controlled in addition to the self-referencing datum features which at least adds some value. Here the only features involved are the self-referencing datum features themselves - in that case I see no advantage over my #1 non self-referencing proposal except to add confusion. What do you think?

greenimi,
I don't see it anywhere in his response that your option #3 is not self-referencing. All he said is the tolerance zone is not necessarily halved (could be any ratio from 0 to 1) and that your #3 is equivalent to #2. I won't speak for him though - lets ask him.

I don't want to speak for him either, but it seems 3DDave agrees with me at least in part:

[...]the undesirable effects of self-referencing surfaces controlled by a profile[...]

pylfrm and 3DDave,
Do you believe as I do that #2 / #3 proposals would be self referencing? If not, why not? My point previously was that I did not differentiate between something self-referencing/valid (ie: shown example #2/#3) and self-referencing/invalid (ie: position of a datum feature to itself) - they are both self-referencing. Perhaps more precise terminology is needed.

Also what do you believe would be the preferred/more proper control in this instance? #1, #2, or #3 ?

 

[greenimi]

chez311,

Agreed. I am curious too.

 

[pylfrm]

Could you expand on this a bit? Are you saying this because its referenced as a secondary datum feature, because it is a tapered feature, or both? Or for some other reason? If it was referenced as primary would that mean the tolerance zone is effectively halved then?

Consider ASME Y14.5-2009 Fig. 4-29 (a). According to the "Means this" section of the figure and the text of para. 4.16.1, the datum feature simulator progresses from the MMB toward the LMB until it makes maximum contact with the datum feature. Unfortunately "maximum contact" is not well-defined, so in this case I interpret it to mean the datum feature progresses until it can't progress any further.* Let's imagine that occurs after progressing 25% of the way through the tolerance zone, resulting in a radius of 14.95 for the datum feature simulator. Now imagine we change the profile tolerance to reference B as the secondary datum feature. The boundaries are still R14.9 and R15.1, so 25% of the tolerance zone will be on the wrong side of the datum feature. This leaves 75% usable. With different as-produced geometry, the usable portion could be anything from 0% to 100%.

I think Fig. 4-29 (a) is rather similar to the case at hand, and none of the differences should harm the analogy. Do you agree?

*I'd be interested to hear other opinions on this. Maybe I should start another thread on the subject though.

Looks like pylfrm thinks that my option #3 is not self-referencing....

I purposely avoided mentioning that because I think the whole debate is a bit ridiculous. ASME Y14.5 does not prohibit "self-referencing". It does not define, use, or even mention the term. I've now been asked directly though, so I suppose I'll try to respond.

I would consider ANY feature which is controlled with a DRF which includes that feature to be self-referencing

As far as I can remember, this is the only definition of the term I've ever seen posted. If we accept it, then I think it's quite obvious that #2 and #3 are self-referencing.

greenimi: Do you accept this definition but disagree with the conclusion, or are you using some other definition that you're keeping secret?

Also what do you believe would be the preferred/more proper control in this instance? #1, #2, or #3 ?

Without fully understanding the application, I would tend to prefer #1. Although I don't think #2 and #3 are invalid, I do think they are more complicated and more likely to cause trouble. I can't think of a case where I would prefer them over #1.


pylfrm

 

[greenimi]

As far as I can remember, this is the only definition of the term I've ever seen posted.

I might agree that self-referencing term is not defined in Y14.5 and the standard does not prohibit “self-referencing”
Has been discussed, at least tangentially in this thread:

https://www.eng-tips.com/viewthread.cfm?qid=438912

I do not know if I agree that the only definition of the term is this
“I would consider ANY feature which is controlled with a DRF which includes that feature to be self-referencing”

Based on how my brain is “wired” I would go with pmarc’s definition (definition of datum self-referencing is that self-referencing is when inspection reading of a geometric characteristic defined relative to a datum is always 0, no matter what).
I do not know why, but fit better my cognitive level.
That being said: I would maintain that #2 and #3 (and we agreed that the schemes are equivalent) are not self-referencing per my definition (well per pmarc’s definition….or the definition I would accept)

greenimi: Do you accept this definition but disagree with the conclusion, or are you using some other definition that you're keeping secret?

No. I would not accept your definition (or chez311), due to the fact does not fit my thinking process/ mental reasoning (if there is one) and yes, I would disagree with your conclusion.
And there is no secrets around this term….just different opinions.

 

[pylfrm]

greenimi,

I didn't say that the definition posted by chez311 is the only definition of the term, or that it happens to be my definition, just that it's the only one I remembered seeing posted. As it turns out, my memory is not worth much in this case.

Although it only became clear with your latest post, I think the discussion about self-referencing in this thread can be roughly summarized as follows:
[ul]
[li]It's possible to have non-zero actual values associated with the profile tolerances in #3.[/li]
[li]The profile tolerances in #3 are applied to datum features referenced by those tolerances.[/li]
[/ul]
I assume everyone here would agree with both statements. The only cause for debate is the use of terminology without an agreed-upon definition.


Getting back to somewhat more practical aspects of the topic, perhaps you could explain what potential advantages you see for #2 or #3 over #1. Are there particular scenarios you can imagine where these schemes seem more appropriate?


pylfrm

 

[greenimi]

pylfrm, chez311 and all,

After giving it some thoughts, I agree with pylfrm that option #1 in chez311 comparison chart is the preferred one.

And No, I cannot think a certain case or scenario where option #2 or #3, hence legal callouts according to Y14.5, would be the preferred choices. Maybe, someone else (let’s say from Metrology) would think otherwise (as #2 and #3 being more friendly for some specific inspection methods)

The main takeaway from this discussion, IMHO, was that there is not only one good way to define the OP drawing. Maybe there are several (limited options), but not only one. And I am glad we have concluded (at least some of us) that option #2 and #3 are viable (not illegal per Y14.5 and not self-referencing per whatever definition you might want to use).

 

[Kedu]

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1540342041/tips/Alternatives_for_Single_Datum_Axis_at_MMB_for_Coaxial_Features_1_aorpkg.pdf[/url]

Chez311,

Are any of those alternatives self referencing? Which one, if any?

 

[chez311]

Sorry for the hiatus - I had quite a bit on my plate.

Pylfrm,
I think I understand and agree with your analogy to 4-29(a). I'm still trying to wrap my head around this, however what the ratio looks like seems to be affected by what types of datum features are as well as what the order/hierarchy looks like in a particular DRF. For example, the difference between your example of Fig. 4-29(a) with a secondary non-planar self-referencing datum feature (tolerance zone = any ratio between 0 and 1) vs. if Fig. 4-22 were modified so that the offset planar datum features were primary and self-referencing (tolerance zone = halved). Would you agree that if the datum features B and C in my example #2/#3 were primary and self-referencing (ie: both with a DRF that was [B-C] instead of [A|B-C]) the tolerance zone would be halved?

The more I think about it as well, the concept you brought up that the tolerance zone could be "any ratio between 0 and 1" seems to be a result of measurement and not actually any geometric constraint on the datum feature. By that I mean in your example showing that only 25% of the zone is usable is only determined AFTER it is measured. In reality the entire tolerance zone is usable and the datum feature is passable as long as it falls anywhere within that zone - it is only after measurement that if that outside 25% of the zone is unused that it is determined that 25% of the zone is unusable, does that make sense? I think 3DDave's statement in the post

https://www.eng-tips.com/viewthread.cfm?qid=438924

is very apt with self-referencing (or whatever we're calling them) datums - "wherever you go, there you are".

To push that concept further, in the situations where as you say the ratio of usable tolerance zone could be anywhere between 0 and 1, if as I noted above that the entire zone is actually usable (that is - if you agree with me), is there actually any difference between my #1/#2/#3 ? It seems that they actually may all result in an identical tolerance zone. I'm interested to hear what you think.

In regards to the discussion of terminology, my apologies for dragging you into trying to define the concept of self-referencing. To me, it makes sense to define any datum feature which references itself as such (self-referencing) even if they result in non-zero actual values and a valid tolerance zone but there are clearly other opinions on the topic. Not being able to define them as such makes it difficult to discuss them because I believe they deserve their own subcategory, however it seems I may have to be more careful in my choice of words in the future to avoid getting into the same discussion of terminology!

greenimi,
Agreed - there is almost always more than one way to skin a cat, however I believe its always best practice to choose the most clear and least confusing solution if possible - in this case I think #1 certainly fulfills that, even if #2/#3 are not "illegal".

Kedu,
The discussion of what constitutes self-referencing notwithstanding I'll try using the terms "invalid self-referencing" (zero actual values) and "valid self-referencing" (non-zero actual values). Since your alternatives #2/#3 do not reference any datums in the DRF (and in fact straightness in #3 cannot reference datums) I would not consider them self-referencing at all. I *think* your alternative #1 would be considered valid self-referencing since datum A is the result of the pattern of 2x UAME's of the two features (vs. if the position tolerance was just a single feature). That being said, since it is referenced at MMB I don't think the result is any different than if the reference was actually datumless position (similar to Fig. 7-59) - to be different it would have to be referenced at RMB. I am however, open to alternative interpretations.

 

[pylfrm]

Would you agree that if the datum features B and C in my example #2/#3 were primary and self-referencing (ie: both with a DRF that was [B-C] instead of [A|B-C]) the tolerance zone would be halved?

Yes. The half of the tolerance zone would be unusable in that case.

In reality the entire tolerance zone is usable and the datum feature is passable as long as it falls anywhere within that zone - it is only after measurement that if that outside 25% of the zone is unused that it is determined that 25% of the zone is unusable, does that make sense?

This would seem to imply that #2 and #3 are equivalent to #1, but that is not the case.

Consider an example that meets the profile tolerance in #1: Datum feature B has a deviation from true profile of -.0005 over 100% of its surface, and datum feature C has a deviation from true profile of +.0005 over 99% of its surface and -.0005 over the remaining 1%. Now evaluate this geometry against the profile tolerance of #2 or #3. The datum feature simulator will contact 100% of feature B and 99% of datum feature C, and it will also happen to end up coincident with the true profile. Datum feature B will now have a deviation from true profile of 0 over 100% of its surface, and datum feature C will have a deviation from true profile of 0 over 99% of its surface and -.001 over the remaining 1%. The full tolerance zone was required to contain datum feature C, but it's out of tolerance by a factor of two now that only half of the zone is usable.


pylfrm