Circular runout 2

[gabimo]

Is circular runout controlling:
1.) Straightness of the cylinder surface?
2.) What about Derived Median Line (DML)?

Same questions for total runout:
Is total runout controlling:
3.) Straightness of the cylinder surface?
4.) What about Derived Median Line (DML)?

In other words, if a cylinder surface (surface feature A) has a circular or total runout shown (in relationship to a datum axis defined by other cylindrical surface, feature B) and a straightness callouts or a derived medial line callouts for the surface feature A are also shown, does the straightness or DML should be smaller than the circular/ total runout?

 

[greenimi]

pmarc and pylfrm,

So, how the new revelation will affect the outcome of the thread, or the answers to the chart I have posted?
Pylfrm said somewhere :".......and because I came up with a different answer for some of greenimi's scenarios"


I have to admit I do not understand
"The cross section (shaded in dark blue) is bounded by two arcs. The first is the size tolerance envelope boundary, has diameter 100.000, and is centered on the datum axis. The second is the minimum circumscribed circle of the cross section, has diameter 99.544, and is centered 4.772 away from the datum axis. Imagine every cross section of the feature is this shape, but the direction of decenter of the second arc varies through 180 degrees or more over the length of the feature.

I believe this feature meets a size tolerance of 5.000 and a total runout tolerance of 5.000, but has DML straightness of diameter 9.544"

Yes, you can explain it to me, but you cannot understand it for me......so I am still working on it

 

[pmarc]

greenimi,
I will leave pylfrm the explanation - after all, he holds rights to this revelation ;-)

And yes, it will change answer to your question. In fact, I think it will change answer to initial gabimot's questions too.

When it comes to your scenarios, looks like the only redundant cases are: A1, B1, F1 and G1.

As shown and explained by pylfrm, the actual DML straightness error can be almost twice as big as actual total (and circular) runout error, but no more. In A1 & F1 it is c=.011 vs. b=005, and in B1 & G1 it is c=.010 vs. b=.005.

pylfrm,
Feel free to correct me.

 

[greenimi]

The main thing I take away from this, and other similar discussions on this forum, is that there is often not much value in trying to interpret the control provided by one type of tolerance in terms of another. Things quickly become more trouble than they're worth.

That is true, but still these are good theoretical exercises. They definitely help to keep a sharp mind and sometimes (like in this case) expose to totally new aspects of the problem.

Like someone said here on the forum: makes a good mental exercise which prevents Alzheimer…allegedly.

I am waiting for pylfrm for his explanation..... how did he get that..............

 

[mkcski]

pamrc:

I stand corrected. I had to sketch your example in my CAD software to see that you are right. The minimum circumscribed cylinder diameter is indeed 99.544, and the DML can behave exactly in the way you described it. Thank you for showing this - really an eye-opener to me.

I have been following this thread. The word discussions alone are a little hard for me to visualize and the related attachments add much to my understanding. You mentioned in the above quote you did some CAD work to convince yourself of pylfrm's analysis. I do not have CAD available. Could you please attach the CAD drawing in JPG format so I can follow your understanding?

Certified Sr. GD&T Professional

 

[pylfrm]

greenimi,

Before I attempt to give answers for the scenarios in your chart, could you please clarify what "DML redundant" means to you? I posted the interpretation I was using earlier, but it appears pmarc is using a different one which does not involve the envelope requirement.

As for further explanation, I'm not sure where more detail is desired. If anyone has a specific question, ask away.

The constraint solver in SolveSpace, which is free software, was used to calculate the numbers and create the image I posted.


pylfrm

 

[greenimi]

pylfrm,

Consider a two steps cylinder--as shown in my attachment from a couple of days ago.
The "big" OD is dimensioned with a feature of size requirement and the size is within .010 (for example), then has a circular runout within .005 (for example again) relative to the small OD (datum feature).
Now, the designer want to add a derived median line, let's say .010 (consider there is a functional reason for that.... do not ask why)

The question is: is the DML (derived medial line) callout even necessary or the combination of the size+ circular runout will cover / will ensure that the DML is smaller than .010?

Does my explanation holds any water? If not, please let me know...

Thank you again for being patient and explain to us what you've found.......

 

[pylfrm]

greenimi,

I agree with pmarc that the DML straightness tolerance is redundant in cases A1, B1, F1, and G1. Your latest question is case B1, so this should answer that. I also agree that the DML straightness tolerance is not redundant in cases C1, D1, E1, H1, I1, J1, C2, D2, E2, H2, I2, and J2.

Your intended meaning of "DML redundant" becomes important for cases A2, B2, F2, and G2:

If the straightness tolerance is not applied and the envelope requirement is in effect, then I think the maximum DML straightness error will be slightly less than 2*min(a,b).

If the straightness tolerance is not applied and the envelope requirement is not in effect, then I think the maximum DML straightness error will be slightly less than b+min(a,b).

Note that I have not thoroughly confirmed these last two statements.


pylfrm

 

[pmarc]

greenimi, mkcski,
Have you been able to visualize what we have been talking about?

mkcski,
I wish I could post a sketch of what I did in my CAD software, but that is unfortunately impossible. The reason is that by "my CAD software" I meant the software I use at work. Due to security reasons I am not allowed to publish anything I create there in the internet. But you should definitely try the link to the SolveSpace solver that pylfrm offered. This thing is for free, does not require installation, and although quite crude (comparing to more advanced CAD systems), it allows to simulate/see things that have been discussed here.

pylfrm,
About your last two statements... for greenimi's scenarios, why are you considering situations in which the DML straightness tolerance is not applied? If I understand the question correctly, he is not asking what happens if the DML straightness is or is not applied. He wants to know at which point exactly applying the DML straightness does not make sense any more.

 

[pylfrm]

pmarc,

I am trying to avoid, for example, a redundant callout!

To determine whether a tolerance is redundant, one needs to know what will happen if it is not applied.


pylfrm

 

[3DDave]

pylfrm,

More important, one needs to know what will happen when the acceptable variant parts are installed. It seems unlikely that the combinations mentioned would be used in the way they have been examined.

Without seeing an application and surrounding analyses it's sort of odd to consider something like this. So far there's no reason to apply any tolerance type, much less values.

 

[greenimi]

Pylfrm and 3DDave,

This is a theoretical exercise .... Lets say a test question, to evaluate the theoretical knowledge in different callouts. No practical application to be provided as I don't have a such in my mind.

 

[greenimi]

And by the way, pmarc got my question right.... Yes I would like to know that point..
"He wants to know at which point exactly applying the DML straightness does not make sense any more."

 

[mkcski]

pmarc:

Thanks for the tip on SolveSpace. I need to see our IT guy to see if I can install this on my PC.

Certified Sr. GD&T Professional

 

[greenimi]

Pylfrm,

Just to restate again:
....... at which point exactly applying the DML straightness does not make sense any more?

Could you, please, explain in the layman therms "your discovery" / little known revelation?


Thank you again for your help and patience.

 

[aniiben]

Just for my own education:
If a DML callout is NOT used, but instead the Independency symbol is shown on the feature controlled with the circular/total runout, what would be the worst surface straightness for the cases shown in the attachment (A1-E1, A2-E2, F1-J1, F2-J2)

 

[greenimi]

I guess this is an excellent question for the certification exam (2009, as that’s the one using the “I” symbol). My opinion.

I am not sure about the “correct” answer: If I have to answer, I would say that the worst surface straightness is limited by the total runout (of course for the total runout case).

However, for the circular runout case, gets more complicated.

Could be the size tolerance combined with the circular runout tolerance? (.010+.005) or
half size circular runout + size tolerance (.0025+.010)
or even double the circular runout tolerance (2x 0.005)

Anyone can help? Any GD&T experts could chime in, please?

Interesting question anyway……….