Tangent Line as datum

[Bux1986]

Hi
1) Can I use the tangent line in red as my third datum?
If not then what would be the correct way?
Please see the attached picture.
Thank you

 

[greenimi]

There are plenty of such figures within Y14.8 (casting standard). I know, different committee, but some people suppose to know what’s in one standard and be consistent within Y14 family.

I don’t know if I agree with “Points aren't features. Not a feature, can't use a datum feature symbol. This also applies to datum target areas, in case that was the next question.”
Again, there are plenty example otherwise within Y14.8

 

[3DDave]

greenimi, I expect zero consistency; consistency reduces training sales. Are you in 14.8? Why is 14.8 inconsistent with 14.5?

 

[pmarc]

greenimi,

The reason I recommended to remove the datum feature C symbol was basically to avoid confusion as to which features the datum center plane C should be established from. Besides, if I wanted to use the symbol, wouldn't I have to add a linear dimension to the view at the extension of which the datum feature symbol could be placed? I am not sure I see a benefit behind doing this.

Generally, my experience has been that when datum feature symbols are combined with datum target symbols, often times there is a confusion as to which features exactly should be used to establish the datum. I guess this was one (if not the only one) of the reasons why ASME decided to adopt in Y14.5-2018 the ISO practice of specifying the datum target letter and number(s) next to the datum feature symbol in such cases.

 

[3DDave]

I see from the 2009 version of 14.8 not one mention of "datum feature symbol"

But it does say "Datum targets and datum features may be combined to establish a datum reference frame." indicating there is a notion they are different.

Is there a statement that a width is required to have a size dimension in order to be used as a datum feature? Seems like RFS/RMB application on a profile should be allowed. For certain I could center the part in a vise and not care what the width "size" is and locate the center as exactly as the equipment and my time would allow just the same.

If centering a part in a vise is not allowed then there is a massive failure in the writing of the standard.

 

[Burunduk]

The standard doesn't say "A datum feature is established by datum targets." Sometimes the terms are used interchangeably, other times as distinct ways to establish a datum.

But it does say that datum features are features labeled by either the datum feature or target symbols. Look at the definitions section under "datum feature".

As clarified in the section on datum reference frames, both symbols can be used on the same drawing to designate the same datum feature.
This is useful when you show a detail view where the datum feature is shown, but the datum targets are not included.

 

[3DDave]

That is useful for creating confusion and should be marked reference to distinguish that is it defined elsewhere, just as any other dimensional control is. However, rules makers are unconcerned with the results anywhere but within the committee meetings.

"a feature that is identified with either a datum feature symbol or a datum target symbol(s)."

Doesn't include "or both." Only one or the other. Doesn't affix a priority. Doesn't require enumerating all the targets at a dstum feature symbol.

 

[Burunduk]

That is useful for creating confusion and should be marked reference to distinguish that is it defined elsewhere, just as any other dimensional control is

A datum feature or a datum feature symbol is not a dimensional control. It is used for a dimensional control, but it is not one in itself. Therefore, when the standard specifies that "Where the same datum feature symbol is repeated to identify the same feature in other locations of a drawing, it need not be identified as reference", it makes sense.
Also, if the applicable datum targets are clarified near the datum feature symbol like Y14.5-2018 requires, I don't see why anyone should be confused.
A datum feature identified by datum targets is still a datum feature, that's why the datum feature symbol is not discounted.
As a drawing user I often prefer seeing as much of the relevant information as possible in one place and not having to look through the entire drawing, that may contain several sheets, to find the datum targets that define the datum feature referenced in the feature control frame I see in the detail view. If the detail view shows a portion of the datum feature, there is no reason not to identify that datum feature by the datum feature symbol and the datum targets indication near it.

 

[3DDave]

" it is not one in itself"

I didn't say it was. And it still doesn't make sense for it to be repeated without calling it reference any more than reproducing a dimension should be.

As a drawing user it is your job to understand the entire drawing. If someone f's up and misidentifies a feature as a datum in a view that is different than using the same identification of a different feature in another view - it's up to you to find that error.

What you are looking for is an reference block that lists where the datum features are defined. Before anything one needs to understand the topology of the part and how the datum features are defined. If you are reviewing a drawing you can make that reminder yourself in all the places you feel the need to; if that's difficult then that's a lack of understanding of the part.

In fact, as part of understanding the part one should be looking at mating parts and identifying related datum features to understand how the tolerances will stack up.

 

[Burunduk]

"I didn't say it was"
Not explicitly but "should be marked reference to distinguish that is it defined elsewhere, just as any other dimensional control" implies that you treat it as another type of dimensional control.
The reference symbol (parentheses) is for "indicating a dimension or other dimensional data as reference". You stated not once that dimensioning and tolerancing symbology is a programming language - then putting the datum feature symbol in parentheses will result in a code that doesn't work.
Applying the datum feature symbol and datum target symbols on the same feature in different places on the drawing will not cause an error in the code. But, it may aid drawing readability for humans, when the geometric tolerance and a basic dimension given from an identified datum feature are shown all in one view, even if the datum targets that specify how exactly to fixture to that datum feature and what to base the DRF on are shown elsewhere.

 

[axym]

Hi All,

The way I look at it is that datum targets define a different type of datum feature simulator (true geometric counterpart).

Instead of the "default" TGC that covers the entire datum feature, datum targets define TGC's that would contact at particular points, lines, or areas on a perfect part. The datum target TGC's have a basic relationship to each other and to the other part features, just like default TGC's do.

The labeling of the datum feature itself seems less critical. Some figures in Y14.5 show the datum feature symbol as well as datum targets, and some do not.
To me the datum feature is the feature the part that the datum target TGC's contact, whether it is labeled with a datum feature symbol or not.

I also never noticed the use of the A1, 2, 3 and B1, 2, 3 annotations in Fig. 7-64 in 2018. I'm still not sure that I see what value this adds.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

I also never noticed the use of the A1, 2, 3 and B1, 2, 3 annotations in Fig. 7-64 in 2018. I'm still not sure that I see what value this adds.

It comes from paragraph 6.3.3.1 (near the end). This requirement was not in paragraph 3.3.3 of 2009.
It's probably because they thought there may be confusion if someone sees the datum feature symbol but fails to notice that it's broken down into datum targets.

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

 

[3DDave]

I would have preferred [A1, A2, A3] as that is something the software can be programmed to find and not depend on some proximity to the callout. Still, it is unnecessary and redundant.

Also, Burunduk, you really know so little about software - constants are defined only once in a program and are not repeatedly given alternate definitions. Compilers bitch if a constant is redefined, even with the same value for just that reason. Since they form the origin of dimensional measurement, they are dimensional controls. You can see this by deleting them entirely from a drawing and seeing if the Feature Control Frames still make sense.

I'm sure some on the committee are happy to have your unquestioning support for some really questionable decisions but they won't send you a coupon for a free coffee.

 

[flash3780]

I think you can use a datum target to establish the tertiary datum.
However, datum C doesn't look very functional to me as you've shown it.

Have you considered using the bolt holes as datum B and locating all of the features on the part relative to the hole pattern?

 

[axym]

J-P,

Thanks, that helps to make sure that the reader notices the datum targets.

One other thing I'm wondering about in 7-64 though. In the section view, the B1, B2 and B3 targets (and the A1, A2, and A3 targets) are shown at the same point even though they're not directly behind each other in that view. Am I missing something?

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[3DDave]

Reading the drawing helps one notice the datum targets. Adding the symbol disguises the datum targets as anyone looking for the datums would see the datum feature symbol and think that is the definition for that datum. Adding proximate, but not embedded information, tries to offset the disguise.

The section was largely unnecessary; a small breakout would do the job, allowing the targets to be shown at the correct locations in that view with the "hidden" line style for the targets on the far side of the material.

For example, PTC Creo software would not group the datum target symbols that way and would show them in the right hand view in the correct locations.

 

[Burunduk]

Also, Burunduk, you really know so little about software - constants are defined only once in a program and are not repeatedly given alternate definitions. Compilers bitch if a constant is redefined, even with the same value for just that reason. Since they form the origin of dimensional measurement, they are dimensional controls. You can see this by deleting them entirely from a drawing and seeing if the Feature Control Frames still make sense.

You got the context wrong.
Any language, computer or human, has rules. If you follow the rules of the language, you can communicate effectively.
The dimensioning and tolerancing language is a symbolic engineering language.
ASME Y14.5 or the ISO GPS standards define this language, which should be both machine and human-readable. Allowing repeat of information in a synoptic manner ("This is the datum feature. Datum targets for this datum feature are defined elsewhere") benefits communication with humans.

Datum features are not dimensional controls, even if they form the origin for measurements. Dimensional controls set the limits for dimensions. Dimensions and dimensional data are numerical values, and this is what the reference symbol (parentheses) can apply to.

 

[Belanger]

Evan, that's a great question and I didn't notice that either. But I think it's because when you create a section view, you only show the visible lines for that exact cutting plane. Since B2 and B3 don't appear down at the 6 o'clock location, there's nothing to be able to point to in the section view. The next best thing is to shown them with B1's cross, so we at least know where the targets are longitudinally. That seems to be what 3DDave is saying above, too.
I checked the previous versions, and the same pic with section view appeared in 2009 and 1994. However, in 1982 (Fig. 114) they didn't have a section view, so they just showed a couple of crosses for B2 and B3 floating in the main longitudinal view.

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

 

[3DDave]

Parentheses directly indicate "look elsewhere."

There is no requirement specified that the datum target identities be nearby the duplicated symbol in the standard, so there is no requirement to indicate that the datum feature is defined elsewhere.

By not replacing the reference inside the datum feature symbol with the target identities there is opportunity for confusion.

When communication rules are created that increase the ability to misunderstand the communication the rules are wrong.

People developing software discovered that eliminating duplication was a good thing years ago. There is easily 100X more (100,000X?) software than dimensioned drawing symbols. Follow the experts at communication, not the ones voting to be polite and keep antique methods.

 

[Garland23]

There is no requirement specified that the datum target identities be nearby the duplicated symbol in the standard

It looks like there is such a requirement in the 2018 ASME standard.
"Where the datum feature symbol is used in combination with datum targets, the letter and numbers (separated by commas) identifying the associated datum targets shall be shown near the datum feature symbol."
From paragraph 6.3.3.1 (mentioned above somewhere).

 

[3DDave]

Correction noted. Also noted is that the requirement is at the end of a long paragraph to add a requirement not part of the previous edition. As indicated before, this addition was to cover for a problem that was self-inflicted. The change went unmentioned in "PRINCIPAL CHANGES AND IMPROVEMENTS" suggesting that it was not communicated to the group in charge of noting differences.

But thanks for getting there first.