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Usage of Datums in Profile of a Line.

Usage of Datums in Profile of a Line.

Usage of Datums in Profile of a Line.

(OP)
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
 

RE: Usage of Datums in Profile of a Line.


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.
 

RE: Usage of Datums in Profile of a Line.

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

RE: Usage of Datums in Profile of a Line.

or laugh, i guess.
Frank

RE: Usage of Datums in Profile of a Line.

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.
www.axymetrix.ca

RE: Usage of Datums in Profile of a Line.

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

RE: Usage of Datums in Profile of a Line.


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.
 

RE: Usage of Datums in Profile of a Line.

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.
www.axymetrix.ca

RE: Usage of Datums in Profile of a Line.

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?

RE: Usage of Datums in Profile of a Line.

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

RE: Usage of Datums in Profile of a Line.

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.
www.axymetrix.ca

RE: Usage of Datums in Profile of a Line.

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.
www.qmsi.ca

RE: Usage of Datums in Profile of a Line.

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.

RE: Usage of Datums in Profile of a Line.

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

RE: Usage of Datums in Profile of a Line.

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
www.d3w-engineering.com
 

RE: Usage of Datums in Profile of a Line.

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

RE: Usage of Datums in Profile of a Line.

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

RE: Usage of Datums in Profile of a Line.

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
 

RE: Usage of Datums in Profile of a Line.

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

RE: Usage of Datums in Profile of a Line.

John-Paul,
It sounds like we will agree that profile of a line is not a purely 2D tolerance if applied to features other than linear extrusions.  

When profile of a line comes up in a discussion I just like to point out that the tolerance zone is required to be normal to the surface, so for any "tapered" features the tolerance zone must tilt out of the plane of a given cross-section.  If we restricted its application to linear extrusion type of features then it would be a 2D spec and I would agree that it could be viewed as a simple way to specify the size or shape of cross-sections of a feature.

We need to extend some of the coordinate system methods introduced in 2009 to allow explicit specification of the orientation of the cross-sectional planes associated with a profile of a line spec.

I would be fine with deleting Concentricity and Symmetry.  Profile of a line may be OK though, if we can just improve its definition a bit.

Dean
www.d3w-engineering.com

RE: Usage of Datums in Profile of a Line.

My concern was that if we say profile of a line is 3-D, then that implies that it controls a feature in the depth direction; it does not.  

I guess it makes sense to be concerned with the orientation of the cross-sectional planes associated with a profile of a line spec.   But we already have a simple solution for that -- datum references.

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

RE: Usage of Datums in Profile of a Line.

Wait ... I'm still thinking.   Are you trying to orient the profile of a line zone to tilt as we move back along the surface?  (i.e., trying to stay normal to a horizon that's curving away.)

I guess I'm having trouble because of the statement "the tolerance zone is required to be normal to the surface."  I don't quite agree with that.  A couple of weeks ago there was a thread where we debated how troublesome that concept was for runout!
http://eng-tips.com/viewthread.cfm?qid=310831

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

RE: Usage of Datums in Profile of a Line.

J-P, Dean,

Just for clarity, per 8.2.1.2 of Y14.5-2009 tolerance zone for profile of a line "is normal to the true profile of the feature at each line element", which in my opinion does not equal to the statement: "the tolerance zone is required to be normal to the surface."

Unfortunately, Y14.5-2009 does not give any example of profile of a line application for features other than linear extrusions, but there is an appropriate example in '94 edition (fig. 6-18). Profile of line control is applied to a view showing true curvature of a surface and the figure shows how the profile tolerance zone looks like. The zone does not tilt at each cross-section, it is normal to the line element and is oriented and located relative to datum reference frame.

RE: Usage of Datums in Profile of a Line.

Belanger,
I think you are reading what is not written there.
The tolerance zone still "flat" and 2-dimensional, the tolerance is MEASURED  normal to the surface, while staying inside of the "flat" plane.
 

RE: Usage of Datums in Profile of a Line.

"I would be fine with deleting Concentricity and Symmetry."

How did that get in there?
I would not.
Frank

RE: Usage of Datums in Profile of a Line.

CH -- right, it's a 2-D zone, not 3-D. Thanks for spelling it out better than I was doing.   I think Pmarc also reinforced my issue with the "normal to the surface" statement.  In my mind the actual surface can do all sorts of crazy things that weren't intended by the designer, so the phrase "normal to the true profile" is the key.

But Dean might still be on to something regarding a continuously curving surface.  Think of a bubble or saddle shape to which we want to apply profile of a line.  Datum references will tell us how to "slice" the part to check cross-sections, but I wonder if we're again crossing into the territory that was previously discussed with runout and the angle of its dial indicator.

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

RE: Usage of Datums in Profile of a Line.

Since we agreed about 2D, I would think about something like CMM probe being oriented to X, Y, Z, rather than "wiggling" the indicator around.
Also, maybe Profile of a surface could be better for heavily curved part? This is what I started with - it's all about the part and what you want to do with it, rather than some formal rule.

RE: Usage of Datums in Profile of a Line.

Dean,

I agree with pretty much everything you stated.  Here's a question though.  If Profile of a Line was applied to a nominally straight continuous extrusion, with no datum features, would the tolerance zones follow the as-produced bend and twist of the part?  I agree that the zones would be 2D slices that would "start out" being oriented to each other, but I'm wondering how they can adapt to the as-produced geometry.  Adapting to twist would requre the zones to "roll" relative to each other but remain parallel, like a deck of cards.  Adapting to bend would require the zones to rotate out of parallel with each other.  I realize that the standard doesn't address this but I wondered what your thoughts were.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Usage of Datums in Profile of a Line.

John-Paul - Please tell me what you think of the three alternative profile of a line call-outs applied in the attached sketch (incomplete-by-intent drawing)...  What is the orientation of the cross-sections for profile of a line for each?

pmarc - You are quite correct that "normal to the true profile" is the better wording...  "Normal to the surface" was sloppy wording on my part...  Apologies to all for that mistake.

CheckerHater - Unless the profile of a line is applied to a linear extrusion, the tolerance zone does not lie in a plane...  It tilts out of the plane as needed in order to be normal to the surface.  There is no other way for the tolerance zone to be normal to the true profile for a tapered or compound curved surface.  I think we will agree that for a linear extrusion profile of a line is OK, but for other features profile of a surface is a better choice (I don't want to deal with a tolerance zone that tilts out of the cross-sectional plane either).

fsincox - Admittedly, the deletion of Concentricity and Symmetry would be a discussion for another thread...  I do have what I think are good reasons for saying this.

axym - The issue you're pointing out is very real and at least for me, makes by brain hurt a bit smile.  If a linear extrusion is a bit wiggly (a "wiggly extrusion", is always the case to some degree I suppose) then I can't say explicitly what the orientation of the cross-sections should be be.  To orient them to the as-produced surface might be a nice ideal, but it's not often that the work needed to do that will be considered justified.  Some sort of median surface is all that I am thinking of that the cross-sections could be oriented to, for more complex feature shapes...  I think I would prefer to limit the application of profile of a line to linear extrusions only, then ignore any wiggliness of the extrusion...  Clamp them is a vice as best you can, cut them, then measure with a vision system with a dxf overlay...  If if the degree of part imperfection is great enough relative to the tolerance value then maybe an uncertainty calculation and guard banding should be implemented..?  If a cmm instead of a vision system is used for measurement then I would still want to orient the cross-sections for the data to a simple "vice" method.  Too crude for Evan's sensibilities/sensitivities? profile smiley.

Dean
www.d3w-engineering.com


 

RE: Usage of Datums in Profile of a Line.

Dean,

Allow me to disagree.
Per Y14.5-2009 Para 8.2.1.2 "Each line element tolerance zone established by the profile of a line tolerance requirement is two-dimensional (an area) and the tolerance zone is normal to the true profile of the feature at each line element."

It clearly sais "two-dimensional", which is "flat" - there is no tilting.

And "normal to profile" does not mean "normal to surface".

RE: Usage of Datums in Profile of a Line.

Dean,
The 3rd option is the only adequate one.
In the first case, there is no datum reference.  The standard won't say that this is illegal, but the obvious problem is that the sections to be sampled for the lines we trace are assumed to be in the plane of the view.  OK, but if that front face that we're looking at is tilted slightly away, then what do we do?  Keep the section cuts perpendicular to the floor instead?

The second case has a datum reference which creates a single point.  This also doesn't help us nail down the orientation of the planes that we wish to sample.

The third case controls all degrees of freedom, and now we know exactly how to sample the sections for profile of a line.  I'm not saying it's required to have datums for all DOF. A single datum reference of the front or back face (the surfaces that intersect the hole) would have been fine.

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

RE: Usage of Datums in Profile of a Line.

Regarding the cross sections of an extrusion; I have always interpreted the tolerance zone of profile of a line, without the use of datums, applied to an extrusion, as being oriented perpendicular to the derived centerline of the extrusion. I understand that a 2D cross section would really constitute a point, to which there really is no perpendicular orientation, but it shouldn't be a stretch to imagine what the orientation of the zone should be.

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

RE: Usage of Datums in Profile of a Line.

CheckerHater - Of course we can disagree since the standard does not make this clear enough.  The issue I am pointing out is one the standard should eventually address.  As written now, Y14.5 specifically allows the application of profile of a line to tapered features.  This is a problem since at some point a feature's taper can be great enough to cause a significant practical issue.  I don't think you will want to apply profile of a line to a feature which tilts at 89.99 degrees from a cross sectional plane...  Even tilting at 80 degrees is likely unworkable, if the tolerance zone is treated as being in the plane of the cross section.  For features other than a linear extrusion, as the angle of tilt relative to the cross sections grows, at some point there would be a significant problem since small form errors will cause very large variation at a given cross section.  Just what is the limit to the angle of tilt that would make profile of a line a poor choice?  If I have a better alternative then I won't use a tool that has issues.  This "tilting issue" with profile of a line is the reason that I believe it should be restricted to linear extrusions.  Even if this issue were resolved I still probably wouldn't ever choose to use it since profile of a surface can be applied on a unit basis (incrementally), the same effect can be achieved without the tilting issue, or profile of a line's other issue of the inability to detect a step in the feature from one cross section to the next.

It's not often that profile of a line is a function driven spec anyway.  I think the functional need is usually for profile of a surface.  

John-Paul - If option 3 is the only one adequate one then we shouldn't use profile of a line on this part...  With both A and B referenced all 6 degrees of freedom are constrained, so the profile of a line tolerance zones would stack into an exact equivalent of profile of a surface.  We should just use profile of a surface here.  If we're dealing with simple rectangular blocks then relying on datum features referenced to provide the orientation of cross-sections might seem to work well enough, but for more complex parts, or if we want cross sections that are at an odd angle to any plane of the datum reference frame, then we're left without a good way to say this.  A method needs to be added which clearly and explicitly specifies the orientation of cross-sections, just as we need for line element straightness applied to any ruled surface other than a cylinder.

If profile of a surface tolerance zones are not "location constrainable" then profile of a line would not ever be equivalent to profile of a surface, but I don't think there is anything in the standard that would clearly make this a trait of profile of a line tolerance zones...  They're not described as being any less constrainable than profile of a surface zones as far as I know.  There may be some implication of this, since the examples shown have combined profile of a line with directly toleranced dimensions, but that is only further illustration of my point that profile of a line is not well enough defined in the standard.

Dean
www.d3w-engineering.com
 

RE: Usage of Datums in Profile of a Line.

Dean, I don't think we're that far apart.  You bring up a good point about the difficulty in being normal to a true profile if the surface is a compound curve.  (I've waged that battle in regards to runout.) And I hope you see why I like the datum references for profile of a line, in order to constrain the orientation of the cross-sections.  However, I didn't intend for the distance to the datums to be basic. (Sorry... I shouldn't have said all DOF; I meant just the rotational/orientation degrees.)  If the location of the curve from the datums is kept as ± tolerances, then profile of a line is not equivalent to profile of a surface.

However, it seems that many people seem to think that profile tolerances must always be located to the datums with basic dimensions. That's reading something into the standard that is clearly not there.

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

RE: Usage of Datums in Profile of a Line.

CheckerHater,

The 2D vs. 3D issue is a tricky one.  "Two-dimensional" is not synonymous with flat.  In mathematics and geometry, a curved surface is considered to be two-dimensional even though it does not exist in a two-dimensional plane.  By the same token, a curved line (e.g. a Derived Median Line) is considered to be one-dimensional.

The standard states that "the tolerance zone is normal to the true profile of the feature at each line element".  I would take "true profile" to mean the as-designed surface, and therefore get zones that may tilt out of the cutting plane that defines the line element.  But I can see how you (and many others) would take "true profile" to mean the intersection of the cutting plane and the as-designed surface, and therefore get zones that are all flat and parallel to each other.

So I would agree with Dean that the tolerance zones would in fact tilt relative to the cutting planes, for geometry other than continuous extrusions.

powerhound,

Your interpretation of Profile of a Line for a continuous extrusion makes it behave in a similar way to Circularity, in which the zones must be perpendicular to an arbitrary "spine" (which is a line that isn't necessarily straight).  This is perfectly reasonable and practical, as it lets the cross-sectional tolerance accommodate the as-produced bend of the feature.  We can go the simple and easy route and choose a straight spine, or we can do the extra work and extract one that follows the feature's as-produced centerline as you mentioned. Our problem is that orienting the zones to a curved spine is one of many possible interpretations, and the standard does not provide any guidance.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Usage of Datums in Profile of a Line.


Dean,
I still don't understand what the big deal about taper is.
Unless we are talking about different things; then please specify the scope of the problem: is it impossible for the mankind in general, or is it impossible for the machines you are selling?

Evan,
2-dimensional means "the one that can be described with 2 coordinates" and yes, indeed it is flat. I would greatly appreciate if you direct me to college-level math book that states otherwise.
 

RE: Usage of Datums in Profile of a Line.

Evan, I'm with CH that 2D means flat, but I might be missing something.  I don't understand how a curved line (think of a drawing on an Etch-a-Sketch) can be considered as only 1D, and a wavy surface with depth can be considered as only 2D.  Then what the heck is 3D?

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

RE: Usage of Datums in Profile of a Line.

Forget the college book, let's get closer to home.

From ASME Y14.5.1M-1994:

"A profile is the outline of an object in a given plane (two-dimensional figure). Profiles are formed by projecting a three-dimensional figure onto a plane or taking cross-sections through the figure."

For anyone resonable, including members of ASME committees, 2-dimensional = PLANE; 3-dimensional figures are projected onto PLANE, to become 2-dimensional.

2-dimensional IS "flat", profile IS NOT surface, one cannot live in denial forever.
 

RE: Usage of Datums in Profile of a Line.

I'm sticking to my guns that 2D is not equivalent to flat.  Part of the problem is that Y14.5 uses certain terms in an inconsistent way, and does not always follow the way these terms are defined in mathematics and geometry.

John-Paul,

A curved line is 1D because it has no thickness or width, only length.  A wavy surface is 2D because is has no thickness, only length and width (and therefore area).  3D entities are solids that have length, width, and thickness (and therefore volume).  I didn't invent this - I'm just the messenger ;^).

CH,

The Wikipedia entry for "surface" has some good statements in the first couple of paragraphs (I realize that this isn't an actual textbook).  Here's a quote that mentions the 2D nature of surfaces:

"In mathematics, specifically in topology, a surface is a two-dimensional topological manifold. The most familiar examples are those that arise as the boundaries of solid objects in ordinary three-dimensional Euclidean space R3 — for example, the surface of a ball."

Armed with this understanding of the proper math concepts and terminology, we can nit-pick the errors in the passage from Y14.5M-1994 that you quoted:

"A profile is the outline of an object in a given plane (two-dimensional figure). Profiles are formed by projecting a three-dimensional figure onto a plane or taking cross-sections through the figure."

An outline of an object in a given plane would be a line, which is a one-dimensional figure.  The figure that is projected onto a plane or has cross sections taken through it would actually be a surface, which is two-dimensional.  So here's a corrected version:

"A profile is the outline of an object in a given plane (one-dimensional figure).  Profiles are formed by projecting a two-dimensional figure onto a plane or taking cross-sections through the figure."

So I agree with you that the statements in Y14.5M-1994, if taken as gospel, would lead a reasonable person to conclude that 2-dimensional is flat.  But I continue to live in denial as you say.  The members of the Y14.5 committee are also reasonable, at least most of the time - right Dean ;^) ?  But they are not mathematicians, and so their application of terms and concepts from math and geometry is not beyond question.  These things are much better dealt with in the Y14.5.1 Mathematical Definitions standard.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Usage of Datums in Profile of a Line.

Not buying it, Evan  :)
A 1D line is just that -- it goes in one dimension only (hopefully we agree on X, Y, and Z as the linear dimensions of space).  So if a line traversing along only the X axis suddenly curves, that means it has deviated into Y or Z (or a little of both).   It's now 2D.  But it still has no thickness.
I'll pose the same question as before: Given the fancy definitions that you want to impose, what is an example of somthing that's 3D?

I'm not saying the topological stuff is wrong. But certain disciplines have certain rigors to their terminology.  And you're trying to blend terminology that might have a place in high-falutin' math into a discipline that deals with real dimensions and tolerances.

It reminds me of Microsoft Word's spell-checker that keeps telling me that the plural of datum isn't datums.  If you're a purist when it comes to grammar/spelling, then yes the plural is "data."   But we know that in the world of GD&T the plural is "datums."   

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

RE: Usage of Datums in Profile of a Line.

I agree that 2D is not necessarily flat because surfaces, in the context of drafting and IGES files, are 2 dimensional. They can follow various complex curvatures but they have no thickness. For the purposes of defining a tolerance zone, I can't see another way to interpret profile of a line other than flat, 2 dimensional, with the cross section oriented with respect to the datum reference frame.

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

RE: Usage of Datums in Profile of a Line.

All right... I'm beginning to see why there's a discrepancy.

I agree that a surface itself is not a 3-dimensional thing.   It has no thickness.   But if that surface wavers at all, it occupies space in a 3-dimensional manner. So both perspectives are correct.

The confusion is that in the world of dimensioning and tolerancing, we are not talking about what the surface is (the ontology of the surface, to borrow a term from philosophy), but we are talking about where the surface lies and what space it takes up.

With this clarification in place, we can say that the standard is still correct to say that a profile of a line tolerance zone is 2-D.

Did I salvage myself?   :)
 

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

RE: Usage of Datums in Profile of a Line.


axym,

First, Wikipedia is rejected as reliable source even in High School - references to Wiki are not accepted.

Second, if you are so fond of Wiki, search for "2-dimensional" instead. You may be surprised.

Third, topology studies objects that have no shape whatsoever, it is called the study of "connectivity and continuity" - the only thing certain about objects is how they relate to each other. Topology is the area of mathematics that has nothing to do with Euclidean geometry, it also has things like "zero-dimensional space" - very useful in GD&T.
Why not refer to fractals – they have 1.5-dimensional objects as well?

Shortly, "2-dimensional" - I don't think it means what you think it means.

RE: Usage of Datums in Profile of a Line.

Guys, no offence, but I think you have just prooved your mastery of the art of splitting hairs. The funny thing is that in my opinion all of you are right - you are just trying to convince each other by looking at the issue from different perspectives and using arguments from different disciplines of science.

Whatever the consensus will be, this will not help in finding an answer to a question: "How should cross-sections for profile of a line tolerance measurements be oriented if there is no datum reference in profile FCF or datum(s) referenced in FCF do(es) not define proper orientation of the measurement plane?".

RE: Usage of Datums in Profile of a Line.

CH,

I thought you might discredit the Wiki reference - fair enough.  It's not that I'm that fond of Wiki, that's just what I happened to find first when I went hunting for things that agreed with my opinion ;^).  I had a look at the Wiki entry for "2-dimensional" and it focuses on 2-dimensional planar space and the geometric figures that exist in it - nothing earth-shattering there.  2D space is indeed flat, but that doesn't mean that all 2D surfaces are flat.

I'm not sure what to say about the references to topology - I'm no expert, but I don't think we should dismiss the content of an article that mentions topology just because topology has some arcane concepts that don't apply to GD&T.

Here's a reference from the Y14 world.  The "dimensionality" of a surface is discussed in Y14.5.1M-1994 in the context of size tolerances, in which a ball is swept along a "spine".  Here's a quote from Section 2.3.1 on page 7:

"A 0-dimensional spine is a point, and applies to spherical features.  A 1-dimensional spine is a simple (non self-intersecting) curve in space, and applies to cylindrical features.  A 2-dimensional spine is a simple (non self-intersecting) surface, and applies to parallel-plane features.  These three types of spines can be more rigorously defined, respectively, as connected regular (in the relative topology) subsets of d-manifolds, for d = 0, 1, and 2."

pmarc,

I realize that we're splitting hairs here.  Do I do anything else on this forum? ;^).  I suppose that I felt that splitting hairs was more justified than usual in this particular debate.  The exact meaning of the term "2-dimensional" determines whether or not a Profile of a Line tolerance zone is restricted to be flat or not.  If Y14.5 is going to use a mathematical term in an ambiguous way, with no other supporting text or figures to clarify the intended meaning, then we have to bring in the proper meaning of the term from mathematics.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Usage of Datums in Profile of a Line.

I think "2D or not 2D..." is just semantics, but still an issue.  The standard should be improved to prevent this issue.

The fact is that the tolerance zones MUST be normal to the true profile (the theoretically exact surface, that is) & therefore will tilt out of the cross-sectional plane for any feature which is tapered from one cross-section to the next, otherwise the result will be erroneous.  Leading me to...

...CheckerHater - The issue with any tapered feature is shown well enough, I hope, in the file I've attached.  If you knew me you would know that any argument I would make about how GD&T should be will have absolutely nothing to do with any type of measurement machine I may be involved in the sales or support of as part of my business, but you don't know me, so maybe that was a fair question to ask smile.

Dean
www.d3w-engineering.com
 

RE: Usage of Datums in Profile of a Line.

Man, it's no wonder this thread has taken on such a life of its own. Dean, why would you try to control that profile like that? It just doesn't make sense to me. Why wouldn't you put the profile callout in the left view? I see now why you think the tolerance zone should be normal to the surface but a datum reference frame is made up of mutually perpendicular planes, thus the tolerance zone in this case would be perpendicular to A, parallel to B. The problem your are having with this seems to be based on a really bad way to control a profile. Not illegal, but bad nonetheless.

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

RE: Usage of Datums in Profile of a Line.

All you're really controlling here is straightness. The straightness tolerance zone would be normal to the surface. I realize that you are only providing an example to illustrate your point. Even though you're showing the implications of maintaining a tolerance zone parallel to datum B, that's the way it is. You can't ignore the DRF just because you don't like what you see. That's why this is a bad way to control this profile. You have to apply the profile callout in the view of the profile being controlled, in this case it's a single straight line. That's also why the J<->K is pretty much meaningless.

Please consider my posts with all due respect. I'm not trying to trash your work. I'm simply disagreeing and telling you why.

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

RE: Usage of Datums in Profile of a Line.

OK, now my head is starting to hurt!    I'm thinking that Dean is on to something, though.

Powerhound, you said that "a datum reference frame is made up of mutually perpendicular planes, thus the tolerance zone in this case would be perpendicular to A, parallel to B."  The first half of that statement is true, but not the second.  Simply having datum references doesn't mean that the tolerance zones will always be perpendicular to those datums.   Plus, the statement in the standard that the tolerance zone must be normal to the true profile nullifies the idea that it will always be normal to the datums.

Overall, the standard could use some work when it comes to profile of a line.  That is evident simply by our discussion on these finer points.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

RE: Usage of Datums in Profile of a Line.

Powerhound - I would never apply profile the way it's shown in the file I attached...  In Y14.5M-1994 Fig 6-18 the cross-sections are cut "parallel to the page" with respect to the view in which the profile of a line is applied...  This is the only reason I applied the profile of a line in the right view as opposed to the left...  To show the issue with tolerance zones that lie in the plane of the cross-sections this seemed like the best approach.  If anyone has a better way to illustrate the problem then I'm all ears & eyes.

I don't expect to ever suggest to anyone that profile of a line is their best approach to controlling anything. This is partly due to the fact that Y14.5 does not define it well enough and partly due to the fact that there is rarely a functional need for a "line control".

The example I attached was purely to address CheckerHater's question about what the big deal was about taper.

Now, on to whether the planes of a datum reference frame should be used to somehow determine the orientation of the cross-sections for profile of a line...  Please tell me what the cross-section orientation would be for the two options shown in the attached file.  My point is going to be that additional specification is needed in order to make the cross-section orientation explicit and clear, just as it is needed to orient the tolerance zones for line element straightness applied to any non-cylindrical part.  Now that we can show X, Y, & Z axes to represent a datum reference frame (per Y14.5-2009), that won't be too difficult, but for now, I think we're missing a necessary element for these specifications.

Dean
www.d3w-engineering.com
 

RE: Usage of Datums in Profile of a Line.

Powerhound - In response to your post about 40 minutes ago...  We could show the same issue with more complex features but if the super-simple, but admittedly not realistic, example illustrates the point well enough then that's all we need.

The only purpose of that example is to illustrate why tolerance zones for tapered features cannot lie in the plane of cross-sections when those cross-sections are not normal to the surface.  I think it does that in a clear enough manner, but as I said above, I'm all ears and eyes if someone has a better way of illustrating the issue.

Dean

RE: Usage of Datums in Profile of a Line.

Dean,
   About your first example: I do get your point but I still see no justification in changing the orientation of the tolerance zone. By your own admission, this is not realistic and the reason it's not realistic is because it's not right to do it this way. It's kind of like saying there's a problem with perpendicularity because you can't use it to control features that are parallel. There's a right way to control parallel features. Creating an example that shows how perpendicularity can't be used to control parallel features doesn't mean I have an argument.
  About your newest example. I don't see an issue with example #1. The zone is perpendicular to A and shaped and located from B by missing dimensions since the drawing is incomplete. Example #2 is the same as before. It creates a problem where none exists. There's a right and a wrong way to control that profile and #2 is the wrong way.

J-P,
  A true profile and a surface are two different things. Being normal to the true profile and normal to a surface is not the same thing. My understanding has always been that the tolerance zone is oriented per the DRF (parallel and perpendicular). The flat, cross section generates a 2 dimensional contour. The tolerance zone is then disposed about that contour WRT to the DRF. When you said "Simply having datum references doesn't mean that the tolerance zones will always be perpendicular to those datums." I didn't mean to imply that every aspect of the zone would be perpendicular to every datum. In the second example that Dean provided, the tolerance zone will curve per the basic dimensions in example #1. In example #2 since the true profile is a rectangle, the tolerance zone is rectangular and thus perpendicular to A, parallel to B.

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

RE: Usage of Datums in Profile of a Line.

PH -- I must have misunderstood your post; sorry.   Sure, the true profile and the actual profile are not the same, and that's part of the difficulty we've been mentioning.  Datum references are there to help orient the tolerance zones, but the zones could be at some crazy angle to the datums, if we have basic angles to establish such (that was my point).  So I think we're on the same page.  

Dean -- I'm not clear on the most recent graphic you posted.  In the RH view you have "between J and K" yet I think of profile of a line as inherently meaning that we are to take the lines in the direction of the given view.
But if you did that on purpose just to get us to think about the difference between the two callouts, then I would say #1 and #2 yield the exact same tolerance zones (they would be parallel to A, not perpendicular).

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

RE: Usage of Datums in Profile of a Line.

John-Paul,
I expect others to have opinions that differ from yours regarding the orientation of the cross-sections for options 1 & 2 in the second file I posted.  I'll let others comment though, maybe there's something I'm not seeing.  The "J <--> K" means the same and is fine to use for both options, and I think the answer is "nobody really knows" regarding the orientation of the cross sections...  The only justification for "knowing" the orientation of the cross sections should require citing words, or at least an example, from Y14.5 that support that opinion.  I don't think the standard puts anyone in a good position with anything in regard to profile of a line, so I think this can only yield unsupportable answers.

Powerhound,
I've already said this, but I'll just try again...  That example is purposely simplistic, just to get only one point across...  Tolerance zones for tapered features must tilt out of the plane of the cross-sections that many people associate with profile of a line in order to get a meaningful measurement result.  The tolerance zone must be normal to the surface along a given line.  If I make the true profile of each of the planar faces that the profile of a line is applied to curvy instead of flat, then the application would look more realistic, but then the simplicity that makes it easy to illustrate the dimensional difference would be lost...  It's not intended to be a realistic example!!!!!!!!!!!!!!!!!

BTW - you say it's like straightness...  How do you know that it's not more like perpendicularity of each line element?  With datum features A and B referenced, one could say that all three rotational degrees of freedom are constrained.  What statement in the standard tells you that orientation of each line element is not controlled?

Does anyone think the current definition of profile of a line in Y14.5 is adequate?  The general point I've been trying to make is that profile of a line should be avoided since there are various interpretations that many have "read into" the inadequate definition in the standard.  The first issue is the orientation of the lines on the considered feature (aka the orientation of the cross-sections).  The second issue is treating the tolerance zones for tapered features as not normal to the true profile (where true profile is a theoretically exact definition of the entire surface, not just a line along it).

Any wording that accommodates an "everything lies in the cross-sectional plane" interpretation of the standard needs to be changed, since that interpretation yields misleading results.   

RE: Usage of Datums in Profile of a Line.

Dean,
  I get that the example is not realistic which is why it's a problem with me. You are arguing a point using an example that no one would ever use in real life, thus my statement that the example creates a problem where there is none.

 Why don't you post a realistic example?

 The tolerance zone is only normal to the surface if you put the profile control in the left view, but that's not because the tolerance zone is supposed to be normal to the surface, it's because it's normal to the true profile. The true profile is NOT the surface and it IS a line along it where the cross section cuts through. Look at Fig. 6-18 in the 1994 standard and read what the example says. That zone is normal to the surface because of the direction the cuts are taken. They are taken across the profile, not along it as in your example.

Point taken on the straightness issue. Change that to perpendicularity.

Again, I fully understand, and have understood, that your example was just that, an example. I just don't think that you can support a real world argument using a non-real world example.

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

RE: Usage of Datums in Profile of a Line.

Quote (axym):

A 2-dimensional spine is a simple (non self-intersecting) surface, and applies to parallel-plane features.

How exactly "parallel-plane features" are not flat?

I am outta here!  

RE: Usage of Datums in Profile of a Line.

CheckerHater,

The sentence was not meant to say that a parallel-plane feature is an example of a 2-dimensional spine.

The term "parallel-plane features" was meant to describe the general feature-of-size type (i.e. slots and slabs) that the 2-dimensional spine applies to.  Strictly speaking, the term should be "nominally parallel-plane feature" since the surfaces of the as-produced part feature will not be perfectly flat and parallel.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Usage of Datums in Profile of a Line.

If we really seriously believed in the ASME principles of: "does not necessarily specify process or inspection method", (cough, cough, RUNOUT!!!). Wouldn't a Profile of a Line be another generic way of specifying a circular runout tolerance, using the toleranced size dimension and all? Maybe it is not my dear concentricity callout that should watch out about being obsoleted, let's get rid of runout. :) (Strategic move here, stop playing defense, take the battle to the other side).  
Frank

RE: Usage of Datums in Profile of a Line.

Frank,

I have better idea.

All tolerance zones have mathematical description, so we can teach computer to calculate them.

This way they can be entered directly into 3D model like in the enclosed file.
We will have precise presentation of where part body can and cannot be.

This way we will eliminate not only Concentricity, Symmetry and Runout, but also ALL of the symbology, ALL of the GD&T, and ALL of the Certified GD&T Professionals.

As the data may be passed directly from $10/hr CAD-monkeys to CMM personnel, there will be no need for middle-man.

So, be careful what you wish for.
 

RE: Usage of Datums in Profile of a Line.

CH,
Nice part. I think that you are right, that is eventually the idea. I just hope it takes them more than 10 years.
Frank
 

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