AVG and F symbol - fig 8-14/2018

[greenimi]

Figure 8-14 Specifying Restraint for Nonrigid Parts

Is F modifier in the circularity needed in figure 8-14?
I am asking this because in fig 8-13/2018 F symbol has been removed (relative to the equivalent figure 5-13 from 2009)
Why F was kept in 8-14?

 

[Garland23]

I don't think the F is needed in Fig. 8-14, because the restraint note is clearly meant only for the circular runout check and nothing else.

 

[jassco]

Circle F is needed. Otherwise, the roundness will have a different meaning.

 

[greenimi]

Otherwise, the roundness will have a different meaning.

What do you mean by "different meaning" ?

 

[Garland23]

jassco, according to paragraph 8.4.3 of the 2018 standard, I think that the AVG notation already tosses R#1 out the window. So I'd still say that the F is not needed.

 

[ctopher]

I agree the F'n circle (lol) probably not needed.
I don't understand the AVG meaning with the dim.

 

[3DDave]

In Figure 8-13 there is a single compound tolerance callout.
In Figure 8-14 there are two separate callouts.

Any "why" questions should be addressed to ASME committee members directly at the ASME Y14 committee meetings. No one but those who attend can possibly have any idea why anything is done in that group. Frankly, I have my doubts that most of them do.

 

[greenimi]

3DDave,
8-13 goes against their own requirements that circularity should be separated from the size dimension.
Am I see it incorrectly?

 

[greenimi]

A callout for circularity shall be specified on a surface and not to a size dimension. The circularity tolerance shall be less than the size tolerance and other geometric tolerances that affect the circularity of the feature, except for those features where Rule #1 does not apply (e.g. "free state" symbol, average diameter, “independency" symbol).

From 2018 paragraph 8.4.3

Therefore do you see at least two problems here or it's just me?
One problem identified at the onset of this thread and the second being the "association" with the size in fig 8-13 (clearly forbidden per the above text).

 

[3DDave]

No one but those who attend can possibly have any idea why anything is done in that group. Frankly, I have my doubts that most of them do.

Did this particular issue come up on a drawing that you are working with? Was it an actual problem?

Is the admonition to apply it to a surface anything but an arbitrary rule? Was there any confusion about what surface the circularity applied to or is it just that it appears to conflict with a rule that is made for no other reason than to have a rule?

 

[Burunduk]

They should've just written one clear, simple rule for the whole topic of placing feature control frames. I've seen drawings where position or profile FCF was attached to a basic locating dimension. The rule should have been straightforward: attach it to a dimension only when the tolerance zone is meant to control resolved geometry like a center plane, axis, or derived median line/plane. In all other cases, when it's a surface control, the FCF should be attached to the surface representation using a leader or directly. Or to another FCF, or in a note.

 

[Burunduk]

F in 8-14 is unnecessary because it is the default. The exception is in the note and it concerns only the runout requirement.

 

[3DDave]

The size dimension applies to the surface; there is no ambiguity.

A basic dimension used for locating a feature does not apply to a surface so whatever fever dream you are finding on drawings are very far from any sense of reason and no amount of rules will fix that.

The (F) is useful for idiot-proofing the manufacture and inspection process to ensure, by being explicit that there would be no confusion as to the status of that measurement when an AVG modifier is also in use.

It remains unclear why there is no perimeter tolerance to ensure flexible parts will fit, something that "AVG" doesn't do.

 

[Burunduk]

The size dimension applies to the surface; there is no ambiguity.

A basic dimension used for locating a feature does not apply to a surface so whatever fever dream you are finding on drawings are very far from any sense of reason and no amount of rules will fix that.

The (F) is useful for idiot-proofing the manufacture and inspection process to ensure, by being explicit that there would be no confusion as to the status of that measurement when an AVG modifier is also in use.

It remains unclear why there is no perimeter tolerance to ensure flexible parts will fit, something that "AVG" doesn't do.

Is it far from any sense of reason to say that a basic dimension that locates a surface for a profile control applies to the surface?
Yet we are not supposed to attach a profile FCF to any basic dimension, it should be attached or pointing to the surface or surfaces.

Perpendicularity, parallelism or angularity has a different meaning when the FCF is associated with the size dimension. So is flatness and straightness. Having a single rule would only add clarity and consistency and make this language more accessible.

 

[3DDave]

Far be it from me to make an assumption, but every basic dimension is between two surfaces/features, except for radii and diameters. Even if one end references an axis, that is the axis of a surface.

Which of the two surfaces would the tolerance apply to?

Join the committee and tell them how to do the right thing. You won't win because it's not your profession to sell training materials or to sell required upgrades to CMM software that are needed to keep in step with the latest changes that, sometimes/often contradict previous parts of the standard.

 

[greenimi]

You won't win because it's not your profession to sell training materials or to sell required upgrades to CMM software that are needed to keep in step with the latest changes that, sometimes/often contradict previous parts of the standard.

.........or is it just that it appears to conflict with a rule that is made for no other reason than to have a rule?

It is the latest. The checker wants to follow the rules and regulations per the applicable standard. And I don't blame him.

If what you said (......"to sell training materials or to sell required upgrades to CMM software"....) is true than is sad.
We can expect than the "forever changes" to keep ASME in bussiness.

But then, at least stay consistent with your own rules. Don't break your own imposed rule in your own created figure.