Correct datum callout? 4

[SeasonLee]

Please adv is this right on the datum system construction? there is no intersection from datum feature A and B, is this correct to change the FCF from A|B|C to A|C|B?

Season

 

[greenimi]

How this part is assembled?
I guess you can even use C as primary, then A secondary and B tertiary.
Again, how this part works and what is suppose to do?

 

[Belanger]

There's no requirement that a secondary datum feature intersect with the primary datum feature. It's all about the degrees of freedom that are to be constrained.

 

[SeasonLee]

There is a mating part go through the hole and works as a pivot pin, it will rotate a certain degrees along the pivot pin, I think its Okay to be the Primary datum feature for the through hole.

Season

 

[greenimi]

Do you have enough clearance on A (cylinder and its mating component + pin) for the surface C to act like primary? If yes, then C should be primary. If no, then A is primary.
"A" is press fit or clearnce fit? What kind of press fit/ clearance fit?

 

[3DDave]

As shown, C does not control any necessary degree of freedom. The tolerance zones are fully constrained by A and B.

 

[greenimi]

3DDave,

As shown, C does not control any necessary degree of freedom. The tolerance zones are fully constrained by A and B.

I know it is Friday afternoon, but are you sure?
what about the translation along the shown cylinder?

 

[SeasonLee]

C does not control any DOF, and no intersection on datum feature A and B, then how to immobilize this part?

Season

 

[drawoh]

3DDave,

Datum[ ]C controls sliding up and down. It does constrain a DOF.

--
JHG

 

[SeasonLee]

There's no requirement that a secondary datum feature intersect with the primary datum feature. It's all about the degrees of freedom that are to be constrained.

This is a new concept that I learned today, there are no such cases in GD&T textbooks that I have read in the past, all examples I learned is that there is an intersection on Primary and Secondary. For this case Primary constrains 4 DOF and Secondary constrains 3 DOF, both of them are separate features and there isn’t any intersection on the DRF estabilishing at this stage, there are intersections only until the tertiary datum feature was added.

Can someone provide a similar case?

Season

 

[greenimi]

Well I would say in your case primary is constraining 4 DOF's and secondary COULD constrain 3 DOF's but in reality is actually constraining only one.

 

[Belanger]

Greenimi and Drawoh -- Dave said that C doesn't control any "necessary" DOF: necessary for the given feature control frame. (If there are other callouts on the drawing that tie something in to that third direction then it would be necessary.)

 

[3DDave]

One could provide a profile to A|B for the 'front' view profile. From what is depicted there is no related need for C.

The axis of datum A is infinite as are the profile zones that are primarily located and oriented to A and oriented to datum B. Placing the part to align the datum feature axis anywhere on that axis and then orienting to B will be enough to verify if the feature(s) is compliant.

There are examples similar to this in the Y14.5 sections on positional tolerance application. The examples look like hinges and don't locate the feature along the two datum planes.

 

[CheckerHater]

Yes, there is DRF created by A|B|C that fully constrains the part.
No, it doesn't look like the best choice.
Considering that merely clocks around [A], changing the order of precedence to A|C|B may be beneficial


"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[powerhound]

In this case, I don't think switching B and C around makes any difference in how this part would actually engage a fixture since A is at RMB (or RFS if this is actually a 1982 drawing).

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[TheTick]

The profile as defined is a 2D feature. Fatima A and B are sufficient. C does nothing.

 

[greenimi]

Still not clear what is the conclusion (or the main takeaway) of all these replays? Did the OP got what he wanted or just added to the confusion?

 

[chez311]

In regards to intersection of datum features there is no requirement that they intersect - refer to figures 4-31/4-32 in the Y14.5-2009 standard, the secondary datum feature is offset/parallel similar to your drawing. It essentially orients the two mutually orthogonal datum planes created by the primary cylindrical datum feature.

Edit:
Okay, I should really clarify what I mean by "no requirement that they intersect" (notice I said datum features and not datums) - the Datum Reference Frame (if it consists of more than one datum obviously) will always consist of two or three mutually orthogonal datum planes that intersect either at a line or a point, respectively. That being said there are often secondary/tertiary datum features that are orienting/clocking datum features (as is in your provided example and the figures in Y14.5 that I referenced) that do not necessarily need to be intersecting/mutually orthogonal to the other datum features as they actually orient the datum reference frame instead of actually "forming" another datum plane. I hope that doesn't muddy the waters too much - of course the concept gets even more confusing when you start adding in things like inclined/angled datum features and patterns as datums.

 

[SeasonLee]

Thanks for all, I got what I want to know, its very clear for me now, really appreciated.

Season

 

[mkcski]

I would recommend adding a control to define the geometric relationship between Datums A and B by adding an profile-of-a-surface to datum A FCF to Datum B.

Certified Sr. GD&T Professional