Planar surface with interruption - single datum or compound? 1

[dtmbiz]

Single or Compound Datum?

1) If I have a block with 2 flanges (mounting features) on either side of a cutout. And those flanges had mounting holes in them, I would/could use the single planar datum from two surface features and call them e.g. A-B to establish a datum plane.

2) If I have a large let’s say .250” thk. plate 48” x 24” with a .125” wide by .125” deep groove across the width of the plate, would this be a single datum feature with a groove? Would it be proper to call datum A as the planar datum or use either side of the groove for A-B?

3)If the groove did not cross the entire part surface but stopped an inch or two short of the part edge, then I would say it is a single surface in that scenario and would use just a single datum letter A to identify the entire surface


I have had this discussion before as described in the 1st example (the block) and the project engineer sees both flanges as a single datum, even after I showed him the A-B from the 1994 standard.

The large flat surface with a "small break" across the width of the part resulting in to distinct surfaces is the interesting thought for me. That is considering "large surface area" with a "small" feature that interrupts that surface.

-Basically is the size of the interrupting feature of any concern or.....
is it the strict technical definition of being 2 separate features requiring the A-B callout regardless of feature sizes involved?

Is there anywhere in the 1994 or 2009 standard that gives more detail on this question?

Comments?

 

[drstrole]

Section 4.12 and Figure 4-23 on page 67 of the 2009 standard addresses this.

Drstrole
GDTP - Senior Level

 

[pmarc]

dtmbiz,

The example from Y14.5M-1994 I believe you are referring to (4-20?) has been modified in 2009 edition. So currently, if 2 or more separated surfaces are shown coplanar on a drawing, it is allowed to use single datum letter (see fig. 4-23). I would recommend reading paragraph 4.12.1 in Y14.5-2009.

Based on this I think that for all 3 cases you described single datum letter may be enough. The difference will be that for cases #1 and #2 you physically have 2 datum feature surfaces so mutual relationship between them, as well as their form, should be controlled by profile of surface tolerance. For case #3, as it is a single datum surface, flatness control is a way to go.

One more thing: please notice that in the figure 4-20 from '94 edition no mutual relationship between two datum feature surfaces A & B was defined. This means that allowable offset between them is not controlled at all. As a result you can have two surfaces offset e.g. 5 or 25 or 100 mm, based on which you will have to establish single datum plane. Not quite sure, but this might be one of reasons why this figure was modified in 2009 version.

 

[dtmbiz]

dstrole,
We are using 1994 std... thanks

pmarc,
Yes I am referring to fig 4-20. Read it again and see it addresses the groove or slot and geometric control consideration for coplanar features... thanks

 

[CheckerHater]

dtmbiz,

If you want to emphasize the fact that your multiple datum features are, in fact, one single feature being interrupted you can use new concept of "continuous feature".

I know, you are using 1994; my copy of 2009 sais right on the front page: "Revision of ASME Y14.5M-1994 (R2004)".
so, I am using 1994, you are using outdated 1994

 

[dtmbiz]

Interesting CheckerHater

Which makes me wonder about the standard reference in our title blocks. In the past we never referred to a revision; just the year.

Wonder if we should consider modifying the ASME ref to indicate that the 1994 is the original and not the rev.

How would that be done?... or if a company moves on to 2009; should the tblk use 2009 or 1994 (R2004).


You like openning cans of worms?

 

[pmarc]

Checker,
Per 2.7.5 of Y14.5-2009 continuous feature symbol <CF> should be used if two or more features of size were considered. Planar faces are not features of size, therefore I do not think it is a good application for this concept.

dtmbiz,
Paragraph 1.1.3 of Y14.5-2009 states that: "Where drawings are based on this Standard, this fact shall be noted on the drawings or in a document referenced on the drawings. References to this Standard shal state ASME Y14.5-2009."

In 2004 revision of 1994 standard there is a very similar statement: "Where drawings are based on this Standard, this fact shall be noted on the drawings or in a document referenced on the drawings. References to this Standard shal state ASME Y14.5M-1994." Notice that there is still 1994 not 2004

 

[dtmbiz]

Thanks Pmarc,

Just received a copy of 2009 and read 1.1.3 just before I saw your comment.

Side Bar:
"Not sure how "CheckerHater" would like being referred to as "Checker".

 

[CheckerHater]

dtmbiz,
Sorry, next time I make a statement that is not 100% serious, I will put 5 or 6 smiley faces next to it, just to be sure.

On serious note, it looks like all the new versions of standards have a sub-title saying in big letters "Supersedes" , "Revision of", etc. to clarify the status.
I just wanted to point out that 2009 is further refinement of 1994 and not "opposing" it in any way, so your company could very well make a transition.
Judging from the fact that you already own a copy, you don't need this kind of encouragement.

By the way, if I remember correctly, Y14.5 is the only standard that must be specified by the year; all other are assumed to be the latest revision/release. I could be wrong though.

pmarc,
If you look at Fig. 7-45 in ASME Y14.5-2009 you will see that definition of <CF>, ( or FOS?) is, should I say, blurry?

Sometimes I call myself "Checker" in my resumes . But I firmly believe "checker" should be assignment, rather than profession.

 

[dtmbiz]

CheckerHater

No reason to be sorry... not offended...
I like to think that I have a pretty good sense of humor or wouldnt have survived the years...

currently "design auditor".... change the names to protect the guilty

on to the next thread......

 

[pmarc]

CheckerHater,
You are right, 7-45 seems to make Continuous Feature definition blurry, because <CF> symbol is obviously applied to surface elements.

Additionally the relationship between two pieces of surface A is not controlled anyhow there, so this makes things even worse. Whether it has been done intentionally or not is a different story.

 

[MechNorth]

Based on the '94 standard, use a surface profile control to relate the multiple surfaces together for coplanarity; it's the only way to control the relationship.

Every standard invoked on the drawing (Y14.41, Y14.8, etc.) should have the full designation, including the year. It indicates how the drawing is to be read and interpreted. Enough changes are made that revision control and therefore revision designation needs to be noted.

If you have the option of going to '09, or invoking the <CF> fromn '09, it's a good way to go. In my seminar today, a participant asked when they should use <CF> rather than a coplanarity control. My thought/philosophy on it relates back to the manufacturing method. If the multiple surfaces were originally fabricated as one surface and some sort of interruption was added afterward , then I would consider those features to be continuous; otherwise, I would use a coplanarity control instead to make sure that the surfaces are in fact co-located.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[pmarc]

Jim,
Even if those surfaces were fabricated as one surface, from complete geometry definition point of view there is still a need of assigning a tolerance which would control mutual relationship between them or at least their form - especially when the surfaces are assigned as primary datum features like in fig. 7-45. <CF> itself is in my humble opinion not enough. It helps to express design intent (in ideal case two surfaces are thought to be coplanar), but it does not give any value for coplanarity deviation.

For two or more features of size (as shown in figs. 2-8 to 2-10) this relation is clearly defined by size dimension and rule #1 envelope.

 

[MechNorth]

Didn't think I implied otherwise, Pmarc. If it's not a FOS, then flatness (or coplanarity by profile) is still needed of course. Just giving a personal thought process on when/why to use the <CF> vs coplanarity by profile.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[fsincox]

Checker Hater,
Excellent catch, I have always thought the ASME was trying for the same concept as the ISO and just bungled it. That example pretty much calls a spade a spade in my book.
Frank

 

[fsincox]

So let me get this straight, You guys do not belive a dimensional relationship is sufficient to control surfaces like shown in fig. 7-45 even if it was dimensioned from a directly opposed surface without using a profile control and just relying on dimensional limits?
Frank

 

[pmarc]

Frank, why do you think we do not believe so?
If there was a dimension like you described on 7-45 and <CF> modifier still present, then the relationship (coplanarity) between interrupted surfaces would be defined and limited by the amount of size tolerance.
The point is this figure shows nothing that controls such relationship and that is why I have a problem with it.

 

[fsincox]

pmarc,
OK, I did not understand it that way. I generally start with the position that the drawings shown in the standard are "supposed" to be "correct" and dimensions and notes are then removed for simplicity ASME Y14.5M-1994, paragraph 1.1.4. I also know that on a "correct" drawing the surface must be defined and then read it as it was just removed from my view. Obviously if the position/coplanarity was to be done with profile the <CF> would not really be required. So, I thought it best to clarify.
Thanks,
Frank

 

[dtmbiz]

It appears from the responses that I didn’t make my question clear. I understand that a geometric control would be required to establish a relationship of two coplanar surfaces on either side of an interrupting feature such as a groove thru the center of an otherwise flat surface.

******************************************************************
4.5.7.1 Simulation of a Single Datum
Plane. Figure 4-20 is an example of a’single datum
plane simulated, as explained in para. 4.5.1, by simultaneously
contacting the high points of two surfaces.
Identification of two features to establish a
single datum plane may be required where separation
of the features is caused by an obstruction, such as
in Fig. 4-20, or by a comparable opening (for example,
a slot) of sufficient width. Where appropriate,
an extension line may be used to indicate a continuation
of one datum feature across slots or obstructions.
For controlling coplanarity of these surfaces, see
para. 6.5.6.

****************************************************************
The second to last sentence is where my question lies. “Where appropriate”… is the wording of concern. If I have a large flat surface of lets say 12” x 1” and I take maybe 6” of material out of the center, thru the whole thickness; I am left with two flat surfaces on either side of the cutout. This is obvious from my view that this is a A-B situation to indicate that I want those 2 surfaces to be in the equation for assembly to a mating surface ..And for inspection to use both surfaces to establish a single datum when I use it as the primary datum.

However if I have the same 12” x 1” surface with a .0625” groove cut thru the center, I still want either side of the groove to establish a single datum. The second to last sentence of 4.5.7.1 leads me to interpret that I would NOT need the compound A-B callout to use the entire surface as a single datum.

It reads to me that a slot or groove thru the entire surface would need a geometric control for coplanar requirements, however what tells inspection that I want points on either side of the groove to be used in establishing one single datum?

4.5.7.1 “….a comparable opening (for example,a slot) of sufficient width…”

What is sufficient width?

Seems subjective to me and was wondering what size feature would make others consider/use the compound callout?

 

[MechNorth]

The "Where appropriate..." statement refers to the use of the phantom line across the gap, indicating the implied/intended coplanarity of the features. "sufficient width" can't be quantified, but essentially it means that the visually they are far enough apart on the drawing that they may not intuitively/visually appear to be coplanar. When in doubt, just add the phantom line establishing the relationship.
I wouldn't use the multiple-datum-feature (compound) callouts unless there were multiple separations between the considered features (e.g. multiple slots and/or tabs between the considered features).

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com