Rotational orientation on circular parts with cylindrical Datum

[ligo]

Here is a simplified drawing of a circular plate with two hole patterns and a single hole. Case A, B show the same hole locations, and case C has the single hole, and one hole pattern, coincident to the 2nd & 3rd datum planes.

Is my interpretation of Y14.5 section 4.4.2, 4.4.3 correct?

There was a similar discussion in thread 169440

http://www.eng-tips.com/viewthread.cfm?qid=169440

But I'm not sure if my understanding from that is correct?

Thoughts?

Thanks

 

[ligo]

pmarc-
Thanks for your input, I completely agree with you about using profile on datum A. It's been a constant struggle for me to convince people not to use flatness in coplanarity scenarios. There is constant concern that fabricators will not understand or ignore a profile callout, but will understand the intention of a flatness callout.
I also like the solution you suggest for Datums B & C, I had looked at that tec-ease example earlier and thought that it might apply, But it's always easier and quicker to slightly modify what you have than to make changes that have to be explained and justified, when you don't have the full picture yourself.

 

[pmarc]

ligo,

I know what you feel. I also very often face the situations when I am somehow forced to not specify something in accordance with the standard only because there is an assumption that somebody (fabricator, inspector or even other designer) will not understand it. And I am pretty sure that a lot of folks here have to face the same dilemmas.

But does this mean we should give up? Of course not. If we specify in a general note that a drawing is in accordance with e.g. Y14.5 std. we have to follow this one. This is a legal requirement for us and for everybody who uses such drawing, so manufacturers and inspectors are equally obliged to be familiar with the standard.

Therefore, as an example, if you specify flatness control of 3 surfaces and in the same time you follow Y14.5 std. you must be aware that in case of any legal troubles, the court for sure will not be on your side. You will be the guilty one, not a guy who told you that he does not understand profile of surface concept.

 

[axym]

Just a word of caution on the "simultaneous requirements" method being equivalent to the "clocking datum" method. This equvalence is only true in certain cases, not in general.

In the Tec-Ease example, the two methods have the same meaning in terms of the gages that would result. No argument there. But they only have the same meaning because datum feature C was referenced at MMB, making datum feature shift available.

But in ligo's first example, datum feature C is referenced at RMB. This means that Case B imposes a different control than Cases A and C. In case B, the pattern of tolerance zones gets clocked to the axis of datum feature C with no datum feature shift.

So choosing a specific datum feature to control clocking is generally more restrictive than controlling the overall pattern "to itself" using simultaneous requirements. The two methods are only equivalent in special cases.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[ewh]

MMB = MMC?
RMB = RFS?

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[pmarc]

So, Evan, you are saying that the meaning of case B is different than A & C in terms of rotational relationship of two patterns relative to the single hole, am I right?

 

[axym]

Sorry, yes, these are terms from ASME Y14.5-2009. Datum features are now referenced at a particular "material boundary" instead of at a "material condition" as they were in '94.

MMB (Maximum Material Boundary) was MMC
LMB (Least Material Boundary) was LMC
RMB (Regardless of Material Boundary) was RFS

The considered feature is still referenced at MMC, LMC, or RFS.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

ewh -- those are the new terms from the 2009 standard. An "M" symbol after a datum reference is now "maximum material boundary," and no modifier after a datum reference is called "regardless of material boundary."

They did this because the datum feature that is being referenced with the "M" could also have its own geometric tolerance. Thus, the size of a functional gage (and any resulting "shift") would be based not on the true MMC, but on a worst-case boundary.

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

 

[ewh]

Now I am really confused... ASME Y15.4-2009 para 2.8 page 29 refers to the modifiers as still being RFS, MMC and LMC.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[Belanger]

But we're talking about the same symbol as applied to a datum reference. See the 2009 standard, Figure 3-11 at the top of page 42. More detail is then given in paragraph 4.11.3 on page 59. I agree it seems confusing, but once I learned the reasoning, it started to make sense.

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

 

[ewh]

My head hurts...
Thanks!

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[SeasonLee]

ewh

I like the article in Belanger's blog, it will help you, take a look.

http://gdtseminars.com/blog/2009/07/30/mmb-a-new-term-in-the-asme-y145-standard/

SesonLee