"Per Unit Basis" For Roundness? 4

[courtjester140]

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!

 

[DeanD3W]

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 ).

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

http://www.d3w-engineering.com

 

[SeasonLee]

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

 

[DeanD3W]

I just think the explanations of what combinations of various call-outs mean gets muddied by terminology. There are a couple, ok a few , 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

http://www.d3w-engineering.com

 

[ptruitt]

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

 

[Belanger]

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

 

[Belanger]

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

 

[pmarc]

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 with[Ø]30[±]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? :-]

 

[SeasonLee]

J-P

Here is a snapshot from Alex training book, diameter is a basic dimension on the conical feature.

SeasonLee

 

[ptruitt]

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

 

[Belanger]

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

 

[courtjester140]

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.

 

[pmarc]

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.

 

[courtjester140]

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!

 

[DeanD3W]

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

http://www.d3w-engineering.com

 

[DeanD3W]

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

http://www.d3w-engineering.com

 

[SeasonLee]

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