Rotating datum feature to align to axis 3

[cmmguy75]

There is a hole labeled as 3rd datum |C| feature located in 1st quadrant @60degree from X axis.
The question is: does it matter if you rotate to align with Y instead of X axis for inspecting? Like if you rotate to align Y axis then it's 30deg. or 60deg to the X axis?

 

[Belanger]

Since the datums are taken from those holes, then there is no "x" or "y" axis as you speak of -- those terms are based on preconceived notions of horizontal and vertical as derived from the outside edges. The true "x" axis will be the straight-line connection between those two datums.
In other words, the 60 degrees is not of direct importance to the holes themselves. (However, it would help to see the rest of the drawing to explain the full effect.)

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[3DDave]

It isn't located in rotation from anything. There needs to be some datum to establish its orientation from. As it is, regardless of where the holes are drilled in the part they can be aligned to any arbitrary angle with any arbitrary precision relative to any random basis of measurement. As a probable tertiary datum it -is- the feature that will be used to establish orientation, and therefore is always perfectly oriented.

 

[CheckerHater]

It all depends on what exactly are you trying to measure wrt your datum frame.

What numbers will you report?



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

 

[emorel22]

You need to assign a datum to one of the edges parallel to your x or y axes. That will give you a set orientation to measure an angle from. I think.

E. Morel
M.E.

 

[cmmguy75]

Please see my updated picture.
Thanks for your guys tips.

 

[PaulJackson]

updated alternate
Paul

 

[PaulJackson]

Oh the stuff you see after you post!
The angle should be 56.31 degrees rather than 60 among other things for that detail to work ...
Paul

 

[Belanger]

No... the 0, 0 origin notations should not be there, nor should the 60 degrees. (What is it 60 degrees from? The edges? No, else those edges should have been identified as datums.)

The new hole is tolerance strictly from A, B, C. That's it.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[PaulJackson]

If [A], , & [C] Orient, Locate, and Stop rotation of the part respectively in their assembled condition ... and the material edge location is dependent upon the DRF [A|B|C] then it is fitting that the origin is at the intersection [A] and and oriented for rotation to [C]. The DRF does not need to be moved to the edges unless they are the functional registry features. The 0-0 reference can be anywhere on the drawing but it works best at the intersection of a fundamentally important DRF.
Paul

 

[Belanger]

Paul, I agree that the origin can be "at the intersection [A] and and oriented for rotation to [C]." But you have introduced a new variable into the discussion that was not present at the beginning: that is the statement that the part's "edge location is dependent upon the DRF." That is irrelevant to the OP's question about the position of the small hole relative to A, B, C. (Perhaps if we added a profile tolerance all around wrt A|B|C then we could speak of some sort of 60º relationship.)

Check out my attached sketch. Notice that the holes are in the exact same location as the other pictures posted in this thread! Thus, the position tolerance that the OP gave in his second upload has been fully satisfied. I simply cranked the outside shape to a different rotation, but that was never part of the given tolerance scheme. Therefore, the 60º was -- from the very start of the discussion -- meaningless to the question being asked.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[CheckerHater]

the 60º was -- from the very start of the discussion -- meaningless to the question being asked

I would take it further and say that the question itself is meaningless.

I already said it before - datum is no more no less than reference point of you measurement. If you don't specify what exactly are you trying to measure, the question "check my datums" is meaningless.
It's like having dimension with only one reference point, leaving the other end dangling. Is it correct dimension? Who knows?

We agreed that set up should include flat surface for datum A. A and B will leave one rotational degree of freedom, so we add feature C. Now, to create rectangular coordinate system we may draw our axis / plane directly thru C, or at the angle. It all depends on the rest of the part and what exactly are we trying the measure. The typical example will be Fig. 4-7 (2009) - angled datum feature used to establish rectangular coordinate system

So let me say it again in different words - you cannot check the "correctness" of datums ripped out of context.

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

 

[geesamand]

I tolerance prints in this way and this is what it means to me:
Set up XY plane on A. Set Origin location on B. Align X or Y with C. Doesn't matter which. The nice thing about this is the Y or X value of C will be dead zero - confirming that alignment - but the nominals for everything else will be rotated and not match the print dimensions.

So from this point you are free to rotate the coordinate system -60 degrees from the actual (not theoretical) location of C. You're doing it right if the coordinates of the actual C fall on a line between the origin and theoretical C. I always check this when reading the CMM report.

Hope this helps,

David

 

[SeasonLee]

If you can understand what J-P said in this thread, you will get the right answer on this quiz question.
Thanks

Season