Datum targets and their relationship

[Andera]

ASME Y14.5-2009 or 2018

Is there any requirements for datum targets to be related (orientated or located) to its higher precedence datums?
Searching for applicable and relevant figures in the 2009 and 2018 standards, I cannot find a single figure where datum targets are used AND the relationship between primary and secondary are shown with geometrical relationships (callouts).

 

[3DDave]

The relationship is considered basic - there's no requirement to use geometric characteristics symbols to control a tolerance for the targets. Depending on the order they are referenced datums derived from datum targets are oriented based on higher precedence datum feature callouts, just like any other datums are.

 

[greenimi]

Depending on the order they are referenced datums derived from datum targets are oriented based on higher precedence datum feature callouts, just like any other datums are

3DDave,

Do you have a figure or an example of such relationship?

 

[drawoh]

Andera,

No.

Your datum targets can be used as your primary datum.

--
JHG

 

[chez311]

4.24.7 Datum Target Dimensions
The location and size, where applicable, of datum targets are defined with either basic or toleranced dimensions. If defined with basic dimensions, established tooling or gaging tolerances apply.

Specification of datum targets defines the simulator geometry and location/orientation, and when the dimensions applied are basic the default tolerances are "established tooling or gaging tolerances" because as I said you are defining the simulator. I suppose the allowance for directly toleranced dimensions would mean one could apply a geometric tolerance to the *datum targets by extension of concept however there is no example of either in the standard and it seems unadvisable - it would also likely override any applicable gage tolerances. Even then, regardless of how the datum targets are defined since as previously noted you are only defining the simulator (or in 2018, the True Geometric Counterpart) - the feature which it contacts still needs some tolerance applied to the feature itself otherwise it is uncontrolled.

*Edited

 

[3DDave]

greenimi

Like a nominally rectangular, but lumpy cast block with 3 target points on one face, two on a perpendicular face, and one on a mutually perpendicular one, establishing three datum planes.

 

[metrologic]

Andera, there are many examples in section 4 illustrating datum targets related to higher precedence datums using basic dimensions. (But no toleranced examples.) Also, in accord with chez311, datum targets appear to be essentially a proxy for datum simulator geometry. And as such, I believe they should have the same requirements for basic location and orientation as datum feature simulators. This implies that lower precedence datum targets need to be orientated and located to other relevant datum feature simulators. Furthermore, look at subsection 4.24.14.(2009) It looks like they want us to specify which lower precedence targets to use for locating the datum targets referenced in a feature control frame, even when there are no secondary or tertiary datum features beyond the primary reference.

 

[greenimi]

So, why do you think datum target A and B are not related with each other?
Should I understand that they are perfectly perpendicular to each other?

Are there any examples within Y14.5 or ISO within which those datum targets ARE related with geometrical tolerances with each other?

 

[chez311]

So, why do you think datum target A and B are not related with each other?

They are - where do you see suggested they aren't?

Should I understand that they are perfectly perpendicular to each other?

Well A and B are points, so I'm not sure how perpendicularity comes into it. Yes the datum targets are defined by a perfect theoretical basic distance to each other. This determines what the theoretical simulators should look like and their basic geometry/location/orientation, and the resulting physical datum target simulators are defined by applicable gage tolerances.

Are there any examples within Y14.5 or ISO within which those datum targets ARE related with geometrical tolerances with each other?

As far as I can tell, not in Y14.5-2009 or 2018. I can't speak to ISO.

 

[greenimi]

https://www.eng-tips.com/viewthread.cfm?qid=449165

I have to disagree slightly with you here.
See the above discussion.
Looks like the opinions are split between the experts.

“ This isn't really a question about datum targets. Regardless if datum feature B is referenced RMB or MMB and regardless if secondary datum axis is derived from datum targets or from the entire feature, it shall have a perpendicularity control relative to A, othwerwise the feature won't be fully defined.

Besides, if you take a look at the RMB definition (1.3.49) it says that: "regardless of material boundary (RMB) indicates that a datum feature simulator progresses from MMB toward LMB...". This means that even in RMB case there has to be a MMB from which the progression of datum feature simulator should start.”

 

[metrologic]

@greenimi

This is a good discussion for me. I'm studying datum targets next week. In Fig 4-53, I don't see an MMB boundary for the B target set. There's a feature of size spec for B. And if I'm not mistaken, there is an implied 90 degree angle between nominal datum features A and B, but it goes un-toleranced. (Technically, I think Y14.5 says there isn't a BASIC 90 until an associated geometric tolerance controlling that angle has been specified.) I think the orientation and location of the B target simulators is clear, but the MMB boundary from which they progress has not been defined due to the missing perpendicularity control on feature B.

 

[3DDave]

2.1.1.4 Implied 90° or 0° Basic Angle "basic dimensions or geometric tolerances have been specified"

Just basic dimensions is enough.

 

[chez311]

greenimi,

I don't see anything in that thread that contradicts what I said. In fact, its right there in your quote "this isn't really a question about datum targets."

Lets read the rest of the statement:

Regardless if datum feature B is referenced RMB or MMB and regardless if secondary datum axis is derived from datum targets or from the entire feature, it shall have a perpendicularity control relative to A, othwerwise the feature won't be fully defined.

If I'm reading it correctly, the "it" refers to the datum feature which must have a perpendicularity control, NOT the datum targets. I said something similar in my responses above:

Even then, regardless of how the datum targets are defined since as previously noted you are only defining the simulator (or in 2018, the True Geometric Counterpart) - the feature which it contacts still needs some tolerance applied to the feature itself otherwise it is uncontrolled.

"The feature" being the datum feature here.

Of course I don't want to put words in pmarc's mouth but thats how I read it.

 

[metrologic]

@3DDave

Good catch! In 2.1.1.3, why is there only an implied 90 degree angle versus an implied 90 and 0 basic in 2.1.1.4?

 

[greenimi]

Chez311,

This statement threw me off "Regardless if datum feature B is referenced RMB or MMB and regardless if secondary datum axis is derived from datum targets or from the entire feature, it shall have a perpendicularity control relative to A"

Should I understand that even if datum targets are used, a perpendicularity between Ø170 feature and the planar surface/ the flange is needed?
I am confused now about your replay.

Thank you for the clarification

 

[chez311]

Should I understand that even if datum targets are used, a perpendicularity between Ø170 feature and the planar surface/ the flange is needed?

Correct.

 

[greenimi]

chez311 (Automotive)11 Oct 21 12:31
Quote (greenimi, 11 Oct 21 10:43)
Should I understand that even if datum targets are used, a perpendicularity between Ø170 feature and the planar surface/ the flange is needed?

Correct.

Then why do you think the standard does not have absolutely any figures to show such?
For so long and for so many revisions....1994, 2009, 2018
Seems strage to me...

Maybe the intent is NOT what we have concluded...I am just saying.
Couldn't be a missed from the standard committee for so long.

 

[chez311]

greenimi,

I cannot say exactly why they may have done this, the only thing that comes to mind is that we can fully define our datum targets without tolerances applied* to the feature themselves and know exactly what our simulators should look like with basic dimensions so the figures were simplified. I guess the only exception would be if MMB is specified for a feature like the one in your thread (Y14.5-2009 fig 4-53) we would need a tolerance applied to B to determine our MMB. Speaking of, I can't think of a good reason why someone would apply MMB to datum targets - seems like it would allow some irregular behavior that could be difficult to quantify without an in-depth study. Perhaps on features that are very large where a fixed FOS gage wouldn't make sense or be practical?

Lets take it from your angle. In Y14.5-2009 fig 4-53 what do you believe to be the orientation tolerance, implied or otherwise, which relates datum feature B to A? Or in other words, what control do you believe is relating orientation of datum feature B to A that keeps it from being undefined?

*Edit to add: Even if the datum features which the datum target simulators contact are undefined.

 

[greenimi]

Chez311,

In Y14.5-2009 fig 4-53 what do you believe to be the orientation tolerance, implied or otherwise, which relates datum feature B to A? Or in other words, what control do you believe is relating orientation of datum feature B to A that keeps it from being undefined?

My personal opinion. The standard speaks with both sides of their mouth.
I would say drawings are "intentionally" incomplete (from this point of view) to not contradict past practices or people's prefferences.
Let's ask commitee members, on why?

See attachment. Even Y14.8-20xx draft does not deal with that question you addressed me. So I have no idea.
Not sure why this issue is not addressed. Shouldn't be clarfied?

 

[chez311]

Well if theres going to be another side, there has to be some substance to the other side/opinion right? I'm not asking why in my question to you, I'm asking what. If you think there is an alternate interpretation, I'd be interested to know what it is.

Theres plenty of places the standard comes up short or contradicts, I don't see this one as one of them. Sure, the drawings could be a bit more complete for clarity but I don't see the omission as suggesting ambiguity.