Datum Modifier Effects - Planar Surface 1

[Viper555]

Hello,

I have been studying for the GDT Senior level exam (2009) and I'm really struggling to wrap my head around a concept laid out and exemplified in Fig 4-31 through Fig 4-33 (equivalent is Fig 7-34 through Fig 7-37 in 2018 (Reaffirmed 2024)).

The first example, Fig 4-31/Fig 7-34 makes sense because RMB is supposed to represent the feature datum simulator expanding until contact is made to the datum feature, thus constraining the part in rotation.

The second example, Fig 4-32/Fig 7-35, sort of makes sense because the BSC feature requires that it be in contact (similar to that of RMB), thus constraining the part in rotation.

The third example, Fig 4-33/Fig 7-36 starts to really lose me because it's defining the datum feature simulator at MMB, yet it states in Fig 4-33 that is must be in contact with one point. It seems that Fig 7-36 refines this illustration a little along with Fig 7-35, but it still doesn't address my misunderstanding and in fact further confuses me by placing part of the datum feature outside of the LMB/MMB established by the profile tolerance.

Here are my main questions:

1. In Fig 4-32/Fig 7-35, is my understanding of the BSC usage correct in that a datum at BSC requires contact, by definition, to the datum feature?

2. In Fig 4-33/Fig 7-36, why couldn't the part rotate CCW, provided the datum feature falls within the profile tolerance LMB and MMB?

3. In Fig 7-37 (there is no equivalent for LMB in the 2008 standard), why is the part profile able to pass when outside of the LMB and MMB limits?

Any help with clarifying this would be greatly appreciated!

 

[3DDave]

The answer - they didn't "do it" pre-2009 because there was never an "it" to "do." This whole section is made up over an internal fight about whether the tertiary datum could override earlier established constraints and they made this up, the translation modifier, and other things that don't appear in real mechanical devices. That last bit? I could be wrong, but after a decade of asking not one example has been brought up that could not have been handled with the pre-2009 set of tools.

As mentioned, one can already define the Maximum Material Boundary with the old-fashioned unequally disposed tolerance zone. The Least Material Boundary is inside the material and it seems like until we can get two chunks of matter to share a common volume there isn't much value to that in a mechanism.

 

[Burunduk]

Then how people did it pre-2009 standard? I guess instead of clarifying and be crystal clear and (there were no need to change it again in 2018) they did a half-ass job back in 09.
Now we are left to pick up the pieces and no idea how to fix it.

Before the 2009 revision, the standard didn’t address this topic at all, meaning there was a lack of guidance on how to handle similar cases.The 2009 version provided a fairly good solution, but apparently someone was bothered by the fact that there was no shift, since people were used to being able to move the part relative to the datum simulator whenever an MMB modifier was applied. This was never actually required by the standard, but a misconception developed that MMB must be linked to a shift. That’s why in 2018, they ruined the definition and made it nonfunctional.

 

[3DDave]

Without a practical use there was no need for guidance.