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# can a 90 degree feature be used as a datum?6

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## can a 90 degree feature be used as a datum?

(OP)
I have a thermal strap, in 2 halves, which straddles a bellows fixture. There is an r51.5 mm feature which provides clearance around the bellows. the (2) halves make up a 180 degree segment, meaning each half is 90 degrees. i am detailing 1 segment, with the other half opposite hand. the drawing is attached as is.

The flat which bolts up to the bellows is -A-. i think the r51.5 should be -B-, with a size tolerance, perp to -a- so it locates the bolt circle pattern. As it is, what is the relationship?

Some say I can't use the arc feature as -B- because is's not a full hole. I say with a CMM, you can find the center axis and verify the size of it, and use it to locate the bolt holes.

Any suggestions? Other than dodge the bullets?

teddykaye

### RE: can a 90 degree feature be used as a datum?

If you have the Y14.5-2009 spec handy then section 4.16.1 with Figure 4-29 (pg 71) shows them calling out the axis of a crescent shaped cut as a datum. It doesn't seem that different from yours.

### RE: can a 90 degree feature be used as a datum?

(OP)
DanStro: Thanks for the quick reply. My supervisor needs to see this reply to see the importance of getting the new standard.

teddykaye

### RE: can a 90 degree feature be used as a datum?

Many here subscribe to the calliper test for using FOS as a datum.

However, I'm not convinced that's what the wording in the 94 version explicitly says.  I'm at home so don't have the standard to hand.

Similar has been discussed before in some detail, even getting heated as I recall, you may be able to find something.

One think though, double check how you've got B identified, if you mean to be using the cylindrical surface as the datum feature I'm not sure what you have is quite right, but like I said I don't' have the standard to hand.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

### RE: can a 90 degree feature be used as a datum?

The way he has B identified does not callout the radial surface it calls out he surface tangent to it.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net

### RE: can a 90 degree feature be used as a datum?

(OP)
To all:
I want to MAKE the 90 degree cylinder datum -B-. How can one inspect the bolt hole pattern without it? That's what i'm trying to convey.
But my boss says I HAVE to have a complete hole! And i disagree with him.

teddykaye

### RE: can a 90 degree feature be used as a datum?

Yes, what you are describing was somewhat debatable prior to the 1994 standard, so I see what your boss might have been thinking.  The way around that might have been to use datum targets.

But with the 2009 edition, the previously mentioned page 71 has a very close cousin to what you seek (thanks Dan!).  So the boss's concern was addressed and it is now kosher to have a curved datum feature that will indeed yield an axis.

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

### RE: can a 90 degree feature be used as a datum?

2
Is your boss one of the members here by any chance? I agree with Ken. I feel function should be the ultimate determinator. 2009 appears to open many doors thrown shut here many times before. I understand making things easier for people in principle, but it hasn't helped the standard's acceptance one bit from my point of view. Accommodate if you can but don't lie to people, if it is important it is important to say so.
I love 2009.
Frank

### RE: can a 90 degree feature be used as a datum?

The shops I worked with would love your print, except all of that "funny" profile stuff. We have a nice little corner datums, edge, edge, edge. Does't look very stable but, they will love it. I don't even need to know how it functions :).
Frank

### RE: can a 90 degree feature be used as a datum?

While it's apparently allowed by the 2009 standard, by ASME Y14.5M-1994 standard it's not. So, if your drawing references the new standard, it shouldn't be a problem.

### RE: can a 90 degree feature be used as a datum?

flash,
I will respectively disagree with your statement. I do not know it you are on the committee or not. I am not. I do know that it is, in fact, a committee of people and that they, themselves, do not necessarily agree on all issues before them. This is discussed in the another group I visit. There are some here in this group that did not feel the '94 interpretation was as clear as you have stated. I have always disagreed with its veracity. I suspect some powers within the committee felt "a feature of size" needed a clearer definition (I call "the caliper guys") and so the text was changed in '94.
Real parts are not always as simple or black and white, as ASME parts. I believe if it functions with a partial radial locator or a taper it should be stated as so. It would appear the argument was made in the committee that this change was being interpereted to loterally and in a typical pc fashion, instead of saying we were wrong, we now have regular and irregular features of size.
Just as the definition of composites was expanded on from the '82 standard in '94. I would maintain the committee has made it's statement of intent with 2009.  I have never looked at the committee as God so I never assume they are always right , I prefer to look at it as their best effort at the time this would include 2009.
Frank

### RE: can a 90 degree feature be used as a datum?

flash,
Sorry, literally. I do not doubt it was clear to you. :)
Frank

### RE: can a 90 degree feature be used as a datum?

Ken,
I like the first one, it seems an example of the extent that some are willing to go to avoid the dreaded GD&T.
Frank

### RE: can a 90 degree feature be used as a datum?

Actually, '94 had the option of using any mathematically defined feature as a datum feature; the radial surface then could be used as a datum feature, with the simulator arc built at the nominal basic 15mm radius.  This would effectively eliminate 4 degrees of freedom.  You could then show the arc center point as the origin of measurement on the drawing, with other features tied to it by basic dimensions.  Quite legal, though unconventional.

A feature of size requires directly opposed points; pre-94, I believe that the caliper rule was shown.  As a result, the radius cannot be used as a datum feature of size.

Now, if you wanted to use the width-center of the part as a datum feature of size, you could, per '09; it's an irregular datum feature of size (see Sect. 4-17).  You could also have used the pattern of mounting holes as a pattern of datum features (see Fig. 4-28 in '09).

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

### RE: can a 90 degree feature be used as a datum?

#### Quote (ASME Y14.5M-1994):

1.3.17 Feature of Size.  One cylindrical or spherical surface, or a set of two opposed elements or opposed parallel surface, associate with a size dimension.

I still don't see that it says 'complete cylinder', neither do I see it saying 'and two opposed...'

So I'm still not convinced from what the standard actually says that using a partial cylinder of less than 180° at worst case tolerance isn't supported, maybe I'm just that dense though.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

### RE: can a 90 degree feature be used as a datum?

But Kenat, you previous post used the adjective "partial" in front of the word "cylinder."   Therefore, it is a a slightly different concept than what the standard envisioned for the definition of a feature of size (in '94, at least).

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

### RE: can a 90 degree feature be used as a datum?

Kenat, Jim, John-Paul

I am afraid we are starting FOS debate again. I think we will all agree that feature of size definition from 1994 standard is not precise enough. 2009 definition, although more detailed, still did not solve the problem.

However this doesn't wonder me at all, as I can imagine Y14.5 committee as a group of GD&T authorities that have to go through similar discussions to ours very very often, and in numerous cases each of these guys has got different opinion on the same subject. Probably FOS definition was one of such issues, therefore we still do not have one, clear and unambiguous term.

I am personally closer to say that FOS must have opposite elements, but I fully understand and respect Kenat's opinion on it. I cannot say he is not compliant with Y14.5 definition of FOS simply because this definition leaves huge field for different interpretations.

### RE: can a 90 degree feature be used as a datum?

Unless I missed something in geometry class, a cylinder is a circle drawn into the third dimension.  Nothing in the standard or any mathematical definition that I've seen for a cylinder indicates a partial arc.  Sometimes wording is taken too liberally, and is diffused by the reader; I've done that myself a number of times, reading more into something than was intended.  I can tell you from participation with Y14.5 and many discussions with its authors, that the directly opposed points interpretation was what was intended.  The extension to irregular features of size as indicated in '09 is a practical extension.

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

### RE: can a 90 degree feature be used as a datum?

Jim,

I am not a linguistic expert (especially that English is not my native language), my knowledge of geometry is quite good, but in this case I understand that users could misunderstood intentions of committee members.
I fully agree with you that reading more than was intended could lead into blind alley, but you must also admit that this is very often caused by insufficient precision of definitions or descriptions. I think in our FOS example simple sentence like e.g. 'Feature of size must have opposite elements' added to the definition would solve all the dilemmas.
And the last thought - drafters/designers also have some intentions when they specify GD&T on a drawing. If they do it correct, readers will not have any problems to figure out what they meant, but if they do something wrong or inprecise, probably only the author of the drawing will fully know what idea stands behind it.

### RE: can a 90 degree feature be used as a datum?

I know better than to get into this, but a from a "cylindrical" surface, you can get two series of three points (if taken on the same planes) to define two arc segments which can then define a centerline.  A complete cylinder is not required to achieve this.  Granted, you can't use calipers on two opposing points to get this info, but this info is inherent in the surface.

"Good to know you got shoes to wear when you find the floor." - Robert Hunter

### RE: can a 90 degree feature be used as a datum?

If it said 'surface of a cylinder' or similar I wouldn't argue.  However, saying 'cylindrical surface' to me introduces an ambiguity, intentional or otherwise, which might allow use of a partial cylinder.  Or at least explains why I and a few others might might percieve it's allowed.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

### RE: can a 90 degree feature be used as a datum?

Jim,
"A feature of size requires directly opposed points; pre-94, I believe that the caliper rule was shown."

I am not sure what you mean pre-94, the '82 definition was:
"1.3.8 Feature of Size. One cylindrical or spherical surface, or a set of two plane parallel surfaces, each which is associated with a size dimension", no opposed, no calipers.
Frank

### RE: can a 90 degree feature be used as a datum?

(OP)
ewh!
Someone else has the same perspective as i do regarding this surface.

Using a CMM, 3 points taken on the same plane gives you a center point. Another 3 points at a different elevation yields another centerpoint. These (2) centerpoints give me an axis from which the bolthole positions can be inspected.

No offense to the GDTP-S experts, but IMHO, the committee has to take computerized inspection and measurement into account regarding the standard. We are in the digital age and our thought processes should align themselves with it. Mentioning the '82 standard, at this point in time, shows that we are not keeping up and we have, more or less, rubber-stamped portions of the standard. Perhaps this is a portion the committee haggled about and let it fly.

I am forging ahead. Thank you all for your valuable input.

teddykaye

### RE: can a 90 degree feature be used as a datum?

teddykaye:

You only take 3 points on radius surface to find the centre point?? Would there not be a chance for error with this limited number of contacts? Why not take 8 or 10 points? Just asking.

By the way, I do agree that the axis can be developed this way but I question you limited number of contacts.

Dave D.
www.qmsi.ca

### RE: can a 90 degree feature be used as a datum?

I've seen an internal cone e.g. 90deg csk used as a datum but never an external 90deg feature.

Tunalover

### RE: can a 90 degree feature be used as a datum?

Repeatability would would be the key issue in using the 90 degree cylindrical surface as a datum.  Finding the center of a partial arc of less than 180 degrees is much less reliable than finding the center of an arc greater than 180 deg.  The CMM could find the axis with 6 points, but if you checked the finished part several times, it would probably find a different axis each time.  For that reason, I don't consider it the best choice for a datum.

Robert Bohot
GDTP-S

### RE: can a 90 degree feature be used as a datum?

Best choice, certainly not.  I agree that even if you accept it's not forbidden then it's not a first choice option and there would have to be a fairly compelling functional reason to choose it.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

### RE: can a 90 degree feature be used as a datum?

Reality of physical geometry supports the use of opposed points vs random points.

Per Y14.5-94, Section 4.5 Establishing Datums, (4.5.3 and others) "A machine element that is variable in size (such as a chuck, mandrel, vise, or centering device) is used to simulate a true geometric counterpart of the feature and to establish the datum axis or center plane."  For a cylindrical feature of size, the true geometric counterpart (TGC) is a perfect cylinder; if the datum feature of size is referenced RFS, then you find the largest inscribed cylinder which makes maximum contact with the wall of the hole.  This is repeatable.

While understanding the industry norm of using CMMs of various types, the fact remains that there is a greater inherent error in a CMM-established datum from the raw feature than there is by using a TGC at least as the basis of establishing a CMM datum.

Anyone that has ever done precision metrology on radial elements has seen that a seemingly "perfect" radial segment in fact is composed of multiple radial segments.  Using the 3-point method (or 6 or 10, or whatever) to find the center of the arc means that you will end up with multiple arc centers and radial measuremnts.  Which, then, do you select?  The one closest to what you want, regardless if it is representative of the majority of the feature?  Then, add a second set of 3 points to establish the axis of the "cylinder"?
I've tried to illustrate this in the file at the end of this link:
http://www.profileservices.ca/files/tidbits/thd1103_267281.pdf
I was at the knife-point of a project where this was a critical factor, costing significant s.

I don't dispute that CMMs are useful tools, even necessary in many cases, however I do get rather frustrated at the CMM salesman's mentality / pitch that you just need a few points to simulate a surface adequately to represent its functionality ... and THAT is what ASME GD&T is intended to do ... represent the design intent / functionality.  There is considerable ongoing dialog on this site & in other venues as to whether the design intent or manufacturing process or the inspection process is to be predominant in the GD&T application.  The standard specifically talks about engineering and omits reference to manufacturing and inspection.  Perhaps, in that light, metrology should be focusing on how to achieve the intentions of the design documentation as established with GD&T rather than expecting GD&T to accommodate the inconsistencies of metrology.  I recognize that the two aspects are symbiotic in nature, but within the ASME standard, design is paramount.  ISO offers the opposite, a GD&T standard that focuses on inspection rather than design intent.

The fact that a center point or axis established by a radial segment without directly opposed points is unrepeatable should guide people to recognize that it is a fundamentally flawed process as far as ensuring design intent.  As with any written work (standard or literary), the text itself gives you a substantial part of the knowledge, but you have to extend, combine and contrast ideas to get a full understanding of the content.

OK, I've beaten my head enough for today.

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

### RE: can a 90 degree feature be used as a datum?

Jim:

You stated about the use of a CMM "Using the 3-point method (or 6 or 10, or whatever) to find the center of the arc means that you will end up with multiple arc centers and radial measuremnts.  Which, then, do you select?"

Most modern computer programmes on a CMM automatically select the "best fit" centre. It is not up to the CMM Operator to say "I like this centre" since I am hoping the product is non-conforming." The higher number of contacts, the better the centre but it is the programme that selects the centre. The calculated arc and centre are developed using the mid-point of all the contacts rather than the theoretical true geometrical couterpart.

I do agree with you that a CMM has error and the repeatability (and reproducibility) would be suspect unless one contact on the exact points which is unlikely. The error would certainly be reduced if one had to find the centre of a hole (feature of size) but, again, more than 3 points should be taken.

You are correct that the standard does not cover manufacturing, inspection or, in fact, suitable applications for GD&T. It is up to the Designer to apply GD&T primarily based on the part's "function and mating relationship" being aware manufacturing capabilities.

Just putting in my 2 cents worth.

Dave D.
www.qmsi.ca

### RE: can a 90 degree feature be used as a datum?

Agreed for the most part, Dave, but the operator DOES select the points (or sampling routine), so in effect the operator does select which radial segment is being used.  Then, the operator also selects the fitting algorithm that processes the data points, again deciding the outcome.

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

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