Usage of Datums in Profile of a Line. 1

[Madhu454]

Hi All,

Few months back I saw a tip on profile of a line with datum referred. The drawing uses profile of a line with datum A to control the line elements of the surface , also the line elements are located to the datum A with basic dimension. Since each line elements are located by basic dimension to datum A, the entire surface of the part is controlled. It is as good as using profile of a surface instead of profile of a line. My opinion is to use profile of a surface itself instead of using profile of a line and confusing the people.

From the above explanation we can come to a conclusion that, using the datum’s in Profile of a line to locate the line elements can be avoided. Does anyone have different opinion?
Now using the datum’s in Profile of a line to control the orientation of the line elements wrt datum’s, In such a case instead of using the profile of a line, we can use any of the required orientation controls say parallelism, and use the text EACH ELEMENT beneath the FCF. This method will be straight forward instead of using profile of a line with datums to control the orientation.

I heard that using the datum’s with profile of a line is very rare, is that true? Also I would like to know any such practical example where we must go for profile of a line with datums?

I would like to know more about the usage of datum’s with profile of a line ? Can anyone help me on this.

Madhu

 

[CheckerHater]

I am sorry, but I really don't see connection between presence or absence of datum and Profile requirements used.

It's all Fit, Form and Function as everywhere else. I don' even know where to start.

 

[fsincox]

A drawing is always helpful, but, I believe it is safe to say most measuring equipment requires the set up of a "reference" datum framework wheither they are stated or implied, the statement of datums on a drawing just makes it "formal" and is usually apreciated by inspection, if it is not done so poorly as to make them want to cry.
Frank

 

[fsincox]

or laugh, i guess.
Frank

 

[axym]

I would say that Profile of a Line is the least rigorously defined characteristic in all of Y14.5. From the text and minimal examples given in the standard, it's hard to determine how the tolerance zones actually work or what effect the datum feature references have.

I would suggest specifying Profile of a Surface if possible.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[powerhound]

Profile of a line works well when the profile of a feature is shown from the respective datum using toleranced dimensions instead of basic ones. Of course you achieve something similar using single segment, or even composite, profile of a surface and using basic dimensions to define the true profile.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[CheckerHater]

As Profile of a line only applies in the view where it is specified, it may be beneficial when you have different requirement applied in different directions. Let say your part is an extrusion, you may want to control cross-section more tightly, and allow greater tolerance for bending along the direction of extrusion.

Even then specifying Profile of a surface may be more beneficial to control the twist of a part.

Another application for Profile of a line may be controlling outline of "flat" parts, like glass or sheet metal.

Now, imagine you have to fit piece of glass into the window. Glass itself functionally doesn't have any "preferred" features to serve as datums. But the window opening in your panel has to be located somehow. So having datums is an obvious choice.

What I am trying to say, I've never heard of rules like "line-no datums, surface-datums" somehow implied or as a "good practice". It's all about you and your part - do you see benefits of using datums from functional, manufacturing or inspection standpoint.

 

[axym]

CheckerHater,

One reason that the use of datums with Profile of a Line is often discouraged or avoided is that the meaning is unclear. The only example shown in the standard combines Profile of a Line with a toleranced dimension.

Another reason is that, as Madhu mentioned, the addition of datum references can make Profile of a Line equivalent to Profile of a Surface. In the case of a fully constrained DRF, the set of cross-sectional Profile of a Line zones would be static in 3D space. This is equivalent to a Profile of a Surface zone.

As you say, Profile of a Line is often used to control the outline of flat, thin parts. But one can argue (and I would) that Profile of a Surface should be used in these applications. The outline of a thin part is still a surface, just a thin surface.

The standard mentions the application of Profile of a Line to parts with varying cross section such as tapered aircraft wings, but it does not provide an example. There are many unanswered questions about exactly how the cross-sectional tolerance zones would be defined, how the zones would be allowed to translate and rotate relative to each other, and what effect datum feature references would have. These issues would affect how (if at all) the Profile of a Line tolerance would control things like the bend and twist of the part.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[CheckerHater]

Evan,

I would understand argument like "Profile of a line is hopelessly outdated and is irrelevant considering todays progress in CMM technology", but you should agree "I don't like it because there is only one picture in the standard" is not really a valid argument.

There is only one picture (Fig.2-6) illustrating Rule 1. Could it be a reason Rule 1 is widely ignored?

 

[Belanger]

I think profile of a line can be quite useful, and not necessarily for thin parts (which is often the scenario given).
However, it should probably have at least one datum reference, in order to provide a plane in which to take the imaginary section cuts for the 2-D lines which are to be measured. Otherwise we are limited to thinking of the plane of the view in which it's shown, but if the part's geometry is not perfect there are issues with finding that viewing plane.

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

 

[axym]

CheckerHater,

I would agree that not liking something because there is only one picture of it would not be a valid argument. If my criticism of Profile of a Line came across this way, then I didn't make myself clear. The main problem is that the meaning of the Profile of a Line is not clearly defined or explained in the text. The lack of examples doesn't help.

The text mentions in Section 6.5.4 that Profile of a Line can be used without datums on a continuous extrusion, to control the profile shape cross section by cross section. But no further explanation is given, and no example is shown. It is not clear how the Profile of a Line zones would be applied to the part if it was out of square, bent, or twisted. These are the very conditions that would us make us want to control the profile cross section by cross section as opposed to as a single surface.

The text also mentions that "with profile of a line tolerance, datums may be used under some circumstances" but does not explain how the datums would apply or what those circumstances might be. The one example that is shown uses a toleranced dimension to locate the feature. This is not a preferred practice, as the meaning of the dimension on a bent or twisted part is not clearly defined. This practice was mildly discouraged in Y14.5M-1994 and more strongly discouraged in Y14.5-2009.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[dingy2]

Evan:

Profile of a Line is 2 dimensional (8.2.1 2009 edition) while Profile of a Surface is 3D. Example 8-27 is not the best example since it uses a linear tolerance on the height rather than basic dimensions. Since Profile of a Line is 2 dimensional, then we probably would have basic dimensions from 2 datums to create contour.

I can remember a couple of good applications.

In a stamping operation, manufacturing can control the cut (1/3 thickness) and not the break (2/3 thickness) and use scribed lines for inner and outer boundary on a checking fixture. I think the Profile of a Line is more appropriate here than Profile of a Surface.

Here is another example. I remember one company that made rolls of vinyl automotive door seals and they wanted the contour of the seal controlled. They would slice off a section of the seal and place it on an Optical Comparator and with a template confirm that the contour was within the inner and outer boundaries reflected with a Profile of a Line requirement.

Dave D.

http://www.qmsi.ca

 

[CheckerHater]

Evan, Dave,

I think we can agree that ANSI/ASME is steering away from Profile of a Line rather prematurely.
From simple Radius gauges to Optical comparators there still plenty of applications for using 2-dimensional outline, and standard could dedicate more effort to it.

This forum could be a good place to keep it alive then.

 

[Belanger]

We've tackled this somewhat in other threads, but I don't see why profile with toleranced dimensions to a datum is so confusing. It's perfectly fine!

Profile always controls form (and size, if all around); no datums needed. If datums are added, then a relationship to those datums must be established:
--If the relationship is toleranced dimensions, then profile controls form and orientation (but not location).
--If basic dims are used wrt to the datum(s), then profile controls form, orientation, and location.

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

 

[DeanD3W]

Hello,
I haven't posted in a few months, only due to a project that has consumed all of my time. That project ends on the 31st, so maybe I shouldn't be doing this until the day after I suppose. It seems like a good time to check in again though, since the very same issue that I left un-responded to has come up again...

1) I think we can eventually make it clear that combining profile of any kind and directly toleranced (+/-) dimensions on a given feature is not well enough defined in any standard to enable it to be a useful or recommended approach... That will take some discussion, so my hope is that someone who thinks this practice is OK can reply, to get us going on this. I will agree that in some simple cases the practice will not cause problems, but in general it is not a good practice to combine profile & +/-.

2) The orientation of the cross sections to which profile of a line should apply is not well defined... The orientation needs to be specified wrt to a datum reference frame and even the 2009 standard does not provide a good method for this.

3) Profile of a Line is not just a 2D control. The tolerance zone for profile is always normal to the surface, so if a surface has any slope or curvature from one cross-section to the next, then the profile of a line tolerance zone must tilt out of the plane of the cross-section in order to remain normal to the surface. This makes the tolerance zone better described as a 3D "thing", and in my opinion should restrict the application of profile of a line, if it is useful for anything, to features that can be formed by linear extrusion.

4) Profile of a line cannot detect steps in a feature, from one cross-section to the next.

5) If profile of a line is applied with datum features referenced that constrain sufficient degrees of freedom, then the constrained profile of a line tolerance zones will stack right into the equivalent of profile of a surface.

These issues are all related to the fact that I don't remember ever seeing a need for profile of a line that wasn't better served by using some other approach.

Merry Christmas to all, and I will hope to join this very good discussion group more regularly after the 31st, so happy New Year too.

Dean

http://www.d3w-engineering.com

 

[ptruitt]

The attached illustration seems as though it would be valid. But I wonder if the answer would be any different or useful in a universe described using the theory of relativity or the universe described by quantum mechanics.

Peter Truitt
Minnesota

 

[DeanD3W]

Peter,
I don't think the illustration you attached is valid... Profile of a Line is described as applying at each cross-section of a part. What cross-sections of the complex ridge in the illustration would be used? Y14.5 doesn't accommodate the intersection of two surfaces (an edge) as a feature.

It looks to me like profile of a surface should be applied to the surfaces on each side of the ridge. Since the basic dimensions on such a complex part would be difficult to depict, I would expect to see a table of points with X, Y, Z, i, j, k information for points on each surface (which can be applied directly in a CMM program), or instead a note to refer to the 3D CAD body for basic dimension information.

Dean

 

[ptruitt]

Dean,

I thought that 'Profile of a Line' was a valid 3-D concept, but I looked in the Standard and my textbook and now entirely agree with you: It is 2-D. And I also agree that 'Profile of a Surface' would be a better approach. But it seems that 'Profile of a Line' could be a valid 3-D concept, theoretically, even in this relativity-quantum universe, if the Y14.5 commitee defined it as such.

- Peter

Peter Truitt
Minnesota

 

[DeanD3W]

Hi Peter,
Profile of a line is already a 3D "thing"... Since the tolerance zone must be normal to the feature's surface, it must therefore tilt out of the cross-sectional plane for any feature that is tapered or tilted, or in other words, one that has a cross sectional shape or "elevation" change from one cross section to the next... So, I say profile of a line has a 3D tolerance zone since it does not lie all in one plane for all features.

I think profile of a line applied to a linear extrusion is purely 2D... I also think this type of feature is the only one that profile of a line should possibly be applied to. I still have reservations about this, since profile of a line cannot detect a step in the shape from one cross-section to the next. Profile of a surface applied on a "unit basis" (incrementally), with a small increment can detect a step in the feature and will otherwise have almost the same effect as profile of a line.

Dean

 

[Belanger]

Dean ... I see what you're saying, but I'd hesitate to call profile of a line a 3-D tolerance.

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