TP without a basic dimension? 5

[rollingcloud]

This was done by someone who I cannot reach out and ask. I am trying to make sense of his measurement drawing, it's not easy since I am a noob as well...
It has 3 axes as datums A, C and E, I am pretty sure that's not correct, one axis (datum C) should be enough IMO.
Also, none of the TP callouts have basic dimension.
Moreover, calling TP on the ball dia does not make sense to me.
Calling two flatness on the race seem to be not needed, especially if both surfaces are controlled by perpendicularity already.
Is form control on the ball OD a good idea?

 

[rollingcloud]

Ok I think I understood this one, TP's true center point would coincide with the datum center point. (The datum center point of the spherical surface datum feature derived by CMM uses same/similar method (for ex, least-squares fit sphere) for finding the true center point of TP.) Since concentricity has multiple scattered points, you can still measure the deviation.

I understand that the diametrically opposed points must lie on the line passes through the center point, you are saying that center point must be a datum center point and not a true center point that was offset from a datum for concentricity? Interesting, I wonder what the reason behind that is.

But if it was a basic and no callout is using that basic dimension, how does that work? It feels like it would just be a reference dimension in that scenario.
When it's not a basic, why can't we interpret it as the distance from datum D to the center of the sphere by ASME standard? Would it make sense to add a note saying that is distance to the spherical center?
Adding a profile control to the vendor's version would prevent unwanted shape, correct?

If the vendor insists on keeping the concentricity and circularity for some legit reasons. Would the attached version be better? I made datum D the primary datum and removed TP on the hole. I am not sure if char 35 is legal since it ties back to datum E which was defined based on datum D.

 

[3DDave]

Can we correspond directly with the vendor and cut out the middleman?

 

[Nescius]

Loosely speaking, pmarc wants a (datum) point to anchor infinite lines, each line containing one of the infinite pairs of opposed points on the nominally spherical surface.

Loosely speaking again, Burunduk says "no problem", there are an infinite number of candidate anchor points along the datum axis. If at least one of these anchor points results in a small enough cloud of median points, the concentricity tolerance is satisfied.

The real problem is that both of these viewpoints are reasonable.

 

[Burunduk]

Nescius,
You are not speaking loosely, you are speaking precisely. In addition, if a secondary datum feature is referenced and a basic dimension locates the "anchor point" along the axis, the concentricity tolerance zone is fixed relative to the DRF.

pmarc,
I will not deny that the intent of those writing the standard might have been that concentricity of a spherical feature is only evaluated relative to a datum center point established from another spherical feature. This could be the intent, but they didn't make it explicit. Since they didn't, I would not rule out the option of concentricity specification for a spherical feature relative to a primary datum axis and a secondary datum plane based on a guess of what the intent of the committee was. That is, if evaluation of median points within a tolerance zone that doesn't coincide with a datum point but is basically defined relative to the DRF truly suits the design intent, and the vendor understands the requirement (although we all have good reasons to doubt it).
BTW, I think that not only that the committee didn't make what you think they intend explicit, the paragraphs we quoted are badly formed. I don't understand the use of parenthesis such as in "(or spherical)" and "(or center point)" - if it's part of the definition and not for clarification only, there should have been no parenthesis.

rollingcloud, admittedly I didn't look at your latest attachment yet, but based on what you ask, I can offer you a clarification regarding the type of the "horizontal .XXX dimension" - it MUST be basic in order to locate the center of a tolerance zone related to a geometric tolerance requirement relative to a datum/datum reference frame. If it is basic, there are no clarification notes required.
Regardless, I suggest you again to consider keeping the original scheme with the combination of position and profile.

 

[pmarc]

Burunduk,
I agree the definition could be better worded. For example, it could have a structure similar to the one used in the definition of circularity (para. 5.4.3 in 2009).

Interestingly enough, the math standard, Y14.5.1, in its both versions, 1994 and 2019, is quite clear that the symmetry rays for the symmetry type Point must emanate from a datum point (see table 5-12 in 1994 and 7-12 in 2019).

rollingcloud,
My apologies for this short answer, but I am not sure I see any valid reasons for the concetricity to stay on the drawing.

 

[rollingcloud]

Thank you all for sharing your knowledge patiently.
I believe as long as datum D is perpendicular to datum E, its ok for datum E to be slanted. So datum E as primary should be fine even if its difficult to grab by the CMM. When using TP, the spherical diameter should be basic which I have missed. I will take out concentricity on the ball. I think profile without datum and circularity are the same when applying to a spherical surface. I am sure people are tired of newbie questions by now, I might pick up a GD&T book for a better understanding, a lot of stuff are similar...surprised they don't teach this when I was in school.

 

[3DDave]

https://www.asme.org/codes-standards/find-codes-standards/y14-5-dimensioning-tolerancing

 

[Burunduk]

When using TP, the spherical diameter should be basic which I have missed

When using a tolerance of Position (BTW, TP or "True Position" is just the basically located and oriented point/axis/plane at the center of the tolerance zone), in the common case the diameter/spherical diameter/width are defined by a directly toleranced dimension, not a basic dimension. Sometimes (rarely) It can be a basic dimension, only if a surface profile tolerance controls the size (and form) of the feature of size.

I think profile without datum and circularity are the same when applying to a spherical surface

Profile, with or without datum references, requires basic size (spherical diameter) and controls the form as well as the size for a spherical feature. Circularity only controls the form of circular elements sampled on the feature.

I am sure people are tired of newbie questions by now

Don't worry, If they were, they wouldn't be here posting answers.