Usefulness of tertiary datum feature

[greenimi]

As I did not want to mess up the previous thread, I would like to open up the conversation about fig 4-53 in a new thread:

In Fig. 4-53 the part will not be fully immobilized using A and B datum targets only. That is why tertiary datum hole C has been defined. I agree, it is not needed for angular relationship of the cutout and the hole (simultaneous requirement would do the job), but I do not agree that in this particular case there would be no geometrical difference between having and not having C in the profile callout.

If datum feature C (fig 4-53) does NOT physically arrest (there is NO dowel pin placed in the Ø12 small hole) any degrees of freedom in the assembly, then, I guess, the design and the drawing is made more restrictive than needs to be (if, of course, C is shown as tertiary in the DRF).
Am I correct?

 

[SwinnyGG]

In order to build a gauge to that print, there would have to be a dowel or other locating feature to locate datum c, and prevent movement in the third degree of freedom.

'If there was no dowel pin in the gauge at datum c' is the same as saying 'if datum c was not there'. In either case, the part in the gauge is unrestrained.

 

[CheckerHater]

If I remember correctly, Simultaneous requirement requires (no pun intended) "same order of precedence and the same modifiers", which is not the case on Fig. 4-53.
Simply removing reference to C will not cut it.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[greenimi]

If I remember correctly, Simultaneous requirement requires (no pun intended) "same order of precedence and the same modifiers", which is not the case on Fig. 4-53.
Simply removing reference to C will not cut it

That is exaclty the intent of my question: removing C from the profile and simultaneous requirement will be in effect with the position of the Ø12 hole. Correct?

 

[pmarc]

greenimi,

Yes, removing C from the profile callout would make both geometrical callouts shown in that figure subject to simultaneous requirement rule (it has been already said in the quote you used).

Yes, if the datum hole C does not play any physical role in how the part is oriented in assembly, then the drawing defines more restrictive geometric requirements than they need to be (although based on experience I would say that to make everyone's life easier, it is sometimes better to define a "clocking" datum feature, even if it is not a function-based choice).

 

[greenimi]

I think I know what your next question is going to be

Is/are the question(s) above the ones you expected or I did not fulfill your expectations?
If not, I am fine with that and we can develop the subject further based on your input.......

Thank you pmarc

 

[pmarc]

I thought you would rather ask for detailed explanation/proof of this:

"I do not agree that in this particular case there would be no geometrical difference between having and not having C in the profile callout."

 

[greenimi]

Well, I wish I have such a good memory for each and every thread on eng-tips, but I remember not too long ago was a discussion between you and Evan about the very same subject “clocking”.

I remember only the conclusion: do not stop the degrees of freedom just for the sake of stopping the degrees of freedom (I know the CMM guys would like to have all of them stopped for measurement purpose), because suddenly the design would be more restrictive than needs to be and some perfectly good functional parts might get rejected.

From each and every thread I am trying (although, I am not promising anything) to remember the conclusion, if the experts reached one.
The rest are details and with my old age I do not remember them.

There is so much good information on eng-tips, just need to find it …..where certain subject was discussed and debated…..that is the hardest part for me.

 

[axym]

greenimi,

As usual, I agree with pmarc. The datum feature C reference makes us line up the part so that the "angular location" of the hole is perfect, possibly making the angular location of the X<>Y cutout worse. If the C reference is not there and simultaneous requirements applies, then the angular location of the cutout can be made better at the expense of making the angular location of the hole imperfect. We can imagine as-produced parts that would fail if C was referenced and pass if it was not.

I don't remember the other discussion, but the conclusion sounds like something I would say ;^).

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[pmarc]

I don't remember the other discussion too, but greenimi is one of the best on the forum in finding old stuff, so I am sure he will remind us the conversation ;-)

In order not to let this thread die a natural death I would like to ask another question:
Say that datum feature C is referenced at MMB in the profile callout in fig. 4-53, and someone did similar excercise and removed C(M) from the profile FCF. Would the two schemes be different too?

 

[greenimi]

http://www.eng-tips.com/viewthread.cfm?qid=398730

I think this is the thread we are talking about.

As far as the second question (pmarc's additional challenge) I have to think about .......I don't want to rush an answer ( if the two schemes are different) and regret that I missed the boat/correct answer.

 

[axym]

pmarc,

I'm not sure that I understand the latest question. Can you clarify what the position FCF and profile FCF are?

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[greenimi]

Let me try to explain what was my understanding of pmarc’s question: (pmarc, if I am wrong, please correct me)

As currently is shown in fig. 4-53, profile has the tertiary datum feature C called at RMB. Let’s call this scheme 1
Now, pretend we remove datum feature C from the profile callout (everything else stays the same, including the existing positional callout), therefore Profile will be with A primary and B secondary only. Lets call this scheme 2.

Based on the replies above (and also on the thread from November 2015) we agreed that scheme 1 is a little more restrictive than scheme 1, Correct?
So, the two schemes are different meaning will NOT provide the same mathematical result. (scheme 1 will reject more parts than scheme 2 or the SAME part will be accepted by scheme 2, but might get rejected by scheme 1)

Pmarc’s additional challenge is:
Pretend datum feature C is at MMB (tertiary datum feature)---scheme 3
Will now scheme 2 and scheme 3 be different ? Or they will achieve the same mathematical result?
Scheme 3: profile …..A, B, C (MMB)
Scheme 2: profile…..A, B

Everything else stays the same, including the existing positional callout for the Ø12 hole (which become datum feature C, in some applicable schemes)

 

[pmarc]

Evan,

Greenimi got it.

 

[axym]

Thanks for the clarification. So we're looking at the difference between referencing C at MMB, and not referencing C at all and letting simultaneous requirements apply.

I would say that there would still be a difference, but not as much of a difference.

If referenced MMB, datum feature C would "partially constrain" the rotational degree of freedom. The amount of constraint would depend on the as-produced condition of feature C. If it was produced at its virtual condition, there would be no clearance with the simulator pin, no datum feature shift, and the effect would be similar to Scheme 1. If it was produced far from its virtual condition (big and near true position), there would be lots of clearance with the simulator pin, lots of datum feature shift, and the effect would be similar to Scheme 2. If it was produced at a condition somewhere in between, the rotation would be partly restricted and the effect would be somewhere in between Scheme 1 and Scheme 2.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[pmarc]

If referenced MMB, datum feature C would "partially constrain" the rotational degree of freedom. The amount of constraint would depend on the as-produced condition of feature C. If it was produced at its virtual condition, there would be no clearance with the simulator pin, no datum feature shift, and the effect would be similar to Scheme 1. If it was produced far from its virtual condition (big and near true position), there would be lots of clearance with the simulator pin, lots of datum feature shift, and the effect would be similar to Scheme 2. 

Okay, but I still don't see how it would be different from the scenario where C was not referenced at all and sim reqt was applied.

 

[axym]

pmarc,

OK I think I see what you're getting at. I think you're right that there would be no difference in the conformance requirements.

In Scheme 2, both the arc profile callout and the hole position callout reference A|B and simultaneous requirements applies. So we have a profile zone of .03 and a VC boundary of 12 - 0.2 - 0.3 = 11.5, that the features must conform to simultaneously. The rotation can be optimized to get both features to pass, but the rotation must be the same for both.

In Scheme 3, the arc profile callout reference A|B|C(M). So we have a profile zone of .03, and a tertiary simulator sized at the hole's MMB of 11.5. The rotation can be optimized to get the profile to pass, but only within the shift allowed by the simulator. The "gage" is exactly the same as in Scheme 2. I agree that there would be no difference in the conformance requirement.

The difference I was thinking of would only come out in the actual values. In Scheme 2, the rotation would be optimized with both the arc profile and hole position in mind. In Scheme 3, the rotation would be optimized with only the arc profile in mind, and the hole/simulator relationship just being a constraint.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[pmarc]

Yes, Evan, this is what I was getting at. Thanks.

Oh, and greenimi, thank you for the link to the other thread.