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Datum Target

Datum Target

Datum Target

(OP)
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

RE: Datum Target

Any change at attaching a sketch?

Certified Sr. GD&T Professional

RE: Datum Target

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

Certified Sr. GD&T Professional

RE: Datum Target

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

RE: Datum Target

Sketch did not attach.

Certified Sr. GD&T Professional

RE: Datum Target

Got it. Will respond this afternoon.

Certified Sr. GD&T Professional

RE: Datum Target

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?

Certified Sr. GD&T Professional

RE: Datum Target

(OP)
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

RE: Datum Target

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).

Certified Sr. GD&T Professional

RE: Datum Target

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.

RE: Datum Target

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.

Certified Sr. GD&T Professional

RE: Datum Target

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.

RE: Datum Target

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

Certified Sr. GD&T Professional

RE: Datum Target

(OP)
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

RE: Datum Target

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.

Quote (rje2019 2 Apr 19 20:08)

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.

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