Not sure myself. Probably because position and perpendicularity are RFS.
Would the calculation change if position and perpendicularity would have been modified at MMC?
At RFS, inner boundary of an internal feature is a single value: MMC minus geo tol.
Looks to be a very similar situation to Fig. 4-16, except the geometric controls on the datum feature D are RFS instead of MMC. I think that at the very least the calculation as shown is not correct.
My gut tells me it should be as you say utilizing the perpendicularity tolerance of .005 to calculate the boundary (1.995-.005 = 1.990) however I'm not really even sure how MMB is to be interpreted when the datum feature is controlled at RFS.
It looks more like Fig. 4-20 (c) to me.
There is probably bad choice of words, value calculated better be called not MMB but "virtual condition"
There is a rule known as “Datum Feature at Virtual Condition": Although referenced in a feature control symbol at MMC, a datum feature of size controlled by a separate tolerance of location or form applies at its virtual condition.
2009 standard doesn't emphasize it, but back in 1973 it was known as "Rule 5"
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
Agreed, 4-20 shows the exact situation better. That being said, in the notes in 4-20(c) they actually do use the term "maximum material boundary" even though the perpendicularity for the datum feature is at RFS.
Actually looking at 4-16 though, it seems like as long as the DRF for the feature of interest is [A|D(M)] the MMB of the datum is 1.990 no matter whether the datum itself is controlled by MMC or RFS position(.01 to [A|B|C])/perpendicularity(.005 to [A]). I think its been said that there are some issues with 4-16 due to violation of datum precedence but this is how I'm interpreting it based on what I see in the standard.
Yes, when gauging feature D wrt A|B|C you gauge it at RFS,
BUT
When gauging hole pattern wrt feature D it is referenced at MMD, meaning possible datum shift, so you gauge D at virtual condition, which is kind of obvious, which may explain why we don't call it "rule" anymore.
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
CH,
I agree with everything you said - I was just pointing out that in the standard in Fig 4-20(c) they actually utilize the term "maximum material boundary" instead of "virtual condition" to refer to the calculated value, even though the perpendicularity is at RFS.
SeasonLee,
No I was just saying I was unsure exactly what it meant in this case. As CH has shown its perfectly valid and acceptable. That said, as I previously stated I think the calculation as shown is incorrect since the DRF is [A|D(M)] only perpendicularity should be taken into account which means that the MMB/virtual condition should be 1.995 - .005 = 1.990, which is actually the same regardless of whether the perpendicularity/position on the datum feature was referenced at MMC or RFS.
In the last case, the MMB should use the perpendicularity and ignore the position tolerance because the related virtual condition does not include the references to the datum features that the position tolerance depends on.
Season: I agree with 3DDave's observation..."In the last case, the MMB should use the perpendicularity and ignore the position tolerance because the related virtual condition does not include the references to the datum features that the position tolerance depends on."
Frankly speaking, I do like all materials from DraftingZone, they give me a lot of unique information that you can't find from any other sources, for examples:
1. How to use a milling machine act as a CMM to check position tolerance.
2. Utilize the parallel to find out the datum centerplane.
3. The reason of a CMM should not probe the actual surface of the part to establish the DRF.
.....
