Orientation constrain in the lower segment of composite position tolerance.

[Woosang]

In Y14.5-2009 Figure 7-39(a), DRF A|B in the lower segment constrains the orientation of FRTZF to datum A and B. And reference to datum C is not required since it does not give any additional meaning to the DRF.
But look at the picture below. I modified Figure 4-9(a), changed datum C to the side face and made the position tolerance for the pattern of four holes to composite position tolerance.
The question is: Does DRF A|B in the lower segment constrain the rotation of FRTZF of four hole pattern as DRF A|B in figure 7-39(a) does? Or does it require DRF A|B|C in the lower segment to do the same?

 

[Burunduk]

Or does it require DRF A|B|C in the lower segment to do the same?

Yes, it does.
Your modified version of fig. 4-9(a) is different than 7-39(a) because of the type of secondary datum feature.

To fully constrain the orientation of a feature relative to the rest of the part, you need to constrain 3 rotational degrees of freedom.
Let's look at it step by step:
In both cases, datum feature A as a primary planar datum feature constraints 2 of the 3 rotational DOF (and one translation).
In fig. 7-39(a), the secondary datum feature B is also planar. It constrains another, third, rotation (and one translation). Contrary to that, in your modified figure 4-9(a) the secondary datum feature B is a hole. It constrains 2 translations, but no rotation. Only the addition of datum feature C, the clocking datum, constrains the third rotation. The orientation refinement by the second segment of the position tolerance FCF remains partially unconstrained with just datum features A and B referenced.
With that said, if your goal was only to refine the perpendicular relationship to datum A, datum reference A only could suffice in the second segment.

 

[greenimi]

Burunduk,

Do you agree that in the modified version "B" as secondary does nothing? It is there just to fulfill the composite position requirements of having the same DRF in FRTZF as in PLTZF?

 

[Burunduk]

greenimi, the modified version could reference A only in the 2nd segment and still fulfill the repeat and order precedence requirement. See fig. 7-38.

I agree that it essentially does nothing meaningful.

 

[greenimi]

greenimi, the modified version could reference A only in the 2nd segment and still fulfill the repeat and order precedence requirement. See fig. 7-38.

Yes, it does (fulfill...….), but in order to stop all DOF's you need "B" as secondary (because you need "C" to do the job of stopping the last rotation in the composite) even technically "B" does nothing as secondary.
Do you agree?

 

[Burunduk]

If it is intended to control orientation in all 3 rotational degrees of freedom, then yes B is required and the appropriate references in the 2nd segment are A, B, C.
If one cares only about refining perpendicularly (2 DOF are relevant), then A only is enough.

 

[greenimi]

So, if one has A|B| on the FRTZF for the modified option (no "C" as tertiary) that DRF should be considered inadequate. I think that was the OP's point. At least his figure shows it as A|B| (with no "C")

Therefore, would be either "A" only either A|B|C|, but no A|B|

Do we agree on this point?

 

[Woosang]

Thanks Burunduk, greenimi,

I understand the sequence of datum A, B and C constraining DOF as Burunduk explained. I was wondering if there is any possibility that even the inherent capability of datum axis B regarding DOF constraint, just repeating A|B can do the same function as planar datum B since all DOF were already constrained by A|B|C in the upper segment. Come to think of it, it would be ridiculous because if that was possible repeating datum A only would do the same.

 

[greenimi]

By definition all the datum features in the FRTZF are allowed to stop the rotation DOF's only (and not any translation DOF's).

 

[axym]

Woosang and All,

Here is how I think of it:

1. The datum features in a composite FCF lower segment work in exactly the same way as they would in a single segment FCF. For datum feature B in the modified Fig. 4-9 figure, the true geometric counterpart would be a solid cylinder at the basic location of datum feature B, that constrains two translational degrees of freedom. The datum axis and DRF axis would pass through the cylindrical TGC as usual.

2. The tolerance zone framework in the lower segment of a composite FCF is allowed to translate relative to the datums and DRF. This is how the end result described in Y14.5 is achieved, that the lower segment datum features only stop rotational DOF's and not translational DOF's. It's because the tolerance zone framework translates, not because the datum features constrain degrees of freedom differently.

This description of composite FCF lower segments was used in Y14.5.1-2019. It works very well even in difficult cases like 7-42 and 7-43 in Y14.5-2009, where three datum features are referenced in the composite FCF lower segment and the secondary datum feature appears to do nothing (it actually does participate in the lower segment constraint, despite only controlling translational DOF's).

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Burunduk]

I agree with the way axym describes it.
I always thought of the lower segments of a composite position FCF governing only the orientation (relative to the referenced datums) as a result of the locating basic dimensions that tie the pattern to the datums not applying to what the lower segments control. It's not that the datum features and simulators (TGCs) work somehow differently for the lower segments. Datum features and their simulators will constrain whatever DOFs they are physically capable of for the part.

 

[axym]

Burunduk,

Right, Y14.5 describes it in terms of certain basic dimensions not applying to the lower segment. This explanation has its limits, however, because it only makes sense if the basic dimensions are laid out a certain way. In the modified Figure 4-9 example, I believe it would be that the 8.7 and 12 dimensions do not apply (and the 40 and 24 still do). However, if the basic dimensions had been laid out as XY coordinates from a corner origin, then there wouldn't be an obvious set of dimensions that would not apply. Also, if model-based definition is used and there are no explicit basic dimensions, then the entire basic definition (the model) still applies. I believe that the explanation that the lower segment tolerance zone framework (FRTZF) translates relative to the DRF stands on its own - we don't need to say that certain basic dimensions don't apply.

The idea that basic dimensions "apply" or "don't apply" also implies the misleading (and incorrect) notion that constraint is somehow dependent on the presence or absence of explicit basic dimensions.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Burunduk]

axym,
I agree for the most part. The concept that some basic dimensions do not apply to the FRTZF can be misleading, although I don't consider it incorrect. It may be misleading if understood as if the basic dimensions that do not apply are some specific explicitly stated basic dimensions. However basic dimensions can also be queried or derived. By derived, I mean that in the modified fig. 4-9 example the 24mm dimension could be replaced by two 12mm dimensions from the common center where datum axes B and C lie. Each of the 12s separately wouldn't apply to the FRTZF but 24 as the sum of them would still apply although not explicitly stated. Differentiation has to be made between locating basic dimensions and orienting basic dimensions, and within the locating basic dimensions category, there are those that locate from the DRF and there are mutually locating basic dimensions that act between the features in the pattern. Any type can be explicitly stated, derived, or queried, but only basic dimensions locating the FRTZF from the DRF do not apply.