2 Single Segment FCF to make Rectangular Tolerance Zone? 3

[JasonSp]

I have seen a number of drawings where a hole ends up with a larger tolerance east-west than north-south, so rather than expand the diametrical tolerance to cover both, I see two reference linear dimensions with separate control frames. (See top example) I was wondering if possible to combine these into two single segment control feature controlf frames on the diameter dimension. (See bottom example). Is this allowed and is it clear?

 

[KENAT]

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[MechNorth]

C'mon, Kenat ... the horse just won't die!

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[ringster]

Whoa,

This horse aint dead yet. Why not apply the symbol for the origin of the dimension and a +/- tol to the location? Forget datum features and PHYSICAL IDENT .

 

[MechNorth]

Please don't take this thread there.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[KENAT]

The OP's illustration was primarily to show the 2 different approches the OP was considering to trying to show 'bi-directional' tolerance zone.

They were perfectly adequate for that purpose.

Were they a complete engineering drawing fully complying to the standards, no. However, when looking at illustrations on this site (as opposed to actual drawings/extracts of drawings) I wish we could treat them like figures in 14.5 and invoke paragraph 1.1.4. It even states "EXAMPLES ARE INCOMPLETE BY INTENT" in the top left.

For what the OP was asking about the second sketch was just plain wrong, it was trying to use terminology/symbology that means something completely different. The GD&T 'format' issues of the zones being different sizes in the wrong order etc were completely irrelevant.



KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[MechNorth]

Kenat, The Y14.5 Section 1.1.4 clause would be a great suggestion to include in a list of etiquette for this forum. Maybe that should be a new thread?

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[KENAT]

I've brought it up before and it's typically the same posters that violate it, I doubt that formalizing it in a forum specific 'rules of use' or similar would stop them.

However, I did have the same thought.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[powerhound]

KENAT,
I don't know if I understand your meaning when you say the GD&T format issues and things being in the wrong order are irrelevant. If I do understand your meaning correctly then are you saying that the appropriate response to the OP should have been a simple "No" with no further explanation and everyone else just move on to the next thread?

Stating "EXAMPLES ARE INCOMPLETE BY INTENT" is one thing and in that light, I wouldn't address issues of incompleteness. The tolerance values being out of order were not a matter of incompleteness, they were a matter of incorrectness and from reading the responses of other posters; some said the example was wrong but no one said why. I didn't want the OP to think that it would be okay to use single segment in that fashion for a different application. I guess it would be like a planar feature of size having a tolerance of +/-.005 and then applying a parallelism callout of .015 to one face relative to the other. That's not incomplete, it's incorrect and most people around here would point it out on a print that was submitted for our review even if it wasn't the subject of the post.

Approaching the issue of intentional incompleteness from another angle, I fully agree. I think it's ridiculous when someone asks a question about a positional callout on a hole in a square part with rounded corners and someone else responds with "Hey, there's no radius callout on the corners."

Powerhound, GDTP T-0419
Production Manager
Inventor 2009
Mastercam X3
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[SeasonLee]

Let’s go back talk more about 2 single segment FCF callout here, if the second example is incorrect to have a 2 single segment FCF callout on the same feature, how about the 2 single segment orientation callout on datum C as shown, Datum surface C is to be perpendicular to datum A within 0.15 and also perpendicular to datum B within 0.12.

Correct me if I am wrong.

SeasonLee

 

[axym]

powerhound,

As far as I know, there are no rules regarding the different segments in a multiple single segment callouts. Each segment is an independent control. The lower segment(s) of a multiple single segment callout do not need to be refinements of the upper segment like they are in composite FCF's.

The usual convention is to make the FCF with the largest tolerance value the upper segment, and go down from there. But I don't know of any rules relating to that.

Where are you getting the idea that there are rules that apply to multiple single segment callouts?

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[KENAT]

Powerhound, my point was that the OP was trying to create a bi-directional tolerance zone. The second example has a different meaning which 2 of the first responders pointed out. Once you've pointed out it has the wrong meaning I'm not sure I see the specific benefit of going into detail of what that meaning is and how it's even incorrect for the wrong meaning. I did give a ref to the std to clear up the confusion with composite.

Maybe I'm just in a bad mood, I don't normally disagree with you Powerhound.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[powerhound]

Evan,
Single segment is a variation of composite (or vice-versa). The difference between composite position and single segment is that single segment maintains basic relationship to the datums in the lower frame while composite does not. Everything else is the same. They are not independent controls. See 5.4.1.3 and also use those links I provided earlier in this thread.


Powerhound, GDTP T-0419
Production Manager
Inventor 2009
Mastercam X3
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[axym]

Powerhound,

5.4.1.3 directly follows the sections on composite FCF's, and shows an example that highlights the distinction between single-segment and composite. This kind of implies that single-segment is a variation of composite, but I don't think it should be taken that way. The Tec-Ease links show similar examples, specifically intended to emphasize the distinction between single-segment and composite. But there are many other applications of single-segment in which the datum features in the lower segment are not subsets of those in the upper segment.

There is a note in Y14.5 which mentions the independence of single-segment feature control frames. This is on page 95, deeply buried in the endless sections on composite positional tolerancing, so it's easy to miss.

Another case in which the explanation in Y14.5 is misleading.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[ringster]

Perhaps I am missing something, but do not composite tolerances apply to PATTERNS OF HOLES, rather than a single hole?

Im confused as to how composite tolerancing has entered this thread.

 

[axym]

SeasonLee,

Sorry we got sidetracked there with the composite FCF thing and didn't address your question.

I would say that the 2 single-segment orientation callout isn't illegal, but it doesn't match the functional intent. Because C is going to be functioning as a tertiary datum feature in an ABC datum referencing scheme, its orientation tolerance should reference both A and B as follows:

PER|0.15|A|B

If C's orientation relative to B needs to be tighter for some other functional reason, a second single segment could be added as a refinement:

PER|0.12|B

Because A and B will not be exactly perpendicular to each other, referencing A and B separately will give a different control than referencing them together. The tangent plane extracted when B is primary might not point in exactly the same direction as the "orientation constrained tangent plane" extracted when B is referenced as secondary to A. A diagram is badly needed here.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[MechNorth]

OK, back to the tangent for a moment;

When the two single-segment position FCFs follow the same DRF or progressive refinements thereof, they are a special variant of the composite positional control. The second FCF refines inter-feature position, orientation of the features & pattern wrt the datums, and also restricts the location wrt the last datum in the DRF sequence. Over-simplified, but accurate.

When the two single-segment position FCFs contain unique DRFs that are not a progressive refinement, they are separate controls, without any refinement functionality. The most common example is on a casting where a hole is positioned wrt the cast datums (e.g. /Z/Y/X), and wrt the machined datums (e.g. /A/B/C). Though this isn't on Tec-Ease's Tips section (yet), I can assure you that it is in our training material.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[ringster]

I believe that even Lowell would find the first example acceptable and would suggest a functional gage for it.
There is a very similar example in one of his texts.

 

[axym]

Jim,

I'm still not sure about the "special variant of composite" thing.

In "regular" FCF's, which multiple single-segment FCF's are an example of, the datum features constrain all of the degrees of freedom that they can and may. In other words, each datum feature constrains all of the DOF's that it is able to and that have not already been constrained by higher precedence datum features.

In the lower segment(s) of composite FCF's, the datum features are only allowed to constrain rotational degrees of freedom. This was presumably devised to provide a means of applying the equivalent of orientation tolerances to patterns of features.

To me, multiple single-segment controls are just regular FCF's with no special rules applying to them. Even if the lower segment has a subset of the upper segment's datum features. In general, FCF's must represent a refinement of any other controls that indirectly limit the characteristic in question.

The composite FCF is the special variant, with special rules applying.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[ringster]

I have a copy of a a letter to the editor of Machine Design, dated 1990, by a committee member, condeming them for the article that appeared in the magazine. The article was "The Problems with Geometric Dimensioning and Tolerancing". The editor was taken to task for being ridiculous and full of opinions and misconceptions.


looking at this thread, and the problems in coming to a consensus on a single hole in a rectangular block, it leaves one to wonder. Maybe they were ahead of their time.

 

[ProfDon]

Interesting thread! I believe the original question has been answered by MechNorth and others. The first example is legitimate. The second example has a couple of problems. However, putting the lower tolerance on top is not one of them. The lack of the diameter symbol in the FCF without indicating direction is a problem. The lack of a diameter symbol indicates that the tolerance is a width or bi-directional tolerance but there is no indication of direction. The assumption that the datum reference clears this up is not supported in the Y14.5 standard.
It was suggested that a drawing of 2 single segments might help clear things up. Attached are two pages from our Fundamentals text that may help. See the 2 position tolerances which are 2 single segments on the second page. Putting the smaller position tolerance on top would not change the requirements of the drawing. I was reluctant to sent this for fear that it will start more tangents but after receiving a couple of emails from your participants, I decided to pass it on. I hope it helps.