Datum Target

[rje2019]

I have a print that has datum targets sitting next two linier dimensions. two pins in an assembly same size on a coaxial line. pin diameter is ref position to A datum targets are A1 and A2. this to me is not a valid way to apply the datum targets

this is what I understand to be correct
Datum targets are attached by a solid or dashed line to point, line or phantom line of the desired shape

 

[mkcski]

Any change at attaching a sketch?

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[mkcski]

Woops. Any "chance" at... (typing and edit skills were off-line)

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[rje2019]

Here is a quick sketch of how they applied the datum target

 

[mkcski]

Sketch did not attach.

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[rje2019]

second try

 

[mkcski]

Got it. Will respond this afternoon.

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[mkcski]

rje2019:

From a symbology standpoint, paragraph 4.24.1 in Y14.5-2009 defines what is allowed. Your sketch, which has direct contact with the "balloon", does not meet the requirement. So I agree with your statement "Datum targets are attached by a solid or dashed line to point, line or phantom line of the desired shape"

I also have concerns with the application of the position tolerancing. Not knowing the design requirement and what someone is attempting to communicate with the symbols, I cannot offer commentary. But does figure 4-24 in 2009 mean anything?

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[rje2019]

ok Thanks for confirming first part

their intention was to establish a datum A thru the pins( they are fairly far apart) as called out the feature frame is referencing datum A that it creates, which make is not valid. I was planning on recommending two options. left side diameter becomes datum A and right side positions to datum A and becomes Datum B. then future frames use A-B. the other is what you listed, standard 4-24, 7.6.2.3 position no datum reference becoming datum A. that last option they should understand a little easier

 

[mkcski]

rje2019:

Your datum B positioned to datum A approach is Ok. But... caution should be used since the projection of datum axis A across the "long" distance to the pin on the other end may not offer desirable results. I might recommend controlling the datum A feature perpendicular to a planar feature (I call it datum C) and then controlling the datum B feature using position to C and A (secondary).

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[rje2019]

Ok adding with your thoughts updating sketch. their is plates labeled G I could be parallel to

 

[chez311]

Datum D is not valid - datums are derived from physical features (datum features) and not from theoretical points/lines/planes or the intersection with theoretical points/lines/planes.

That being said I'm not even sure what you're trying to accomplish with that or what intersection you would be referencing since datums B and C look to be coplanar.*

*Edit - looks like theres two datum features referenced as "B", a 24.00 width and a 2.00 diameter, this would need to be fixed. Regardless my first statement still applies.

 

[mkcski]

rje2019:

It look like you "missed" my example. I was thinking: the left face of the 24.00 width would be specified as datum A. The 2.00 hole on the left would be perpendicular to datum A and be designated datum B. The 2.00 hole on the right face would be positioned to A primary and B secondary and could be specified as datum C. Hole XXX would be positioned to B-C. Note: There may be other relationships between part features that impact the final GDT tolerancing schema. I would need an in-depth understanding of part function to even attempt an approach. And this is beyond the purpose of this format for me.

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[chez311]

Also this seems to be a very similar scheme to what you posted previously (

https://www.eng-tips.com/viewthread.cfm?qid=451052)

and my suggestion at the end of that thread (which remains unanswered) still applies about referencing features of a common size as a pattern (ie: 2x dia 2.000) as a single datum.

 

[mkcski]

chez311: Thanks for the input. I didn't follow the other thread you mentioned.

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[rje2019]

Mkcski Ok I stand corrected with your example and yes this is a more complex part there is a lot going on with it

chez311 yesterday I had to ask what they intended with D. they were looking at datum D as the last step to the degrees of freedom. A axis, B datum on 24.00 and D. thier example C already does this.
yes one of the options is two datum features single datum axis. have not heard back on what they think of that yet

 

[chez311]

rje2019,

There is no need to specify a datum D as the intersection (nor is it allowed) of B (on the 24.00 width) and C. If referencing B|C in a FCF the width datum feature establishes a centerplane which is orthogonal to the 2x orthogonal planes established by C at the point shown.

yes one of the options is two datum features single datum axis. have not heard back on what they think of that yet

Something to keep in mind - if it as shown first dia 2.00 is datum A and second dia 2.00 is datum B referencing A and then referencing the pair as A-B on other controlled features you are adding an additional step which results in stacking of tolerances. This becomes a little more ambiguous when utilizing MMC/MMB but you will certainly see the effects at RFS/RMB.