Clocking cylindrical parts with datum feature

[Tenkan]

When positioning features on a round part, like a flat disk, should one of the features be made a tertiary datum C to clock the location of the rest of the features?

Do not all features orient to the X and Y plane that establish the datum axis, eliminating the need for a tertiary datum to “clock” other features with?

Reference drawing attached.


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[fsincox]

I raally do not understand why, since the simultaneous requirement should cover it. That is one of the many irrational-illogical things you learn here, the simutaneous requirement covers you, but it seems, most do not trust it. Just like the envelope principle, it is ignored in practice though it is clear ASME law.
It is certainly not "wrong" to add a clocking feature.

 

[CheckerHater]

On big complicated part with several features it may be easy to overlook which pieces exactly fall under sim reqt.
Also, there is slightly different view on it between ASME and ISO.
So, if you are in ASME world, technically, yes, you can do without clocking datum. But adding one to be safe and better understood, is not entirely wrong.

 

[KENAT]

If you want simultaneous requirements to cover it doesn't the datum structure in the FCF need to be the same - in the attached drawing the 2 FCF aren't calling up exactly the same datum structure.

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[randy64]

Can someone explain "simultaneous requirement" to me, and how it alleviates the need for a clocking datum?

We use a tertiary/clocking datum (we're still under 1994), and feel that it is necessary, to the point of many times adding construction holes just for that purpose. However, if there is a way to get out of using it, I'm eager to hear about it.

Thanks

 

[CheckerHater]

To Randy64:
Start here:

http://www.tec-ease.com/gdt-tips-view.php?q=161

http://www.tec-ease.com/gdt-tips-view.php?q=126

 

[fsincox]

I just want consistant application of the rules or why bother to make those rules! I do not really like that engineering has one understanding , but, the shop has another. Use of a clocking datum to the uninformed helps only perpetuates the misunderstanding, IMHO.
To be clear, I am not a fan of simultaeneous or envelope.
Frank

 

[CheckerHater]

How about another buzzword, “functional requirement”?
When you make one feature a datum and control another feature wrt the first, you somehow imply that functionally first feature is more important than the second one.
Why not use it where it makes sense?
Typical sim req example shows shaft with two keyways – fine with me.
But if one of them is really somehow “primary” to the other, why not use a datum?

 

[fsincox]

CH,
I am all for functional requirements. I am not sure you can easily say which has a precedence with two keys or two round dowel pins.
Frank

 

[pmarc]

Keep in mind that "simultaneous requirements" and "one of features as clocking datum feature" result in different geometric requirements. The latter tightens mutual relationship between considered features in comparison to what SIM REQT requires.

 

[fsincox]

pmarc,
Good point to make, we should not leave the impression they are completely equivalent! If a datum feature of size or pattern of features of size is referenced MMB, I thought that the datum virtual condition rule (rule #5) makes it the same, though, true or not?
Frank

 

[pmarc]

Not sure what you mean Frank. Rule #5? What the heck is this?

 

[fsincox]

pmarc,
Sorry, we are still at 1982, I believe it is the same statement as section 2.11 in 1994. I am not sure in 2009.

 

[pmarc]

Frank,
I do not think that MMB concept changes anything. There will still be a difference between "simultaneous requirements" and "one of features as clocking datum feature".

 

[Tenkan]

The reason is because I am being asked to put the datum C on the drawing for QC inspection. It’s as if they can’t determine the location of the features otherwise? What I would like to do is not include datum C at all for simplicity.

It sounds like the correct answer is it could go both ways…?


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[powerhound]

Yes. Your drawing shows one way to do it. If you reference both features to datums A and B. That would do it too via the simultaneous requirements rule. As mentioned by others here, they do not say the same thing but the outcome could be identical anyway.

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