Will straightness affect the circularity and cylindricity? 2

[SeasonLee]

The figure below is a straightness callout on a shaft with a MMC modifier, but here I'm not talking about straightness, but circularity and cylindricity.

May I ask What are the maximum errors of circularity and cylindricity respectively.
Thanks for your help in advance

Season

 

[Wuzhee]

Circularity is 2D and by definition, each circular element of the surface must be within the specified limits of size.
In this case, it's 0.4 in circularity because 18.4-18.0=0.4

The VC of the pin is 18.4+0.4(MMC)=18.8
Cylindricity directly controls surface straightness and not the other way around. Your greatest cylindricity error would be 0.8 which corresponds to your DML error.

 

[SeasonLee]

Thanks, Wuzhee
The straightness control is associated with the size dimension, the tolerance zone applies to the axis, so you mean this axis error will affect the surface (cylindricity) error?

Season

 

[SeasonLee]

Further thoughts
When the straightness control is associated with the size dimension, the Rule #1 is overridden, will it affect the result as well?

Season

 

[greenimi]

I am really not sure if what Wuzhee said is correct. I don't think so.
Adding those values ("Your greatest cylindricity error would be 0.8 which corresponds to your DML error.") to get the cylindricity seems not right to an untrained individual.
Has been discussed on this forum before and it is NOT useful to place/specify a callout / requirement and ask what is the maximum error for other requirement based on the requirement specified.

 

[greenimi]

I think here

https://www.eng-tips.com/viewthread.cfm?qid=423429

has been discussed (at least tangentially)
But if rule#1 is no longer in place then should we understand that the circularity and cylindricity is no longer confined to their size requirements? So is DMLS overide everything OR just the straigthness (to rule#1)?

Look for more threads where pmarc participated and give us his valued opinion.
Like this one:

https://www.eng-tips.com/viewthread.cfm?qid=344229

 

[Wuzhee]

greemini,

By controlling the surface with cylindricity, aren't we controlling the axis straightness also?
If we control the axis straightness, aren't we controlling the surface straightness thus cylindricity also? Although it's unorthodox and indirect, but there's a little correlation between the two.

There's also the difference between the metrology for the two. One is rotation and a dial gauge, the other is two-point measurements usually by CMM.

By the way I agree with you on this:

it is NOT useful to place/specify a callout / requirement and ask what is the maximum error for other requirement based on the requirement specified.

 

[Garland23]

it is NOT useful to place/specify a callout / requirement and ask what is the maximum error for other requirement based on the requirement specified.

I don't see why this statement is true. I can think of many scenarios where such a question is viable.

 

[SeasonLee]

Well, I would be grateful to those who can give me a guide /light on my questions.

Season

 

[3DDave]

I am curious to know what performance factors depend on these controls. Is this input for a stress analysis? What functional overlap is there between variations allowed for by these controls?

 

[SeasonLee]

It's just an in-depth study of geometric tolerances, in order to understand how they relate to each other, just like the thread I posted a few years ago.

In worst case, the flatness is

Season

 

[3DDave]

What have you worked out so far?

 

[SeasonLee]

Have read the definition of the four geometric tolerance(Form) over and over again, but I just can’t figure out a clue and my head is a bit stuck. That's why I'm here for help.

Season

 

[3DDave]

Draw pictures of each allowed variation and compare them. With no practical application, understanding can only come from personal effort. Others can help with evaluating your interpretation of tolerance vs variation, but it's like teaching without any feedback from a student except a random nod if you don't show your work.

 

[Burunduk]

By controlling the surface with cylindricity, aren't we controlling the axis straightness also?
If we control the axis straightness, aren't we controlling the surface straightness thus cylindricity also?

The former is true, the latter is not.
A cylinder with barrel-shaped variation could have accurately straight derived median line, but may have very poor cylindricity.

 

[greenimi]

So you guys agree that the OP answer, as indicated above, is max cylindricity of being 0.8?

 

[greenimi]

And, by the way, we know this question is for academic purpose only

 

[3DDave]

The numeric answer is a waste of time. It fails to convey understanding and is of no academic value. There is a good reason good teachers require students to show their work.

 

[greenimi]

3DDave,
I agree with you, but if this question is from a training material what the student should answer?
Or (your favorite subject) if this question comes up for the certification exam, what is the "correct" answer?

 

[SeasonLee]

It seems you are a night owl, I have to sleep for a whole day work.

This part only has FOS size and straightness callout, and there is no information about circularity and cylindricity, so I am thinking what will be the circularity and cylindricity error of this part?

Per standard, Rule #1 will automatically control the form, so the circularity and cylindricity will be 0.4(18.4 – 18.0).But when the straightness control is associated with the size dimension, the Rule #1 is overridden, so what will control the circularity and cylindricity right now? Is the FOS size that controls the circularity and cylindricity? Or the straightness error on the axis will affect the circularity and cylindricity?

My question is if taking FOS size and straightness both into account, what will be the results of max error possible on both circularity and cylindricity?

I will say 0.4 for both circularity and cylindricity, since the max straightness error [highlight #FCE94F](0.8 corrected)[/highlight] on the axis will not affect (or control) the Form (circularity and cylindricity).

Please correct me if I am not right.

Season