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Two Datum Features, Single Datum Axis.
4

Two Datum Features, Single Datum Axis.

Two Datum Features, Single Datum Axis.

(OP)
Hi Friends,
Need a help in reg. Two Datum Features, Single Datum Axis.


Is my assumption correct?

Thanks.

RE: Two Datum Features, Single Datum Axis.

No. They cannot be added.

RE: Two Datum Features, Single Datum Axis.

(OP)
Pl. help in how to interpret the drg.



RE: Two Datum Features, Single Datum Axis.

I agree with 3DDave. One is the datum feature shift (datum feature mobility) and the other is the controlled feature.
It is more like a basketball hoope analogy.
The (M) on the left -close to 0.6- will make the ring smaller or bigger.
The (M) on the right -close to A- will just make the ring move slightly with no effect on its size.

RE: Two Datum Features, Single Datum Axis.

Given the function of typical cross holes (retaining pin), applying MMC to the diameter seems counterproductive. As the diameter of A grows, there is a greater need for precision of the cross hole location.

Also, I don't understand how a position with no datums is valid.

RE: Two Datum Features, Single Datum Axis.

Quote (TheTick, )

Also, I don't understand how a position with no datums is valid.

Because it controls the relationship between each other. You have 2X meaning two features (not just one).

Was not valid in 1994, but that was almost 30 years ago....time to upgrade.

RE: Two Datum Features, Single Datum Axis.

ASHWA,
As mentioned above, the (M) for MMB (Maximum Material Boundary) applied to A in the Feature Control Frame allows for datum feature shift. It is NEVER a bonus, but it allows the datum feature to deviate from a simulator of fixed size and the part to "shift" when applied to the gauge. If your cross-hole comes in at 8.1 there is no more tolerance allowed than 0.6+(8.1-8.0) = 0.7, it cannot violate your virtual condition of 7.4 regardless of the size of A (ie: on a simulator a pin of size 7.4 fixed at basic location/orientation to datum feature simulator for A).

An excellent thread that breaks down the difference is (https://www.eng-tips.com/viewthread.cfm?qid=439000).

TheTick,
Position's main "job" is to control location - whether to a specified datum feature or between features. As long as one or both of these are being accomplished then the control is valid.

RE: Two Datum Features, Single Datum Axis.

Knowledge upgraded. Thanks.

RE: Two Datum Features, Single Datum Axis.

(OP)
Thanks SME's....

But, chez311 (Automotive),

Quote (As mentioned above, the (M) for MMB (Maximum Material Boundary) applied to A in the Feature Control Frame allows for datum feature shift.)


How to calculate the shift?

RE: Two Datum Features, Single Datum Axis.

Quote (ASHWA)

How to calculate the shift?

In order to "calculate" the datum feature shift you do need the part to be produced and measured.
It is the difference between U/ RAME and MMB (VC)

Am I correct, chez311?

But since A is primary I guess UAME in this case (not RAME).

Please correct me if you would like.


RE: Two Datum Features, Single Datum Axis.

The primary datum A constrains two rotational and two translational degrees of freedom. Referencing A at MMB will allow for a relaxation in those constraints such that some amount of rotation and translation is allowed between the datum feature and the MMB. The rotation/translation will allow for the feature Ø8.0 - 8.2 to be optimized within it's specified tolerance zone.

The amount of rotation/translation allowed will be dependent upon the part that is produced and is not a simple calculation like bonus tolerance.

RE: Two Datum Features, Single Datum Axis.

Also keep in mind that if there's enough datum shift from A, then the face of the part will flatten out upon datum feature B. That means that some degrees of freedom normally constrained by datum A will be tackled by datum B instead.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: Two Datum Features, Single Datum Axis.

Lacking a specific example in the 1994 version does not mean it was not valid. It was certainly used as a secondary entry in a composite FCF which means it was interpretable.

Berlanger - isn't having the end of the part flat against the datum simulator the nominal case? The inspection fixture for this is a blind hole that the part is dropped into with a sliding gage cross-pin. I expect that most quality shops would make parts such that they didn't need to touch the sides of the fixture and would rest on the flat.

RE: Two Datum Features, Single Datum Axis.

Quote (3DDave)

isn't having the end of the part flat against the datum simulator the nominal case?
While that would be the case for a perfect part, we have to consider the possibility of datum feature B being tilted. And the drawing does not give a tolerance relating datum feature B back to datum A (perpendicularity or a similar tolerance). So I don't think we can come up with a threshold number of when the part would be oriented by A vs. B.
It's like Fig. 7-20 (c) and (d)... If there's little to no shift, then the primary datum constrains the orientation of the part (figure d). But given enough shift, as in (c), the secondary datum becomes responsible for orienting the part.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: Two Datum Features, Single Datum Axis.

If there is the maximum primary datum orientation shift then the secondary datum reference has no necessary orientation control. Of course it's all dependent on ratios of length and diameter and the control on orientation of datum feature B. No matter what, there will be overlap, but in the NOMINAL (aka PERFECT) case we can see that the part will rest flat on datum feature B and not need to contact the datum A feature simulator.

I don't have a copy of the latest, so I have no idea what pictures "we" put in it.

RE: Two Datum Features, Single Datum Axis.

(OP)
Thanks SME's,

Since iam in early understanding of terms, so getting to key concept takes time for me,

Jacob Cheverie (Aerospace),

Quote (The primary datum A constrains two rotational and two translational degrees of freedom)


can you please explain which are the 2 Rotation and 2 translation, anything with a simple sketch pl.?

Quote (Referencing A at MMB will allow for a relaxation in those constraints such that some amount of rotation and translation is allowed between the datum feature and the MMB.)


can i know how pl.?

RE: Two Datum Features, Single Datum Axis.

ASHWA,

Of the six available degrees of freedom (DOF), it's easier to mention which DOF a cylindrical primary datum feature does not constrain. It does not constrain rotation about it's own axis (1 DOF) and it does not constrain translation along it's own axis (1 DOF). All other DOF are constrained.

Referencing A at MMB will require the use of a fixed datum feature simulator (true geometric counterpart). This means that as datum feature A departs from it’s MMB [MMC size plus any applicable geometric tolerances (form for a primary datum feature)] it is allowed to translate and/or rotate within the MMB. The feature is dimensioned to the MMB of A, so if the MMB of A is larger than A, A has some “wiggle room” which can be taken advantage of.

Think of a pin inside of a larger sleeve. The pin can translate and rotate in ways that it couldn’t if the sleeve were a tight fit.

A more precise example: A cylindrical feature has an MMB of Ø1.2 and the feature is produced at Ø1.1. The feature can shift within it’s boundary by .050 in any one direction because that is half of the difference in size [(1.2 – 1.1)/2 = 0.1/2 = 0.05]. This shift is purely translational. Imagine that the feature is .445 in length. What is the maximum rotation angle allowed between the two cylinders? How would you quantify shift if it was rotated a partial amount and translated a partial amount? You can see that it becomes complicated.

RE: Two Datum Features, Single Datum Axis.

(OP)
Thanks Jacob Cheverie (Aerospace), for your effort...!!

So,
TGC for A will be Ø16.56+0.15 = Ø16.71.
&
"A" can rotate/Translate, when its value is less or equal to Ø16.56 (MMB) to place the Hole.

Is my understanding correct?

RE: Two Datum Features, Single Datum Axis.

ASHWA,

You are correct in that the MMB will be Ø16.71.

It isn’t so much that deviation from MMC will allow “shift”, but rather once the two surfaces making up datum feature A are measured, they are allowed any available “shift” (together) within the Ø16.71 boundary. For example, the first surface may measure Ø16.54 and the second may measure Ø16.52 and they may be out of position with respect to one another by .008. There will certainly be some rotation/translation allowed within the MMB that can be taken advantage of to place the hole as you mention but the surfaces must shift together as one feature.

Belanger brings up an interesting point about the interaction with datum feature B "overriding" some of the established constraints.

RE: Two Datum Features, Single Datum Axis.

3DDave -- If you have the 2009 version of the standard, it's like Fig. 4-21 (c) and (d)... If there's little to no shift, then the primary datum constrains the orientation of the part (figure d). But given enough shift, as in (c), the secondary datum becomes responsible for orienting the part.

Quote:

in the NOMINAL (aka PERFECT) case we can see that the part will rest flat on datum feature B and not need to contact the datum A feature simulator.
Who says the part is going to be at NOMINAL (aka PERFECT)? Think beyond that a bit.
Consider if datum feature B is tilted a fraction of a degree; that end face may not rest flat (if there isn't enough shift) or it may rest flat. I'm pointing out that the orientation of the entire part is not driven by a specific datum in every case. Study the figures I referenced in the standard if you need a visual.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: Two Datum Features, Single Datum Axis.

I take shift to mean between the datum feature and the datum feature simulator, not between one datum feature and another. The greater the shift of datum feature A, the less likely datum feature B is to be co-planar with its simulator. Also, unlike 4-21C, there are no bolts forcing planar contact.

For all I know a real part generally like the example has a convex end and can never drive orientation no matter what one considers shift to be. That means it can never be responsible.

RE: Two Datum Features, Single Datum Axis.

Hi All,

Fig. 4-21 (c) in Y14.5-2009 is a classic example of a condition that is implied but not explicitly stated. As a cylindrical primary datum feature, A is responsible for constraining two rotations and two translations (including the CW/CCW rotation in the view shown). As a planar secondary datum feature that is nominally perpendicular to the primary, B is only responsible for constraining one translation (up/down in the view shown). B only needs to be in contact with its TGC at one point, as it is in 4-21 (d). Yet in 4-21 (c), B is shown in full contact with its TGC and effectively constraining rotational degrees of freedom. The question is: is there a requirement that B establish full 3-point contact with its TGC if it can? Or is full 3-point contact just one of many allowable candidate relationships? The fact that the figure shows full contact implies that this is a requirement, but there is nothing in the text that states this or addresses the issue at all.

The issue of "inherited constraint" can be exapressed as a more general question:

Q: If a higher precedence datum feature only partially constrains a degree of freedom it is theoretically capable of constraining, are the lower precedence datum features required to constrain that degree of freedom?

Here's a hypothetical scenario to illustrate the point. Let's say that the as-produced part in 4-21 (c) was made such that the 4 holes were nicely parallel to feature A, but skewed relative to feature B. The 4 holes were all made at MMC size and with some error in their relative spacing. The gage has 4 2.9 mm pins that the 4 holes have to fit over. Let's say that the 4 holes fit if feature B is in 1-point contact with its TGC and feature A is inside its TGC, but the 4 holes don't fit over the 4 pins if B is pushed down into full 3-point contact to constrain the rotational DOF's. Should I call this a nonconformance because I can't get the feature to pass with the part contacting the gage in the way that is shown in Fig. 4-21 (c)?

I would say that the answer is no. The feature satisfies the position requirement while feature A is inside its TGC and feature B is in 1-point contact with its TGC, so the feature conforms to the tolerance.

Short story long, I am saying that the condition shown in 4-21 (c) is allowable but not mandatory.

Opinions?

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Two Datum Features, Single Datum Axis.

I agree in that the secondary datum becoming responsible for the constraint is not a firm requirement but rather a possibility.

Another example/argument that may help would be if we were to look at 4-21 (a) with only the left and right holes (as drawn). Starting at the case of 4-21 (d), assume that the two holes were drilled with a negative slope (in a coordinate system that gives the surface vector of the top-most surface a positive slope). Progressing toward 4-21 (c), as datum feature A departs from it's MMB we are allowed to rotate the part clockwise to optimize the position. If we allow datum B to override the rotational constraint once datum feature A departs "enough", we are no longer allowed to optimize that rotational DOF and we have thrown out a potential reason for the MMB modifier. We would go from bad to better to best to worst. That doesn't make much sense, except for maybe in the cases that the holes are bolt holes forcing the two parts together (in which I would think B would be primary anyway) or we are considering gravity.

What is the purpose of datum B in 4-21? If B is only responsible for constraining translation along the axis of datum A then it does nothing to further define the tolerance zones of the hole pattern. If B is allowed to override the rotational constraint then it actually has significance in the development of the four tolerance zones when the override happens, which would be dependent on the amount of departure from MMB on A and the orientation error between A and B.

In the topic posted here, the tolerance zone is dependent on B with the [14] basic.

RE: Two Datum Features, Single Datum Axis.

In 4-21, datum B has no apparent use. It's an extension diagram for 4-20 where in 4-20(c) it is applicable, but is contrived somewhat poorly to show that there is another potential assembly condition. In fact the axes of the screw holes in 4-21(c) should be parallel to datum feature A.

RE: Two Datum Features, Single Datum Axis.

Jacob,

Good example. I agree that datum feature B really doesn't have any purpose in the FCF in Fig. 4-21.

Here's another scenario. Let's say that the 4 holes in the as-produced part in 4-21 (d) are parallel to feature A, and just barely fit over the 4 VC pins in the gage. Feature A is at MMC, there is no datum feature shift, and the 4-hole feature conforms to the position tolerance. Everything is good.

Now let's say that the part from 4-21 (d) is reworked, and feature A is turned down to LMC size. There is now plenty of datum feature shift available, but we don't need it because the feature conforms when the axis of feature A is aligned to the TGC. Would we say that the 4-hole feature no longer conforms to the position tolerance, because we need to establish 3-point contact on datum feature B? Surely not. That would mean that the feature would pass if the FCF specified |A|B| but fail if it specified |A(M)|B. To me, that would not make sense.

I realize that there would be major practical difficulties with following the |A(M)|B| sequence properly. I think that this highlights the risks and unpredictability involved with specifying higher precedence datum features that don't constrain their assigned degrees of freedom very well.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Two Datum Features, Single Datum Axis.

It seems like there are a handful of similar examples that can demonstrate this strange situation. You make a good point, Evan, in that a part conforming to a datum structure at RMB should conform when there are MMB/LMB modifiers invoked. That logic seems to imply that datum feature B should not override the rotational constraint, even in an extreme example in which conformance is only guaranteed when B happens to make 3-point contact. The part may appear to be constrained in rotation by B, but it would still only be partially constrained by the MMB of A.

I would assume that these types of datum reference frames are best left to software gaging, but at the end of the day I think the software developer would have to answer to these unclear questions.

I wonder if it'd be possible to categorize the datum reference frames in which higher precedence datum features "don't constrain their assigned degrees of freedom very well".

RE: Two Datum Features, Single Datum Axis.

Evan,

Quote (Evan)

I realize that there would be major practical difficulties with following the |A(M)|B| sequence properly. I think that this highlights the risks and unpredictability involved with specifying higher precedence datum features that don't constrain their assigned degrees of freedom very well.

That is why I think ISO allows you to use A(M)-B in shown configuration. Don't you think is better?



RE: Two Datum Features, Single Datum Axis.

(OP)
Thanks Friends,

3DDave (Aerospace), Kedu (Mechanical), Jacob Cheverie (Aerospace), TheTick (Mechanical), Belanger (Automotive), axym (Industrial).

Learned how to interpret the drg. much more, than the level before.

Interaction with each other, gives the exposure to learn the concept with ease.

Thanks all...!!

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