Referencing Centerline Datums
Referencing Centerline Datums
(OP)
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
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





RE: Referencing Centerline Datums
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
RE: Referencing Centerline Datums
RE: Referencing Centerline Datums
RE: Referencing Centerline Datums
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
RE: Referencing Centerline Datums
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).
RE: Referencing Centerline Datums
Thanks,
Pete
RE: Referencing Centerline Datums
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 www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Referencing Centerline Datums
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
RE: Referencing Centerline Datums
thanks again,
pete
RE: Referencing Centerline Datums
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Referencing Centerline Datums
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 www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Referencing Centerline Datums
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
RE: Referencing Centerline Datums
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
RE: Referencing Centerline Datums
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?
RE: Referencing Centerline Datums
"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
RE: Referencing Centerline Datums
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 www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Referencing Centerline Datums
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
RE: Referencing Centerline Datums
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 www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Referencing Centerline Datums
But now I at least get what your thinking was :)
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Referencing Centerline Datums
1st cent - datum self-referencing
I keep thinking why Jim is trying to covince us that OP's sketch (with D instead of C in profile FCF) makes sense and is legal. And I think I am starting to see a logic behind. Jim, please correct me if I am wrong, but is your interpretation following; datum plane D is derived from cutout width which is in fact a two-opposite-planar-faces feature of size. But the profile callout controls each face independently, so in fact it is not that the FOS is being controlled to itself. Is it what you are trying to say, Jim?
2nd cent - relationship between datum features
I absolutely agree that the relationship between D and C should be somehow specified, and I would most probably go with profile for D since position applied to basic .11 does not look good (although we could argue whether it is legal or not).
Plus, I am again with J-P - this time that Tec-Ease tip is not reflecting the situation we have here and that datum plane D will always go together with datum feature D because nothing else contrains the part's movement in vertical direction during setup for profile callout inspection.
P.S.: I wonder when we come to a conclusion that it would be much easier for everybody if datum D was not here at all and only profile callout wrt C|B|A (or composite profile FCF with lower segment tightening relatioship to B or C) was specified.
RE: Referencing Centerline Datums
C|B|A was in the other thread about controlling symmetry
RE: Referencing Centerline Datums
Too many things are missing from the picture.
For example relationship between datums [C] and [D].
Should it be explicitly specified? Because otherwise General Tolerances (you know, +-5 degrees) will imply; won't that give the feature too much freedom?
RE: Referencing Centerline Datums
In general, I don't like the idea of FCF's in which the considered feature is referenced as a datum feature (or as part of a multiple datum feature). I would say that these applications are generally flawed and are seldom, if ever, functional.
Jim, I know of the runout application you referred to - where the datum features are included in the runout tolerance. There's an example in the standard that shows it - where the datum axis is defined using A-B, and features A and B both have runout tolerances to A-B. I think I've ranted on this one before. I understand that there is nothing impossible going on here, that A and B could be checked back to A-B using a CMM or with some awkward chucking technique. But I don't think that the concept makes sense. A and B just have to have good runout to some axis - it doesn't have to be axis A-B. But there's currently no way to say that in Y14.5.
In the OP's example, I would say that the reference to datum feature D doesn't add any value. Referencing A and B would suffice. The profile zone can float in the X direction parallel to B, and if we can find some location for it that allows the feature to pass then we're good. The profile zone doesn't need to be centered on the feature's oriented AME - this is an unnecessary restriction and is very likely not functionally required.
That said, it is sometimes convenient to add extra restrictions like this when using CMM's. Some software packages do not work well with FCF's that leave degrees of freedom open - they like to have a fully constrained DRF. So it often makes sense to fill in any open DOF's using the considered feature. This gives the CMM software the fully constrained DRF that it needs, without introducing artificial constraints to other features.
But these extra restrictions do not belong on the drawing.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Referencing Centerline Datums
I still don't think any value is added by mentioning D (see the lower picture on my sketch), but maybe I'm off on a weird tangent from what you intended, Jim. Either way, I'm stumped as to why it's ever logical to have a self-reference of any kind.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Referencing Centerline Datums
For the need to locate the simulator at its basic location, pls reference datum C in Fig. 4-18 in '09, in particular the last sentence in the note on the top right. Similar in Fig 4-15, -17. This contrasts with the situation for a datum translation modifier where the simulator gets to move to make the best fit.
J-P, I was thinking more of a similarity with cast & machined datums; a feature is realted to the cast drf and to the finish-machined drf with a refinement.
As for whether or not a datum feature can be controlled wrt the datum established by it, please someone explain why not, as opposed to it just not looking or seeming right. As the most extreme case(?), consider a workpiece with nothing but a general surface profile control (allover). The datum features are thus controlled by the general control, are tney not?
Good discussion(s), gents.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Referencing Centerline Datums
I racked my brain and thought up a scenario in which the self-reference would actually make a difference (see attached sketch). The surface of the cutout is both tilted and asymmetric, but will fit in the profile zone if optimized in the X direction. If the self-reference is added, the zone must be centered on Datum D (the surface's oriented actual mating envelope). With this extra constraint added, the feature no longer conforms. Darn - I just noticed that I drew the simulator lines on the outside of the cutout and they should be on the inside. But you get the idea - just imagine that it's a boss instead of a cutout ;^).
So the self-reference provides a control that is real and definable, and does make a difference in certain cases. But I'm having a hard time envisioning a functional situation that would require this.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Referencing Centerline Datums
RE: Referencing Centerline Datums
That's correct. In the first case, the cutout isn't being controlled in the X direction at all. This would make sense if D was intended to define the X location of other features. For example, feature C might have a profile tolerance to ABD.
I fixed the error in my earlier sketch, and the result changed (see updated sketch). With Datum D established from the correct side of the surfaces, the diagram no longer shows the effect I was trying to show ;^(. The cutout still conforms to the profile zone, even with the constraint to Datum D. I'll have to make a new sketch and cook the surface geometry differently.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Referencing Centerline Datums
Again, I struggle to see an application in which the self-referencing would be functionally necessary.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Referencing Centerline Datums
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Referencing Centerline Datums
Tks, J-P. I don't recall the details of the applications I've used it for; too many clients back to remember all of their applications. A general surface profile is the easiest example that comes to mind for a practical use.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Referencing Centerline Datums
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Referencing Centerline Datums
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca