Datum callout question 2

SeasonLee · Thursday at 3:17 PM

I have some questions about this print, and I tried to make some changes as shown below. The reasons for making these changes are:

The common datum(A-B) is constructed after the individual feature is designated as the datum feature.
The existing DRF refers to the datum feature itself, normally we will consider it as a coaxial part, but here is a multiple skewed cylinder.

Do you agree with the changes on the datum callout? Please correct me if I am wrong here.
Thanks for your help.

Season

3DDave · Friday at 5:11 PM

The one without FCFs on the datum features do not control the datum features relative to each other. They could be individually angled at 45º and the part could still be accepted.

pmarc · Friday at 5:40 PM

SeasonLee,
If you don't like the A(M)-B(M) option in the position FCFs for datum features A and B, you can specify position FCFs without datums and with SIM REQT note associated with them.

But, as was already said, you need to have a position tolerance (or other tolerance that will is able to control mutual location of the datum features) to have the relationship between the features fully defined.

Burunduk · Friday at 6:17 PM

pmarc said:
SeasonLee,
If you don't like the A(M)-B(M) option in the position FCFs for datum features A and B, you can specify position FCFs without datums and with SIM REQT note associated with them.

But, as was already said, you need to have a position tolerance (or other tolerance that will is able to control mutual location of the datum features) to have the relationship between the features fully defined.

Or as shown in these examples...

The first one is the same as using a Continuous Feature modifier for the 35+/-0.1 diameter.

CheckerHater · Friday at 7:05 PM

I have strong suspicion that simulated datum feature will rather look like this:

SeasonLee · Saturday at 11:42 PM

Thanks again on all comments.

pmarc said:
If you don't like the A(M)-B(M) option in the position FCFs for datum features A and B, you can specify position FCFs without datums and with SIM REQT note associated with them.

Is this your idea? hope I am right.

Season

pmarc · 2025-03-30T17:23:43+0100

SeasonLee,

Yes.

Of course, depending on the function, you may still add MMB modifiers after both datum feature letters in the position FCF for the large cylinder.

The other option suggested by Burunduk may work as well, but only if the datum cylinders can be defined with the same size callout.

greenimi · 2025-03-30T20:51:14+0100

pmarc,
Is it true that figure 10-58 in Burunduk's laytest replay will have an interpretation problem IF the callout is RFS?
Otherwise stated 2x size xxx.xxx ; position Ø0.15 RFS -datum feature A is ambiguos? Or is okay to be used?

mfgenggear · 2025-03-30T23:28:28+0100

greenimi said:
pmarc,
Is it true that figure 10-58 in Burunduk's laytest replay will have an interpretation problem IF the callout is RFS?
Otherwise stated 2x size xxx.xxx ; position Ø0.15 RFS -datum feature A is ambiguos? Or is okay to be used?

why?
it would require a more precise machine.
there has been times I had true poison of .0005 at rfs. but the cost increase

greenimi · 2025-03-31T01:17:21+0100

It's a pattern at RFS so I am asking how that works?

mfgenggear · 2025-03-31T02:00:26+0100

True position. 6 mm (.026)
It doesn't get any bonus
But that is liberal

Burunduk · 2025-03-31T04:22:55+0100

greenimi,
RFS would be interpretable.
In terms of the requirement it's two dia. 0.15 mm coaxial cylindrical tol. zones in which the two unrelated actual mating envelope axes must fit. In terms of the measurement It's two measured zones fitted over the two unrelated measured mating envelope axes.

greenimi · 2025-03-31T11:18:45+0100

Burunduk said:
greenimi,
RFS would be interpretable.
In terms of the requirement it's two dia. 0.15 mm coaxial cylindrical tol. zones in which the two unrelated actual mating envelope axes must fit. In terms of the measurement It's two measured zones fitted over the two unrelated measured mating envelope axes.

Ok. I understood. Or I guess....I understood.
The question was: are there two features (and consequently two simulators, one of each end) or just one simulator?
Because if the former, then if one simulator stops contracting to get the UAME - smallest perfectly formed cylinder circumscribed around the actual "as made" feature--- what is going to happen with the other simulator? This second simulator will stop too or will keep contracting?
That was my main question about the interpretation.

Do you have any suggestions on how to interpret (per the standard) this RFS scenario?
Do we need a note to explain to the end users or the standard has already clarified this RFS case?

Burunduk · 2025-03-31T15:06:40+0100

greenimi said:
Ok. I understood. Or I guess....I understood.
The question was: are there two features (and consequently two simulators, one of each end) or just one simulator?
Because if the former, then if one simulator stops contracting to get the UAME - smallest perfectly formed cylinder circumscribed around the actual "as made" feature--- what is going to happen with the other simulator? This second simulator will stop too or will keep contracting?
That was my main question about the interpretation.

Do you have any suggestions on how to interpret (per the standard) this RFS scenario?
Do we need a note to explain to the end users or the standard has already clarified this RFS case?

So, you are asking about RMB (the datum feature material boundary modifier), not about RFS (for the mutually locating position tolerance specification).

ASME Y14.5-2018 states:

" 7.12.4 Pattern of Features of Size RMB
When RMB is applicable in a feature control frame to common datum features of size used to establish a single datum, the true geometric counterpart of each feature shall be fixed in location relative to one another. The true geometric counterparts shall expand or contract simultaneously from their worst-case material boundary to their LMB until the true geometric counterparts make maximum possible contact with the extremities of the datum feature(s). When irregularities on the feature(s) may allow the part to be unstable, a single solution shall be defined to constrain the part "

It's two coaxial (in the discussed case) datum feature simulators, and both simulators must be brought to the condition of max. possible contact with the actual datum features.

greenimi · 2025-03-31T18:08:09+0100

Burunduk said:
So, you are asking about RMB (the datum feature material boundary modifier), not about RFS (for the mutually locating position tolerance specification).

ASME Y14.5-2018 states:

" 7.12.4 Pattern of Features of Size RMB
When RMB is applicable in a feature control frame to common datum features of size used to establish a single datum, the true geometric counterpart of each feature shall be fixed in location relative to one another. The true geometric counterparts shall expand or contract simultaneously from their worst-case material boundary to their LMB until the true geometric counterparts make maximum possible contact with the extremities of the datum feature(s). When irregularities on the feature(s) may allow the part to be unstable, a single solution shall be defined to constrain the part "

It's two coaxial (in the discussed case) datum feature simulators, and both simulators must be brought to the condition of max. possible contact with the actual datum features.

Yes, I was assuming (rightfully or not) that if position of the 2X features is RFS, THEN the location of the Ø8mm hole is also RMB, hence my simulator questions.
I don't know, but thinking about a functionality I most likely consider this scenario true.

Can we use (logically) MMC on the 2X outside diameter features and RMB for the Ø8mm hole?
Or can we use RFS on the 2X OD's and MMB for the 8mm? what would be the functional requirements for such a "mix and match" ?

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