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"Per Unit Basis" For Roundness?
4

"Per Unit Basis" For Roundness?

"Per Unit Basis" For Roundness?

(OP)
I have a problem with press fitting a circular component into another circular piece.  They need to be as flush as possible, but I'm not sure how I can control it most effectively.  I'll try to explain my problem as simply as possible.

Suppose I have two toilet paper rolls (and lets assume they're rigid).  I want to press fit one toilet paper roll into the second toilet paper roll.  I want the fit to be as flush as possible, so I don't want any gaps between the two rolls once they're mated.

I can use a total runout callout on the ID of the female toilet paper roll, but the tighter I make the tolerance, the more expensive it gets.  There's also the risk that all of the runout will be on once side of the ID, while the rest will be fine, causing the roll to pass inspection but create a leakage path.

What I am looking for is something like total runout per unit basis, where that unit would be some number of degrees.  So say the nominal total runout value would be 2.5, but for each 15 degree segment it can't exceed .5.  That would solve all of my problems, but as far as I can tell it isn't valid.

What else can I do here?  I want to minimize gaps between the two components.  I also can't change the male component, so it has to be something for the ID of the female component.

Thanks for taking the time to read this!

RE: "Per Unit Basis" For Roundness?


Circularity, or Cylindricity, depending on shape and size of the parts.

The tolerance zones are formed by concentric circles or cylinders respectively, so that prevents "bulge" from passing inspection. Cylindricity will also control Straightness.

If you are in ANSI/ASME part of the world you can rely on Envelope rule: your surface should always lie between LMC and MMC. So you just specify tolerance tight enough.

In ISO part of the world you can use (E) in circle symbol to invoke Envelope requirement.

I hope this will help.
 

RE: "Per Unit Basis" For Roundness?

(OP)
Thanks for the reply!  Circularity or concentricity would be fine, except I would need to use a very very tight tolerance to prevent any major bulges.

I'm looking for something that looks like the attached image.  I'm sure this doesn't exist, but maybe there's something similar that I can use?  I know it's possible while inspecting to check a tolerance for a specific range of degrees.  Doing this would ensure the part follows a smooth path within the tolerance zone.

RE: "Per Unit Basis" For Roundness?

I agree with CheckerHater. You can probably specify ASME Y14.5-1994 or -2009 in your notes and give the size you want because of Rule #1 in ASME Y14.5-1994 or -2009. Have you read Rule #1? You could add 'Profile' to Circularity and Cylindricity as acceptable alternatives, although most folks on this forum will probably say that you only need to specify size if it is defined per a recent ASME Standard.

You can look in the Machinery's Handbook for recommended fits.

You might also need to add ASME B46.1-2002 or -2009 to your notes and specify a surface finish per that Standard that will prevent leaks. The surface finish may need to be developed through testing.

Peter Truitt
Minnesota

RE: "Per Unit Basis" For Roundness?

Hi,
I agree with ptruitt, surface finish of the parts have greater segnificance in providing proper seal between mating surfaces.

Also in the drawing attached by you, u have used datum for circularity control which is illegal. Roundness does not have any datum references.

Also there is no option in ASME to specify roundness on unit basis as shown in your drawing.
This is an illegal control.
 

Madhusudhan Veerappa
Mechanical Engineer

RE: "Per Unit Basis" For Roundness?


Real Puzzler

Normally part surface can do anything as long as it stays within tolerance zone.

The only control more restrictive then that is Controlled Radius, it actually prevents "reversals"; but I have never seen it used on cylindrical parts.

I am 99.9% sure there is no standard symbology for what you are trying to accomplish, so you may have to create a verbal description like "Surface curvature not to be less than so-and-so", but it may be difficult to actually measure.

I am out of ideas by now, but will follow this thread with great interest. :)

RE: "Per Unit Basis" For Roundness?

(OP)
Madhu,
My mistake, I had concentricity in my head while writing that.

Surface finish won't solve the problem.  The bulges would still be there, but with a smooth surface.  The magnitude of surface finish compared to the tolerance zone doesn't compare enough.

If there are no "per unit basis" tolerances for circular surfaces, then I'll have to settle of a tight total runout.  It will be expensive, but it would have to do.

ptruitt,
Are you saying that I should use MMC on the ID of the female part?

RE: "Per Unit Basis" For Roundness?

The 2009 standard now allows something called NON-UNIFORM. By putting that word in the feature control frame, it opens up unlimited possibilites for whatever crazy tolerance zone you want.  You just have to be very careful in defining the zone with basic dimensions. (I don't have Y14.5 in front of me so I don't know the paragraph or figure number.)

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

RE: "Per Unit Basis" For Roundness?

J-P,

My example is an equal-bilateral tolerance (uniform). I do wonder how it could be measured, though. As far as I know, SmartProfile is the only GD&T analysis software that conforms to the ASME Y14.5.1 math standard (according to Medtronic), but it does have limitations. I am guessing that it would not be able to do it.
 

Peter Truitt
Minnesota

RE: "Per Unit Basis" For Roundness?

courtjester140,

Out of curiosity. You said male part can't be modified, but could you share with any simplified sketch of it so we could see how it is dimensioned and toleranced?

I am asking because if dimensioning on male part is not grasping intended function as well as on female part or - even worse - if the dimensioning is anyhow vague or incomplete, whatever you specify on female side will not make much sense since the failure (leakage) will most likely be caused by male side.

RE: "Per Unit Basis" For Roundness?

(OP)
pmarc,
I don't have the drawings in front of me since I'm at home at this point, but attached is a finely crafted piece of art from MS Paint that illustrates what's going on.

The intent here is for the softer graphite to conform to any imperfections in the shell.  The back surface is bonded with a non-porous epoxy.  The problem with this particular piece is that it's larger than anything else we've done, so it seems small leakage problems that we didn't detect before are now amplified.

changing the male part won't really help here, since it's role is to conform to the shell's shape anyway.  Without going into too much detail, leakage through the mating surfaces needs to be minimized.

RE: "Per Unit Basis" For Roundness?

2
The concentricity on the male part will allow a lobed shape, as long as there are an even number of lobes, so that would be one concern.

It has been pointed out that Circularity cannot be applied on a unit basis (incrementally)...  That is not true.  Y14.5 provides concepts and those concepts can absolutely be extended to applications other than those shown in examples provided by the standard.  I think cylindricity applied on a unit basis may work well.  A size or profile tolerance can be used to control size and a form refinement on a unit basis, even with an angular increment, should be fine.  Something like |Cylindricity|Tol Value/x degrees|.  I think the measurement results may require some manual analysis, but it is possible that asking for cylindricity with only a limited data sample, from a given increment, may work.

This could even be done with a linear increment, so with a distance in the axial direction.  The effect could be combined and turned into a square or round patch on the surface, with the cylindricity value within any patch required to be within the specified very tight tolerance value.  If you want a fixed curvature tolerance zone a unit basis profile of a surface would provide this.  If you want a variable curvature tolerance zone the unit basis cylindricity can be used.

Please trust me on this.  There is absolutely no problem with extending a principle shown in the standard to other applications.

Dean
www.d3w-engineering.com
 

RE: "Per Unit Basis" For Roundness?

courtjester140

We have seen straightness, flatness and profile application on a unit basis, and we haven't seen any "per unit area" used on roundness. The "per unit area" applied on a profile surface means to refine the original requirements, it will limits abrupt changes on the surface such as bumps, dimples, but this application may not solve your leakage problem between the mating surfaces.

I will not to settle a tight total runout as you mentioned, since total runout is a surface to axis control only, it will control the form (cylindericity), orientation and axis offset, but not for the size control, so I will recommend to use multiple single-segment profile control on your application :

1. The upper segment is a profile of a surface control, it controls size, location and orientation relative to the DRF.
2. The lower segment is a profile of a line control, it controls the line elements for form and orientation relative to datum axis, it's a refinement of surface profile control, the tolerance is less than the upper segment.

This design, the profile of a surface controls limits the size and location of the cylinder surface, the profile of a line control limits the form and orientation of the line element of the cylinder.

SeasonLee

RE: "Per Unit Basis" For Roundness?

Season Lee,
Multiple single segment feature control frames really shouldn't be referred to as an upper and lower segment.  They're not segments.  Each is a feature control frame which imposes a tolerance just like any other.

The upper feature control frame you describe, with profile of a surface, will control size, form, orientation, and location, assuming capable datum features are referenced.  I don't know why any datum features would be referenced for courtjester140's case though, since I believe he is dealing with a single cylindrical feature's form and size only.

The lower feature control frame you describe, with profile of a line, will impose tolerance zones on each cross section of the feature and those tolerance zones will control size, & form of those cross sections...  You left off the size control that will be present.  Also, since profile of a line cannot detect steps in a the considered feature from one axial location to the next, it doesn't seem like a good candidate to achieve the seal courtjester140 needs.

A unit basis cylindricity or unit basis profile of a surface would likely address the need though.

Dean
www.d3w-engineering.com


 

RE: "Per Unit Basis" For Roundness?

Dean

Multiple single segment feature control frames is very common on GD&T, its easy for us to say the upper or lower segment, I just follow what Alex said, you may find out this term from Advanced Concepts of GD&T by Alex Krulikowski on oage 25-2, you may also find out the same usage on page 20-2, I can send you a snapshot if you need them.

On the upper FCF, you are right , relative to "DRF" better update to relative to "datum axis".

But, I am interesting on what you said "profile of line cannot detect steps in the considered feature from axial location", each cross section will limited the tolerance within the specified tolerance zone, it's a series of cross section to control the size to a tighter tolerance zone, I can't image why cannot avoid the steps.

If the step is a concern on the axial direction, we may use straightness control instead on the cylindrical surface, my multiple single-segment FCF will updated to :
1. The upper segment is a profile of a surface control
2. The lower segment is a straightness control

SeasonLee
 

RE: "Per Unit Basis" For Roundness?

(OP)
All,
I should clarify that the concern is bumps in the radial direction, not axial.  Inspecting a component, that's where these bumps come from.  Really I'm just having trouble putting this on a B/P.

Even though I say that a radial direction is my concern, that doesn't mean that I can omit the axial direction as well...although controlling axial imperfections is much easier.

RE: "Per Unit Basis" For Roundness?

SeasonLee,
I'm completely aware that some people use the terms "upper and lower segment" with regard to stacking the feature control frames.  I'm telling you that it's a poor practice to do so.  The point is that there is no special meaning at all with regard to this stacking...  One could "un-stack" the feature control frames and apply them with separate leaders and they will have the same meaning as when they were stacked.  The requirements for each feature control frame are completely independent of the others.  You may choose to create a series of controls that have tighter values and fewer degrees of freedom constrained as you progress down the stack, but that doesn't mean that you should refer to any of those feature control frames as a "segment".  To do so implies a systematic link, when there is none.

With composite feature control frames there are restrictions regarding what may be specified with the 2nd or lower segment(s) that are based upon the upper segment, so the segments are not independent of each other.  So, for composite feature control frames the "upper segment" and "lower segment" terminology has some meaning, but if you're not using a composite feature control frame no such meaning exists.

Regarding the inability of profile of a line to detect steps, that is true if no datum feature which constrains translation normal to the considered feature's surface are referenced in the particular call-out...  The tolerance zone for each cross-section is fully independent, wrt translation, of those for adjacent cross-sections, so there is no ability to detect a step.  If you reference datum features such that the tolerance zones don't have that freedom, then the tolerance zones form a fixed stack that creates the equivalent of profile of a surface.  So, when profile of a line is used in a way that justifies its existence (when the tolerance zones have some degrees of freedom remaining, so they don't stack into a profile of a surface equivalent) then steps in the considered feature will not be detected.

Another problem for the specific case proposed is that the profile of a line in the lower feature control frame will control size, so the effect intended will not be present.

Dean
www.d3w-engineering.com

RE: "Per Unit Basis" For Roundness?

Dean, I think the question arose because of a slight inconsistency in your earlier post.  You wrote: "Multiple single segment feature control frames really shouldn't be referred to as an upper and lower segment.  They're not segments."
Why can't we called them segments if it is a multiple single segment feature control frame?   And if they are indeed called segments, it's not a huge leap to resort to the terms of upper and lower simply for ease of discussion, though you are correct that there is no hierarchy.  

When different symbols are involved (the example given was profile of a surface and then profile of a line), they really shouldn't be called multiple single segment feature control frames anyhow... I think this was your main point.

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

RE: "Per Unit Basis" For Roundness?

John-Paul,
You're right, I shouldn't say "multiple single segment" if I have a problem with "upper segment" and "lower segment".  My apologies to SeasonLee and anyone else confused by that conflict.

I still assert the the word "segment" should not be used when speaking of stacks of feature control frames...  Whether same symbols or different symbols within the stack, the requirements imposed are separate and independent, whether "stacked" or "unstacked".  I just prefer to keep the term "segment" confined to composite feature control frames.

Maybe I shouldn't even bring this preference of mine up...  It's not really a big deal.  My concern is the confusion I've often encountered when people new to the subject are trying to understand what the difference is between composite and just a stack of feature control frames (those "multiple things", with the the same symbol smile).

The bigger deal is finding something that is helpful for courtjester140...  I think a unit basis profile of a surface or cylindricity, with the increment defined as possibly an angle and an axial length (fairly short axial length, since he states that "axial undulations" are not so important) would work.

Dean
www.d3w-engineering.com
 

RE: "Per Unit Basis" For Roundness?

I think the upper or lower segment is not the poster question, but when I try to find out the definition of the terms I was surprised to find that these terms had been changed silently on 2009 standard.

On Y14.5M-1994 Standard, you may notice that the term lower segment and upper segment used on Fig 5-23a and 5-23b respectively at page 116. However they are replaced with Segment 1 and Segment 2 respectively on Fig 7-47 page 143 from Y14.5-2009 standard. Very interesting!

SeasonLee
 

RE: "Per Unit Basis" For Roundness?

I just think the explanations of what combinations of various call-outs mean gets muddied by terminology.  There are a couple, ok a few smile, things in the standard that I think could be said better (yes, I know that we all have opinions about how the standard could be improved...  Please pardon the time I'm taking to mention a couple of my items)...

Not only could stacks of simple feature controls frames be described as simply separate and independent requirements, but also "PLTZF" is a term that has mislead many, to the point that if one buys a particular measurement system that I have run across, then goes to the system manufacturer for training, they will be taught that the upper segment of a composite feature control frame applies to the centroid of a hole pattern.  The upper segment of a composite feature control frame is absolutely exactly the same as a normal position or profile call-out & that's all that needs to be said about it.  To say "Pattern Locating Tolerance Zone Framework" implies that something different is going on, when in fact there is a tolerance zone for each feature in pattern, just as a normal position or profile tolerance would impose.

OK, back off of soap box now.  Again, please pardon the transgression...  My interest is that confusion regarding GD&T be reduced, so the terminology matters, I believe.

Dean
www.d3w-engineering.com

RE: "Per Unit Basis" For Roundness?

Speaking of confusing terminology, There was mention of cylindricity in earlier posts. I don't see any difference between what cylindricity does and what profile of a surface does. If they are identical, I would think that it would reduce confusion to eliminate one or the other. Does cylindricity add anything to the GD&T table beyond what profile of a surface provides?

Peter Truitt
Minnesota

RE: "Per Unit Basis" For Roundness?

Peter -- cylindricity only controls form.  The diameter (size) can vary from piece to piece within the given size limits.

Profile of a surface is more stringent because it must be applied to a true profile, meaning that the diameter must be a basic dimension.  This means that it controls form and size.  (Of course, if datums are referenced, then orientation and/or location is involved, too.)
 

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

RE: "Per Unit Basis" For Roundness?

Follow-up:  given my previous post, this is why I think that the diameter in Fig. 8-17 of the standard should be a basic dimension, since profile must be applied to a "true profile"  (see paragraph 8.2).  I see what they're trying to do (control only the form of the cone) but then they should modify the wording in para. 8.2 so there is some wiggle room in cases where the feature needs to be a toleranced dimension.

Sorry to digress, but I got the soapbox from Dean  :)

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

RE: "Per Unit Basis" For Roundness?

J-P,

At the end of paragraph 8.2 there is a sentence:
"Where used as a refinement of a size tolerance created by toleranced dimensions, the profile tolerance must be contained within the size limits."
For me this clearly means that profile control does not have to be always associated with basic dimensions.

I would go even further...
I think this dimensioning scheme might be very useful. Some time ago there was a thread in which we were thinking about how to apply location tolerance to conical features - by position or by profile control. As far as I remember there was a strong belief that the profile is more suitable for this kind of applications. My objection was, however, that profile control would not allow bonus tolerance. But imagine the dimensioning as in fig. 8-17 with the addition of position tolerance at MMC associated withØ30±0.2 dimension. It would do the thing, I believe. What's your opinion about it?

I am wondering whose digression will be the most off original topic? :-]

RE: "Per Unit Basis" For Roundness?

Digressing further: When the nominal size does not fall at the mean of the virtual and resultant boundaries, (hello, bonus tolerance), the amount of available tolerance is not as effectively used. Composite profile call-outs provide equal-bilateral tolerances for both size and form, so the means of the inner and outer boundaries of form and size coincide. Nominal CAD geometry with coinciding nominal size and nominal form would increase the chances for good parts in many circumstances. (Especially with cones, I would think.)

Peter Truitt
Minnesota

RE: "Per Unit Basis" For Roundness?

pmarc -- yes, Alex got it right.  And I did see that sentence at the end of 8.2.  But the earlier sentences clearly state that profile must be applied to a "true profile."  It then defines a true profile as one defined by basic radii, basic dimensions, CAD data, etc. The sentence about toleranced dimensions makes sense to me functionally, but my point is that it flies in the face of the earlier stipulation requiring profile to be on a "true profile."

Again, I don't disagree with the notion, I'm just picking apart the contradiction in the verbiage.  Fig. 8-17 would be fine if they straighten up the contradicting verbiage.

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

RE: "Per Unit Basis" For Roundness?

(OP)
Based on the discussion here, is it safe to assume that there is no existing callout for a roundness check on a "slice" of a circular component?  If not, I'm going to have to modify the actual machining process, which is something I wanted to avoid.

RE: "Per Unit Basis" For Roundness?

courtjester140,

As DeanD3W said in the second paragraph of his post from (4 Jun 11 0:57) "Y14.5 provides concepts only and those concepts can absolutely be extended to applications other than those shown in examples provided by the standard".

I agree with this statement especially that it is obviously impossible to create a standard containing strict and precise rules for all dimensioning and tolerancing scenarios in the world.

However I see at least one weak point of such approach. Even if you specify circularity on angular unit basis, you would have to be aware of a fact that a reader of your print might not understand what you tried to say. If this person takes a look to the standard and finds no similar figure to yours, you will probably be forced to spend quite a lot of time on explaining what was your intent.

What I mean is that this 'philosophy' may work really fine if both sides are at certain GD&T knowledge level and are aware that conscious extending of principles is available. Otherwise I would rather suggest putting this requirement as a kind explanatory note.

RE: "Per Unit Basis" For Roundness?

(OP)
pmarc,
Thanks, I missed that post.  I've always considered Y14.5 a concrete standard, for which there was no independant interpretation or modification allowed.

Here is what I have.  It follows the same format as the "per unit basis" format for flatness, so it doesn't deviate too much from Y14.5.  The next step for me is to work with quality on a note to explain exactly what it means.  Following that will be process capability studies etc etc.  So I have gotten the information I came here for.

Thank you all so much for your help!

RE: "Per Unit Basis" For Roundness?

courtjester140,
I think your call-out should work fine and it sounds like you're right on track with your plans for a note to clarify its meaning and further work with data gathering and analysis.  It is an unusual application of unit basis (I wish it were called "incremental") tolerancing, so it makes perfect sense to me to add the note.

One point I'd like to mention is that I think these controls would be better shown using two simple feature control frames, rather than something that looks like a composite feature control frame.  I have always thought this about the unit basis flatness examples in Y14.5...  Maybe some day this can be simplified in Y14.5.  The approach you show is consistent with unit basis applications in the standard.  An an example to clarify my position on the type of feature control frame used would be if, in the figure you posted, the size tolerance was +/-.005, then the upper cylindricity tolerance of .01 would be unnecessary and only a simple feature control frame with the unit basis .001/15 degrees would stand alone.  BTW - Are you happy with incremental zones the cover the entire length of the cylinder all at once?

Dean
www.d3w-engineering.com
 

RE: "Per Unit Basis" For Roundness?

SeasonLee,
Regarding the cone tolerancing figure you posted, that does work, since datum feature B is not referenced in the profile of a line call-out...  The profile of a line tolerance zone is then free to translate in the axial direction, which effectively yields a variable diameter tolerance zone when applied to a given cross-section of the conical surface.  This would not be the case for the same tolerances applied to a cylindrical feature, for which the profile of a line would control size, and form and coaxiality & its tolerance zones will stack into an equivalent of profile of a surface (as long as "A" is referenced, keeping all the zones coaxial).

We are on the same page if you agree that the profile of a line tolerance in the cone tolerancing figure you posted has exactly the same effect as circular runout would have.  Do you agree?

Dean
www.d3w-engineering.com
 

RE: "Per Unit Basis" For Roundness?

Dean

Thanks for your interpretation on my last post, it really makes me clear, this forum is a nice place to learn GD&T, a star is granted to each one of you, Dean, J-P and pmarc.

SeasonLee

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