Perpendicularity to 3 datums

[The Fahj 122]

Greetings all,
I'm working on a part that has a perpendicularity callout referencing 3 datums. Datum -A- is a flat face, datum -B- is a hole perpendicular to -A- and datum -C- is also a hole on the same face as -B-. This seems incorrect how they have it called out. I understand they are constraining the part through |A|B|C| but, seems counterintuitive since |B|C| in no way, shape or form create a perpendicular feature to the surface in question. To me, I would think a composite profile of a surface would be more appropriate and less confusing. ASME y14.5-2018 is the standard stated on the drawing. Is the drawing correct or am I overthinking this?

 

[pmarc]

Greetings,
What is the toleranced feature? Are you able to attach a rough sketch showing the feature and the datum features?

 

[The Fahj 122]

Here's a rough drawing of the callout in question. Let me know if you need more info and thanks for your help

 

[Burunduk]

With orientation tolerances you often want to constrain all 3 rotational degrees of freedom to have the orientation fully defined. The A,B,C datum reference frame, from what you described, does that. Like pmarc suggested a sketch would help to give a better answer, but in principle it doesn't sound like there is an issue.

 

[The Fahj 122]

With orientation tolerances you often want to constrain all 3 rotational degrees of freedom to have the orientation fully defined. The A,B,C datum reference frame, from what you described, does that. Like pmarc suggested a sketch would help to give a better answer, but in principle it doesn't sound like there is an issue.

I attached a rough drawing

 

[Garland23]

You didn't describe the feature that the perp tolerance is applied to. But if it's a surface then yes, profile of a surface might be an alternate way to do things. But if it's a hole/pin or other "feature of size" then I wouldn't change the symbol to profile because that would change the meaning of the callout (profile might then control size, form, and location, which perpendicularity of a FOS doesn't do).

 

[greenimi]

I would say you need a location relationship between the top surface and the datum features hence profile is the correct callout instead of shown perpendicularity (which by the way offers only orientation relationship and not location relationship)

 

[greenimi]

By the way CF symbol is incorrectly applied. You can use 2X before the "profile" (well future correct profile callout). You can use even composite profile to tight the relationship between the "two" top surfaces (or to make it one continuous feature).
CF symbol should not have been attached to the basic dimension.

 

[The Fahj 122]

By the way CF symbol is incorrectly applied. You can use 2X before the "profile" (well future correct profile callout). You can use even composite profile to tight the relationship between the "two" top surfaces (or to make it one continuous feature).
CF symbol should not have been attached to the basic dimension.

It's not my drawing, I'm making the part. The is a "rough" recreation on the drawing since I cannot share the actual drawing. The perp callout and the way they're referencing the 3 datums is what I am questioning. I believe they actually have the CF with the perp callout.

 

[pmarc]

Here's a rough drawing of the callout in question. Let me know if you need more info and thanks for your help

No further info is needed to answer your question.

Leaving the other problems with the drawing aside for now, the |A|B|C| in the perpendicularity callout is semantically correct, as it enables to fully constrain all 3 rotational degress of freedom of the tolerance zone. With only |A| specified, which could also make sense, the perpendicularity tolerance zone would be free to rotate around a vector normal to datum plane A, and so this would create a different geometrical requirement.

 

[Burunduk]

I attached a rough drawing

OK, I see it now, somehow missed it the last time.
The perpendicularity callout with its datum references is "legal" and fully defines the orientation of the surface. The tolerance zone is perpendicular to datum A (primary), but also at a direction set by B (secondary), C (tertiary): B gives a rotation axis and C stops that rotation, finally allowing the establishment of a single specific coordinate system.

However as others noted, there are other issues on the drawing. This callout COULD be replaced by profile if both location and orientation are to be controlled within the same value - but that depends on the design intent. <CF> must have been applied differently. The perpendicularity requirement could be a refinement of profile modified by <CF> (profile for location could be applied as a "2X" pattern). The role of the 18 non-basic dimension is not clear - if there is an attempt to apply it for location with some default title block tolerance that's not a supported practice.

 

[The Fahj 122]

Thank you everyone for the clarification. I've been a machinist for 38 years and have never seen a perpendicularity callout like this. It's always been relative to a single plane or axis.

@Burunduk the 18 basic should not have been basic in my crappy recreation of my customer drawing. It's a FOS with an oval to denote it as a critical dimension and they have the <CF> on the actual perp callout. Sorry for the confusion

 

[Burunduk]

The Fahj 122,
Dimension 18 is not basic in your recreation, and since you say that isn't basic in the orginal drawing either, that's part of the problem. It's not "FOS" since the distance from the hole to the two surfaces is not a across a feature of size.

 

[ghostdigital]

The specification does not appear to incorrect but awkward and uncommon..

Datum "C" is not required as an orientation of Datum's A and B is not a required refinement.

 

[greenimi]

Datum "C" is not required as an orientation of Datum's A and B is not a required refinement

I am not understanding this statement.......could you please explain it further?

Leaving the other problems with the drawing aside for now, the |A|B|C| in the perpendicularity callout is semantically correct, as it enables to fully constrain all 3 rotational degrees of freedom of the tolerance zone.

pmarc,
Probably strictly talking about the question asked, you are correct.
I looked a little deeper in my response (I guess you agree that the feature's location is not controlled, but should have been)
Or at least, is not controlled by what the current attachment is depicting.

 

[pmarc]

greenimi,
Yes, I agree that the location of the interrupted feature is not clearly controlled. In my response, I tried to focus only on providing an answer to the original question.