Justification for Best fit alignments

[Fuutgut]

I am trying to find a justification in ASME Y14.5 for doing best - fit alignments when there is a true position on a datum feature.
For example: a simple disk with a face as datum A, OD as datum B, a slot as datum C. (Ie. ASME Y14.5-1994 Figure 4-6)
A true position call out on datum C should allow to do a best fit on the rotation, correct?
I need to be able to reference where Asme y14.5 allows this.
Thanks,

 

[CheckerHater]

@pmarc:
It is not limited to para. 4.3.3, but also mentioned in para. 4.3.2 as well. It is simply a definition.
If the tertiary datum constrains only one last remaining orientational degree of reedom, then guess what? The datum has to be orientational.
Unfortunately the creators of Y14.5.1 chose a backwards way of explaining "degrees of freedom" thing later in para. 4.4

 

[pmarc]

If the tertiary datum constrains only one last remaining orientational degree of reedom, then guess what? The datum has to be orientational.

In that case take a look at fig. 4-7 or fig. 4-16 in Y14.5M-1994. It is clearly shown there that: "True geometric counterpart of datum feature C (MMC virtual condition width perpendicular to datum plane A. Center plane aligned with datum axis B). Compare it with fig. 4-2 and associated text in the Y14.5.1, and tell me, please, why do these two standards differ?

I realize there are different opinions about that concept amongst GD&T experts (commonly known as the "Tertiary Datum Problem"), but I do not really understand why both standard, Y14.5M and Y14.5.1M are not coherent. Shouldn't the math standard be a support for concepts defined in the Y14.5?

 

[CheckerHater]

Fig. 4-7 refers to Fig. 4-6 where datum C is referenced MMC (MMB in modern terminology, but we refer to 1994 standard(s))
When it comes to MMC, 14.5.1 para. 4.34(b) states “For secondary or tertiary datum features, the parallel planes are constrained to to be basically oriented and, as applicable, basically located to the higher precedence datums.”
So, there is no contradiction in Fig. 4-7. Same thing with Fig. 4-16.
When it comes to RFS on the other hand, Fig. 4-15 disagrees…

tell me, please, why do these two standards differ?

Sorry, but I am not a committee member.
You mentioned yourself that it is indeed a murky area where not everybody agrees.

Shouldn't the math standard be a support for concepts defined in the Y14.5?

I see it the other way around – The 14.5 standard should try and not violate mathematical definitions just like Engineering should comply with the laws of nature.
But it’s just me.

 

[pmarc]

Okay, so as you see, there is still a conflict between fig. 4-15 (Y14.5) and fig. 4-2 (Y14.5.1).

Honestly speaking, I have no idea why Y14.5.1 distinguishes between RFS and MMC when it comes to basic location of tertiary datum wrt to secondary datum. This is illogical to me.

As to which of the standards is superior, that does not really matter - all that matters is they disagree with each other, which is, in my opinion, really serious issue. And I am quite surprised that this has not been solved yet.

 

[CheckerHater]

I guess the major concern was to avoid over-constraining the part. They probably saw RMC / RMB as more dangerous because expanding datum simulator(s) could “fight” each other or create other confusing situations.
Unfortunately they didn’t find easily understandable way to convince users of the standard that under certain condition datum has to “let go”

On the bright side 2009 has not one but two tools to turn off unnecessary degrees of freedom if they seem to cause trouble or confusion.

 

[3DDave]

The likely reason no one notices is that '14.5.1 is, deserved or not, largely ignored.

It's $80 and supposed to represent exactly what '14.5 does, so how many would see the need to get a copy?