Circular runout control; understanding potential surface effects

All are true.   

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

Thank you for commenting John Paul

Much appreciated

~dtm~

I may be having a "late Friday" moment, but if the runout control exceeds the size control, then are you not violating the limits of size?  

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

Jim, I don't think any of the 3 questions implied that the size of the part itself can ever exceed the maximum allowed by the size tolerance.

I suspect you are thinking about question #1, but that merely asked if the runout tol (for example, 0.6 in the FCF) can be larger than the size tol (which might be ±0.1).  That is allowed; it just means that the circularity aspect of the diameter will be caught by the ±0.1, and the runout's job then consists of only orientation and location.

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

John-Paul, your interpretation of my question is exactly what I intended to ask.

Mech North touches on why I asked these questions. In the past I have been lead to believe that the runout control tolerance must be less in value than the size tolerance for the feature it controlled. (and instructed to apply tolerances less than the size tolerance)

I haven't used this callout much in the past. Now with my current employer, the circular runout is the default control in this company's general notes.

I reviewed the control in ASME Y14.5m 1994 and concluded what my questions posed as true. Noteably that the circular runout control is in addition to a feature's size limit; not a refinement of a cylindrical feature's MMC.

The main problem that I have been concerned with is that the circular runout is not only used widely here on virtually every drawing, but that it is the only control used for sealing features. Typically the sealing feature in these products are o-rings. I have tried to point out that circular runout does not control the sealing surface in a way that IMO is sufficient.


 

Think of a run-out as sum of your out-of round and out-of-center combined. (This is not entirely thru, but helpful)

When you tighten-up runout, you tighten-up roundness and position together, as they have to split the amount of run-out tolerance between each other (and then some, but I won't go there right now)

O-ring grooves are often tight crumped spaces and run-out usually is both economical and powerful enough to control them.

Tks J-P.  I'm so used to using restrictive runout controls that I tend to forget that the tigher of the two controls (size or runout) restricts the form of the feature.  For some reason I keep slipping into the mindset that runout behaves like straightness of a derived median line in that it overrides Rule #1, which is not the case for runout.

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

This brings up an interesting point about how the ASME standard sort of explains runout differently than all the others.  It's almost as if they ran out of gas near the end of the book!  The runout pictures never show size dims (which would have helped answer this question) and the descriptions are all based on the inspection method (TIR, oops I mean FIM), whereas the other chapters have detailed pictures of the imaginary tolerance zones. Just an idle thought...

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

Not so idle, J-P.  Been raised many times, including in-committee.  Disappointed to see it's still done this way with no improvement in documentation.

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

dtmbiz - this is off the subject of runout but addresses your primary concern which is the adaquate specifying of o-ring sealing surfaces.

The other vitally important control for o-ring sealing surfaces is surface texture.  You need to specify lay as well as the roughness parameters.  The measurement is perpendicular to the direction of lay.  So if you have a revolved groove with circular lay the roughness trace is parallel to the axis.  This will prevent steps or grooves in the sealing surface.  The only problem is when you get to really small o-rings.  Then the tracing length available forces you to use short cutoff lengths that will filter out the steps.   

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.
 

J-P,

The runout tolerances are still explained in terms of readings on an indicator, instead of in terms of tolerance zones.  But I don't think it's because they ran out of gas near the end of the book (I know you're kidding anyway).  I think it's just because the indicator explanation is so straightforward, and tolerance zone explanations for runout get very complex.  The indicator definitions were considered by many people to be not broke, so they weren't fixed.

But the philosophical disadvantages of an inspection-based definition have been recognized, and there is a desire to make the definitions for the runout tolerances consistent with those of other characteristics.  A while back, a few of us from various Y14 subcommittees informally looked at converting the indicator-based runout definitions into corresponding tolerance zone definitions.  I did some of the figures and believe me, it gets complicated.  I had to model the surface-indicator interactions in CAD in order to puzzle out what the resulting zones would be and how they would have to behave.  But I will say that I learned a quite lot about GD&T from studying this.

I'm cautiously optimistic that the next revision of Y14.5 will have tolerance zone definitions for the runout tolerances.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

Yes, Evan.  I agree that the inspection-based explanations are the easiest.  So I guess I wasn't complaining, but just noticing a paradigm shift or whatever you want to call it in how the approach to the symbols changes when we get to runout.

Another thing is that when we do tolerance stacks, most of us (at least this is how I teach it) will treat the runout tolerance as residing around the axis.  It makes the stack easier by just thinking of its error as being entirely due to location.  But the actual tolerance zone resides around the circumference.

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

Think of it this way - Circularity has simple "proper" definition.

And then there is 27-pages book - ASME B89.3.1 - that explains how exactly to measure it.

You decide which is worse. I will take inspection-based definition anytime.
 

You'll take the 27-page book over the simple ASME handling of circularity?

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

I'm not sure I understand the Circularity comment either. The Y14.5 definition of the tolerance zone is relatively straightforward, but evaluating Circularity in a repeatable way on an actual feature can be very difficult. This is especially true if the feature is warped or bent.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

Belanger, axym,

You both get it right.

When you have "simple" definition of control that is difficult / impossible to gauge / measure, why do you consider it valuable?
What is so "simple" about it?

Unless the part in question is extremely expensive / important, I will take run-out over concentricity or roundness thank you very much.

 

CH,

Ok, I think I see your point.

There are several Y14.5 characteristics that are relatively simple to define and specify, but are very difficult to measure repeatably and cannot be gaged.  Circularity is one of them.  Surface line Straightness, derived median line Straightness RFS, Concentricity, Symmetry, and Profile of a Line are others.

The problem that we have is that giving more tolerance usually makes the inspection more complicated.  The comparison of Circularity and Circular Runout is a good example.  Circularity is less restrictive than Circular Runout, but is more difficult to inspect.  The lack of a datum axis means that the location (sorry) of each Circularity zone can be individually optimized.

So if the real requirement is just for the cross sections to be circular, what should the designer specify?  Should they specify Circularity in order to give the most tolerance?  Or should they specify Circular Runout so that the inspection will be straightforward?

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

Evan,

This (and your other examples as well) depends primarily on design of the part and secondary on the manufacturing process you anticipate.

Let say, you have round part turned and/or ground in centers.
Checking runout to same centers will give you (almost) as good results as roundness.

Sometimes difference between "best" and "good enough" gives you huge advantage.

This doesn't mean we have to abandon controls we don't like.
 

Taking advantage of the fact that the circular runout is debated here I'd like to ask 2 questions related to fig. 9-2 of Y14.5-2009:

1. When applied to a cylindrical feature, circular runout controls each cross-section's circularity and axis offset. But when applied to a conical feature, circularity in its standard meaning (according to paragraph 5.4.3 and right picture in fig. 5-10) is not controlled because the measurement is not taken in a plane perpendicular to feature's axis. Is that right?

2. According to para. 9.4.1 when verifying circular runout, the indicator is fixed in a position normal to the toleranced feature. Now let's assume that conical feature in fig. 9-2 consists of 2 segments where one is at an actual angle 43° to datum axis while the other is at actual 47°. Does this mean that indicator has to be oriented at right angle to the surface individually for each segment of the cone?

Thanks for a feedback.    

I don't think the premise you give in the first question is true.  I would say that both runouts always control circularity, unless:
the size tolerance on the diameter is tighter
it is applied to a feature perpendicular to the datum (such as an end face)
if you try to do something goofy like apply runout to a non-circumferential circle, such as tips of gear teeth

But on a cone, I think circular runout would control circularity. The dial indicator might be on an angle, but the rotation about the datum still creates a plane that is perpendicular to the axis of rotation.   (Since we're talking about a theoretical ring, it has no width.)

I have to think more about your 2nd question.  But taking the words at face value, I would agree that the dial indicator has to be readjusted when it goes to a part of the cone which has a different angle.  Ugh ... that might be a can of worms depending on how many rings you want to check on that cone.

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

To pmarc:

I think you have to set your indicator to 45˚, or whatever the blueprint sais.
Just to be consistent with general rule - you build your fixture to basic dimensions within gage-makers tolerances.
 

CH,

I agree that checking the Circular Runout to the same centers that the part was turned/ground on will generally be a very good approximation of Circularity.

In theory, the characteristics and tolerances should be based on the final part requirements and not influenced by how the part is made.  In practice, overall costs can often be lowered by specifying a process-related requirement that is more restrictive but is easier to verify.  This is one of the eternal struggles of GD&T application, and different people have different philosophies on it.

pmarc,

I don't have Y14.5-2009 in front of me, but I can briefly comment on your questions.  You're opening two different cans of worms here!

1. I believe you are right.  For a conical feature, the cross-sectional elements that Circular Runout would control would be different from those that Circularity would control.  This difference is generally insignificant for gently tapered cones, and would be much more significant for very steeply tapered cones.  To me, the meaning of Circularity becomes dubious for steeply tapered cones.

2. This is a nasty issue, and the answer (if there is one) has far-reaching effects on other tolerances that control line elements (Straightness, Circularity, Perpendicularity of line elements, etc.).  It boils down to whether the indicator (and thus the tolerance zone for that element) must remain normal to the as-designed toleranced feature, or whether it can be oriented to the as-produced toleranced feature.  There is still debate over this, but the general opinion I've seen is that the indicator must remain normal to the as-designed toleranced feature.  So whatever the basic angle of the cone is (was it 45?), the indicator must remain normal to that cone.  There are practical difficulties that result from this, and I'm sure you will have further questions and comments.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

Evan, CH,
Just for clarity:
The angle of a cone in fig. 9-2 is not basic - it is directly toleranced 45˚+/-2˚.   

Right, pmarc.  If the intent were to hold the dial indicator at the desired angle, then we'd be required to show it as a basic angle.  Thus, the can of worms...

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

pmarc,

I was afraid you would say that.  The issue of how to treat directly toleranced angles has not been resolved either.  Its another clash between the simplistic, non-rigorous plus/minus world and the complex, rigorous GD&T world.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

Guys,
I had a hidden reason in raising these two issues. In my opinion the root cause of ambiguity here is an unclear Y14.5's explanation of how both circular and total runouts must be verified.

Let's take another example with total runout applied to a cylindrical surface as shown on fig 9-3. I think we all agree this is very typical case shown in almost every GD&T book. The problem (or I should say simplification) of this figure is that it presents a cylinder with perfectly straight surface line elements that are parallel to datum axis A. In such case an indicator can really slide along the surface staying perpendicular to datum axis and normal to the probed surface at the same time.

But what if the actual line elements of surface were like sine. According to my understanding of the requirement stated in last sentence of para. 9.4.2, the indicator would always have to be normal to the toleranced surface, so automatically could not always be at right angle to datum axis. It would not be a case for this particular example if the term "toleranced surface" was undestood as as-designed feature, but it is not written anywhere so is really confusing - at least to me.

And even if the term "toleranced surface" was indeed undestood as as-designed feature, what would happen if we took the example with the cone from fig. 9-2 but instead of specifying 45+/-2 we placed limit tolerance 47/43. How would one know what was as-designed angle and therefore at which angle to set the indicator for runout measurement?

My point is that it would be much more consistent if the requirement for verifying runout of cylindrical surfaces was to always place indicator normal to a datum axis not to a toleranced surface. Then it would go perfectly in line with statements like: "Where applied to surfaces, constructed around a datum axis, total runout may be used to control cumulative variations such as circularity, straightness, coaxiality, angularity, taper, and profile of a surface" (para. 9.4.2.1)

Extending this approach to planar surfaces normal to a datum axis it could be said that indicator must always be parallel to a datum axis, not perpendicular to measured surface. This again would play nicely together with statements like: "When applied to surfaces at right angles to a datum axis, total runout controls cummulative variations of perpendicularity (to detect wobble) and flatness (to detect concavity or convexity)." (para. 9.4.2.2)

I think the approach could work for conical features as well with some additions though. I would not like to talk about it in details now, because I am really interested in your opinions about runout verification in general.

BTW: Happy Thanksgiving Day to All!

I guess I completely agree with all this. Perhaps the standard should say that the indicator is to be normal to the datum axis, thus avoiding issues of surface variation, especially a sine wave.

It would be easy to change the wording in the standard for a cylinder ("normal to the datum axis") or a flat shoulder or end face ("parallel to the datum axis").  But as you say, the problem is for cones: rather than 45 ± 2º, what if Fig. 9-2 had the angle as 44 +3/-1 ?   Should the angle of the indicator be adjusted simply because someone rearranged the nominal value of the angle?

On to some turkey!

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

Imagine a corkscrew: constant diameter, crazy runout.

pmarc,

I see your point, that the explanation might be more clear in terms of the indicator's relationship to the datum axis.  This is true for runout applied to cylinders and planes, because the relationship between the indicator and the datum axis is simple and constant.  But when we get to cones and curved surfaces of revolution (which we must, even though these applications are much less common), the indicator's relationship to the datum axis is not so simple.  The real requirement is that the indicator must be normal to the as-designed geometry.  This applies equally well to cylinders, planar surfaces, cones, and curved surfaces of revolution.  I prefer explanations that are based on the general case, as opposed to special cases with unique properties.  This approach usually makes the complicated things easier but also makes the simple things harder ;^).

Having said that, I would agree with you that the Y14.5 explanation needs work.  It needs to clarify that the "toleranced surface" is the as-designed surface and not the as-produced surface.  It also needs to define what the as-designed surface of a cone is, when the angle is directly toleranced with plus/minus or limit dimensions.  This is a debate in itself, because it is an attempt to impose rigor on decades-old loosely defined practices.

Traditionally, the runout tolerances have been applied to single cylindrical features and single planar features.  Applying the runout tolerances to more complicated features, or groups of features, stirs up all sorts of issues that are generally glossed over in the simpler applications.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

The standard does say that the indicator is to be oriented "normal to the true geometric shape".

I believe this should be interpreted this as the "true geometric counterpart"; which IMO takes away the confusion over the +/- size tolerance and the variations of a surface questions.  I understand this to mean that the indicator is not oriented to the actual surface(s) or surface elments but rather to an axis determined by the true geometric counterpart.  

After the indicator is oriented it will actually touch the surface feature for measurement.

It has been mentioned that basic dims are a better way to dimension cylindrical features when applying a runout control; regardless if the feature has been dimensioned with basic dims or +/- size tolerance (even for a cone) determining the indicator orientation based on the true geometric counter part of the produced feature eliminates that confusion for me.


 

dtmbiz ... the standard says that the indicator is to be normal to "the toleranced surface" (para. 9.4.1 and 9.4.2). I don't see any mention of being normal to the "true geometric shape" (although it would be nice!).

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

As indicated above, runout controls are nice for simple cylindrical surfaces about an axis and for planar faces perpendicular to an axis; beyond that, profile (line and surface) controls should be used.  The recurring problem is that some (too many / most?) people are not adequately trained in profile controls and therefore can't extend their uses to non-documented cases (i.e. within the standard).  It is frustrating because it's like having a 6-speed transmission in a performance vehicle, but never leaving fourth gear.  What's the point of having the performance vehicle?

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

John-Paul,

This is where I find "true geometric shape" in the standard(s);

"At any measuring position, each circular element of
these surfaces must be within the specified runout
tolerance (0.02 full indicator movement) when the
part is rotated 360' about the datum axis with the
indicator fixed in a position normal to the true
geometric shape."

2009 Std fig 9-2 and 9-3 page 181
means this....

1994 std fig 6-47 fig 6-48 page 190
means this...

 

dtmbiz,
Nicely noticed. I think the issue is that proper paragraphs in the text do not say that (9.4.1 & 9.4.2).

This however does not solve the problem in case of directly toleranced angles for conical features.     

Thanks, dtmbiz.  I did a search in the PDF and those words never popped up because they are embedded in the graphic.

But that shows an inconsistency in the language used for runout. We're all OK with the true geometric counterpart stuff until we get to a cone, though...

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

...again...just use Profile and avoid those nasty plus-minuses ...  

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

It would seem to me that a cone would have the indicator positioned to the angle of the cone's true geometric shape. The indicator angle for a plus/minus 2 deg as shown in the example would be the 45 deg in the example shown. The plus/minus 2 deg only controls the size for one of the cone's diameters.

If the cone is dimensioned with 2 diameters, then those diameters control the size and the true geometric  shape driving the angle of the indicator is trig'd out using the 2 diameter's values before applying the tolerances. (e.g.  a cone with a base diameter of 1.0" -/+.02" and a top diameter of .50" -/+.02", with height of 1.0" +.02"; use the 1.0" base dia, the .50" top dia, and the 1.0" hght to trig the indicator angle.)


How does this not follow the standard in reference to the true geometric shape?
 

dtmbiz,

Let's use fig. 9-2 again.
What if the angle of cone was not 45°+/-2° but 46° +1°/-3°? This would create the same limits (function has not been changed) but the angle of the indicator would theoretically be different. So does it mean you would have to change a direction of measurements depending on the way how the angle was expressed? How could one know a direction of runout verification if the angle was expressed e.g. 43°-47°.

You were using symmetrical tolerances in your example and then one can more or less assume at what angle the indicator has to be set up. But that does not have to be always a case and that is why we are saying that for cones the procedure of runout verification is inconsistent.    

pmarc,

Regardless of the angular tolerance whether bilateral, unilateral, or equilateral distribution of the tolerance; looking at this as the true geometric shape, the angle is 46 deg in your latest example. The angular plus/minus tolerance is the limiting size of the cone. If I took those angles and replaced them with high / low limits expressed by equivalent diametrical values (values that are more or less equivalent to the +1°/-3°) the true geometric angle would still be 46°.

A cylinder seems to be easy for most to accept regarding the true geometric shape. We know that the angle of the cylindrical surface is parallel to the cylindrical axis. That is theoretical exact geometry.


In the case of a cone there needs to be the angled surface. It is the theoretical exact surface which includes the angle drawn or modeled. It would be the true geometric shape that the angle indicator is positioned to for runout inspection.

The true geometric counterpart references the actual mating envelope. In the case of an external feature the actual mating envelope is...

... A similar perfect feature counterpart of smallest size that can be circumscribed
about the feature so that it just contacts the surface at the highest points.

 If the produced cylindrical feature has a taper as a result of tolerances, the actual mating envelope is still a "...similar perfect feature...." still a cylinder and not adjusted to be a cone. The actual mating envelope contacts the high points of the actual size cylinder.
 
A cone's surface is not parallel to its axis and therefore the actual mating envelope would include the angle of the surface in the true geometric shape.

Too me, if this is not an adequate interpretation as to how to position the indicator for checking runout on a cone; then what angle would you use for a surface built at a right angle to an axis. It is implied by the standards fundamental rules as 90 deg. If the right angle surface is produced within tolerance yet has a 5 deg taper to it; then what angle is the indicator positioned to for a runout control inspection?



 

Man, this is like peeling an onion!
For one thing, A.6.5 of the appendix seems to say that the term "true geometric counterpart" was nixed in the standard as of 2009. Yet they still use it in para. 4.6.15, and also parenthetically in 3.3.3. (Maybe in some figures too, but my search function only looks at the text.)

I guess you make a good case, dtmbiz, for the angle of the indicator on a cone of 46º +1º/-3º  to be different from that used on a cone of 45º ±1º.  Interesting to see what others think.

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

dtmbiz,

Apologies for this, but I have to repeat my question:
At what angle would you set up the indicator for runout verification if the angular dimension for cone in fig. 9-2 was expressed as 43°-47°?

 

John-Paul,

I like onions dont you? Good source of antioxidants (:

Anyway, the more I exercise my interpretation of the standard with you folks, the more I learn how others interpret...ultimately helping me and hopefully others to understand the concepts and the practical considerations necessary.

I taveled home for the weekend (work out of town) and didnt get the chance to offer my thanks for thanksgiving for this forum and you folks that participate!!

Very very valuable resource and I really appreciate you John-Paul and the rest of you guys and (gals?)that take your valuable time to participate! It's a great place to get various points of view, deeper understanding, correction and affirmation.
Thank you  all very much.



 

pmarc,
Definitely like the question.

45°

Considering the standard's concept to accept as many functional parts as possible, it is most probable that I would be accepting as many parts as possible.

As in my earlier comments I mention what was the angle that was modeled or drawn. Someone chose an angle. What is it? Why? You want me to make the call? It's 45°.

In this case let's say that I had a $50,000 part and it failed inspection at the 45 indicator setting and then I find out with further inspection that it passes with a 43° indicator setting, there is a discussion with the responsible parties involved. The important part is that the part functions as designed and the responsible parties would need to make that decision.  The part is accepted or rejected on that decision.

I believe I see your point and would ask why would limit dimensioning be used in light of the ambiguity that it causes in this instance? Engineering / Designers are supposed to apply the standard to provide design intent.  As you have pointed out with your question, I don't see limit dimensioning capturing that philosophy in this case. I personally limit, limit dimensioning (: Don't particularly like it for this reason. Baseline dimensioning is legal in the standard also; however I am not an advocate for it. IMO it does not capture design intent either. As we know there are always questions surrounding application of the standard, and it does not give us the "best practices" within it.

I find that comes from on the job training....experience.
IMO and learning; ASME Y14.5m is a language to express relationship and function for communication of design intent. It is not an answer to eliminate the design process... particularly the human element. Other communication in various forms will always be needed to support engineering drawings.

Your question can be argued from various points of view. Because those arguments are note worthy, it doesn't stop me from using runout controls on a cone because there can be a question as to what angle the dial indicator should be used to set. If the control is needed then figure out the best way for your organization to communicate "what means what to who", regarding the runout control. Liven up your day and bring it up in your design reviews. (:
 

That is a good example of problems with limit-based tolerances.  As written, there is no guidance as to the preferred value unless there is a note such as "UNLESS OTHERWISE NOTED, GEOMETRIES MODELLED AT MEDIAN NOMINAL".   

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

Jim,

If the conical feature with the directly toleranced angle had a Surface Profile characteristic, would this be legal in your opinion?  Would it make any difference if it was a plus/minus tolerance or a limit tolerance?  

I'm thinking that your answer to both questions is no, but I'm not 100% sure.

Also, what's your opinion on the use of profile tolerances in conjunction with plus/minus tolerances in other contexts?  Y14.5 has several figures that show this and they have always been a pet peeve of mine, particularly the one in which Surface Profile is combined with a plus/minus size dimension to control "conicity".  What do you think of those?

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

axym,

Not sure of your proposed scenario. I am working to the 1994 standard. It says that a profile tolerance is a uniform boundary and if you have a plus/minus angle dimension that has a "pie" shaped tolerance zone, then I cant see how that could work at all.

Fig 6-18 shows a plus/minus dimension that appears to be the min/max height of the profile. The profile tolerance is within in that range. It still has a uniform boundary.

6.5 of the standard states...

..... With profile tolerancing, the true profile
may be defined by basic radii, basic angular dimensions,
basic coordinate dimensions, basic size dimensions,
undimensioned drawings, or formulas....

According to this the profile always needs to be defined with basic dimensions. I dont see how your scenario could be "legal".


 

Evan, I'll let Jim chime in, but recall how much buzz that caused in a past thread a few months ago.  I used to be pretty vehement that a ± tol on a cone is never allowed if profile was to be used (because profile must be used on true profile with basic dims).   But the discussion brought to light that this is sometimes possible -- ONLY in the case of a cone and if no longitudinal datum is given -- because the cone could be a perfect size and shape, but if it shifts left/right then the diameter at any given point would change.

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

There was a statement that I recall in '94 (sorry, don't recall where exactly) indicating that when profile is applied to a feature whose size is controlled with a limit based dimension, the profile can control location, orientation and form, but not size.  So, in the case of a cone, the profile control wouldn't limit the size but would effectively control the runout (circular and total) and "conicity" of the cone.  I don't see any difference between +/- and limit tolerances in this situation.  On the otherhand, I don't use nor advocate the use of all "options" provided in the standard.  When I teach, I explain the issue with wedge-shaped tolerance zones to my students and they balk at the concept and readily step away from +/- angular tolerances.  I also teach them that optics & acoustics are probably the only two situations that I use +/- angles.  The better way, in most cases, is to use basic dimensions and profile.
As for the tolerance zone description of uniform or wedge shaped, consider this; the profile control would be uniformly offset from the wedge shaped zone established by the +/- angular tolerance, resulting in another wedge shaped zone.

One other point; when using +/- angular tolerances, you need to us a dimension origin symbol or other indication to establish where the inflection point of the tolerance zone is.  A minor, but critical addition to the drawing.

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com