Datum callout question 2

[SeasonLee]

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]

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]

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]

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]

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

 

[SeasonLee]

Thanks again on all comments.

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]

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]

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]

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]

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

 

[mfgenggear]

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

 

[Burunduk]

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]

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]

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]

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" ?

 

[Burunduk]

greenimi,
The function of MMC position is to allow clearance fit with a feature that is fixed in a mating part that interfaces with the datum features. The function of RFS position is to locate a mating component. In the standard example the function of the 8 dia. mm hole could be to clear a pin fixed to the mating part that has a virtual condition of 7.4 mm relative to the interface with datum features A and B. However if it was for locating a press fit pin the position would make more sense RFS. I would agree that if the location of the press fit pin was really important in the bigger system, RMB for the datum reference would be the more probable functional choice as well.

 

[mfgenggear]

greenimi,
The function of MMC position is to allow clearance fit with a feature that is fixed in a mating part that interfaces with the datum features. The function of RFS position is to locate a mating component. In the standard example the function of the 8 dia. mm hole could be to clear a pin fixed to the mating part that has a virtual condition of 7.4 mm relative to the interface with datum features A and B. However if it was for locating a press fit pin the position would make more sense RFS. I would agree that if the location of the press fit pin was really important in the bigger system, RMB for the datum reference would be the more probable functional choice as well.

yes Burunduk
if the hole had to be precise for a press fit , then RFS would be prefered

 

[greenimi]

yes Burunduk
if the hole had to be precise for a press fit , then RFS would be prefered

Just to make sure you understood me.
I am talking about MMB versus RMB for the 8mm hole.
I am not talking about MMC for the 8mm hole

I don't really know why, but I am thinking that if the OD's (again fig 10-58, Ø16.51-16.56 outside diameters) are modified at MMC (hence being a clearance fit application--I know very simplistic way of thinking--) then if those two diameters are now datum features (call it "A") for the Ø8mm hole THEN the most logically this primary datum feature "A" should/ shall be modified at MMB. What would be the physical reality of an assembly to act as "clearance" fit in one place and then a "non-clearance" fir (press fit) in the other place on the same drawing for the same physical part?

Do you guys understand my question?

 

[mfgenggear]

Just to make sure you understood me.
I am talking about MMB versus RMB for the 8mm hole.
I am not talking about MMC for the 8mm hole

I don't really know why, but I am thinking that if the OD's (again fig 10-58, Ø16.51-16.56 outside diameters) are modified at MMC (hence being a clearance fit application--I know very simplistic way of thinking--) then if those two diameters are now datum features (call it "A") for the Ø8mm hole THEN the most logically this primary datum feature "A" should/ shall be modified at MMB. What would be the physical reality of an assembly to act as "clearance" fit in one place and then a "non-clearance" fir (press fit) in the other place on the same drawing for the same physical part?

Do you guys understand my question?

Yes and it's a PITA, it will confuse people.
There's plenty of online clarification.
But personally don't want to tackle it.

I would ask my customers engineering and QC quality engineer, department to clarify .
Which happens often.
That said it is said that it is better for assembly

 

[greenimi]

Yes and it's a PITA, it will confuse people.
There's plenty of online clarification.
But personally don't want to tackle it.

I would ask my customers engineering and QC quality engineer, department to clarify .
Which happens often.
That said it is said that it is better for assembly

Well, I guess that is exactly the purpose of this forum. To find arguments and provide potential solutions.
(If you don't want to tackle this teoretical subject- with no consequences of being right or wrong- then how are you going to handle a real life example where the consequences could be way more dramatical?)

Should I say that there is a CAPOUT?

 

[mfgenggear]

Well, I guess that is exactly the purpose of this forum. To find arguments and provide potential solutions.
(If you don't want to tackle this teoretical subject- with no consequences of being right or wrong- then how are you going to handle a real life example where the consequences could be way more dramatical?)

Should I say that there is a CAPOUT?

Yes it is , I am retired gear guy, I can