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

[tk369]

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

 

[DanStro]

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.

 

[tk369]

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

teddykaye

 

[KENAT]

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.

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[PeterStock]

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

http://www.infotechpr.net

 

[KENAT]

Thanks Peter that's what I thought, but given my discussion with someone the other day on similar wanted to verify.

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[tk369]

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

 

[Belanger]

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

 

[fsincox]

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

 

[fsincox]

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

 

[flash3780]

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.

 

[fsincox]

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

 

[fsincox]

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

 

[KENAT]

Having trouble finding the thread that looked at this in depth but try these.

thread1103-258957

thread1103-260566

thread1103-235809

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[fsincox]

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

 

[MechNorth]

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

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TecEase, Inc.

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[KENAT]

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.

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[Belanger]

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

 

[pmarc]

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.

 

[MechNorth]

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
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