Validity 4

[aniiben]

Q1 - referring to a datum feature in a way that it was not initially constrained is a potential problem for understanding the condition. See previous discussion where I make clear that Figure 4-16 (c) is clearly evaluated incorrectly in the standard.

Inspired by a statement made in a previous discussion and also by some of my misunderstandings of the standard’s intent on dealing with datum features of size called at RMB and also at MMB, I would like to ask the members of this forum how would you consider the datum reference frame shown in the embedded picture.

Case 1: A│B│C(M)│
----Datum feature B at RMB (shown in the picture) ---

Is this datum structure or DRF valid? If yes, are there any issues with the violation of datum feature precedence order?

Case 2: A│B(M)│C(M)│
----Datum feature B at MMB (NOT shown in the picture) ---

Same open ended questions as for case#1.

 

[Sem_D220]

chez311, you probably missed the edited version of my opening statement which represents the idea better (sorry for not identifying the edit - but it was done pretty soon after the post): it says now:
"It is not mandatory to start by constraining just v (or w) in order to finally constrain X "everywhere".

Would you still disagree?

 

[chez311]

It is not mandatory to start by constraining just v (or w) in order to finally constrain X "everywhere

I apologize, I did miss that - I would agree with this revised statement.

I stand by the remaining second part of my post on (30 Apr 19 16:48) in regarding sequential constraint of A and B in your theoretical example with no revisions needed.

 

[Sem_D220]

I would still apply the same logic. B alone CAN constrain [v,w,x] however since it is secondary, primary datum feature A already constrains [u,v,z] - secondary datum feature B only constrains [w,x] so A is involved with constraint of [x] "everywhere".

I suppose that by the same interpretation, datum feature simulator of datum feature A in fig. 4-21 illustration (b) is involved (at the end result) in constraining the degree of freedom parallel to the A axis?

 

[chez311]

Sem D220,

If we take [z] to be parallel to the A axis, and translation in [z] is the DOF parallel to the A axis to which you are referring, then yes.

This should be the same as my previous example utilizing 4-24, *excluding my modified addition of a tertiary datum feature. Repasted below:

In this case we could say the following is the end result:
The primary datum feature A alone constrains [u,v]. The primary and secondary datum features A|B are involved in constraint of [z].

*Edit - clarified intent about omission of my modification adding a tertiary datum feature to the existing figure 4-24

 

[pylfrm]

If you eliminate the primary, I would say that those degrees of freedom are unaffected.
The only way I can understand this is that the primary has an influence on the constraint of w, X, Y indirectly by constraining the orientation of the datum feature simulators of the lower precedence datum features which in turn constrain those degrees of freedom.

I can fully see how in 4-9 and 4-19 primary datum feature A is involved in constraint of [x,y], however I am now not so sure about [w].

If the higher-precedence constraint is removed or changed, the interaction between the lower-precedence datum feature simulators and the corresponding datum features will change. This will affect the resulting constraint of [w,x,y].

That being said, and I'm going to preface this by saying that I am not 100% sure about this or that its a control that even makes sense, if we were to take 4-18 and apply a translation modifier to C (allowing translation in [y] ie: up/down) would the same still hold? My instinct says yes, but I have some doubt in my mind that is saying your spherical bearing example is a special case because of the arrangement of the datum features (datum feature simulators would surround the point/axis of rotation vs. offset as in 4-18).

To simplify the Fig. 4-18 example, let's also remove the MMB modifiers and consider a position tolerance referencing |A|B|C>|. The primary datum feature alone constrains [u,v,z]. The primary and tertiary datum features are both involved in the constraint of [w] because the basic orientation relationship between their simulators must be maintained. The primary, secondary, and tertiary datum features are all involved in the constraint of [x,y].

The secondary datum feature surrounding the point of rotation does not make the spherical bearing example a special case. The behavior would be the same if it were completely offset. What does make this example special is the fact that the secondary datum feature does not have a defined basic orientation with respect to the primary datum feature. Then the translation modifier removes the requirement for basic location of the secondary datum feature simulator, leaving it completely unconstrained with respect to the primary.


Sem_D220,

For your latest two cases, I agree with chez311 that A is involved in the constraint of translation normal to B. It is indeed possible for B to constrain this translation alone, but not in the same way.


pylfrm

 

[Sem_D220]

For your latest two cases, I agree with chez311 that A is involved in the constraint of translation normal to B. It is indeed possible for B to constrain this translation alone, but not in the same way.

I have an issue with the implied meaning of "not in the same way". I say that in the context of DOF constraints, different ways of constraining a DOF lead to an identical final result - not in terms of the consequential tolerance zones established, but in terms of what a DOF constraint is by definition.
I am taking a risk here at being scorned, but I would say that when we talk about a full constraint of a degree of freedom, it is either just constrained or it is just not constrained (Note that by including the word "full" I exclude the differences between RFS constraints and MMB/LMB constraints from the discussion). With different datum feature sequences or different specified datum reference frames for the same datum features (I am talking about |A|B| vs. |B|A| vs. |B|, etc), the actual part will end up oriented differently at the fixture and the tolerance zones will end up located and oriented differently relative to the part, but the specific common degrees of freedom which are constrained in all cases are constrained just the same in comparison between the cases. The constraint of a degree of freedom is merely the blocking of movement of the part in a given direction, by the datum feature simulator that will do so physically in a fixture.

For the Y14.5-2009 fig. 4-21 for example, if Z is the axis perpendicular to B, Z is either constrained - as in |B| or |A|B| or |B|A| or it is not, as in |A|. There are no different "types" of Z constraint. In this example, the constraint of the Z degree of freedom is always done by datum feature simulator B alone. There is also no value in dissecting Z to the rotational "components" so to speak, that contribute to constraining this translational direction in every point on the part, and checking if the primary datum feature constrains them.

Edit: rephrased for clarity.

 

[chez311]

If the higher-precedence constraint is removed or changed, the interaction between the lower-precedence datum feature simulators and the corresponding datum features will change. This will affect the resulting constraint of [w,x,y].

The primary and tertiary datum features are both involved in the constraint of [w] because the basic orientation relationship between their simulators must be maintained

Thank you, these were the puzzle pieces that I was missing. I assume we can say that B does not have a basic orientation relationship with C because it does not constrain any rotational DOF because [u,v] is already constrained by A, correct? And if the DRF was instead |B|C>| then B (now primary) would be involved in constraint of [w] as well as [x,y]?

I am trying to put this into one overarching statement - below I have repasted and modified my initial statement. Do you think this is an accurate summary? And if not, maybe what you would change? I feel like theres a better, more precise way of stating the bolded portion about translation modifiers but I'm coming up blank.

[x] cannot be fully constrained "everywhere" until [v,w] is constrained, same with [y] and [z]. Likewise a datum feature that constrains [v] or [w] is also involved in fully constraining [x] (again, rinse/repeat for [y] and [z]).

If [v] or [w] has not already been constrained by a higher order datum feature, datum feature that constrains [x] is also involved in constraining [v] or [w], unless the location relationship between the datum feature which constrains [x] and the datum feature constraining [v] or [w] is removed by use of the translation modifier and the datum feature constraining [v] or [w] can do so without involving the datum feature which constrains [x]. If a higher order datum feature has an orientation relationship to the datum feature which constrains [v] or [w], the higher order datum feature is involved in constraining [v] or [w] (because the relationship between the datum features and their simulators would change if it were removed). Again repeat with the applicable rotational DOF with [y] and [z].*

Thanks again for your help in grasping your vision of sequential DOF constraint, this has been hugely educational!

*Edit - added that it is repeated with [y] and [z]

 

[chez311]

I have an issue with the implied meaning of "not in the same way". I say that in the context of DOF constraints, different ways of constraining a DOF lead to an identical final result - not in terms of the consequential tolerance zones established, but in terms of what a DOF constraint is by definition.

For the Y14.5-2009 fig. 4-21 for example, if Z is the axis perpendicular to B, Z is either constrained - as in |B| or |A|B| or |B|A| or it is not, as in |A|. There are no different "types" of Z constraint.

As you noted there are NOT** different "types"* of constraint in any translational/rotational direction, I believe the distinction is in that the relationships between the datum features and the datum feature simulators changes. As in your example |A|B| and |B|A| constrain the same DOF but the relationship to the datum feature simulators changes, which is shown when you break down the DOF constraint sequentially as we have been doing.

but I would say that when we talk about a full constraint of a degree of freedom, it is either just constrained or it is just not constrained

In the 4-9 example if we consider |A|B|, B constrains [x,y] but only along the axis of the simulator for B, so it is unconstrained everywhere else since [w] is left unconstrained - this would be partial constraint. The addition of C in |A|B|C| constrains [x,y] through all axes parallel to B - this would be full constraint. At least this is my understanding per pylfrm's explanation.

*Edit addendum - here I am not considering partial/full constraint to be "types" of constraint

**Edit - for clarity

 

[pylfrm]

With different datum feature sequences or different specified datum reference frames for the same datum features (I am talking about |A|B| vs. |B|A| vs. |B|, etc), the actual part will end up oriented differently at the fixture and the tolerance zones will end up located and oriented differently relative to the part, but the specific common degrees of freedom which are constrained in all cases are constrained just the same in comparison between the cases.

This difference in the resulting spatial relationship between the actual part and the theoretical geometry is what I am referring to as different constraint. Perhaps "not in the same way" was a questionable way to phrase that.

I assume we can say that B does not have a basic orientation relationship with C because it does not constrain any rotational DOF because [u,v] is already constrained by A, correct? And if the DRF was instead |B|C>| then B (now primary) would be involved in constraint of [w] as well as [x,y]?

I agreed with the second sentence. I'm not entirely sure you mean with the first, but I don't imagine it's worth worrying about.

I am trying to put this into one overarching statement - below I have repasted and modified my initial statement. Do you think this is an accurate summary? And if not, maybe what you would change? I feel like theres a better, more precise way of stating the bolded portion about translation modifiers but I'm coming up blank.

I don't see any glaring inaccuracies in your summary. Perhaps it could be improved by avoiding discussion of specific degrees of freedom, and instead concentrating more generally on whether a particular datum feature reference has any influence on the interaction between lower-precedence datum features and their corresponding simulators.


pylfrm

 

[chez311]

I'm not entirely sure you mean with the first, but I don't imagine it's worth worrying about.

I was just trying to clarify in explicit terms why B was not involved in the constraint of [w] in the 4-18 example. From your explanation I believed that to be because C does not have a basic orientation relationship to B that must be maintained. I was reasoning that this is because the control of C is based upon |A|B| and in |A|B| the constrained rotational DOF [u,v] are fully constrained by A with no involvement of B.

I don't see any glaring inaccuracies in your summary. Perhaps it could be improved by avoiding discussion of specific degrees of freedom, and instead concentrating more generally on whether a particular datum feature reference has any influence on the interaction between lower-precedence datum features and their corresponding simulators.

Glad to see I'm at least mostly on track! Thanks again for laying it out for me. I'd have to think about how to pare that down and remove references to specific DOF without becoming ambiguous - for right now thats the best way for me to consider it (though maybe not the most simple way) but I see what you mean.