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Features Of Size Question

Features Of Size Question

Features Of Size Question

(OP)
Gentlemen,
I am curious if the interpretation of the ASME Y14.5 standard is the same for all others but me on the issue of Features of Size (ASME Y15.4-1.3.17, 1992). I had never interpreted the "or a set of two opposed elements or opposed parallel surfaces". To mean the surfaces had to be "directly" opposed, so to speak. This is the way one of the big names teaches it. I also know there is dissent among the ranks on various issues in the standard. I just wondered what others think. Say something like this:

_____________
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                         \_____________
 

RE: Features Of Size Question

The Y14.5 standard thinks of a "feature of size" as being something where the elements are directly opposed.  In the 1982 edition, they didn't mention "opposed," so that was included in 1994.  Some were hoping that the word "directly" would be added in the 2009 edition, but it was not to be.

The 2009 edition did embellish things by separating FOS into "regular" and "irregular" features of size, but even the irregular FOS doesn't help with your sketch.

A helpful hint for me is that something is a FOS if it can be measured with calipers or a micrometer (but without creating an extension of the surface to make in opposed).

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

RE: Features Of Size Question

By the 1994 standard, your example is not a feature of size.

Powerhound, GDTP T-0419
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Inventor 2010
Mastercam X3
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Taji, Iraq OIF II

RE: Features Of Size Question

A figure is worth than thousands of words, please look at the attached fig for more information about the opposed side.

As mentioned by Belanger, caliper rule is a good way for you to identify the FOS, you may get further information on the link below.
http://www.tec-ease.com/tips/jul-07.htm

SeasonLee
 

RE: Features Of Size Question

(OP)
Thanks, guys for your input. Seasonlee, I enjoyed the illustration you added as it almost helps me make the point that it seems like a very fine line.
I think this was also the issue with my confusion in the post on "How to measure a feature of size".
Belanger, thanks also for your mention of the '82 standard. The '82 standard is the one I was trained on and the one we reference at the place I currently work, I will not even go into that now. This subtle difference, the addition of the word opposing, did not really change my interpretation, as it does not say "directly opposing".
As with all legal issues, I understand that fine lines sometimes need to be drawn and should that, in fact, be the correct interpretation, as a long time supporter of the standard, I will be forced to accept it. Please understand, it is my nature to oppose reading things into places that are not really there.  As far as people who write books to tell you what the "bible" says, they are entitled to have their opinions. I have heard of meetings of the committee and I know there is sometimes dissent on issues brought up there. I believe the purpose of leaving something like that unstated, as you seem to indicate, may be to be to allow for some flexibility.
 

RE: Features Of Size Question

(OP)
Well, I must admit after reviewing the '82 standard and the '94 I am forced to the conclusion that they must have meant something by adding that word in.
 Thanks all.

RE: Features Of Size Question

Matt -- I agree that "opposed" should be clear, but with legal language you want to make it impossible to misinterpret.

For instance, Jack could be facing west and Jill could be facing east, so they are "opposed."  But if I add another detail that Jack is in New York and Jill is in Atlanta, they cannot be "directly opposed."

Just felt like being the Devil's advocate today...

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

RE: Features Of Size Question

During a discussion today with a few colleagues, another detail of the new standard came to light that makes me want to amend our explanation above of a "feature of size."

It might be easy to say that a feature of size is anything with opposing sides that can be "grabbed" with calipers --  I've often explained it that way.  But the new standard is extra careful to say that it must also be something that is "associated with a directly toleranced dimension."

This means that you can have something that passes the calipers test for a FOS but it's not really a FOS because it lacks a directly toleranced dimension.  I'm thinking specifically of a width where a basic dimension is used for the width and then a profile tolerance is used on one or both sides to control the width.

So we can never really say that anything is a FOS until we see the drawing for it.  Thoughts?

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

RE: Features Of Size Question

Belanger, your last post re: directly toleranced dimensions raises a question in my mind on SeasonLee's sample above. Wouldn't you say that dimension A is a directly toleranced dimension in all three examples, directly opposed or not?
 I can envision fitting dimension A on all 3 examples into a slot feature on a mating part, thus giving a "feature of size" element to deal with.
 What do y'all think?

RE: Features Of Size Question

Let me throw another monkey wrench into the works. What if I need to make dimension A width a symmetry datum for other symmetrical features, but the the 2 sides of dimension A are not directly oppoded so that I can put a calipers or mic across them. Isn't it still a feature od size?   Bottom line is that I think figures 1 and 2 in Seasonlee's sketch are also features of size for dimension A. I can clamp them between two gage blocks on a surface plate and measure them---Right?

RE: Features Of Size Question

Using the new word fcsuper just taught me, are the 3 dimension A's just tautologies?

RE: Features Of Size Question

Belanger,

   I agree with your Nov 02 post.  Much of this is design intent.  If I apply a dimension and tolerance to a rectangular hole, I am controlling the size of the hole separately from the location of the hole.  There are all sorts of ways to do this, but it results in a feature of size.

   If I dimension and tolerance both sides of the hole from a common datum, then you can assume I am not very interested in the size of the hole.   

               JHG

RE: Features Of Size Question

CheckerRon,

I would say that in SeasonLee's picture, the first two examples are still not features of size.  There are two conditions that have to be met: the surfaces must be directly opposed, and they have to be directly toleranced (the title block tolerance counts as directly toleranced for what we're talking about). His first two examples fail the first test -- I think that was the point of the graphic.

Also, it's not possible to make the width a datum in those first two examples, since a datum must be derived from either a surface or a feature of size.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

RE: Features Of Size Question

Thanks, John-Paul. In kicking these ideas around with my colleagues, we came to the same conclusion as you, that the first two examples are really not features of size. Para 1.3.17 of 1994 ed. seems to say that, and the expanded explanations in 1.3.32 of 2009 clarify it for us.

RE: Features Of Size Question

Quote (CheckerRon):

Let me throw another monkey wrench into the works. What if I need to make dimension A width a symmetry datum for other symmetrical features, but the the 2 sides of dimension A are not directly oppoded so that I can put a calipers or mic across them. Isn't it still a feature of size?   ...

   Again, we are up against design intent.  There must be some reason I would want to use the feature as a symmetrical datum other than sheer perversity and/or cluelessness.  Presumably, there is a mating feature on the final assembly to which this discussion also applies.  

   What is wrong with using a height verner to measure each face from the datum and then calculating the distance between the faces?   

               JHG

RE: Features Of Size Question

(OP)
the ASME definition also includes "elements" by that definition wouldn't example 2 be OK because it has opposing points?

RE: Features Of Size Question

(OP)
I want to add to what I said above. We use a lot of tapered cone features, most are defined on the drawings by dimensions to directly apposed points we call "gage points". So, according to what I am reading here these are not features of size then. Personally, I suspect this is why the ASME committee needs to leave flexibility to definitions and simple caliper definitions are not used.  I hope they are trying to define standards to work for all kinds of parts not just the simple parts.

RE: Features Of Size Question

(OP)
I disagree with this whole "a diameter is a feature of size but a radius is not". Talk about confusion, We all knew the circumference of a circle is pi*d or 2*pi*r they were interchangeable now they are not, are you kidding? Until it is stated as such in the standard I am not buying. Give me reason, not someone's rule of thumb. I know it is harder to measure if it is not a whole circle, hey life is tough.  

RE: Features Of Size Question

(OP)
fcsuper,
Thanks, It does look like they are preparing to placate people like me. We will have to see how it is generally interpereted.

RE: Features Of Size Question

(OP)
fcsuper and J-P,
I believe, I was taught in college engineering mechanics that opposed forces, acting not on a common line, create a couple, if opposed really meant to all "directly opposed" there would be no couple created.
Frank
 

RE: Features Of Size Question

As I understand it ... ISO standards would suggest that dimension A in fig-1 and fig-2 is redundant and in fig-3 would only apply over the section that is (directly) opposed.

As a result, could you potentially get the condition highlighted in my attached PDF?
Is that correct or am I missing something?

Thanks,
Jon

RE: Features Of Size Question

(OP)
JonSelby,
I am sorry that I am not as familiar with the ISO standards, I am interested in learning.  From what I had read of the ISO (assuming "principle of independency") I thought there would be there is no assumed condition at all. My impression is it would all based on 2-point measurement unless specifically stated on the drawing or as otherwise, say normal process tolerances for a class if this is specified on the drawing. In this case we have only the implied angularity tolerance which may also apply to the parallel surfaces? This example was intended to show the requirements of the ASME standards.
Frank.
 

RE: Features Of Size Question

Jon,
Quite arguably, that's also true for the ASME definition of feature of size (regular).  I've worked with many companies who didn't realize the subtlety ... then went back and checked ... it was in fact the source of errors because you are projecting a surface out to make a directly opposed point.   

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

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