How you read this drawing?

[Kedu]

I've always been under the impression that if the drawing creator missplaced the datum feature symbol for a cylindrical surface, the datum is still the axis (yes, even the datum feature symbol is not aligned with the size dimensions, AGAIN, on a cylindrical feature and not on a width/ tab).

Anyone else seeing this otherwise?
I will learn from anyone.

Again, as I stated before, for me if someone misplaced the datum FEATURE SYMBOL on a cylindrical surface (not on a width or tab) I will interpret it just as I stated, A MISSPLACEMENT and NOT that the datum is now a plane (or a line) instead of the axis.

opposite opinions are welcomed and encouraged.

 

[Belanger]

Woosang -- for the most part I agree. But the language does leave some wiggle room. It simply boils down to this: When a datum feature symbol is attached in ANY way to a cylindrical object, and does not employ datum targets, then wouldn't the datum have to be the axis?

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[greenimi]

J-P,

When a datum feature symbol is attached in ANY way to a cylindrical object, and does not employ datum targets, then wouldn't the datum have to be the axis?

Looks like Tec-Ease said NO, it does not have to be the datum axis! Could be "datum plane" and a "line in datum plane"

Go figure!

 

[Belanger]

greenimi -- that was discussed above. They are not showing a good example. If they want a "datum plane" and a "line in datum plane" then they should have used a datum target.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[ctopher]

Either way, it's difficult these days to find engineers/machinists/inspectors that understand GD&T anyway.
I have to use less of it for this reason. I get into arguments all the time because they want to remove it. They think it costs $ more with it.
Frustrating.

ctopher, CSWP
SolidWorks '17
ctophers home
SolidWorks Legion

 

[Burunduk]

ctopher,
Do you feel that the situation used to be better in past years?

 

[ctopher]

Yes, it used to be better. In the area of creating and reading engineering drawings, more people are less educated and lazy.

ctopher, CSWP
SolidWorks '17
ctophers home
SolidWorks Legion

 

[chez311]

JP,

Thats the next sentence of the tip.

In the view below, because the datum target feature is offset from the size dimension line, the datum may be interpreted as a line lying in a plane tangent to the feature indicated. If line contact is desired a datum target line should be indicated.

To me, it doesn't seem like they're arguing a tangent plane would be a valid interpretation - only that its possible that someone could interpret it that way and to avoid placing your datum feature symbol in such a manner. Hence the statement about datum targets. You would get my full agreement that such a specification as shown (without a datum target line) is absolutely not a valid way to specify a tangent plane/datum line.

Not trying to hold your feet to the fire on a topic discussed a long time ago (

https://www.eng-tips.com/viewthread.cfm?qid=334165)

but I agree with what you said in that thread "just make sure the drawing is clear". I would not say that the notation shown in the OP/tec-ease tip is clear - the fact that this discussion keeps coming up (and I've seen other questions on here about it, one I believe where the OP actually intended to derive a tangent plane) tells me as much.

 

[TheTick]

I see the issue now. Yes, ambiguous, at least at a technical level.

I don't think I've ever used a silhouette as a datum.

 

[3DDave]

I don't recall a Y14.5 standard example of finding the tangent to a surface and using it as a datum target.

 

[TheTick]

This could be viewed as a "brown M&M" type of indicator...

https://www.insider.com/van-halen-brown-m-ms-contract-2016-9