How to denote a curved or surface radius a datum feature and not the axis?

[SMO]

How can I legitimize the drop-dimension shown in the attached?

From what I've read, Tangent plane is not used for this, I guess.

Regards,
SMO (NX11)

 

[mkcski]

My suggestion: Leave the basic dimension as it is. Apply profile of a surface from datum B to the top of the boss. The profile tolerance will control the "+/-" distance from datum B as well as the flatness and parallelism for "free"

Certified Sr. GD&T Professional

 

[drawoh]

SMO,

Take the case that your flange outside diameter has a tolerance of [±].050", and that you want to locate your boss within [±].010" of the outside edge. The centre-line defined by the diameter is not a functional datum. You can apply a datum target to the outside flange next to the boss, and locate from that. You need to combine this with an end face, or a datum target on the other flange.

Is this what you want?

--
JHG

 

[SMO]

mkcki - thank you,

From what I understand, Basic dimensions have to be derived from references datum features (aside from tertiary in the case of orientation), directly (one Basic) or indirectly (chain of Basics) and since datum feature B is the axis of the flange diameter, does it make sense to have a Basic dimension from the flange surface itself? Also, that surface diameter is tolerances, as is the ID, so I'm sort of confused how this works with a Basic in between. Our shop wants this dimension so I was at least trying the define feature origin for the Basic dimension, hence why I was wondering if I could establish a tangent datum feature (although, I'm still not sure how that would work with the stack-up).

drawoh - thank you,

I was actually wondering about using datum target(s) (I would have to create one for each section-to-feature locally). Then it seems I'm okay with having a Basic dimension from that point. What I was suggesting was to eliminate that drop dimension all together. Not sure it's needed or how inspection would hit that number when the diameters have larger tolerances than the location of the Boss face.

Regards,
SMO (NX11)

 

[mkcski]

SMO: My earlier post was totally bogus. I missed the diameter symbol on datum B.

I assume the surface of the boss is flat and not an arc (diameter). I would still apply profile of a surface from datum B to the top of the boss. But I would move the BSC dim to the plan view and show a linear basic dimension from the center of B (the datum)to the boss. This will indirectly control the distance you show in the sketch. The shop will have to do some math do get the distance from the flange OD. It will be a range of values depending on the MMC and LMC of diameter B.

Certified Sr. GD&T Professional

 

[SMO]

mkcski - thanks again,

I agree with you completely about changing the origin of the Basic dimension so that it's coming from the Datum Simulator (axis of datum B), but I'm getting push-back. So am I safe in telling the shop there's a fundamental error with the current arrangement w.r.t. that dimension? I can at least have a record of this.

Regards,
SMO (NX11)

 

[SMO]

By the way, not a record from your response. I would just create one based on sound advise and research.

Regards,
SMO (NX11)

 

[pmarc]

This is quite common question in the environment I work. First thing I usually ask is, what is the functional reason behind it?

 

[mkcski]

I totally agree with pmarc. The drawing is for part definition based on function-and-fit-up and does not specify how to make or inspect the part. But realize, you gotta make the part and inspect it at a profit. So VAE needs to be considered. And the part has to "work" when mfg is done.

Certified Sr. GD&T Professional

 

[SMO]

Thanks for all the feedback

Regards,
SMO (NX11)