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# What's your approach for tolerancing concentricity of a polygon, see attached image2

## What's your approach for tolerancing concentricity of a polygon, see attached image

(OP)
I'm new to GD&T. What's your approach of tolerancing a polygon shape.

'Profile of a surface' as in option 1 locks down a lot.

At first I thought it looked down too much so I tried Method 2 using symmetry, which in the end pretty much did the same as profile of a surface.

Then I thought of some dotted inscribed reference circle with concentricity tolerance, along with parallellism tolerance.

Brain is confused to say the least. Suggestions?

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

I would have to state the ubiquitous "Depends on the function of the part" first... Secondly, I would tell you that symmetry is rarely used in my experience, and Profile is much easier to understand, measure, and more easily captures design intent in most cases. Concentricity is interesting... Each of those methods looks theoretically sound, but would depend on function and/or the condition of the mating part. In the industries where I've practiced GD&T (firearms, Power, communications, defense, aerospace), profile of a surface would most likely be the default tolerance to use there.

EDIT: Also! Concentricity and symmetry have been removed from the 2018 standard, so there ya go... Was this a test?

I'm not a vegetarian because I dislike meat... I'm a vegetarian because I HATE PLANTS!!

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

Perka,

Here is a good discussion of control of polygon shapes and their use as datum features. Not sure if it is super helpful to your particular case, but definitely interesting (https://www.eng-tips.com/viewthread.cfm?qid=446869).

As Hallec mentioned, profile would definitely be the first choice. I would avoid symmetry/concentricity at all costs for the same reasons that were mentioned - they are very rare except in very specific circumstances and are being phased out in the new release of the Y14.5 standard as a result. If profile results in a tolerance zone which is too tight - I am not 100% acquainted with the specifics yet as it just came out but the 2018* version has a new "dynamic profile" modifier that could be useful, which I believe works like a runout control for shapes other than those of revolution (ie: allows the tolerance zone to uniformly expand and contract). If the actual form tolerance of the profile can be tight, but location of the feature to the DRF can be loosened then multiple single segment profile or composite profile tolerance may be an option - keep in mind though that depending on the DOF constrained by your DRF some unintended consequences may arise (ie: if rotation is unconstrained - polygons with an increasing number of sides nears approximation of a circle and depending on the size of the tolerance zone/exact shape of the polygon the feature could rotate further than you might expect within its tolerance zone).

The shape could also be specified with directly toleranced (+/-) sides and angularity controls to a common DRF, or you could even do something like a wide profile tolerance and tighter angularity controls on each surface, however you have to keep in mind the variation allowed by these methods - the tolerance zone will not expand/contract uniformly.

*EDIT: oops - 2018 is the new standard, I'm a year ahead apparently (or behind, not sure which)

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

(OP)
Thanks! Didn't know they were removed. That's ASME right? Couldn't find anything (free) concerning ISO so will have to lookup if it is removed there too.

Facet-to-Facet tolerance is the most critical form, maybe I can combine method 1 with a 60°±0.01 dimension for the polygon sides. Or might be bad practice since the cone shape of the polygon could be signifantly compared to the tight facet-to-facet angle tolerance. Need to take a courses in GD&T since this is on another level than what I've done previously.

I ran some numbers to for 'profile of a shape' tolerance and how it correlates to the facet-to-facet I want.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

#### Quote (Perka 26 Feb 19 14:35)

Facet-to-Facet tolerance is the most critical form, maybe I can combine method 1 with a 60°±0.01 dimension for the polygon sides
By method 1 do you mean the profile tolerance? Combining directly toleranced dimensions (whether angular or linear) to define the true profile for a profile tolerance is not allowed. You could do 6x individual profile tolerances but I would probably resort to a flatness tolerance since that is all it is controlling at that point.

#### Quote (Perka 26 Feb 19 14:35)

Or might be bad practice since the cone shape of the polygon could be signifantly compared to the tight facet-to-facet angle tolerance.
Not sure what you're trying to say here.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

Method 1 looks like the best choice if you intend to control the form only. Since the all-around profile doesn't reference any datums, it will not control the size of the feature. If that is the intention, remove datum feature symbol A from the side view. Also, the angle should be basic.

Method 2 won't be well received from the reasons already mentioned in previous replies, but please note that it isn't applied correctly either: you need to derive datum A from some datum feature of width on the part, not from the 2 flats you control with reference to this same datum. Additionally, you need 3 datum center planes - one for each control.

Method 3 - as mentioned, just like symmetry, concentricity was removed from the latest ASME Y14.5 standard. If you follow the 2009 or previous revisions I think you can apply concentricity on a polygon (I've seen this shown in a book), but the control should be applied on the physical surfaces, not on the inscribed virtual cylinder. That is because the inspection involves deriving median points from physical surfaces. You will also need a datum axis derived from an actual datum feature. Do not connect the datum feature symbol to any center lines. The downsides of this method were also mentioned in previous replies.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

(OP)

I have a new image with some corrections on the 3 methods. I will have a closer look at this when I get home.
I followed some advice here and put some basic dimensions and center lines in there.

Some questions in the image.

For example, center lines between opposing sides of polygon, good or bad practice?

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

Perka,

"does that mean all opposing sides (3x) are controlled by the profile of a surface all around 0.1?"
Yes. Any form deviation mus fall within a 0.1 wide tolerance zone equally disposed about the true profile defined by the 3x 40 and 6x 60deg basic dimensions.

To note, I don't think anyone suggested adding center lines - they can be useful depending on what you are doing, but in your case are extraneous, unless perhaps you are dimensioning to/from them. The inscribed circle in your method 3 is the one case they might be expected to be used though. What you need to remember is that datums are derived from,and controls are applied to, actual physical features - its a common error for example to define a width dimension and then attach a datum reference to the center line between that width because the desire is to define a datum center plane between this width, this is NOT allowed. The datum reference is applied to the width dimension/leader lines and by the rules in the standard it defines a center plane.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

Regarding the corrected image:
Method 1 - 40 and 60° are now basic dimensions. If you add "3X" before the 40 the whole shape will be fully defined with the basic dimensions and a profile tolerance. The size and form will be controlled. I should correct my previous reply and say - the reason the size was not controlled previously by the profile is that 40 wasn't basic, not because of the lack of datum references.

Method 2, 3 won't become viable as long as there are no datum features for symmetry and concentricity. The datum features for symmetry can't be the controlled features themselves, and the datum feature symbol for concentricity can't be attached to a centerline. Attached is a fragment from Dimensioning and Tolerancing Handbook by Paul J. Drake, Jr. (McGraw-Hill) that might help you specify concentricity correctly for an irregular symmetric feature.

Also, I would remove all the center lines you showed on the drawing. I think that the only cases were center lines should be shown are axes of surfaces of revolution or when you dimension location of features from center planes.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

One more thing - there is no purpose for the basic 60° in methods 2, 3. Symmetry and concentricity don't control orientation (edit: between the controlled feature and anything but the datum), and thus the angle remains uncontrolled without a direct tolerance.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

(OP)
Thanks Hallec, chez311 and SEM_D220. Lots of great response at this forum, glad I registered

Reason I added center lines on Method 2 was due to symmetry, but then I thought center lines might make method 1 easier to read as well, and wanted to hear your comments on it.

Did not know that '...the datum feature symbol for concentricity can't be attached to a centerline' (SEM_D220), might have some GD&T documents on my desk I should throw away, cause I'm pretty sure that they used centerline for concentricity datum, along with another example where the datum pointed at the dimension arrows, and said it had the same meaning.

Method 2:
Thanks SEM_D220, I read up on how to measure symmetry and realize that (like you said) two opposing side, where left side look like this ) and right side like this ( They would result in a "vertical line/zero assymetry" if you took some symmetry measurements with the caliper (measuring at different heights along vertical axis with the caliper jaws opening/measuring horizontally.
As suggested, remove basic from 60° dimension

I'd also need to add parallelism to control the form. And from what I've read tonight I have to have the datum feature on a surface (instead of axis) if I want to control form with parallelism, in other words surface parallelism. On the image it was pointing at the 40 dimension arrow (axis in the center of polygon), which would not be correct in this case I believe (lots to learn). Need to figure out if I need surface parallelism or flatness on the faces... hmmm.

Back to symmetry for method 2, how would one go about placing a symmetry datum, is it possible given I only have "one shape/feature, extruded polygon"?

#### Quote (SEM_D220)

The datum features for symmetry can't be the controlled features themselves, ...

*edited Good quote that I will try to remember from now on :)

#### Quote (chez311)

What you need to remember is that datums are derived from,and controls are applied to, actual physical features - its a common error for example to define a width dimension and then attach a datum reference to the center line between that width because the desire is to define a datum center plane between this width, this is NOT allowed. The datum reference is applied to the width dimension/leader lines and by the rules in the standard it defines a center plane.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

It seems like another case where the OP hasn't got copies of the ASME standards. I understand the politics of the cost, but it's a shame that it costs so much. Whether it's "worth" that amount is debatable.

In ASME Y14.5.1-1994, there is support for 6-way symmetry. It's not limited to planar mirror conditions. It's still described that way in the current draft.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

Peka,
Glad you found our responses helpful. I have to say that the credit for your last quote belongs to chez311, not to me

Before you throw away the GD&T documents on your desk, make sure you know which standard your company is supposed to work by. I know very few about the ISO GD&T standards - only what I read online in and in this forum, but I'm sure I read somewhere that they allow attaching datum feature symbols to center lines. There are members in this forum who can confirm or deny this.
If ISO allows this, I don't know if there are any specific conditions for it, but it sounds to me like a bad idea regardless of being allowed or not. For a part that has 2 or more coaxial diameters or 2 or more symmetric widths, if the datum feature symbol is attached to a center line that goes through the entire part, no one will know which feature is the datum feature that must be used for holding the part in the inspection fixture and for simulation of the datum axis or center plane.

As for surface parallelism, its main purpose is to control orientation, but it does control form (flatness) as a side effect. Parallelism and the other orientation controls can also apply to a feature of size rather than to a surface, and in this case, it is correct to align relate the parallelism feature control frame with the dimension line (adjacent to an arrow or replacing one)., by placing the feature control frame under the dimension of width or diameter (sorry for the corrections, some concepts got mixed up in my head when I wrote that), The controlled element, in this case, is the center plane that should be derived from the feature which is dimensioned and toleranced.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

#### Quote (Perka)

Thanks! Didn't know they were removed. That's ASME right? Couldn't find anything (free) concerning ISO so will have to lookup if it is removed there too.

It is more like another case where some advice is being given based on ASME while it seems like it is the ISO that should be brought to the table in the first place. OP should know that both systems have their own sets of rules, concepts and modifiers.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

(OP)
I raised the question about which standard we work on (recently hired)
I need a center hole through the part and thought this might be a good way to control the geometry. The hole sets the datum A which is then referenced for symmetry, perpendicularity and parallelism in different view orientations (which may imply I need to have a new datum feature for each new view?)

If symmetry is allowed in the standard we work with, do you see anything being overdefined/conflicting/missing with this method?

I see a potential mistake, perpendicularity would only be needed in one view?

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

Perka, the datum derived from the hole is an axis, not a plane. All parallelism controls and the 2 perpendicularity controls can reference this datum. You can do that by simply adding "6X" before the parallelism control if the paralleliam control should be applied to each of the 6 flats. Make sure that the hole is a functional datum (what role does the hole play for this part?). By the way, I added a small correction to my reply above, make sure you noticed it and sorry if it was confusing.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

(OP)
The hole axis is the axis that the polygon need to be concentric about.

Thanks for clearing up the datum reference feature.
Flatness in combination with parallelism does not seem to be controlled on my drawing?
Parallelism is measured over the whole surface but the angle does not control the shape of a polygon face, right? Perhaps make 60° a basic dimension and add angularity tolerance(s).

^scratch that (if the angle at some point on the surface were to be much larger than 60°, the shape of the face wouldn't pass the parallelism tolerance)

Here is the progress so far

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

I'd suggest buying a copy of the applicable standards for the place where you work. Otherwise it will be a game of 2000 questions.

I'm more curious about the stress analysis that's used to determine the tolerances and the need for symmetry.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

#### Quote (Perka)

if the angle at some point on the surface were to be much larger than 60°, the shape of the face wouldn't pass the parallelism tolerance
The parallelism tolerance does not control the 60 degrees angle. That angle is controlled by the direct +-0.1° in your drawing. Since you control each facet with reference to an axis, each facet can rotate about that axis to any orientation and still be parallel, hence no mutual orientation is controlled by parallelism. 6X can be above the feature control frame, not an issue. 3X should stay as it is and the feature control frame for symmetry should be placed under the dimension. Not sure about the two leaders that originate from a single perpendicularity control. All these are just technical tips, but you are strongly advised to consider the Profile scheme again. You could reference the datum axis at Profile and achieve the desired orientation of the external surfaces with reference to the datum axis. You are also advised to consider replacing symmetry with Position. Position is much easier control to inspect, and if the prime goal is centering of features, it is considered the preferred one.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

#### Quote (Perka 27 Feb 19 09:00)

I raised the question about which standard we work on (recently hired)

pmarc - sounds like OP is unsure as well, you are correct that should have been our first step to determine.

Perka,

As pmarc noted the standards can differ in their treatment of datum definition and controls/modifiers. You should definitely determine what standard you should be working to before proceeding much further.

Just to build off of what was mentioned by Sem, I too have limited knowledge of the rules surrounding the ISO rules for datum definition but there is a good reason why the ASME spec does not allow attaching datums to theoretical centerlines/axes/points. In addition to my (and others) previous comments about those theoretical features being derived from physical features - Sem's comment about ambiguity is another driving factor. See the below figure - without attaching it to a specific width it is impossible to determine which feature the datum is derived from (multiple widths or hole?) and what exactly the datum looks like (centerplane or axis?).

This was drawn from a comment by JP back in 2011 from this (https://www.eng-tips.com/viewthread.cfm?qid=292841) thread - all credit goes to him, I hope he doesn't mind my referencing it. Theres some good posts there about the ASME rules regarding datum definition/specification, however your best friend would be again to determine which standard applies in your workplace, and read through the applicable sections - theres no better reference than the primary source.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

(OP)
Dimension according to ISO 129 and GD&T according to ISO 1101

from ISO 1101

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

This is probably where part of us, me included, have to "cede the floor" for those who are knowledgeable in ISO.
I will only allow myself to add that for the ISO case, my comment about Position vs. Symmetry is probably not relevant. You can probably keep the symmetry control because one of the few things I know about ISO is that ISO Symmetry is a special case of Position and is widely used (unless that too is not up to date - but not likely).

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

Knowing it's ISO, this is how I would do it with the symmetry approach:

As it was mentioned, symmetry can be used here without any ASME-related issues because in ISO it's just a special case position tolerance. What is important is the CZ (combined zone) modifier that ties the 3 symmetry tolerance zones together in terms of mutual basic orientation. Without it the tolerance zones would be totally unclocked to each other. This is the consequence of default independency principle in ISO that doesn't exist in ASME.

Also, as shown, circle E modifier (envelope requirement) is not a must for the widths, but most likely is required for the datum feature A - without it, form of the hole is totally out of control. Again this is the consequence of the defualt independency principle in ISO. In ASME, form of the hole would be controlled within the size tolerance through Rule #1.

Additionally, tolerance value and datum reference A in the symmetry tolerance indicator could technically be referenced at MMR (Maximum Material Requirement) or LMR (Least Material Requirement) by using circle M or circle L modifier after the letter A, because in ISO, unlike in ASME, it is not prohibited to use M or L modifiers for the tolerance value and the datum letter in a symmetry or concentricity tolerance (but then the use of circle E modifiers for the widths and the datum hole would become questionable at best).

Side note: From ISO vs. ASME comparison perspective even more interesting would be talking about approach #1 (profile all around), because it would show even more differences between the two systems. Unfortunately, for now that's all I am able to say about it and I can't promise I will find some time to prepare some sketches showing what I mean.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

pmarc,
Does the Actual Mating Envelope principle exist in ISO and does it play the role that it plays in ASME Position in both ISO Position and Symmetry?

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

By default, ISO Position, Symmetry and Coaxiality (concentricity applied to a 3D feature) don't use the concept of Actual Mating Envelope. These three tolerances control imperfect derived features (extracted median line, extracted median plane) which aren't derived from feature's AME.

There are modifiers - different for internal and external feature of size - that are able to change the type of controlled derived feature from imperfect (extracted median line, extracted median plane) to perfect (axis, center plane) derived from ASME-like mating envelopes, but they can only be used if the tolerance value in the position, symmetry or coaxiality tolerance indicator is specified RFS. That is because in ISO there is no axis / center plane interpretation for tolerances specified at MMR (or LMR) basis.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

Thanks pmarc, that is very interesting.
Those "extracted" median lines and median planes must be derived differently than ASME median lines and median planes. Per ASME, the UAME of the feature is involved in the definition of these elements (center points are sampled at cross sections perpendicular to the axis / center plane of the AME...) I suppose that this is the main difference between ASME and ISO Symmetry - the method by which the controlled median plane is derived. Is that right?

I hope the OP doesn't mind the side topic evolving here...

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

#### Quote (pmarc)

From ISO vs. ASME comparison perspective even more interesting would be talking about approach #1 (profile all around), because it would show even more differences between the two systems. Unfortunately, for now that's all I am able to say about it and I can't promise I will find some time to prepare some sketches showing what I mean.

I know pmarc wouldn’t open up this subject if he was not willing to help us with the full understanding of the details (differences between the two systems)
I am willing to “help” moving this subject (of the differences) just a little bit forward by posting these two attachments (profile ASME versus profile ISO) I found here on the forum. Not sure who should I give credit to....

I just hope pmarc will come back and clarify if I am on a right track (profile all around differences) or he was talking about something else.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

I'm very interested to learn about the differences in profile between the two standards - thanks pmarc for noting it and greenimi for posting the attachment. FYI greenimi - if you're looking for credit that looks almost like something by tec-ease, looks very similar to their other materials but I could be wrong.

In regards to the attachment posted by greenimi, I see that the example says to imagine "dragging the center point of a 4mm sphere around the true profile". Not to nitpick, but I have to imagine that a better analog would actually be a 4mm cylinder, not a sphere - unless the treatment of profile is that radically different.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

Sem,

As I said I don't know too much about ISO, but from what I've read I seemed to get the impression that ISO does not deal much with mating envelopes of the types so commonly utilized in the ASME standard. From what I gathered typically ISO relies on envelopes which are usually some sort of averaging which don't necessarily reflect the mating conditions of the features whereas ASME mating envelopes involve the extreme points of the features involved, which would actually contact when mated together. Now I guess which one is actually more representative due to deformation when mating might be another topic.

#### Quote (pmarc 28 Feb 19 05:34)

There are modifiers - different for internal and external feature of size - that are able to change the type of controlled derived feature from imperfect (extracted median line, extracted median plane) to perfect (axis, center plane) derived from ASME-like mating envelopes, but they can only be used if the tolerance value in the position, symmetry or coaxiality tolerance indicator is specified RFS.

pmarc - are these similar analogs or identical?

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

Thanks chez311.
The "averaging envelopes" practice that ISO seems to require sounds less friendly for physical gauging and datum simulation.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

2

#### Quote (Sem_D220)

Those "extracted" median lines and median planes must be derived differently than ASME median lines and median planes. Per ASME, the UAME of the feature is involved in the definition of these elements (center points are sampled at cross sections perpendicular to the axis / center plane of the AME...) I suppose that this is the main difference between ASME and ISO Symmetry - the method by which the controlled median plane is derived. Is that right?

Yes, that is right. In ISO, LSQ (least sum of squares) features, and not minimum circumsribed or maximum inscribed features, are involved in the definition of the extracted median plane and line.

#### Quote (chez311)

pmarc - are these similar analogs or identical?

The modifiers I mentioned basically change an element controlled with a geometric tolerance, like position, symmetry or coaxiality.

For example, circle X modifier used in a position tolerance frame applied to a hole changes default controlled element from imperfect extracted median line to a perfect axis of the maximum possible cylinder expanded within the hole. So it defines the same requirement as position callout in ASME.

Circle N modifier used in a position tolerance frame applied to a pin changes default controlled element from imperfect extracted median line to a perfect axis of the minimum possible cylinder contracted about the pin. So again, it defines the same requirement as position in ASME.

#### Quote (greenimi)

I just hope pmarc will come back and clarify if I am on a right track (profile all around differences) or he was talking about something else.

Your attachment refers to an outdated ISO standard for profile tolerancing. This is what I meant:
https://files.engineering.com/getfile.aspx?folder=...

First, I hope it's clear. Second, I hope that even without trying to understand the content of the document, it sufficiently shows how different both systems are. And third, I hope that those who believed otherwise will at least consider changing their mind.

EDIT: Link edited - the document attached originally contained too many pages.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

Thanks, pmarc!
That is a very informative review of the different Profile applications by ISO.
Lots of modifiers. Looks like they are on a mission to standardize every design intent ever possible. I don't know if it's good or bad, but it certainly sparks an interest to get more familiar with their set of GD&T standards.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

That’s why I came here often. Time well spent for education. Thank you again pmarc.

I am trying to learn ISO as much as possible. Or, at least, the main differences between ISO and ASME.
Looks like the profile section is very different despite the “rumors” (and I get a lot of less than educated opinions at my work place that the systems --ASME and ISO---are almost the same or the converging time is near). Nothing further from the truth.
Thanks again for your excellent piece of work.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

#### Quote (pmarc)

Additionally, tolerance value and datum reference A in the symmetry tolerance indicator could technically be referenced at MMR (Maximum Material Requirement) or LMR (Least Material Requirement) by using circle M or circle L modifier after the letter A, because in ISO, unlike in ASME, it is not prohibited to use M or L modifiers for the tolerance value and the datum letter in a symmetry or concentricity tolerance (but then the use of circle E modifiers for the widths and the datum hole would become questionable at best).

Pmarc,

I self-admitted I don’t have much knowledge in ISO (but I am learning) so I might want to ask you some quick questions about ISO concepts, hopefully easy: “yes-no-maybe” questions.
Are you saying that for a feature of size definition “a combo” size with E + position modified at MMR is an invalid callout?
If a FOS is used as a datum feature and modified at MMR in other FCF’s, then its size cannot be used with E symbol?

Looks like the straightness of a FOS must be refinement of the position, symmetry, coaxiality if the case of this combo of geometric tolerances are applied to the same feature (straightness-position, straightness-symmetry, straightness-coaxiality/concentricity). Is my understanding correct?

Thank you for your answers and, as usually, I appreciate for input and contribution on this forum.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

#### Quote (greenimi)

Are you saying that for a feature of size definition “a combo” size with E + position modified at MMR is an invalid callout?

No, I didn't say that, but it is hard to disagree with the following statement from ISO 2692:2014:
"Use of the envelope requirement (E) (previously also known as the Taylor Principle) [in conjuction with a geometric tolerance at MMR] usually leads to superfluous constraints regarding the function of the feature(s) (assembleability). Use of such constraints and size definitions reduces the technical and economic advantage of maximum material requirement, MMR."

#### Quote (greenimi)

If a FOS is used as a datum feature and modified at MMR in other FCF’s, then its size cannot be used with E symbol?

If a FOS is controlled with a size tolerance only and then is referenced at MMR in the datum portion of a geometric tolerance, for some reason ISO says that maximum material virtual size is equal to the MMS size of the feature. See a snapshot from ISO 2692:2014 below:

This means that in such case the envelope requirement is imposed to the datum feature even though the envelope requirement has not been explicitly specified on the face of the drawing. To me this rule is a gap in their logic, and I know from the discussion with Henrik Nielsen, former chairman of ISO TC 213, that they have been working on changing that.

#### Quote (greenimi)

Looks like the straightness of a FOS must be refinement of the position, symmetry, coaxiality if the case of this combo of geometric tolerances are applied to the same feature (straightness-position, straightness-symmetry, straightness-coaxiality/concentricity). Is my understanding correct?

Yes, that is true. In ASME this is also true but only because Y14.5 aribirtarly says so. From theoretical point of view it wouldn't have to be.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

What does not seem to me very intuitive is that even when E is used for a FOS dimension, the form tolerance can be bigger than the size tolerance on an ISO drawing.
Lack of my education in ISO and lack of full understanding of the default Independency principle makes this concept hard to grasp.

So, the straightness (one of the form possible errors) must be refinement, but circularity (or cylindricity) does not.

Do you know, by any chance, which ISO standard has this form/ size/ location relationship explained?
I am just trying to train myself to think in ISO world, with no much success (so far).

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

#### Quote (greenimi)

What does not seem to me very intuitive is that even when E is used for a FOS dimension, the form tolerance can be bigger than the size tolerance on an ISO drawing.

[...]

So, the straightness (one of the form possible errors) must be refinement, but circularity (or cylindricity) does not.

Not sure what makes you think that. Could you clarify?

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

Thank you again for engaging.
So, if a feature of size specified on an ISO GPS defined drawing has the envelope E symbol associated with its size dimension, then how it is possible to have cylindricity bigger than the size tolerance?
Ø 14 -0.016 / -0.034 E
Cylindricity 0.05
Coaxiality Ø0.05 to a DRF

Now, if a straightness of that FOS is added, that specific tolerance straightness value should be smaller than the coaxiality Ø0.05 value?

Am I missing something?

Which of my statement(s), from this post or the previous one, is not true? (and I am not pretending that all are)

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

For the given size callout and with the envelope princile in charge, I don't think cylindricity tolerance of 0.05 makes sense. I know it was taken directly from Nielsen's handbook, but in my opinion this is a mistake.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

Okay. Thank you pmarc
So, looks like I was not too far away in the weeds, or questioning it.
In general, when I am learning, I have to start from something that I assume is a correct baseline. If I start to query everything, probably I just go nowhere.

Maybe Mr Nielsen thinks in terms of the tolerances cannot "see each other" (same as discussed here: https://www.eng-tips.com/viewthread.cfm?qid=402945) ...Me.... trying to justify

I would say it is bad enough that the official ISO GPS has over 100's standards, but is even worse when the commercial books (books designed/written to "translate" heavy technical language in simple terms) have mistakes to this level.
Really confusing.

I might have more questions related with the subject, but for today I already abused you enough. (and I don't want you to get mad:) )

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

Pmarc,

I hope you are still around as I have quick follow up questions. (As I promised yesterday I did not ask my follow up questions as I did not want to stretch too much your patience)

If yesterday’s feature of size: Ø 14 -0.016 / -0.034 , does not have the envelope E (and also the drawing does not specify ISO 2768) am I correct in saying that Cylindricity within 0.05 (cylindricity size bigger than the tolerance size) and Coaxiality Ø0.05 (RFS or MMR) to a DRF “combo” makes logical sense?

In other words, I am trying to find what requirements (form allowances or location errors) interfere with each other in ISO world.

Combo 1: size + cylindricity>size + coaxiality (RFS) > size
Combo 2: size+ cylindricity> size + coaxiality (MMR) > size

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

#### Quote (greenimi)

If yesterday’s feature of size: Ø 14 -0.016 / -0.034 , does not have the envelope E (and also the drawing does not specify ISO 2768) am I correct in saying that Cylindricity within 0.05 (cylindricity size bigger than the tolerance size) and Coaxiality Ø0.05 (RFS or MMR) to a DRF “combo” makes logical sense?

Yes, I would say this is legal combination of callouts in ISO.

#### Quote (greenimi)

In other words, I am trying to find what requirements (form allowances or location errors) interfere with each other in ISO world.

Combo 1: size + cylindricity>size + coaxiality (RFS) > size
Combo 2: size+ cylindricity> size + coaxiality (MMR) > size

Sorry, but I am not sure I understand this question.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

#### Quote (pmarc)

Quote (greenimi)

In other words, I am trying to find what requirements (form allowances or location errors) interfere with each other in ISO world.

Combo 1: size + cylindricity>size + coaxiality (RFS) > size
Combo 2: size+ cylindricity> size + coaxiality (MMR) > size

Sorry, but I am not sure I understand this question.

Pmarc,

I was questioning the relationship between (of lack thereof) of cylindricity and coaxiality in ISO (coaxiality either at RFS, either modified at MMR).
As far as I understood from this discussion (and other threads here on the forum) when straightness of a FOS (straightness of the axis) is used in ISO, its tolerance value shall be smaller than the coaxiality (shall be refinement) regardless if the coaxiality is RFS or MMR.

But straightness is just one of the possible form errors and I was just inquiring about the other possible form error (in the other direction) such as cylindricity (maybe even circularity)
Looks like, you just confirmed that for cylindricity / circularity is not such a requirement in ISO GPS to be a refinement (to be smaller than coaxiality tolerance).

Is my understanding correct now or I am still missing the ISO concept of defining FOS's?

Thanks again

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

#### Quote (greenimi)

Looks like, you just confirmed that for cylindricity / circularity is not such a requirement in ISO GPS to be a refinement (to be smaller than coaxiality tolerance).

Yes, that is correct.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

Pmarc,

I have prints showing those ISO combinations. Now, knowing that ISO 2692 is invoked (as being an ISO drawing that is calling 8015) should I understand that:

- any linear sizes will have their envelope concept by default also invoked.

- there should be no coaxiality (concentricity, symmetry) callouts modified at MMR or LMR (at least per the theory specified in ISO2692).

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

#### Quote (Kedu)

- any linear sizes will have their envelope concept by default also invoked.
Yes, that is true.

#### Quote (Kedu)

- there should be no coaxiality (concentricity, symmetry) callouts modified at MMR or LMR (at least per the theory specified in ISO2692).
ISO 2692 says that: "Use of the envelope requirement (E) (previously also known as the Taylor Principle) [in conjuction with a geometric tolerance at MMR] usually leads to superfluous constraints regarding the function of the feature(s) (assembleability). Use of such constraints and size definitions reduces the technical and economic advantage of maximum material requirement, MMR."

This means that according to the standard, when applying a geometrical tolerance at MMR, (E) modifier should not rather be used, as it "reduces the technical and economic advantage of maximum material requirement".

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

May I force one more question: how ISO allows some linear sizes to take exception of the envelope requirement on prints shown with notes as such (previous posted picture) in order to use MMR/LMR?
Do we have to put E near other linear sizes? (And get rid of the general note E). Or there are other ways around it?

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

ISO 14405-1 defines LP modifier (two-point size) that, when applied to the size specification, basically cancels the default envelope requirement for particular size dimension in cases like yours, and makes the independency principle in charge again.

But I would like to repeat: ISO does not prohibit use of (E) modifier for features controlled at MMR. They just say that this combination isn't good idea from technical and economic perspective.

### RE: What's your approach for tolerancing concentricity of a polygon, see attached image

Pmarc,
Thank you for the info. Really helpful and enlightening discussion.

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