simple question - datum reference frame 1

[cervantes]

hello all,

During studying ASME standard one question came to me:

please see attachment:

there is a page from ASME and my drawing

is there any chance to create exactly the same reference frame as given in ASME but without using second hole axis as a base C? (i moved it to right to make it even easier to understand)

datum C can be anywhere else, but the second hole can not be used as a datum

there is no special purpose for this, it is just a theoretical question

how to lock it to achieve the same requirements as in ASME example?

thanks!

 

[powerhound]

Sure. You can make the width datum feature C. You can also make one of the walls datum feature C.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[mkcski]

Cervantes:
The motion controlled by the sequence of the Datums in the Feature Control Frame (FCF) takes priority. In that example the C-hole stops rotation of two planes that intersect at 90 degrees (by definition) on cylindrical Datum axis B. Answer: Any feature can be specified as Datum C to control the rotation of the two planes around the datum axis as long as its BASIC relationship with the higher precedence Datums is defined.

 

[cervantes]

That was my point - how to stop rotation of two planes without using another hole's axis.
But what do you mean by "basic relationship" ?... I didn't "catch" it.
So no matter what i choose as C, it will lock these planes perpendiculary/paralel to datum A and chosen C?

 

[mkcski]

Cervantes:

May of the BASIC relationships on the drawing are controlled by the default 90-degrees - para 1.4(k). But as an example see fig 4-18 . One of the two plance is shown passing through the keyway. But suppose the keyway was offset downward was 12mm. The 12mm offset would have to be BASIC to define how far the plane is from the centerplane of the keyway - a non-cylindrical (width) feature. Does this help?


I say "yes" to you last question, but only in this example. If the higher precedence datums are NOT axes, then there are no planes that require rotational control (and assuming the function and fit-up requirements require the rotation to be controlled)

 

[mkcski]

Cervantes: Sorry for the bad "English" I hit submit too soon.

 

[cervantes]

thanks for your patience because now I see that my English does not help me to fully understand it
I must use drawing language

I fully understand fig 4.18 but let's go back to basis

see my next drw

left drw - no C datum specified, does para 1.4(k) states that in tis situation orange planes are default setup or blue ones?

right drw - I made one side as datum C - it setups two planes around axis automatically correct (assuming 1.4(k) does not) ?


by higher precedence you mean order of datums (planes go first)?

 

[mkcski]

Cervantes:

To be clear, I am new to the Forum but not to GDT (30 plus years with a Senior GDTP certificate). Typing is not my best skill either.

left drawing: I agree. The orange planes can rotate 360, which means the 4 hole pattern and the single hole can rotate 360 deg around B.

right drawing: I agree. As there are no symbols nullifying 1.4(k) (like a BASIC angle), the vertical plane passing through Datum axis B - will be oriented "parallel" to the side C. Consider a "cartoon" gauge that would be used to inspect the features. It would have a flat plate to simulate Datum plane A - 3 motions; a pin at the virtual condition of B relative to A (6.50 diameter) - 2 motions controlled; and a plate that would be movable, left and right due to the tolerance on location distance between surface C and hole B - 1 motion controlled.

I hope this helps. Sorry I am not a CAD "guy" to give you example drawings.

 

[mkcski]

Cervantes:

Woops. I missed your last question. Yes a datum or datums in the FCF that come before (to the left of) the datum under consideration.

 

[cervantes]

thank you,
that would be enough for me i think

 

[mkcski]

Cervantes:

You are most welcome. The "secret" to understanding the application and interpretation of datums is to understand the "6-degrees of freedom" that can datums control. The number controlled is based primarily on function and fit-up requirements. I recommend you review Section 4.4, especially the related figures.

 

[cervantes]

During studying advanced examples, sometimes I have some kind of "complete blackout" regarding fundamentals and I start to doubt.
But I think that's more or less normal situation.

 

[mkcski]

Cervantes:

Agreed. At some level GDT is "use it or loose it".

Remember too examples rarely expose function and fit-up information. So it can be impossible to appreciate why the features were selected as datums. Although interpreting the GDT is a fist step - you have to read GDT before you can write it, application (writing) is critical to a deeper appreciation of the symbolic technology. At its foundation I believe GDT is really a philosophy as it requires you to "think" about what you are trying to communicate at much more detailed level than just using +/- dimensioning.

Also be aware that the Y14.5-2009 Standard is for "product definition" only. It does NOT define how to manufacture or inspect parts - an entirely different subject!

 

[cervantes]

yes, fully agree

what's really interesting for me are the situations when theoretically easy question on this forum is placed and forum experts starts to discuss different interpretations
this forum is really a treasure for me

 

[mkcski]

Yeah. I am new to the Forum and I too am amazed at the extensive knowledge that is "donated" to the public on topics surrounding GDT.
Response to your other post about the hardened shaft tolerances is really enlightening. Keep up the effort to better understand GDT.