Straightness based GD&T understanding 1

[thatdanielxy]

What's the correct understanding on below straightness definition?

 

[CheckerHater]

It is often forgotten that the purpose of GD&T is to help real people make real parts.
If it doesn't work, mistake was made somewhere.
Just couple cents.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[pmarc]

@pmarc: Like I mentioned in my post to greenimi, it's ambiguous, that makes it is not fully legal.

The fact that you think it is ambiguous does not mean it is not fully legal. Up to that point it was not even mentioned that there may be something else on that drawing (that we don't see) that controls the minimum thickness of the feature.

And you still haven't answered why you think it is "impossible to properly calculate virtual condition" of that feature. In my opinion, the lower size limit for external feature of size (whether explicitly or implicitly defined) has no influence on what the size of the virtual condition for that feature can be.

 

[CheckerHater]

@pmarc: it is "impossible to properly calculate virtual condition" because size tolerance is not specified on the drawing. Period

Single limits as described in Para. 2.5 are not the same as limits of size as described in Para. 2.7. They are different things.

Now, according to Para. 2.5 "Single limits are used where the intent will be clear, and the unspecified limit can be zero or approach infinity and will not result in a condition detrimental to the design."

In OP example unspecified limit WILL result in a condition detrimental to the design, so it is used improperly. End of story.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[pmarc]

In OP example unspecified limit WILL result in a condition detrimental to the design, so it is used improperly. End of story.

How do you know that without seeing the rest of the drawing or without at least knowing that there is nothing on the drawing that controls the unspecified limit?

@pmarc: it is "impossible to properly calculate virtual condition" because size tolerance is not specified on the drawing. Period

Single limits as described in Para. 2.5 are not the same as limits of size as described in Para. 2.7. They are different things.

As long as we agree that the way the concept of size works in both cases - single limits and traditionally toleranced size - is the same, then I do not have to know the lower size limit to be able to calculate the size of a boundary that will never be violated by the surfaces of the toleranced feature controlled with the DML straightness at MMC tolerance.

This is where I am putting my period.

 

[CheckerHater]

Excuse me? Are you saying that ANY size tolerance will result in THE SAME virtual condition?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[greenimi]

Are you saying that ANY size tolerance will result in THE SAME virtual condition?

I would say that if the upper limit (or the maximum material limit) is the same then the size is irrelevant.

 

[greenimi]

Since CH and pmarc (two excellent ISO gentlemen) are here, could you, PLEASE, convince me that kedu is correct in this thread. PLEASE!
I know, I know……. sneaky tactics.

https://www.eng-tips.com/viewthread.cfm?qid=445355

THANK YOU !!

 

[pmarc]

Excuse me? Are you saying that ANY size tolerance will result in THE SAME virtual condition?

No, I am not saying that. I am saying that if the maximum size limit is the same (regardless if it is defined as a single MAX value or is calculated from nominal size and +/- tolerance expression), the virtual condition will be the same.

So for example:
.024 MAX with DML staightness callout .010 @MMC will result in VC = .034
just like
.020 +/-.004 with DML staightness callout .010 @MMC will result in VC = .034.

In other words, to calculate size of virtual condition one does not need to know lower size limit for the feature. Only the upper size limit must be known.

 

[CheckerHater]

pmarc, greenimi,

Your interpretation allows for 0 thick part to be bent for .034

Just because you can formally make some calculation, doesn't make for good drawing and doesn't tell us how to make good part.

This is why the drawing may be technically legal but useless. I call it "not fully legal". If you don't like it - it's your opinion.

I am out of here.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future