Reference datum on position and profile callout 2

[SeasonLee]

Please ref to the attached sketch, there is no doubt a reference datum should be on the perpendicularity callout, my questions :

1. Are there any problems on the 2nd hole referenced datum? Why not include the planar datum B?
2. Can we call out the profile with referenced datum A and C as shown?

Thanks for your assists.

Season

 

[fsincox]

Since we do not know the function it is hard to second guess. If we assume the person knew what they wanted, then, I suspect only small portions of B are used in the assembly so a primary reference to B would be more for manufacturing purposes. That leads me to the same question I always grapple with in these situations; do I do it anyway just because, well, it is actually made that way?

The perpendicularity on the bores it seems to indicate they are already leading in that direction, otherwise why not just do a parallel to A on the second hole.
Frank

 

[CheckerHater]

Doesn’t make much sense to me.
It looks almost like creator of the drawing was afraid to invoke unnecessary simultaneous requirements.
I would reference everything to (perpendicularity), and [A|B|C] (the rest), and apply SEP REQ if needed. Possibly added perpendicularity of feature C to B.

 

[CheckerHater]

Let's imagine the part passing the inspection.

I have small doubt that "the person knew what they wanted"

Although it's possible.

 

[Tarator]

Am I missing a point?? How can you have a PERPENDICULAR (or any angular) call-out without any datum?? Perpendicular to what??
The GD&T scheme to me is not very "repeatable" either. I wouldn't choose that surface as Datum C to constrain the rotation. Again, hard to assume without knowing the function of the part.

 

[fsincox]

CH,
Calling "B" out as a secondary datum, in this case, means is just for show then, does it not? I am interested to see what others generally do in a situation like this, if we assume, the majority of surface "B" is just clearance.
Frank

 

[pmarc]

The drawing looks messed up a little.

My feeling is that B should be referenced as primary datum feature in positional FCF for the 2nd hole. It should also be used as a datum feature reference in both perpendicularity callouts, like you already mentioned.

As for controlling entire contour of the part with profile tolerance wrt |A|C|, I also have some doubts. One of them is: why is A referenced at MMB in positional callout on the 2nd hole and in the same time is referenced at RMB in the profile callouts? That looks at least strange.

There are a couple of ways to dimension this part. Mentioning just two:
1. Use datum features B and A only and tie the 2nd hole with part outline through simultaneous requirements, so that the tertiary datum feature is not needed at all. The hole would be controlled by position FCF wrt |B|A|, and the contour by two profile FCFs wrt |B|A|. A could be referenced at MMB, if it was functionally justified. (This is my favorite here).
2. Use datum features B, A and C, control the contour with two profile FCFs wrt |B|A| and the 2nd hole by position FCF wrt |B|A|C|. Similar to #1, but not exacly equivalent.

At the end of the day, the proper choice of datum features selection and dimensioning approach should be based on part's functionality. To me it looks like the author of the drawing did not take this factor into account.

 

[fsincox]

Tarator,
I agree that the perpendicularity must imply "B", I am just taking that as given!
Frank

 

[CheckerHater]

Frank,

B is not "for show", B arrests degrees of freedom.

A and B (or B and A) together arrest 5 out of 6.

 

[Tarator]

If you define Datum B, but don't use it in any FCF, it basically is useless, aka does not arrest anything.

 

[fsincox]

CH,
Not as a secondary datum, it better not! It stops translation along one axis, it is a high point of a surface.
Frank

 

[fsincox]

Sorry, I guess if "A" (primary) is referenced MMB it is not as clear.
Frank

 

[SeasonLee]

Thanks for all comments.
For the function of the part, please ref to the left figure.

Season

 

[Tarator]

Do you have a pic/sketch of the assembly (mating components)? What version are you using, 1994 or 2009?
Does datum feature B mate/touch with any component?

 

[SeasonLee]

They are using 1994 std. As Frank said only a small portion on both sides of pivot hole used for assembly and function, sorry for no picture or sketch provide.

Season

 

[CheckerHater]

Not as a secondary datum, it better not!

Judging from A being called perpendicular, it's definitely primary. [A|C] frames should become [B|A|C] (did you notice "B and A" option in my post from 16:09?)

Also Frank, I don't see datum targets or any other symbology proving your theory that "only a small portion on both sides of pivot hole used for assembly and function".

 

[fsincox]

CH,
I said it looked like they wanted to use "B", but didn't and "SPECULATED" that that is because the whole surface is mosty clearance. Then, I was asking if others use "B" as a primary, do mostly then to it's importance as a manufacturing datum in making the part.
Frank

 

[CheckerHater]

Well, they demanded the entire surface of B to be flat, so I “speculated” that entire surface of B is equally important.
Perpendicularity of A could only be wrt B, everybody agreed.
Now, could you imagine functional requirement that A is perpendicular to B, and second hole is parallel to A but NOT perpendicular to B?
Same logic applies to profile callout. If functional requirement actually was for profile NOT to be square to B, I would explicitly specify it somehow.
I am not implying you are wrong; just my chain of speculations is going in slightly different direction,

 

[SeasonLee]

CH

Thanks for you comments and agree what you said.
One more question : will you use planar feature B or the pivot hole datum feature A as primary datum on both position and profile callout FCF?

Season

 

[CheckerHater]

I would use B.
I mentioned order A-B-C in my first post in the hurry, but I take it back. :-(
B-A-C like in post from 6:18