Referencing Centerline Datums

[PRuggiero]

Guys,

Ran across a situation that I am unsure of. Is what I show in the picture legal? Specifically can I call out datum D (plane representing the centerline of that dimension) and then use that same datum in the control frame for a profile of that same feature?

I was asked to do something like this and it doesn't seem right I just can't verbalize why I think its illegal. For one, if a fixture was used to test the part it would be hard to check that profile since the fixture would have a feature that fit into the square hole.

I understand that this part is missing some stuff, I just used it to make an example.

Thanks,
Pete

 

[MikeHalloran]

There ain't no picture.

Similarly, there ain't no 'there' there, associated with a theoretical plane. Since you can't butt a fixture against it, you can't use it as a datum.



Mike Halloran
Pembroke Pines, FL, USA

 

[PRuggiero]

My bad

 

[PRuggiero]

Dammit

 

[Belanger]

What you have in the picture is perfectly fine. You use A, B, and C to establish a framework for the rectangular cut-out, and then the centerplane of that cut-out becomes datum D.

I think you meant something different than what the picture shows: You asked if datum D can be referenced in the profile tolerance that points to that rectangular cut-out, right? Maybe tweak the sketch, because I wonder what happens to A, B, and C if you decide to add D. But in general, it is illogical to have a datum feature reference itself.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[pmarc]

PRuggiero,

Keeping in mind that this is only a rough sketch, I would like to ask for clarity if .11 dimension is basic or directly toleranced and - if the second guess is correct - what are the exact tolerance limits for this dimension. I am asking, because if upper or lower tolerance limit (or both) is outside .105-.115 range, there will be a conflict between dimension tolerance and profile tolerance value.

And regardless of your answer to the question above, I do not see any datum self-referencing on the print. But I am with J-P in saying that such self-referencing is illogical (as well as illegal).

 

[PRuggiero]

guys, sorry again I was rushing. You are correct, the profile tolerance should have been A B D and yes the dimensions are all basic. So it sounds like this is not a "good" thing to do and perhaps illegal. Any chance someone has a section in 14.5 that states such a thing? I will be looking today.

Thanks,
Pete

 

[MechNorth]

Pete, there's nothing wrong with it as long as that's your design intent. Something that people tend to forget is that the "feature" is controlled wrt the "datum". In this case, the perimeter of the rectangular cutout is located wrt A/B/D ... datums, not features A/B/D. Datum D is simulated by whatever means in your inspection setup, then the feature is verified wrt that simulated datum.

J-P, I often use total runout controls on datum features, related back to the datum or a compound datum (multiple datum feature). As for inspection, it's common to use surrogate datum features once the actual datums have been established; picture using tooling centers as surrogate datum features after chucking down on two coaxial bearing surfaces. You can then verify the total runout on the bearing surfaces wrt the datum.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Belanger]

Jim, I agree that you can use total runout of a single feature back to a compound datum where that feature is one of the items making up the compound. But it makes no sense to have total runout back to the datum where that feature itself creates the datum (that one's not just illogical, but illegal).

The OP's question never ventures into the "multiple datum features" territory, so I still maintain that there isn't any real logic to doing what was proposed there. I didn't say his suggestion was illegal, but let's just say that referencing D in that case doesn't add any value to the FCF: Wherever the rectangular cutout is, that's where the true datum plane will be. Ergo, no value added to the overall profile tolerance.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[PRuggiero]

appreciate all of the responses. Still curious to whether this is actually illegal per the standard or not. After thinking about it a little more it seems that the only thing having datum D in there does it muddies up the location part of form, orientation, location that the profile controls. Like Belanger says, wherever the slot is, so is datum D. But the slot could be tilted but Datum D will still be orthogonal to the other datums, so it would still control orientation well, right?


thanks again,
pete

 

[Belanger]

I don't know of any paragraph of the standard which explicitly prohibits a feature referencing a datum created from itself. The book says lots of stuff about establishing a tolerance with respect to a "datum reference frame," and that is what Jim was emphasizing. But as mentioned before, it's like saying "the rectangular slot shall be positioned around a datum created by its own width." I see no logic in that (particularly with a tertiary datum) ... because you're always positioned correctly in relation to yourself!

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[MechNorth]

It's not illegal, Pete. For your stated intent of symmetry based on a center location (datum plane), your callout is the correct one.
J-P, your statements suggest that you are measuring a feature back to itself ... no, you measure a feature (which is imperfect) back to the datum (simulator) which is perfect. I have used this method a number of times, without issue. It's easier in a CMM environment, but only marginally more challenging in an open-setup environment as you need to add a surrogate datum simulator.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Belanger]

Jim, I think your first sentence is meant for the other thread about symmetry.

For this thread, of course I realize the difference between a datum, datum feature, datum feature simulator (which is not perfect), etc. And I know that we measure a feature's actual surface back to a theoretically perfect datum. But one of us seems to be missing something ... and I'm not saying it ain't me

But let's consider the graphic given by the OP (with the correction of the tertiary datum reference to be D). If the rectangular cutout deviates to the left or right, then we should agree that the datum plane, which is derived from the related AME of that width (related to A and B), also deviates left or right. Nothing has been gained, right?
Or if the cutout's shape is irregular or skewed in orientation, the datum plane will again be derived from the related AME. The actual surface might wobble in and out of the profile's tolerance zone (making it out of spec), but that is because it fails wrt datum B, not D. Try a few sketches and I hope you'll agree.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[fsincox]

Guys,
I have periodically run into this kind of thing, more than once actually. I would tend to agree that it is not technically illegal.

For example, I worked with an chief inspector who wanted to use the parallelism of the sides of a shaft keyway to control it's location (indirectly) establishing the axis from the shaft dia (primary), aligning by centering on the keyway (itself) and then inspecting the resulting parallelism of the sides in this state. His argument went something like: that it was more important the key sides are parallel in this condition than that the size is right, since the keys are ground to fit. Basically the bad issue was the tapered sides, when forced into position. Can you guys picture this? I actually saw his point.
Note! I am not a machinist/assemblier and do not play one on TV.
Frank

 

[pmarc]

Not going too much into details for the moment I would just like to give a link to Tec-Ease tip about datum self-referencing:

http://www.tec-ease.com/gdt-tips-view.php?q=191

Look at very first two sentences. Isn't this in opposite to Jim's standpoint?

 

[Belanger]

The gist of the Tec-Ease tip is correct, because it is referring to multiple datum features, which is perfectly fine. But there is an error in the tip... the second sentence:

"Datum features referenced in a feature control frame are establishing a datum reference framework which serves as the origin of measurement."

The second word of that sentence should be stricken, so that it reads, "Datums referenced in a feature control frame..."

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[MechNorth]

J-P, yes, I did cross threads, but looking at it, there's still symmetry being controlled (indirectly, granted) by the profile control wrt A/B/D, at least on the two faces drawn parallel to datum-D.
Ok, I've taken a long look at this and mostly my thoughts are as per my original trajectory, but another issue has arisen. There really must be a relationship established between datum D and datum C, otherwise the cutout can be anywhere on the part without restriction; a refinement (actually a second profile control) wrt datums A/B/D would then be more value-added. Given a relationship established between D and C, if the slot shifts left or right, the simulator is still centered at the basic location (not provided, but required) and may end up contacting only one side of the cutout as a result. The datum plane thus does not shift right nor left with the feature.
Sorry if I came across as reminding you about datum vs datum feature vs datum feature simulator; it's just a way of reminding others whom are less fluent in the terminologies and distinctions. Thoughts, J-P?

Pmarc, the Tec-Ease tip is actually supporting my point.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Belanger]

Yes, it makes sense that D needs a relationship to C. That could be done with profile, as you suggest, or with position. And if the only datum reference given is C (or A and C, more likely), then the right side wall might be tilted wrt B, and the slot will tilt (rotate) with it. I follow the logic this far. The next question is about adding a refining profile tolerance that references A-B-D. We need to home in on whether this reference to D adds any value. I'm still inclined to say no; I think a sketch will have to be created on this specific point, though.

However, I'm not following this statement: "Given a relationship established between D and C, if the slot shifts left or right, the simulator is still centered at the basic location (not provided, but required) and may end up contacting only one side of the cutout as a result."

That doesn't sound right to me ... if the datum feature moves to the right, the datum feature simulator will expand upon the high points of that feature at its new location, not the basic distance from C, which is now irrelevant to the callout. To make C relevant, you'd have to have a FCF that references both datums C and D, and that can't be done.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[MechNorth]

The value of controlling the cutout wrt datum D may come in if there is a distinct difference between the value of locating datum feature D wrt to C (position works fine for me too, here) and a refinement wrt to D. If no significant difference, then I wouldn't see much value either, at least not given what we're provided for intent so far.
For the issue of the datum simulator being centered on and expanding from the basic location of datum D, that's only true if datum D is located (i.e. controlled) wrt datum C. A similar situation is shown in the first figure of the Tec-Ease tip linked here (

http://tec-ease.com/gdt-tips-view.php?q=240).

Extending to a rectangular feature of size means that the simulator would grow from MMB toward LMB, centered on the basic location.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Belanger]

I see what the tip is saying, but that in no way carries over to our situation here, because our datum feature C is a surface. It's impossible to reference our C and D in the same FCF, in the manner that the tip shows A and B referenced in the same FCF. It all has to do with degrees of freedom, etc.
But now I at least get what your thinking was

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems