Legit GD&T ???

[[To...]]

Would anyone challenge the BASIC location of this call-out?

Screenshot 2021-12-01 094711.jpg

[[Jo...]]

I've seen similar prints. If the Basic is not dimensioned from a Datum, it really is irrelevant and is not controlling per Datum precedence. Design intent is the approach I would take to customer. It does seem goofy way to call position. Can I get a profile ? lol

[[Ia...]]

I'd honestly challenge everything shown. But especially the basic callout.

[[To...]]

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It is my understanding that BASIC dimensions don't need to be attached to a datum feature but need to be measured as though they were attached to the datum feature, i.e. do the math to get the nominal location.

[[To...]]

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Does the standard require the location to be made to center?

[[Jo...]]

Now my brain hurts. Idk about legality but in practice it doesn't make much sense. If the surface used in Basic isn't a Datum ,it has a location tolerance. Additionally, if dimensioned to edge with a diametric zone tolerance is also challenging. You end up with a variable target(basic) in one axis if it follows part edge. You also have a baked in modifier of size in ONE axis since the diameter also has a tolerance. I think the print is lacking, even if legal.

[[Ia...]]

These screen clips are all from ASME Y14.5-2018 standard. Nothing outright claims the basic must go to the center, but I haven't seen any examples where the basic doesn't.

[[Ke...]]

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Isn't that what 10.2 a is stating?

[[Ke...]]

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I have seen similar.
I would report the size & position to ABC, and use the CAD nominal Basics; and report the step height (or whatever you want to call that) as a reference dimension... and then explain to the engineer/customer, and suggest a print change or notation (yes, I realize that print changes are often difficult or impossible, depending on the situation)

[[To...]]

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Yes, my brain hurts, too. Fortunately, we are very early in the inspection process so I can voice my concerns.

[[To...]]

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This was my thinking, as well. I just wanted to have a solid reasons to challenge the legitimacy. We are at a point where they can tell me what they want, in writing, ahead of time.

[[To...]]

Oh. I forgot to mention. The drawing is to Y14.5-1994

[[Ro...]]

Hi Tom,
The requirement of BASIC dimensions is that they define a theoretical perfect nominal that can be referenced back to the datums the feature references. There is no rule about how you apply them (I wish there were) as long as they ultimately define a perfect nominal.

For example, you have a square plate with a hole pattern in it. A is the top of the plate, and B & C are 2 perpendicular edges. Pretty common. You'll commonly see these dimensioned with chain dimensions, dimensioning between each hole working back to the datums. But the location of each hole is independent of the other holes. But as long as those chain dimensions add up to a perfect nominal for each hole, it doesn't really matter how you dimension them. You could run a BASIC dimension 2 ft off the part to a point in space, put in an angle, etc and come back to the part. As long as those BASICS ultimately define a perfect nominal they're legit.

From your pic, as long as the surface that the .030 comes from is defined somewhere else and you can get back to the datum structure with BASIC dims, then it's legit. Doesn't matter if it's the center of the hole or not, as BASIC dims have no tolerance and are just defining nominal location relative to the Datums.

Just to add. If the surface the .030 comes from is one of the datums referenced, then you have a direct connection to the datum and that surface is relevant to the feature location. If that surface isn't one of the datums referenced, then it doesn't matter where it is, because the feature is independent of it. It only matters that you can get back to the datums called out to get a nominal. If they are using a model and have a note that the model is the design master, then you don't even need a basic dimension. You can pull it all from the model.

[[Ia...]]

Were you supplied a CAD model Tom? Does the drawing reference the use of a 3D model in the general notes somewhere?

[[To...]]

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Yes. The notes state "ALL MISSING DIMENSIONS ARE TO BE ASSUMED BASIC AND TAKEN FROM CAD MODEL USING PROFILE TO ABC....

[[To...]]

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Yes, that surface is a ± dimension back to B.

[[Ja...]]

I'm in no way an expert here so take everything I say with a grain of salt.

My understanding is that position is for controlling the axis of a feature which needs opposing points to find. This feature is an arc so I wouldn't think position would be the best choice here. And unless I'm missing something you need to use the radius of the arc to find its center point. The center should be controlled with basic dimensions and that radius isn't basic. And depending on what kind of features A, B and C are, and what their tolerances are, it might be possible for the "center" of that arc to be so far out that it's radius doesn't even cut into that surface. Or makes an arc so small that you can't really measure it.

[[Ja...]]

Short answer, this position control application does not follow the ASME Y14.5 standard and would be better controlled with profile of a surface. If using profile of a surface, the size of the radius must be basic, and not a directly toleranced ± size.

In the context of position control:
1. For a cylindrical feature, position would control the axis of the cylinder, not the surface. However, this particular feature does not have sufficient arc length to derive an axis based on the UAME (unrelated actual mating envelope). It must be a feature of size to use position control, and this partial cylinder is not a feature of size.

2. Basic dimensions shall be traceable back to the datum reference frame. Where ever the datum features for A, B and C are, basic dimensions should originate from them to the center axis of this partial cylinder.

[[Cl...]]

PROFILE! I know that doesn't answer the question, but I think the callout should be profile.