FOS (Parallel edges as Feature of Size) ???

[dtmbiz]

Input please....

(See attached drawing)

A drawing uses a position tolerance control for centering a flat by using 2 edges identified as a Feature of Size.

Do straight line edges constitute “opposing elements” as mentioned in the 1994 version’s definition of feature of size?

 

[DeanD3W]

Evan,
That's all fine and in many cases MMC may be the right choice for the hole... In other cases the hole in sheet metal part (datum feature B) could be lightly pressed onto a pin... In that case RFS should be the choice for the Perpendicularity that would be considered. I don't see that as a design that I would question...

If the material is .030 thick, I don't think I'd apply a Perpendicularity tolerance at all at all. If it is .125 thick or thicker then I possibly would & it would be Perpendicularity at RFS.

All this discussion applies to any part with a short feature, sheet metal or not. I see the "grayness" of this as similar to a pair of partially opposed parallel planar surfaces... Starting with them fully opposed they're definitely a feature of size once a directly toleranced dimension is applied between them... Reduce their overlap from 100% to 98% and I think it can still be considered a feature of size... Reduce the overlap entirely, to the point that the surfaces are not opposed at all then the feature definitely cannot be a feature of size... Somewhere between 100% overlap (fully opposed parallel planar surfaces) and 0% overlap the feature should no longer be considered eligible to be a feature of size, but only a designer who understands how a feature of size functions and how their particular feature will function can make the "feature of size or not" call. It's the same for short features and orientation control... A line cannot be drawn, in general, that specifies when a feature becomes too short to no longer justify a meaningful orientation control... It's up to a well informed design team to determine whether it's worthwhile or not. This is part of why I think GD&T is part science and part "art". I think GD&T enables effective management of the risk of dimensional issues. That risk management should include applying meaningful and useful controls on the more critical features on each part, and the rest can be left up to a general profile tolerance that is specified in the notes on the drawing.

Getting back to the OP for this thread... If a position tolerance applied to the width of the bottom face of the tapered slot makes functional sense, then I think it's OK, maybe with some explanation in a flag note. If it does not address function well enough then profile tolerances on each side and the bottom of the slot may be a better choice. Since I think 2D features of size should be acknowledged I don't see profile as the only option though. "Artful Position" may be OK for this case.

Dean

http://www.d3w-engineering.com

 

[MechNorth]

Dean, in your example where the two surfaces are partially overlapped (say 65%?), then is the entirety of the two features considered a FOS? I would argue "No", only those 65% of the surfaces with direct opposition. Then what controls the other 35% of each of the surfaces?

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[CheckerHater]

in your example where the two surfaces are partially overlapped (say 65%?), then is the entirety of the two features considered a FOS? I would argue "No"

Enter "Irregular feature of size" and ASME Y14.5-2009 Fig.4-34

 

[DeanD3W]

CH - I don't think Irregular FoS applies directly to this issue.

Jim - What if the surfaces are designed with 99.5% overlap. Do you then say that 0.5% (0.25% per side, I suppose) would be uncontrolled? What if a cylindrical pin has one end cut at an angle? Since there is no opposition at the angled end of the cylindrical pin would you say that a profile tolerance must then be applied to the portion of the pin affected by the angled end? What if that beveled end is designed at 1 degree from perpendicular? Since dealing with this subject in a black and white manner is problematic, due to the possible slight variations in the geometry that a given practice was developed for, I say if the application is very close to the standard practice, all should be fine, and if it deviates "too much" (according to the opinion of the decision maker of the day), then maybe additional risk of issues can be mitigated with some more definition in a flag note, or maybe another method of control should be used... Maybe we need to shift to a surface control only language so fewer of these issues would exist..? .

Dean

http://www.d3w-engineering.com

 

[MechNorth]

Dean, that's why True Geometric Counterparts are so useful in the pin example. The "real design" world is mostly in that vast grey zone. Minimizing the impact of the "minor differences" is what makes us the big bucks.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[DeanD3W]

Jim,
Yes, I agree... "Mother nature" and the associated creativity of design is what creates the complication that GD&T can be used to deal with. This is why the desire to simplify GD&T won't ever truly be satisfied IMO... All the complication comes from the designed geometry and the potential imperfections... To deal with all those possibilities, combined with manufacturing process capabilities, and inspection capabilities and is not so simple. Said another way, simple is good and the objective, but when it comes to GD&T it's not always so easy to achieve.

Knowing which method should be used, and when the particular case is too far from fitting the method's intent takes experience and judgement. This is part of the reason why the subject (GD&T) should get more Senior level coverage (design electives at least) in the curriculum of all mechanical engineering programs. Mechanical design engineers should have enough background in "their" language to fully understand the tolerance specs that are needed, even if they don't deal with the language often enough to always remember and develop the best approach.

Dean

http://www.d3w-engineering.com

 

[MechNorth]

Agreed, Dean. Unfortunately most programs don't spend more than an hour or so on GD&T. There are exceptions, but rather few, and less every year. I'd like to see "Dimensional Engineering" and "Design Engineering" added as specific streams in the Mechanical curriculum at some universities.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[CheckerHater]

I don't think Irregular FoS applies directly to this issue.

I don't think you ever bothered to look at Fig.4-34 or Fig.4-33 for that matter.

14.5-2009 explicitly states that Feature of size can be created from NON-OVERLAPPING SURFACES, in black and white.
All that "caliper principle" is being down the drain for years now.

 

[Belanger]

Dean and Jim,
Sorry that my posts here and in the other thread seem to be the same (somehow both threads got into the discussion of orientation tolerance on a secondary datum feature).

Dean, your answer of having the simulator be perpendicular to datum A is obvious. But I'm referring to the size of a simulator for datum B at MMB. Exactly what size would this pin be? (This has nothing to do with Frank's question. It has to do with virtual condition.)

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

http://www.gdtseminars.com

 

[fsincox]

Philosophically I like the "what is the chance it is actually out of perpendicular argument”. As such, according to statistical process control, it is not necessary to check. Then it is just an education issue. That is really a key part of the general tolerance argument anyway. The fact that SPC was created here but only the Japanese understand it is an education problem. Even when you listen to the shop people they will make reference to how they make the old +/-.010 “everyday” they just really don't.
Frank

 

[fsincox]

Then if it becomes a legal issue, making the gage to show it would be the least of your problems, would it not?
Frank

 

[DeanD3W]

CheckHater,
You're killing me :-\... I assure you that I've looked at those figures "a time or two".

They don't directly apply to this discussion because
_ _
_

is different from
_
|_

The irregular features of size in Figures 4-33 and 4-34 will constrain a mating envelope just fine, but the non-overlapping-and-not-helped-by-additional-non-overlapping surfaces may or may not reliably constrain a mating envelope, depending upon how much overlap they have. That difference is the reason why irregular features of size are not directly related to this issue.

John-Paul - I didn't intend to sound short with my earlier response... I'm speaking of RFS cases, so I wasn't thinking of simulator size considerations. Sorry that I was missing your point.

Dean

http://www.d3w-engineering.com

 

[MechNorth]

CH, I think that Dean covered my point as well. And yes, I have spent considerable time with those graphics and the paragraph. I did check the graphics again to make sure that I hadn't missed something; apparently I had not this time. The concept of the Caliper Rule is still very much alive, even with Irregular FOS.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[axym]

Dean,

For some reason I'm still not convinced about applying Perpendicularity at RFS to the hole in thin sheet metal. Even if the intended fit was a light press. The reason I question a spec like that is that inspecting it requires finding the axis of the unrelated AME. On thin material, this will generally be very subjective and not repeatable. If the axis has functional importance on a feature like that, then I question the design.

Perhaps it's because we're using the traditional approach to controlling features of size, in which we specify a size tolerance and then another tolerance to control the orientation/location. In this case, diameter and Perpendicularity. Both of these characteristics involve the unrelated AME. But if the hole is a secondary datum feature that is going to be pressed onto a pin, we really don't care about the unrelated AME. We only care about the related AME. If it gets too small there is too much press, and if it gets too large there is not enough press.

We need two cylindrical boundaries for the surface of the hole, that are oriented to the primary datum. I would say that Profile of a Surface would be very appropriate for this type of application. This would control the size and form of the surface, as well as its orientation to the primary datum. Without the need to find the troublesome unrelated AME.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca