Sequence in Referencing Datums
Sequence in Referencing Datums
(OP)
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





RE: Sequence in Referencing Datums
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: Sequence in Referencing Datums
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
RE: Sequence in Referencing Datums
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
RE: Sequence in Referencing Datums
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.
RE: Sequence in Referencing Datums
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
RE: Sequence in Referencing Datums
RE: Sequence in Referencing Datums
Han primo incensus
RE: Sequence in Referencing Datums
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
RE: Sequence in Referencing Datums
Frank
RE: Sequence in Referencing Datums
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Sequence in Referencing Datums
RE: Sequence in Referencing Datums
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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.
RE: Sequence in Referencing Datums
“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV
RE: Sequence in Referencing Datums
"Wildfires are dangerous, hard to control, and economically catastrophic."
Ben Loosli
RE: Sequence in Referencing Datums