datum question 1

[caseynick]

Per the 1994 standard
A cylinder is perpendicular to datum B. It is controlled by a profile requirement to B. Can that feature be used as a datum in other fcf's ?

 

[drawoh]

caseynick,

Yes.

If Datum[ ]B is a flat face and the primary datum, all the profile will do is control the diameter. The diameter is a perfectly good reference feature for the secondary datum.

If Datum[ ]B is your secondary datum, I would have to see your drawing.

JHG

 

[Belanger]

JHG ... the profile tolerance will control diameter (size, form) as well as the perpendicularity to B. But the general answer to the OP is indeed yes.

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

 

[MechNorth]

yup, and good addition, J-P.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[caseynick]

more questions for clarification please:

Should a feature controlled by a profile tolerance and referenced in a fcf be considered RFS in that fcf unless otherwise modified with MMC or LMC?
Can RFS, MMC, or LMC even apply if the cylinder is controlled by a profile tolerance?
Does a cylinder whose boundaries are controlled by a profile tolerance meet the definition of a feature of size?

 

[drawoh]

caseynick,

It depends on what you want to do. If you use a feature of size as a datum at MMC, it is easy to design a fixture to pick it up. This may not be your requirement.

My crude rule of thumb is that I worry about MMC if my datum feature has more than 1/10th the allowance of the features I am trying to control. If you need extreme accuracy between different features, maybe a feature of size is not appropriate.

One of the nice things about the profile tolerance is that MMC is the maximum profile. If your feature has XY coordinates and a diameter all controlled by +/-tolerances, MMC is a tricky concept.

JHG

 

[dingy2]

I agree with others that one can use a profile of a surface on a cylindrical feature that is perpendicular to datum B. But why? Perpendicularity is usually utilized and since the feature is a feature of size, perpendicularity could be used in either RFS or MMC mode. Size of the feature would have to be confirmed separately.

Once this feature becomes a datum, it can be referenced in either RMB, MMB or LMB since it is a feature of size. It doesn't matter whether the drawing complies to the 94 or 2009 standard.

Dave D.

http://www.qmsi.ca

 

[MechNorth]

Dave,
One reason for using profile instead of a toleranced size and perpendicularity is that you don't have to check the size and perpendicularity independently. In design reality, in most cases, designers aren't worried about the actual size separate from actual orientation/position, they typically are worried about violation of boundaries. I don't think there was support in '94 for using a feature controlled by profile as a datum feature at anything other than RMB. With the introduction of irregular features of size and the ability to modify a datum at Basic, MMB, LMB, or a specific size/dimension, things have opened up significantly to reflect real applications that couldn't be communicated before.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[dingy2]

Hi Jim:

Long time between discussions here.

I believe that perpendicularity at MMC would give one a boundary at virtual condition size. It also can be checked on the shop floor with fixtures. I don't think that an inner boundary using profile is beneficial but I could be wrong.

A cylinder is not only a feature but a feature of size. Whether it was controlled using a profile or not, it can be referenced at MMC (94 standard) which would be its largest allowable size. I did not find anything in the 94 standard stating otherwise. If it didn't have a geometric qualifier (perpendicularity or profile), we could still reference it in the FCF at MMC.


Dave D.

http://www.qmsi.ca

 

[MechNorth]

Hi Dave,
Sometimes these threads take a while to unfold, don't they.
You are right about the boundary at vc, and checking on the shop floor with the fixture. Inner boundary is useful as a limit of size and location combined; it's at least as useful as the inner limit of size and the inner limit of position, but again it eliminates the need for a full form check at MMC and a 2-point check at LMC. When a boundary IS the concern, profile is a great option.

As for a cylinder being a feature of size, it is if it has a limit applied and directly opposed points; features of size have size controlled separately for location/orientation. There is no support that I've seen in '94 for using it otherwise, with the exception of an enclosed boundary whose size is controlled by a profile control and whose position is controlled at MMC, establishing a BOUNDARY. Arguably then, if a cylinder has a profile control limiting the size, but no datum references, then it could be considered a feature of size.

There is nothing in the '94 standard that I can recall that guides the user on how to address a datum feature controlled by a surface profile, when modified at MMB or LMB. A cylinder is a fairly simple extension of principles, and I can buy that. The '09 standard extends the principles beyond simple geometries into irregular surfaces and irregular enclosed boundaries. Definitely not saying that most of us couldn't envision it, nor that we haven't used it, but there was no support for it in the standard (that I can recall).

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[caseynick]

Yes. They do sometimes take a while to unfold. But now we are getting to the meat of the discussion.

RMB, MMB, and LMB do not enter the standard till 2009 so I'd say it definitely does matter which version the drawing is intended to comply with.

As for the feature of size issue I can see it both ways. Clearly there are opposed elements and the size (as well as form) are limited by the profile tolerance . But does the phrase "associated with a size dimension." on the end of the definition mean a +/- tolerance? I don't know. If it does that would mean a cylinder controlled with a profile tolerance is not a feature of size. Following that how could you use it as a datum? How can you treat something regardless of feature size that is not a feature of size? And as Jim said above there is no guidance in the 94 standard regarding datum modifiers when the datum itself is controlled by a profile.

The 'extension of principles' mentioned above would make the fos issue moot. And a slightly further 'extension of principles' as regards the profile limits would make the RFS, MMC, LMC as it relates to a profile controlled datum feature, in the 1994 standard, moot too. Just how far anybody is willing to extend his/her principles is up to them.

 

[dingy2]

caseynick:

You are so right on this one and I gave you a star for that. I forgot that this cylindrical feature with a profile tolerance has a basic size dimension and thus, it does not have MMC size. It cannot be referenced in the FCF at MMC.

Is it a feature of size? According to ASME Y14.5M-94, 1.1.17, page 3, which states "One cylindrical or spherical surface, or a set of two opposed elements or opposed parallel surfaces, associated with a size dimension." It does not state that it must have a +/- tolerance on the size dimension so one might assume that it does meet the definition of feature of size. Since we cannot reference it at MMC, we have only RFS left (94 standard). The 2009 standard it different.

What a mess. I would suggest keeping with perpendicular at MMC.

Dave D.

http://www.qmsi.ca

 

[Belanger]

I don't have either standard with me today, but why does a cylinder with basic diameter and a profile tolerance not have an MMC?

MMC is the size of a FOS when it has the most material. Doesn't the cylinder with profile meet that minimal definition?

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

 

[dingy2]

J-P:

If one had cylinder with a basic diameter of 10 mm, what is the MMC size? It doesn't have one. The outer boundary reflected in the profile of a surface is not truly a size variation but a boundary where the combination of size and tilt must meet full length of the feature.

Dave D.

http://www.qmsi.ca

 

[Belanger]

But you can't throw out the 10 basic without giving the accompanying profile tolerance. Then we can answer the question of what the MMC is.
Sure, the profile represents a boundary, but the point of MMC is to identify the worst-case maximum size (for an external feature). So it's possible that the entire boundary is used up with size (meaning that its perpendicularity or location or whatever is perfect).

Again, I don't feel 100% solid on this since I don't have the standard with me, but I'm just throwing the idea around...

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