when to use hole pattern as datum feature
when to use hole pattern as datum feature
(OP)
I see many times, from function or assembly standpoint, hole pattern (bolt holes, pin holes) constrains degree of freedom of a part, but they are not datum feature. So I'm wondering if there is kind of rule of thumb or tip as to when to use hole pattern as datum feature. Another question, is hole pattern manufacturing or inspection friendly as datum feature?





RE: when to use hole pattern as datum feature
Second question: If using a hole pattern as a datum feature, the easiest way is to reference it at MMC. That's the only way it is shown, or even talked about, in the standard. This doesn't mean referencing a hole pattern at RFS is wrong but it's definitely a tough check. That's been hashed over in this forum before.
Powerhound, GDTP S-0731
Engineering Technician
Inventor 2010
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: when to use hole pattern as datum feature
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: when to use hole pattern as datum feature
Y14.5M-1994 only permits a pattern of features to establish a datum when they are referenced at MMC. Not sure why, but suspect a bias toward hard-gauging. That requirement was lifted in 2009.
I don't know about friendly or unfriendly for manufacturing or inspection; there are so many different fabrication techniques (simultaneous vs individual feature fabrication) and metrology techniques available that it is a case-by-case question.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: when to use hole pattern as datum feature
All in all each of these methods begin by modeling the functional relationship rather than a surrogate.
Paul
RE: when to use hole pattern as datum feature
I prefer to use the part base and outline as datums. I strongly prefer to use accurate features as datums.
Often, I want to apply sloppy tolerances to my part outlines, and I have an accurate hole pattern. The part outline stays sloppy.
--
JHG
RE: when to use hole pattern as datum feature
I would appreciate it if anyone can show me an example of using hole pattern as datum feature.
RE: when to use hole pattern as datum feature
Notice that the basic dimensions of 20 and 45.6 could have been tagged to any of the four holes, since they all help form the datum.
The trick is that -- because datum D is made up of a pattern -- it's helpful (I was going to say required, but that's too absolute) to reference datum D with the "M" modifier. That way, if any of the four holes are a little off center (and they will be) then there is slop or "shift" to allow for that.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: when to use hole pattern as datum feature
RE: when to use hole pattern as datum feature
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: when to use hole pattern as datum feature
I like these two pages from Alex's "Advanced concept of GD&T", hope this is what you are looking for.
SeasonLee
RE: when to use hole pattern as datum feature
Attached is graph shows my case. There are two relatively tiny pin holes (ø25 vs. ø2964) on the flange surface. The other one is on opposite side of vertical centerplane. In this case, I doubt the pin hole pattern is qualified to be a datum.
RE: when to use hole pattern as datum feature
Unless there's a really strong reason to use the hole pattern I'll often do as drawoh - always with a nagging doubt in my mind that I'm giving in though.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: when to use hole pattern as datum feature
Sorry for being a little bit sarcastic, but your reply reminds of a statement I often hear:
"I will not apply GD&T on a print, because manufacturer and/or inspector does not understand this stuff at all."
RE: when to use hole pattern as datum feature
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?