I'm working on a machining drawing and one of the features is a hole with multiple diameters (from 15, 18, 20, and 22mm) all along the same axis. Do I have to set a perpendicularity control frame for each dimension or can I set it for the top most dimension and have it apply to entire axis for all the holes?
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Perpendicularity on multiple holes along single axis
- Thread starter Raddy13
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pmarc, chez311,
I suppose I should try to identify the basis of my interpretation:
It seems to me that the basic 20 dimension in Fig. 4-22 is an integral part of the definition of the true profile for the 0.8 tolerance. Concluding that the relationship between datum features A and B is uncontrolled without the "2X" would seem to require accepting one of the following claims:
[ol 1]
[li]The true profile used to construct the tolerance boundaries can differ from the basic geometry specified by the drawing.[/li]
[li]It's not necessary for the entire considered portion of the part's actual surface to be within the tolerance zone at once. Different alignments between actual part and true profile can be used for different subsets.[/li]
[li]The single feature control frame specifies two profile tolerances, not one.[/li]
[/ol]
The first two options seem quite strange, so I'm guessing you're going with the third. Is that correct?
How does adding the "2X" change this?
pylfrm
I suppose I should try to identify the basis of my interpretation:
ASME Y14.5-2009 para. 8.2 (emphasis added) said:
ASME Y14.5-2009 para. 8.2 (emphasis added) said:
ASME Y14.5-2009 para. 8.2.1.1 (emphasis added) said:
ASME Y14.5-2009 para. 8.3.1 said:
It seems to me that the basic 20 dimension in Fig. 4-22 is an integral part of the definition of the true profile for the 0.8 tolerance. Concluding that the relationship between datum features A and B is uncontrolled without the "2X" would seem to require accepting one of the following claims:
[ol 1]
[li]The true profile used to construct the tolerance boundaries can differ from the basic geometry specified by the drawing.[/li]
[li]It's not necessary for the entire considered portion of the part's actual surface to be within the tolerance zone at once. Different alignments between actual part and true profile can be used for different subsets.[/li]
[li]The single feature control frame specifies two profile tolerances, not one.[/li]
[/ol]
The first two options seem quite strange, so I'm guessing you're going with the third. Is that correct?
How does adding the "2X" change this?
pylfrm
All,
Sorry I'm late coming into this thread. Some very interesting discussion, on patterns, grouping, and simultaneous requirements.
Here are a few initial comments:
I would say that leader lines are not directly involved in pattern creation. Fig. 8-16 in Y14.5-2009 unfortunately contradicts this, because the two surfaces are described in the text as controlled to each other with only leader lines connecting the profile FCF to the two surfaces. The consensus is (and I agree) that this was an error and there should have been a 2X annotation in Fig. 8-16. This was corrected in the latest version of the figure, Fig. 11-18 in Y14.5-2018.
The profile tolerance on features A and B in Fig. 4-22 is a good example to study. Here is my take on it:
-there are two features. In other words, the profile tolerance does not combine the two surfaces into one feature
-there is a true profile for each feature. The true profiles have a basic relationship to each other (parallel and offset by 20 mm)
-there is a tolerance zone for each feature. The tolerances follow the true profiles, and are thus parallel and offset by 20 mm
-the existence of basically related tolerance zones does not in itself constrain the two features relative to each other. There also needs to be a mechanism to require the zones to apply "simultaneously".
Unfortunately the word "simultaneous" has a special meaning in Y14.5, and it's confusing if we use this term when describing situations that do not involve datum features and datum reference frames
-Y14.5 uses some loose terminology to describe this. A "pattern" is defined in 1.3.43 as "two more features to which a locational geometric tolerance is applied and are grouped by one of the following methods: nX, n COAXIAL holes, ALL OVER, A<->B, n SURFACES, simultaneous requirements or INDICATED." So when Features are "grouped" by one of the methods listed, a "pattern" is created.
-I'll refer to these methods as "pattern-creating mechanisms".
-I like pylfrm's use of the term "alignment" to represent a spatial relationship between the actual part and the basic geometry
-the effect of a pattern-creating mechanism is that it imposes the requirement that the tolerance zones for all of the applicable FCF's be evaluated in the same alignment
-the 2X annotation is the pattern-creating mechanism in the Fig. 4-22 case. If it were not present, then there would not be the requirement to compare the actual part to both profile zones in the same alignment. In other words, each profile zone could be evaluated in a different alignment.
The effect of a pattern-creating mechanism is the same as the effect of a simultaneous requirement, which requires tolerance zones to be evaluated in the same datum reference frame. In fact, simultaneous requirements is one of the pattern-creating mechanisms listed above. I believe that the most difficult thing about this is the terminology.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
Sorry I'm late coming into this thread. Some very interesting discussion, on patterns, grouping, and simultaneous requirements.
Here are a few initial comments:
I would say that leader lines are not directly involved in pattern creation. Fig. 8-16 in Y14.5-2009 unfortunately contradicts this, because the two surfaces are described in the text as controlled to each other with only leader lines connecting the profile FCF to the two surfaces. The consensus is (and I agree) that this was an error and there should have been a 2X annotation in Fig. 8-16. This was corrected in the latest version of the figure, Fig. 11-18 in Y14.5-2018.
The profile tolerance on features A and B in Fig. 4-22 is a good example to study. Here is my take on it:
-there are two features. In other words, the profile tolerance does not combine the two surfaces into one feature
-there is a true profile for each feature. The true profiles have a basic relationship to each other (parallel and offset by 20 mm)
-there is a tolerance zone for each feature. The tolerances follow the true profiles, and are thus parallel and offset by 20 mm
-the existence of basically related tolerance zones does not in itself constrain the two features relative to each other. There also needs to be a mechanism to require the zones to apply "simultaneously".
Unfortunately the word "simultaneous" has a special meaning in Y14.5, and it's confusing if we use this term when describing situations that do not involve datum features and datum reference frames
-Y14.5 uses some loose terminology to describe this. A "pattern" is defined in 1.3.43 as "two more features to which a locational geometric tolerance is applied and are grouped by one of the following methods: nX, n COAXIAL holes, ALL OVER, A<->B, n SURFACES, simultaneous requirements or INDICATED." So when Features are "grouped" by one of the methods listed, a "pattern" is created.
-I'll refer to these methods as "pattern-creating mechanisms".
-I like pylfrm's use of the term "alignment" to represent a spatial relationship between the actual part and the basic geometry
-the effect of a pattern-creating mechanism is that it imposes the requirement that the tolerance zones for all of the applicable FCF's be evaluated in the same alignment
-the 2X annotation is the pattern-creating mechanism in the Fig. 4-22 case. If it were not present, then there would not be the requirement to compare the actual part to both profile zones in the same alignment. In other words, each profile zone could be evaluated in a different alignment.
The effect of a pattern-creating mechanism is the same as the effect of a simultaneous requirement, which requires tolerance zones to be evaluated in the same datum reference frame. In fact, simultaneous requirements is one of the pattern-creating mechanisms listed above. I believe that the most difficult thing about this is the terminology.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
I don't disagree with this, but I don't see any reason they can't also be considered one feature.
The paragraphs quoted in my previous post refer to a single true profile and a single tolerance zone, making no exception for non-contiguous cases. Combined with "The actual surface [...] must be within the specified tolerance zone", it seems like a sufficient mechanism to me.
Could you please explain why you believe this to be true? It would be great if you could point to some justification in the standard.
pylfrm
Since there are two leaders in '2009 8-16, why would there need to be 2X?
The null datum is a legitimate case as seen in '2009 figure 7-51. This fits within the definition of "simultaneous" in '2009 p4.19.
I expect the original exclusion of orientation tolerances is that, unlike position and profile, one cannot simply drop a part onto a fixture to verify the general case for orientation tolerances. They might require moving elements or other inconvenient adaptations that gage makers might not care to introduce. Better to handle them with separate set-ups that might not duplicate the DRF-to-part alignment/orientation from measurement to measurement.
The null datum is a legitimate case as seen in '2009 figure 7-51. This fits within the definition of "simultaneous" in '2009 p4.19.
I expect the original exclusion of orientation tolerances is that, unlike position and profile, one cannot simply drop a part onto a fixture to verify the general case for orientation tolerances. They might require moving elements or other inconvenient adaptations that gage makers might not care to introduce. Better to handle them with separate set-ups that might not duplicate the DRF-to-part alignment/orientation from measurement to measurement.
pylfrm,
Thanks for the useful comments. There is a lot going on here.
I found that considering the surfaces as 2 distinct features has a much better overall consistency with Y14.5 text and figures than considering them as one feature:
-the 1.3.42 definition of Pattern mentions "two or more features or features to which a locational geometric tolerance is applied and are grouped by one of the following methods: nX, ...". I would say that this strongly indicates that geometric tolerances and pattern-creating mechanisms act on collections of features, but there is no mention of treating the features as a single feature
-the Continuous Feature <CF> concept that was introduced in 2009 actually does treat multiple features as a single feature
-the distinction becomes significant when a form or size tolerance is applied. For example, the 2 coaxial cylinders in Fig. 7-59 have 2X a size tolerance, and a position tolerance applied to them. They are still treated as 2 distinct features - we would evaluate the size of each cylinder individually, and find the axis of each feature (not a combined axis). So by the same token, the two surfaces in Fig. 4-22 are treated as 2 distinct features. If there was a flatness tolerance applied in addition to the profile tolerance, we would evaluate the flatness of each surface individually.
-the consistency is not perfect, however. The pocket surface in Fig. 8-19 is described as an "irregular feature", even though the All Around symbol is applied. For some reason, All Around was not included in the list of pattern-creating mechanisms (but All Over was). This was corrected in Y14.5-2018 - All Around is now included (as I believe it should be). Believe me, feature definition is still a contentious subject in ASME subcommittees including Y14.5.
I freely admit that the multi-feature breakdown I'm suggesting is not consistent with the Y14.5 statements you referenced on profiles and true profiles. We are forced to choose between the consequences of those statements versus the consequences of the statements I mentioned above - they are in conflict with each other. My conclusion was that the definition of the tolerance zone is driven more by the specific indications on the drawing than by the generic definitions of profiles, true profiles, and profile tolerance zones.
Regarding my proposed effect of a pattern-creating mechanism, this is an overall conclusion I reached based on pondering Y14.5's text and definitions-by-example. Over a period of years, with countless discussions with committee and industry colleagues. It isn't explicitly stated, and not everyone I've talked to agrees with it. But I found that when I tried to distill Y14.5 into a relatively concise set of concepts, the "common alignment" effect for patterns was an essential ingredient. I believe it to be true because it gets the "correct" results - it is consistent with the results shown in the vast majority of "means this" figures for position and profile tolerances. Further to that, the alternative explanations I looked at gave results that were not consistent. I should also clarify something else that I should have mentioned - most of the mechanisms also have the commonly understood meaning of acting as a number-of-places indicator. In Fig. 4-22 the 2X tells us that the profile tolerance applies to 2 surface features, in the same way that the 4X in Fig. 7-2 tells us that the size and position tolerances apply to 6 cylindrical features. This has been one of the more difficult aspects to unwind - these annotations have a straightforward meaning, and also an implicit effect that is only really defined by example and the cryptic language of grouping and patterns.
3DDave,
Fig. 8-16 in Y14.5-2009 needs the 2X because the two leaders do not create a pattern and the requirement for a common alignment. 2X is one of the pattern-creating mechanisms listed in 1.3.42.
There are two schools of thought in the GD&T community with regards to how constraint happens with position and profile tolerances with no datum features. Your comment represents one camp - that the position tolerance with no datum features in 7-51 represents two position tolerances with the same DRF and thus become a simultaneous requirement with a common DRF. Those in the other camp (myself included) would say that the there are no datum features and no DRF, and so the simultaneous requirement from 4.19 does not apply. Instead, the TWO COAXIAL HOLES annotation creates a pattern and this is what requires a common alignment. To me, the consequences of the pattern interpretation is much more self-consistent than the consequences of the null-datum interpretation. But there are well-respected Y14.5 members in both camps, and the debate is still going on.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
Thanks for the useful comments. There is a lot going on here.
I found that considering the surfaces as 2 distinct features has a much better overall consistency with Y14.5 text and figures than considering them as one feature:
-the 1.3.42 definition of Pattern mentions "two or more features or features to which a locational geometric tolerance is applied and are grouped by one of the following methods: nX, ...". I would say that this strongly indicates that geometric tolerances and pattern-creating mechanisms act on collections of features, but there is no mention of treating the features as a single feature
-the Continuous Feature <CF> concept that was introduced in 2009 actually does treat multiple features as a single feature
-the distinction becomes significant when a form or size tolerance is applied. For example, the 2 coaxial cylinders in Fig. 7-59 have 2X a size tolerance, and a position tolerance applied to them. They are still treated as 2 distinct features - we would evaluate the size of each cylinder individually, and find the axis of each feature (not a combined axis). So by the same token, the two surfaces in Fig. 4-22 are treated as 2 distinct features. If there was a flatness tolerance applied in addition to the profile tolerance, we would evaluate the flatness of each surface individually.
-the consistency is not perfect, however. The pocket surface in Fig. 8-19 is described as an "irregular feature", even though the All Around symbol is applied. For some reason, All Around was not included in the list of pattern-creating mechanisms (but All Over was). This was corrected in Y14.5-2018 - All Around is now included (as I believe it should be). Believe me, feature definition is still a contentious subject in ASME subcommittees including Y14.5.
I freely admit that the multi-feature breakdown I'm suggesting is not consistent with the Y14.5 statements you referenced on profiles and true profiles. We are forced to choose between the consequences of those statements versus the consequences of the statements I mentioned above - they are in conflict with each other. My conclusion was that the definition of the tolerance zone is driven more by the specific indications on the drawing than by the generic definitions of profiles, true profiles, and profile tolerance zones.
Regarding my proposed effect of a pattern-creating mechanism, this is an overall conclusion I reached based on pondering Y14.5's text and definitions-by-example. Over a period of years, with countless discussions with committee and industry colleagues. It isn't explicitly stated, and not everyone I've talked to agrees with it. But I found that when I tried to distill Y14.5 into a relatively concise set of concepts, the "common alignment" effect for patterns was an essential ingredient. I believe it to be true because it gets the "correct" results - it is consistent with the results shown in the vast majority of "means this" figures for position and profile tolerances. Further to that, the alternative explanations I looked at gave results that were not consistent. I should also clarify something else that I should have mentioned - most of the mechanisms also have the commonly understood meaning of acting as a number-of-places indicator. In Fig. 4-22 the 2X tells us that the profile tolerance applies to 2 surface features, in the same way that the 4X in Fig. 7-2 tells us that the size and position tolerances apply to 6 cylindrical features. This has been one of the more difficult aspects to unwind - these annotations have a straightforward meaning, and also an implicit effect that is only really defined by example and the cryptic language of grouping and patterns.
3DDave,
Fig. 8-16 in Y14.5-2009 needs the 2X because the two leaders do not create a pattern and the requirement for a common alignment. 2X is one of the pattern-creating mechanisms listed in 1.3.42.
There are two schools of thought in the GD&T community with regards to how constraint happens with position and profile tolerances with no datum features. Your comment represents one camp - that the position tolerance with no datum features in 7-51 represents two position tolerances with the same DRF and thus become a simultaneous requirement with a common DRF. Those in the other camp (myself included) would say that the there are no datum features and no DRF, and so the simultaneous requirement from 4.19 does not apply. Instead, the TWO COAXIAL HOLES annotation creates a pattern and this is what requires a common alignment. To me, the consequences of the pattern interpretation is much more self-consistent than the consequences of the null-datum interpretation. But there are well-respected Y14.5 members in both camps, and the debate is still going on.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
"A simultaneous requirement applies to position and profile tolerances that are located by basic dimensions, related to common datum features referenced in the same order of precedence at the same boundary conditions."
The word "pattern" is not in that requirement. By default, each member of a pattern (ones mutually located with basic dimensions) is automatically qualified as sharing a simultaneous requirement with all other members. Simultaneous requirement is one of the means given to identify a pattern.
The null datum requirement is what allows a profile tolerance to be applied to all the individual surfaces of a profile controlled boundary without allowing them to fly off individually into space. I don't have to say 4X profile around a rectangular opening to have a simultaneous requirement apply. The "pattern" definition doesn't include using the "all around" symbol. Does that mean it would be acceptable to readjust the position of the part relative to the datum feature simulators to get each surface to fit the tolerance while pushing the others out of tolerance? That does not seem reasonable.
In the case of '2009 fig 8-5, the profile only has 3 degrees of freedom constrained, leaving 2 degrees of translation and 1 of rotation in the plane the outline is defined in; those individual surfaces are mutually constrained by the null datums. If not, only the individual form and orientation would apply, piecewise, to the outline.
I assume you have an example where the null datum interpretation fails; one in which it is not possible to sketch out the true positions and true profiles without identifying an explicit DRF. I think you will run afoul of '2009 fig 4-3(f), but a good counter example must exist if the null datum is inconsistent.
The word "pattern" is not in that requirement. By default, each member of a pattern (ones mutually located with basic dimensions) is automatically qualified as sharing a simultaneous requirement with all other members. Simultaneous requirement is one of the means given to identify a pattern.
The null datum requirement is what allows a profile tolerance to be applied to all the individual surfaces of a profile controlled boundary without allowing them to fly off individually into space. I don't have to say 4X profile around a rectangular opening to have a simultaneous requirement apply. The "pattern" definition doesn't include using the "all around" symbol. Does that mean it would be acceptable to readjust the position of the part relative to the datum feature simulators to get each surface to fit the tolerance while pushing the others out of tolerance? That does not seem reasonable.
In the case of '2009 fig 8-5, the profile only has 3 degrees of freedom constrained, leaving 2 degrees of translation and 1 of rotation in the plane the outline is defined in; those individual surfaces are mutually constrained by the null datums. If not, only the individual form and orientation would apply, piecewise, to the outline.
I assume you have an example where the null datum interpretation fails; one in which it is not possible to sketch out the true positions and true profiles without identifying an explicit DRF. I think you will run afoul of '2009 fig 4-3(f), but a good counter example must exist if the null datum is inconsistent.
3DDave,
The simultaneous requirement definition includes "related to common datum features referenced in the same order of precedence at the same boundary conditions". The null datum interpretation requires the assumption that a set of two or more FCF's with no datum features meets this criteria. I don't have a dog and my neighbor doesn't either - using that logic, wouldn't this mean that we both have the same breed of dog? To me this is a quite a stretch to think that this is what Y14.5 intended here, but I've talked to quite a few people who are fine with it.
The omission of All Around from the list of methods in the pattern definition was definitely problematic and muddied the waters, especially since All Over (which works in very much the same way) was included. I agree that it's not reasonable that the part could be readjusted to get each surface to fit its tolerance zone. Y14.5 was inconsistent on this - All Around should have been included in the pattern definition in 2009 and this was corrected in 2018. In the case of the datumless profile tolerance for a rectangular opening, I would say that you need either the 4X or the All Around symbol in order for a common alignment to be required and constrain the surfaces relative to each other.
You bring up an interesting reference in Fig. 4-3. In (f), the profile tolerance on the round-end slab (made up of 2 flats and 2 hemi-cylindrical end surfaces) has an All-Around symbol on its leader. If the shape was considered to be one feature, then the All-Around symbol would not be necessary. In (g), the round-end wedge (made up of 2 flats and 2 hemi-conical end surfaces) just has a leader line pointing to one of the end surfaces. To me, this is very inconsistent.
I don't know of examples where the null datum interpretation fails outright and would require an explicit DRF to be identified. It just seems to me that the consequences of the null datum interpretation are not optimal and would make some major things in Y14.5 inconsistent or unnecessary. If there is an underlying null datum simultaneous requirement, then why would the other methods for patterns and grouping even be mentioned? If nX, n SURFACES, A<->B only function as number-of-places indicators, why was there a need to point them out in 1.3.42 as methods of grouping features into a pattern when they are not?
Another consequence of the null datum interpretation is that if a drawing had multiple instances of datumless coaxiality (as in Fig. 7-59) or datumless coplanarity (as in Fig. 8-14) then these would be simultaneous requirements. The entire collection of cylinders and planar surfaces would be mutually located to each other. I don't think that this would be required (or desired) in most situations. To override the overall simultaneous requirement from the null datum, each FCF would need a SEP REQT annotation. Y14.5 could define things this way, but I don't think that this would be optimal and I can't believe that this was the intent.
There is even a specific reference that indicates that the null datum simultaneous requirement was not intended, 7.6.2.3 Coaxial Features Without Datum References. It mentions that the FCF is supplemented by a notation such as TWO COAXIAL FEATURES (this notation would be the pattern-creating mechanism). Then it states "a positional tolerance specification with no datum reference creates a relationship between the toleranced features, but implies no relationship to any other features". The null datum interpretation would directly contradict this, because there could be an implied relationship with other features (if there were other datumless coaxiality or coplanarity tolerances on other features).
Evan Janeshewski
Axymetrix Quality Engineering Inc.
The simultaneous requirement definition includes "related to common datum features referenced in the same order of precedence at the same boundary conditions". The null datum interpretation requires the assumption that a set of two or more FCF's with no datum features meets this criteria. I don't have a dog and my neighbor doesn't either - using that logic, wouldn't this mean that we both have the same breed of dog? To me this is a quite a stretch to think that this is what Y14.5 intended here, but I've talked to quite a few people who are fine with it.
The omission of All Around from the list of methods in the pattern definition was definitely problematic and muddied the waters, especially since All Over (which works in very much the same way) was included. I agree that it's not reasonable that the part could be readjusted to get each surface to fit its tolerance zone. Y14.5 was inconsistent on this - All Around should have been included in the pattern definition in 2009 and this was corrected in 2018. In the case of the datumless profile tolerance for a rectangular opening, I would say that you need either the 4X or the All Around symbol in order for a common alignment to be required and constrain the surfaces relative to each other.
You bring up an interesting reference in Fig. 4-3. In (f), the profile tolerance on the round-end slab (made up of 2 flats and 2 hemi-cylindrical end surfaces) has an All-Around symbol on its leader. If the shape was considered to be one feature, then the All-Around symbol would not be necessary. In (g), the round-end wedge (made up of 2 flats and 2 hemi-conical end surfaces) just has a leader line pointing to one of the end surfaces. To me, this is very inconsistent.
I don't know of examples where the null datum interpretation fails outright and would require an explicit DRF to be identified. It just seems to me that the consequences of the null datum interpretation are not optimal and would make some major things in Y14.5 inconsistent or unnecessary. If there is an underlying null datum simultaneous requirement, then why would the other methods for patterns and grouping even be mentioned? If nX, n SURFACES, A<->B only function as number-of-places indicators, why was there a need to point them out in 1.3.42 as methods of grouping features into a pattern when they are not?
Another consequence of the null datum interpretation is that if a drawing had multiple instances of datumless coaxiality (as in Fig. 7-59) or datumless coplanarity (as in Fig. 8-14) then these would be simultaneous requirements. The entire collection of cylinders and planar surfaces would be mutually located to each other. I don't think that this would be required (or desired) in most situations. To override the overall simultaneous requirement from the null datum, each FCF would need a SEP REQT annotation. Y14.5 could define things this way, but I don't think that this would be optimal and I can't believe that this was the intent.
There is even a specific reference that indicates that the null datum simultaneous requirement was not intended, 7.6.2.3 Coaxial Features Without Datum References. It mentions that the FCF is supplemented by a notation such as TWO COAXIAL FEATURES (this notation would be the pattern-creating mechanism). Then it states "a positional tolerance specification with no datum reference creates a relationship between the toleranced features, but implies no relationship to any other features". The null datum interpretation would directly contradict this, because there could be an implied relationship with other features (if there were other datumless coaxiality or coplanarity tolerances on other features).
Evan Janeshewski
Axymetrix Quality Engineering Inc.
If the standard was a work by a single author who carefully examined the effect of each piece I'd be concerned about their mental health from the lack of consistency.
As it is, the standard tends to mix decorative requirements with mathematical elements. Decorative ones are depictions for the benefit of the reader, such as requiring a space after a quantity. "2X" is wrong. "2X_" is the only "right" way. Completely arbitrary and solely decorative. As is "TWO COAXIAL FEATURES", which is a condition that is otherwise observable as they share a common axis and their count is obvious.
The problem is when decorative requirements are taken as mathematical semantics.
The null datum just means that the mutual locations and orientations of the related features are not required to be aligned to any other features. Datum references are additional restrictions; it's allowable that the restriction count is zero. So it's not a matter of "breed" at that point; and dogs is not a good example because restrictions on mutual locations and orientation are a fundamental part of the relation between features and not a decorative add-on.
So, yes, a consistent evaluation of null datums would make a few decorative elements unnecessary.
Why would the section 7 group feel empowered to be the arbiters of datum evaluation when the section 4 group was responsible for it? My guess - the internal battle for control at the expense of users of the standard. If that was the intent, that one sentence far flung from the datum section, that the default, unlike other datum references, is independent, then that is a contradiction and would effectively say, every feature can be located and oriented at random when there is no datum explicitly referenced unless a decorative element is applied.
The more consistent reading is that null datum FCFs are independent of other non-matching DRFs, while retaining simultaneous evaluation to each other, just like the rule ordinarily states.
That rectangular shape could have 4 separate profile tolerances, perhaps each with a different magnitude tolerance, possibly with different unequally disposed tolerances, that would only make sense if they were simultaneously applied and make no sense if they were presumed to be entirely independent.
But then my preference is for a simple rule rather than a complicated one that requires other, even more complicated rules that need even more maintenance from version to version.
As it is, the standard tends to mix decorative requirements with mathematical elements. Decorative ones are depictions for the benefit of the reader, such as requiring a space after a quantity. "2X" is wrong. "2X_" is the only "right" way. Completely arbitrary and solely decorative. As is "TWO COAXIAL FEATURES", which is a condition that is otherwise observable as they share a common axis and their count is obvious.
The problem is when decorative requirements are taken as mathematical semantics.
The null datum just means that the mutual locations and orientations of the related features are not required to be aligned to any other features. Datum references are additional restrictions; it's allowable that the restriction count is zero. So it's not a matter of "breed" at that point; and dogs is not a good example because restrictions on mutual locations and orientation are a fundamental part of the relation between features and not a decorative add-on.
So, yes, a consistent evaluation of null datums would make a few decorative elements unnecessary.
Why would the section 7 group feel empowered to be the arbiters of datum evaluation when the section 4 group was responsible for it? My guess - the internal battle for control at the expense of users of the standard. If that was the intent, that one sentence far flung from the datum section, that the default, unlike other datum references, is independent, then that is a contradiction and would effectively say, every feature can be located and oriented at random when there is no datum explicitly referenced unless a decorative element is applied.
The more consistent reading is that null datum FCFs are independent of other non-matching DRFs, while retaining simultaneous evaluation to each other, just like the rule ordinarily states.
That rectangular shape could have 4 separate profile tolerances, perhaps each with a different magnitude tolerance, possibly with different unequally disposed tolerances, that would only make sense if they were simultaneously applied and make no sense if they were presumed to be entirely independent.
But then my preference is for a simple rule rather than a complicated one that requires other, even more complicated rules that need even more maintenance from version to version.
Wherever position without datum references is used such as in fig. 4-24 or 7-51, it is applied to more than one coaxial features. The notation "2 COAXIAL HOLES" is no more "decorative" than the "2X" in fig. 4-24. Both do the grouping of features into patterns.
Unlike position, profile can be applied without datum references to a single feature. In that case, the location of the feature to any other features or datums is uncontrolled by the profile tolerance. In the rectangular opening example, applying a profile FCF without datum references to each face of the opening would create a separate form-only requirement for each face. The mutual location between the faces could be controlled by directly toleranced size dimensions. Perhaps not recommended, this would essentially be an alternate practice to four separate flatness controls.
An example of a legitimate case where there are two separate profile requirements without datum references applied on two independent sets of features is attached.
Unlike position, profile can be applied without datum references to a single feature. In that case, the location of the feature to any other features or datums is uncontrolled by the profile tolerance. In the rectangular opening example, applying a profile FCF without datum references to each face of the opening would create a separate form-only requirement for each face. The mutual location between the faces could be controlled by directly toleranced size dimensions. Perhaps not recommended, this would essentially be an alternate practice to four separate flatness controls.
An example of a legitimate case where there are two separate profile requirements without datum references applied on two independent sets of features is attached.
Too bad:
A simultaneous requirement applies to position and profile tolerances that are located by basic dimensions, related to common datum features referenced in the same order of precedence at the same boundary conditions.
The use of directly toleranced dimensions invalidates that as a suitable counter-example.
A simultaneous requirement applies to position and profile tolerances that are located by basic dimensions, related to common datum features referenced in the same order of precedence at the same boundary conditions.
The use of directly toleranced dimensions invalidates that as a suitable counter-example.
3DDave,
Take a minute to re-read your own quote, which says:
"related to common datum features".
I am sure you know that datum features are physical portions of a part.
I also think you know that "related to common datum features" is not the same as "related to common lack of referenced datum features".
Therefore I assume you have an example of common datum features related to a common "null datum".
Take a minute to re-read your own quote, which says:
"related to common datum features".
I am sure you know that datum features are physical portions of a part.
I also think you know that "related to common datum features" is not the same as "related to common lack of referenced datum features".
Therefore I assume you have an example of common datum features related to a common "null datum".
Here it is ->
Can you see the null datum feature? It's right there. As identified whenever no other datum reference is given.
Besides - going by your misreading, they can only be simultaneous when two or more features are referenced; just one feature would not meet the pluralization. But you knew that.
I did miss your admission that your example was bad; your response was not a good try at deflection. I think you know that.
Can you see the null datum feature? It's right there. As identified whenever no other datum reference is given.
Besides - going by your misreading, they can only be simultaneous when two or more features are referenced; just one feature would not meet the pluralization. But you knew that.
I did miss your admission that your example was bad; your response was not a good try at deflection. I think you know that.
If this is what you want to get into, it is common to use plural nouns when generalizing, even when the single is included in the generalization. The standard was worded by humans, for the use if humans that understand that. Perhaps some don't.
3DDave said:
Sure I can see it. Null datum features are very useful when tolerancing null parts. Null customers love them.
Perhaps this can help to clear up some of the confusion:
"Where a geometric tolerance is related to a datum, this relationship is indicated by entering the datum feature reference letter in a compartment following the tolerance." (Para. 3.4.2)
"Where more than one datum is required, the datum feature reference letters (each followed by a material boundary modifier, where applicable) are entered in separate compartments in the desired order of precedence, from left to right."(para. 3.4.3)
The reasonable conclusion is that where there are (is) no compartments (compartment) dedicated to datum feature reference letters in a feature control frame, the geometric tolerance is not related to a datum. The "null" tag added before the term datum feature doesn't change that.
3DDave said:
Not before you admit that a common reference to non-existing datum features (feature) doesn't hold water.
3DDave,
I would agree that communicating mutual constraint through implied meanings of drawing indications (or "decorative elements") that already have a different well-defined meaning is very questionable. Especially if the implied constraint is only defined in loosely worded terms and through definition-by-example. A large part of it relies on what is meant by "grouped", a term which is only mentioned in the standard once (I think) and is not actually defined. It's incredibly confusing, and in hindsight definitely could have been done differently. But this appears to be how it was done - constraint in the absence of datum features relies on the vaguely defined pattern concept and is specified through the explicit indications such as nX, n SURFACES, A<->B. Mutual constraint could have been defined in a self-consistent way using a null-datum concept, but to me it appears that they didn't do that.
There are also further issues that we haven't even touched on yet - for example, the consequences of the different constraint interpretations in the context of composite FCF's (whose lower segments are not subject to simultaneous requirements by default). I spent a lot of time (and probably a lot of brain cells) pondering the various tolerance zone frameworks in Fig. 7-38 (composite position FCF's on multiple hole patterns) and 8-23 (multi-feature composite profile with independent size/form control). I could not get the null-datum interpretation to work on these at all.
Whatever the intent of Y14.5 was, I would say that the overall collection of text and figures that ended up in the standard is much more compatible with the pattern interpretation than the null-datum interpretation. There are definitely arguments to make on both sides, and I'm not even sure which side most people would be on.
Burunduk,
Your drawing brings up some more interesting issues. Mixing directly-toleranced dimensions with profile tolerancing is always problematic, and usually results in questions that we really can't answer.
I wonder how the 60 +/- 1 width tolerance affects the simultaneous requirement between the two profile tolerances. With the pattern interpretation, the A<->B creates one pattern and the C<->D creates another pattern. There is no simultaneous requirement, so the lack of basic dimensions between the two collections of surfaces doesn't cause an issue. With the null-datum interpretation, I'm really not sure. On the one hand, one could say that the 4.19 criteria is not satisfied because the two geometric tolerances are not located my basic dimensions. On the other hand, one could also say that there is a simultaneous requirement between the two geometric tolerances and so the 60 +/- 1 dimension should be basic.
Let's say that one of the large flat faces was labeled as datum feature B. If both of the profile FCF's referenced B, there would definitely be a simultaneous requirement and we would say that the 60 dimension needs to be basic.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
I would agree that communicating mutual constraint through implied meanings of drawing indications (or "decorative elements") that already have a different well-defined meaning is very questionable. Especially if the implied constraint is only defined in loosely worded terms and through definition-by-example. A large part of it relies on what is meant by "grouped", a term which is only mentioned in the standard once (I think) and is not actually defined. It's incredibly confusing, and in hindsight definitely could have been done differently. But this appears to be how it was done - constraint in the absence of datum features relies on the vaguely defined pattern concept and is specified through the explicit indications such as nX, n SURFACES, A<->B. Mutual constraint could have been defined in a self-consistent way using a null-datum concept, but to me it appears that they didn't do that.
There are also further issues that we haven't even touched on yet - for example, the consequences of the different constraint interpretations in the context of composite FCF's (whose lower segments are not subject to simultaneous requirements by default). I spent a lot of time (and probably a lot of brain cells) pondering the various tolerance zone frameworks in Fig. 7-38 (composite position FCF's on multiple hole patterns) and 8-23 (multi-feature composite profile with independent size/form control). I could not get the null-datum interpretation to work on these at all.
Whatever the intent of Y14.5 was, I would say that the overall collection of text and figures that ended up in the standard is much more compatible with the pattern interpretation than the null-datum interpretation. There are definitely arguments to make on both sides, and I'm not even sure which side most people would be on.
Burunduk,
Your drawing brings up some more interesting issues. Mixing directly-toleranced dimensions with profile tolerancing is always problematic, and usually results in questions that we really can't answer.
I wonder how the 60 +/- 1 width tolerance affects the simultaneous requirement between the two profile tolerances. With the pattern interpretation, the A<->B creates one pattern and the C<->D creates another pattern. There is no simultaneous requirement, so the lack of basic dimensions between the two collections of surfaces doesn't cause an issue. With the null-datum interpretation, I'm really not sure. On the one hand, one could say that the 4.19 criteria is not satisfied because the two geometric tolerances are not located my basic dimensions. On the other hand, one could also say that there is a simultaneous requirement between the two geometric tolerances and so the 60 +/- 1 dimension should be basic.
Let's say that one of the large flat faces was labeled as datum feature B. If both of the profile FCF's referenced B, there would definitely be a simultaneous requirement and we would say that the 60 dimension needs to be basic.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
axym,
I see no null datum controls on '2009 figure 7-38 and 8-23 specifies "INDIVIDUALLY" on the null datum referenced features.
It's a given that lower levels of composite tolerances are independent by default; not sure how far down one might get with a top level null reference.
I see no null datum controls on '2009 figure 7-38 and 8-23 specifies "INDIVIDUALLY" on the null datum referenced features.
It's a given that lower levels of composite tolerances are independent by default; not sure how far down one might get with a top level null reference.
Evan said:
My answer is simple: since there is no simultaneous requirement between two profile controls that don't share common datum references by virtue of not referencing datum features at all, there is nothing to affect and no dilemma.
Evan said:
This is where I have to admit that as long as the 60 dimension is directly toleranced, there is no simultaneous requirement per the 4.19 criteria because the tolerance zones are not basically located (the basic dimensions only specify the form of the two separate patterns).
3DDave said:
I don't see a single "null datum" used in the standard. What I see is features related to each other either by common datum references related to actual datum features or by one of the explicit means to specify a pattern (with the exception of fig. 4-22 which apparently was corrected in the 2018 version). "Null datum" has a null meaning.
B; you would have argued that the number zero should never be used because that would mean that zero cows was the same as zero sheep and sheep and cows are completely different and zero could therefore not represent them both. Thankfully that was worked out or you'd be in real trouble.
3DDave,
It's true that Fig. 7-38 doesn't have any datumless FCF's. But part of the null-datum interpretation is that the 6X, 4X and 3X indications are only number-of-places shorthand and don't apply mutual constraint. This would mean that if the drawing had six composite position FCF's for the upper set of holes, one for each hole, this should be equivalent to the 6X with one FCF shown in Fig. 7-38. The upper segment is no problem, but the lower segment would be a different story. The lower segments of composite FCF's are not subject to simultaneous requirements, so we would get 6 independent tolerance zones and lose the mutual constraint.
Simultaneous and separate requirements is conspicuously absent from the text explaining Fig. 7-38 and the 7.5 Pattern Location section in general, even though it plays a major role in several of the examples. We don't see any mention of it until Section 7.5.4. There is one figure that deals with separate requirements on multiple patterns, 7-54, and it has no "means this" figure.
You're right that in Fig. 8-23 the annotation INDIVIDUALLY is specified on the null-datum referenced features. If these features have mutual constraint because of a simultaneous requirement from the null-datum reference, why not just specify a SEP REQTS annotation to cancel the mutual constraint? Why do you think there was there a need to invent a completely different annotation instead?
Evan Janeshewski
Axymetrix Quality Engineering Inc.
It's true that Fig. 7-38 doesn't have any datumless FCF's. But part of the null-datum interpretation is that the 6X, 4X and 3X indications are only number-of-places shorthand and don't apply mutual constraint. This would mean that if the drawing had six composite position FCF's for the upper set of holes, one for each hole, this should be equivalent to the 6X with one FCF shown in Fig. 7-38. The upper segment is no problem, but the lower segment would be a different story. The lower segments of composite FCF's are not subject to simultaneous requirements, so we would get 6 independent tolerance zones and lose the mutual constraint.
Simultaneous and separate requirements is conspicuously absent from the text explaining Fig. 7-38 and the 7.5 Pattern Location section in general, even though it plays a major role in several of the examples. We don't see any mention of it until Section 7.5.4. There is one figure that deals with separate requirements on multiple patterns, 7-54, and it has no "means this" figure.
You're right that in Fig. 8-23 the annotation INDIVIDUALLY is specified on the null-datum referenced features. If these features have mutual constraint because of a simultaneous requirement from the null-datum reference, why not just specify a SEP REQTS annotation to cancel the mutual constraint? Why do you think there was there a need to invent a completely different annotation instead?
Evan Janeshewski
Axymetrix Quality Engineering Inc.
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