Sequence in Referencing Datums 3

[SpaciouS]

In most cases, that I've seen, when referencing datums for a hole (for example), typically the datum perpendicular to the hole is the first datum referenced in the FCF, then the subsequent datums. Is there a particular reason or benefit in doing this?

Thanks,
Sean

 

[Belanger]

It's often that way because a hole is often used for a fastener or some sort of connector that will force the part to flatten out on a face perpendicular to the hole, thus making that the main contact surface over any side walls. Of course, there are situations where that wouldn't be true, but that's why it's common.

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

http://www.gdtseminars.com

 

[SpaciouS]

So the reason I asked is someone here is telling me to "Avoid creating multiple datum systems (A-B-C, D-E-F…)". This is not the school of thought that I was originally trained to think.

I think it's entirely design intent specific, and if my hole or hole pattern does not warrant a simultaneous requirement with another hole or hole pattern (for example) I'm not concerned with maintaining A-B-C over and over. I do realize that when changing the order of the datum references, that also means that's a new set up for inspection, but is that enough of a reason to make a hard-fast rule to avoid multiple datum systems?


Thanks,
Sean

 

[Belanger]

The GD&T should be based on function, and on the same part the functional requirements of one hole may dictate an A, B, C datum reference frame, and another hole may require a D, E, F datum reference frame.

Sure, it's nice when everything on the print refers to the same datums in the same sequence, but doing what's nice is not always the best method. So I agree that we shouldn't really make a hard-and-fast rule about avoiding multiple datum systems.

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

http://www.gdtseminars.com

 

[pmarc]

Here is a link to Tec-Ease tip showing that primary datum plane in positional requirement for a feature of size does not always have to be perpendicular to axis of the feature (see datum feature E control).

http://www.tec-ease.com/gdt-tips-view.php?q=202

As for using different datum reference frames on a drawing, I absolutely agree with J-P -- function is number one. If your drawing shows more than just typical-handbook-geometry type of a part, it is often desired to use different datum reference frames. Otherwise, apart from doing a seeming favor to an inspector dealing with the part's measurements, you gain rather nothing.

Of course there are certain limits in playing with DRFs. In most cases refering to for example |A|B|C| for one feature, and then to |A|C|B| for other, and to |C|A|B| for another, and to |A|B| for another, etc., immediately reveals that author of the drawing incorrectly identified functional requirements. But there are exceptions even to that (see link below, CheckerHater's post from 11 Jan 13, 9:44).

http://eng-tips.com/viewthread.cfm?qid=336912

So, no hard rules in this area, common sense is all we need.

 

[looslib]

When I was taught GD&T, the instrustor said to treat the datums as how you would place the part to inspect it.
You usually set the part down on the largest surface area face, thus datum A. This also requires '3 points' of contact to stabilize.
Then you slide the part over to get one edge, Datum B, and '2 points' of contact.
Finally you slide it perpendicular to Datum B to a stop, Datum C, and a 'single point' of contact.

The more datums you use in defining your part, the more inspection fixtures/setups are required to inspect the part.

Functionality and form will always dictate the geometric design, but there is nothing wrong with making things simply, too.


"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

 

[pmarc]

You usually set the part down on the largest surface area face, thus datum A. This also requires '3 points' of contact to stabilize.

Take a look at fig. 2-5 in Y14.5M-1994 or Y14.5-2009. This is how placing a part on the largest area for inspection, without considering what is functionally important, ends.

 

[DanStro]

I look at this way. It is good to avoid using multiple datum reference frames but 'avoid' doesn't mean 'never do'. So use as many as the application requires, just not more than it requires.

Han primo incensus

 

[SpaciouS]

See that's my problem with the word 'avoid', it does mean never, much like “avoiding the plague”, or “avoid at all cost”.

As I said earlier, "I do realize that when changing the order of the datum references that also means that's a new set up for inspection”.

I just think words should be chosen carefully when you're trying to teach others, especially when teaching people with no experience. Eager learners will, and should, hang on to every word, for this is a technical subject and every word should have a specific meaning. The standard itself is VERY selective in with its wording.
The appropriate word would really be to ‘minimize’ not to’ avoid’...? That’s all I’m saying. I just like getting a second opinion from professionals and experts before I challenge anyone (not that I’m challenging anyone in a big way on this matter).

Thanks for all your help!


Thanks,
Sean

 

[fsincox]

You are not going to express the true function of many parts if you do not switch frameworks somewhat, more so, with complex parts. I do feel the function of most part features can be tied somehow to a "master" reference framework. this is true because the part must serve some function in the assembly installation all at the same time. To only use one framework in all cases is just pure manufacturing speak (confusing process with function).
Frank

 

[KENAT]

"avoid unless really necessary" is probably the phrase they should use.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[CheckerHater]

Who is to decide if it's "really necessary"?

 

[dgallup]

The function of the part dictates if it is really necessary.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[ewh]

The function of the part dictates if it is necessary... what dictates that it is "really" necessary? ;-)

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[looslib]

Pmarc, I agree that in those examples in Y14.5, you are not using the laergest area face, but you have also specified an origin surface to inspect from.


"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

 

[pmarc]

But at the beginning the drawing is empty. You decide where to put the origin or datum feature symbol. If you were to follow the rule you had been taught, the origin symbol would land on the larger surface and the part would be checked against non-functional requirement.