Centerlines as Datums 4

[ozzy1]

I am fighting with some of my engineers that insist on using a centerline as a datum. I see nothing in gd&t that allows this. Am I missing something?

 

[Belanger]

Well, first we need to distinguish the terms. A datum is a perfect plane, axis, or point. So on that count, yes a centerline can be a datum. However, on a drawing that centerline should never be identified with the datum feature symbol (the triangle with the letter) because a centerline is imaginary, and we can't touch it. There can also be problems with clarity: sometimes several features share the same centerline! (See the attached graphic for the good and bad examples.)

The other term is "datum feature," which is the actual physical feature that is grabbed in order to derive that perfect plane, axis. or point. This is something we can touch, and this is what should be tagged with the datum letter on the actual print.

So I imagine that is what your colleagues are referring to and in that case they're right. But if you have the correct definitions in mind, it's OK to say verbally that a centerline is a datum. See the ASME Y14.5 standard paragraphs 1.3.13 through 1.3.16 and then especially 3.3.2. (Or if you're looking at the older 1994 edition, it's paragraphs 1.3.3, 1.3.4, and 3.3.2.)

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

 

[Belanger]

Sorry -- in my last paragraph I meant to imply that I agree with you, not your colleagues. To sum up, a datum can indeed be a centerline, but it is never to be labeled as such on a print. That's the sticking point in your debate.

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

 

[ozzy1]

As head of QC mechanical I take the position as explainrd in your first para. A centerline is imaginary and that they should specify the centerline of a feature and then associate other features. Or add in the notes that "all diameters on a common centerline should be concentric within ......."

In your second para. you talk about a "datum feature" and then you say that this is what my colleagues are probably referring to and that in that sense they are right. But I take it that you are still saying that as a centerline you can talk about it but not call it out as a datum on the print. If not than I am confused.

 

[Belanger]

You got it right -- sorry if I created some confusion.

A datum is a perfect thing. But nothing on an actual part is perfect. So what the datum letter is attached to is called the "datum feature" and this is what you grab onto for inspection. You can't grab onto a centerline, so in inspection you never really touch a true datum.

So again, I can verbally say that the centerline of a shaft is a datum for checking runout, but on the drawing the datum symbol (triangle) should be tagged with the diameter of the shaft, not its center. So I meant that if your colleagues are very precise in their terms, they can say that a centerline is to be a datum. But I suspect that your colleagues didn't know the nuances of the term and they were trying to convince you that a datum symbol should hang on the centerline directly.

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

 

[fcsuper]

Simply, a datum must be associated with some real world feature, and that feature should have something to do with the design intent. (Don't create new features just to be datums; yes, I've seen someone try to pull that stunt before.) As Belanger noted, a centerline is imaginary and therefore cannot be used as a basis for a datum. The way to look at datums is that they are constraints on your part. How do you constrain your part with an empty point in space or from inside the material? You cannot.

So a centerline cannot be a datum.To perhaps clarify Belanger's other comment "To sum up, a datum can indeed be a centerline, but it is never to be labeled as such on a print." This is actually not 100% true. A datum is always associated with some real world feature. I believe Belanger's comment refers to the fact that you can use the center of a feature as your datum, but that center is the result of clamping the feature (retangular) or pinning it (hole). Either way, it's the engage of the real world surfaces that creates the center. The center does not magically exist by its own right.

Matt Lorono, CSWP
Lorono's SolidWorks Resources & SolidWorks Legion
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[ozzy1]

Thank you gentlemen you have made my day.

 

[Belanger]

Matt, good comments, but I must defend my statement. The ASME standard defines a datum as a "theoretically exact point, axis, line, plane, or combination thereof..." So a centerline can indeed be a datum! But it is the centerline of the theoretical datum feature simulator (i.e., actual mating envelope), so my statement still stands. You're right that a datum is not a derived median line, but that's not what I meant. I think with these more proper terms invoked that we are then saying the same thing to the OP.

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

 

[TheTick]

Don't let the fact that you are right fool you into believing this is an easy fight.

Round 2: concentricity vs. runout!

 

[fcsuper]

Belanger, yes, 6 one way, 1/2 dozen the other. My concern was that the OP might have interpretted that there is some quesiton about datums.

Tick, don't forget symmetry versus positional.

Matt Lorono, CSWP
Lorono's SolidWorks Resources & SolidWorks Legion
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[DeanD3W]

I'm feeling the need to support J-P here... Datum Features are identified with datum feature labels. Those labels are absolutely not datum labels, many people may say the terms that way that is not at all what the standard says.

Datums are theoretical and can never really be "found" whether they're in the center of a feature or not. As J-P said, datums are found within datum feature simulators, not within datum features. If we're using a CMM and we have not made any physical datum feature simulators then our job is to model the concept of the datum feature simulators so we get a datum reference frame that is oriented and located as closely as possible to that which a physical datum feature simulator set would provide.

A datum feature label may not be applied to an axis, center plane, or center point on a drawing because none of those things are datum features. This doesn't mean that an axis, center plane or center point cannot be a datum... With the datum feature label applied to a cylindrical or spherical feature that datum will be the axis or center point of that feature's datum datum feature simulator. For a slot or its inverse, often oddly called a "width", if the datum feature label is attached to the end of, or side of, dimension line applied to the feature then the datum is the feature's center plane... When there is a desire to label a center line on a drawing that coincides with where a datum will be then Y14.5-2009 includes section 4.21 (page 79) and figures 4-43 through 4-45... That center line can likely be labeled as either the X, Y or Z axis of the datum reference frame.

 

[fcsuper]

Dean,

We aren't contradicting each other. I think you may have simply misunderstand my approach. Someone that doesn't understand how to document a datum may not even fully understand what a datum is. The approach to my comment is to address the lowest common denominator. Once someone understands what a datum is, then they understand why to document it correctly (per the standard).

For those who had no formal training in GD&T, it takes a long time to bring them up to speed on the job, particularly if they've been taught to do it wrong but don't understand why its wrong. Baby steps.

Matt Lorono, CSWP
Lorono's SolidWorks Resources & SolidWorks Legion
Follow me on Twitter

 

[DeanD3W]

Hi Matt,
I understand... We're all trying to help make the picture more clear, but in differing ways. Thanks.

Dean

http://www.d3w-engineering.com

 

[ptruitt]

Would others agree that GD&T is moving away from going in search of virtual centerlines and moving towards actual surface profiles?

Peter Truitt

 

[Belanger]

I think that the "boundary" interpretation has always been preferred over the "axis" interpretation.

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

 

[drawoh]

(
Would others agree that GD&T is moving away from going in search of virtual centerlines and moving towards actual surface profiles?

Was ASME Y14.5 ever there? Read the chapter on establishing datums. It is practical, requiring real features to establish datums.

JHG

 

[fcsuper]

drawoh, your comment represents my approach to this conversation.

Matt Lorono, CSWP
Lorono's SolidWorks Resources & SolidWorks Legion
Follow me on Twitter

 

[drawoh]

fcsuper,

I agree that we all are agreeing. The problem is that datums are an obvious source of confusion about GD&T. You can misinterpret them, and either fail to produce good drawings, or fail to understand someone else's drawings.

ASME Y14.5 shows a set of planes that represent theoretical datums, but in reality, you apply your datums to features. There is no point in describing it any other way.

JHG

 

[DeanD3W]

When speaking of position tolerances, at MMC or LMC (with an MMC or LMC modifier following the tolerance value), yes I agree that the Y14.5 and Y14.5.1 say that the surface interpretation overrules the axis interpretation... It depends who you talk to whether they will evaluate the axis or the surface for this case... This point alone is possibly the most confusing in all of GD&T. Position tolerances (called "True Position" prior to 1982) were originally focusing all attention on an axis, center plane, or center point... With the consideration that the surface interpretation overrules the "axis" interpretation here's a possible conversation. "So, the axis is what we should evaluate, right?" "Well, maybe not, since the surface interpretation overrules." "Oh, so then if the surface interpretation overrules then why shouldn't we use Profile of a Surface?" "That's because we only want that virtual condition boundary on one side (MMC or LMC) and the other is specified only with a local size requirement." "So, why don't they provide a way to just apply profile with only one boundary, since it's the tolerance that leaves no confusion that it controls a surface and not an axis?" "Uh, ..."

My opinion is that it's bad to have two interpretations of a single tolerance. The language could have been written a different way to avoid this point of confusion and extra work/discussion. I do think we would all be better off if some changes were made, but of course we have to work with the standard as it is.

The original post was regarding center planes, axes, or center points as datums though... Regarding these elements, their is no movement towards or away from their use as datums. In every case though, it is not really the center plane, axis, or center point of the datum feature that is the datum... The datum is the center plane, axis, or center point of the datum feature simulator. If no physical simulator is being used then the concept of a simulator should be modeled as the DRF is established, at least to a high enough level of precision that the error is not significant relative to the tolerance values involved.

Datum features are never more than datum features. Datums are always theoretical things that exist only within a datum feature simulator. I'm not saying that datum reference frames cannot be established by probing datum features directly... When this is the approach though, a lot of knowledge is needed, and care should be taken to model the result a datum feature simulator would provide. That might mean looking for residual values on the datum features that would be inside the material of a real simulator set, then adjusting the origin location or DRF orientation to come closer to a "correct" datum reference frame. All of this is why it's sometimes harder to measure a part (correctly) than it is to make the part .

Dean

http://www.d3w-engineering.com

 

[ptruitt]

Thanks, Dean. I might hope for a simple answer, but I think your complex one is probably the pill that needs to be swallowed. (And swallowing it may be easier than trying to do without GD&T.)

Peter Truitt
Minnesota