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

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

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

(OP)


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?




 

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

My opinion is that while this does meet the definition of a feature of size, it is a difficult check because a CMM will yield varying results (but not by much) and the "caliper test" will be dependent on the perfect state of the ID jaws and a perfectly sharp corner.

I believe that this is legal but not very inspection friendly.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

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

This is a tough one.

I viewed the width as part of the intersecting points for the two surfaces with the 1 inch dimension the other component. While I can see the parallel elements of both inclined sides as a feature of size, I have a hard time acknowledging that a couple of interesting points are a feature of size.  

Dave D.
www.qmsi.ca

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

I agree with you Dave. I have a hard time with it too, but it does satisfy the "opposed elements" requirement. There has to be a better way to do this.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

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

Strictly speaking, it may be a FOS per the definition in Y14.5, however the INTENT is for the points to be directly opposed per what used to be called the Caliper Rule.  In real terms, this isn't an FOS, and position shouldn't be used.

We don't know the function of the part, so we can't say whether the center of the flat is actually critical.  That being said, here are some thoughts based on my own experiences with similar parts & features;
- with tapered features, generally the location of the tapers has always been more relevant than the center plane of the flat at the bottom
- if the center plane at the bottom is critical, then there should be minimal vertical walls at the junction with the tapered sides to ensure that the flat does the centering rather than the tapers; generally manufacturing really hates sharp internal transitions, and will apply a tooling radius with or without a vertical wall also added
- assuming that the tapers provide the centering effect, the best way to control them is to use a surface profile (with a basic angle); a composite profile will link the two tapered surfaces even more tightly

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

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

Righto, Jim.  I was just about to say "profile of a surface."

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

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

(OP)
I agree that the edges do appear to fit the definition of FOS, however it doesnt appear to meet a "practical" defintion in terms of inspection (IMO).

The flat at the base of the angled surfs is important in this case Jim. My suggestion of a profile callout was validated by your comment. I am glad that you pointed out the "INTENT" and Caliper rule. Also interesting comment about the minimum vertical walls.

I have been told that, "... this has been the callout on a customer dwg for over 15 years". I hate to hear that argument because if it isnt in compliance with the standard and 'practical' application, I really dont care how many years the callout existed!!!
(Anyway, 15 years ago would have probably taken the callout back to the 1973 version...)


My thanks to all for your greatly appreciated comments!!!
Hearing from you folks helps me to keep my sanity at times.
Ignorance of the standard and/or an unwillingness to think thru a callout in practical terms is extremely frustrating to me at times.

 

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

Jim,
I am confused, We seem so close, assume for my discussion this is a "V" block, I would then argee "we are concerned about the centering effect", but, how we leap from there to profile of a sufrace. This is a big leap for me so we will ignore the center completely? If I was locating somthing to the "V" wouldn't the center be just the point I want to determine?
Frank

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

(OP)
fsincox
 
The problem in this case is reality. The drawing shows sharp corners, however in reality there are radii in those corners. It is a machined part with a general note of .010-.030" inside radii.
Now how is it a feature of size when we go to the part, not the drawing?

I suggested to create a target line on each of the angled surfaces above the edges. "They" insist that the dimension stay where it is shown.

 

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

"... this has been the callout on a customer dwg for over 15 years".

I think everyone of us on this BB has dealt with that philosophy at one point or another. They get good parts in spite of the drawing, not because of it.

A large company here in my city went through a huge cost saving exercise several years ago. They sent all their prints out to get requoted and they went with the lowest bidder in each case. The result was that all the tribal knowledge that their previous vendors had developed over the years in order to give this company what they needed (despite what the prints said they needed) was lost and they had a massive influx of parts that met print but were non-functional. This company is still one of the worst offenders I've ever dealt with in terms of terrible prints.

I am not exaggerating when I say that I would see things like "flat within a diameter" or "profile of a surface at MMC" on a regular basis. Anyway, they are still like that and they insist their print are good, it's just a matter of getting a machine shop to correctly interpret GD&T.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

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

Frank, how do you locate an inclined plane (or 2) wrt the datums?  Not +/- (not an FOS).  For the V-block fixture example you cite, the two tapered sides establish a center datum plane).  You then are comparing the location of the centered feature to the datum plane established by the v-block. Or am I missing something in your question?

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

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

Jim,
That is kind of what I am exploring, here. I want a way to specify define/control of the center derived by the "V". I am maintainining this is what many of these V's are often used for defining a centerplane as you mentioned. I understand you want me to use profile on the tapered faces but how should I reference the centerplane?
Frank

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

Two ways to do this that I can think of;
1) The datum feature callout would be in-line with the basic dimension of the included angle, then the profile FCF would point to the two inclined surfaces and reference the primary datum.  CAD systems don't typically hold associativity between the datum callout and the dimension leader for angles ... don't know why.
2) Use a single profile FCF with two leaders pointing to the two inclined surfaces, and hang the datum feature callout off of the FCF.  The FCF would reference the primary datum.

Arguably, either profile FCF could reference the secondary (centerplane) datum as well, though that will inevitably bring up the self-referencing datum, and isn't worth the effort in this case.

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

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

I agree with Frank that I think the surfaces of the symmetrical V is not as important as its center plane. Profile of a Surface is doable but using that requirement does not emphasize its center plane. The positional tolerance at MMC shown on the drawing for the intersecting points is inappropriate since, I believe, we are interested in the center plane of the symmetrical V taken from the surfaces of the V.

The drawing is somewhat incomplete since an angle has not been specified nor the dimension to the top of the V. I am going to go a different way.

From 2009 edition page 156
"7.7. Symmetry Tolerancing to Control the Median Points of Opposed or Corresponding Located Elements of Features
Symmetry is that condition where the median points of all opposed or correspondingly located elements of two or more feature surfaces are congruent with a datum axis or center plane."

Notice that the above statement did not state feature of size but surfaces. I have never used nor ever recommended that use of symmetry but I think this is a great example of its value. I have attached a bit of drawing in a word program so the arrow heads on the angle are not the best.

To confirm symmetry, I would use a CMM and take 2 opposing points on the V surfaces parallel with the bottom and find the median point. I would compare this to the datum center plane using a +/- .005 tolerance. Take about 4 or 5 pairs on the surface at either the same height from the bottom or at different heights. All should be within the .010 symmetry tolerance.
  
What do you think?

Dave D.
www.qmsi.ca

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

Don't think so, Dave.  What happens as the "V" increases in width along the length of the part?  The point-to-point averaged centerplane (derived median) would still be straight, but the V would be useless.  You need to control the faces of the V as well, and I would argue that the control of the faces of the V is what establishes the control of the centerplane (indirectly).

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

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

Jim:

If the feature was a slot (parallel sides), we would place a positional tolerance on it to control its location. Would we place a profile of a surface on each of the sides? Probably not. We would probably use a vernier and check the slot width in a few spots and that is it. The location would appear to be vital to its function and mating relationship rather than the surfaces.

The original drawing had linear tolerances for the intersecting points for the angle. The angle was not given so I made one up and there would probably be a default angularity tolerance of, say, +/- 30 minutes. The surfaces are controlled similar to the surfaces on a slot. It may not be the tightest control but controlled.

I agree that the V surface may not be perfectly flat nor the angle perfectly on 45 degrees just like the surfaces of a slot. It is the center plane location that the Designer is interested. The application of symmetry on the V surfaces directly controls the center plane relative to datum B.

 

 

Dave D.
www.qmsi.ca

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

As a veteran designer, I don't tend to position the centerplane of a rectangular slot either.  If the slot is horizontal, I am most commonly concerned about the bottom face.  For a vertical slot, I look at the functionality/stack to see which side is more critical (there always is one).  That's the reality of physical assembly & function as opposed to design function.  In both those cases, a profile control is more functionally accurate than a position control.  I would probably use a +/- width to control the second face.

In the case of a V-block, the centerplane is established by both faces.  In your proposal, there is a tolerance on the 45deg +/- 30'.  There is also a tolerance of +/- 30' on each face as it goes into the page [y].  So, the derived median plane (not actually a centerplane if you use symmetry) may still be dead center, but the mating part being aligned by the slot may now deviate to either side [x] by +/- 30' ... in reality, it WILL be at the extreme in [+xy] or [-xy] because the inspector will set it up to minimize rock.  As a result, your mating part will not ever be properly oriented wrt the intended centerplane.

Profile controls (composite perhaps, to refine the relationship between the two inclined faces) controls the location and orientation (and form) of the two faces wrt the datums and wrt each other.  It is an indirect control on the centerplane, but far more repeatable and controllable.

As for inspection, using 4-5 sets of opposed points on the inclined faces for a symmetry control ... well, that doesn't tell you what the entire face is doing, does it.  For the profile controls, you put a feature simulator (true geometric counterpart) on each of the surfaces and probe off of the simulator, giving you an accurate reflection of the net effect of the entire feature.  

Plus, symmetry really sucks.

 

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

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

Jim:

I agree that symmetry really sucks too but in this situation, I think it would work better than profiles. This is the first time that I have seen a symmetry application rather than using positional. It depends upon the Designer in this case. Is the Designer really interested in the surfaces of the V or the derived center plane? The original drawing indicated the relation between the actual center plane of the feature and the datum.

Dave D.
www.qmsi.ca

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

There are times that we have to do things indirectly in GD&T; to me, this is one of them.  I do understand your reasoning, I just don't support it because of the overwhelming issues that I anticipate with using symmetry.   

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

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

"What happens as the "V" increases in width along the length of the part?  The point-to-point averaged centerplane (derived median) would still be straight, but the V would be useless."

Jim,
I think the "V" would still function fine it would just be a knife edged locator if I have it pictured right in my mind?

"Arguably, either profile FCF could reference the secondary (centerplane) datum as well, though that will inevitably bring up the self-referencing datum, and isn't worth the effort in this case."

Jim,
If I am picturing this one correctly, this is very similar to the parallelism callout we use in my thread on orientation locates?
thread1103-315082: Spinoff2: Using orientation to control position
Frank

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

Everyone seems to be speaking of the angled sides of the slot...  dtmbiz shows a drawing that applies a width (size) dimension to the face at the bottom of the slot with a position tolerance, so the OP is not about v-blocks or slots.  I think it's about whether a 2D feature of size exists.

If one is dealing with a so-called "width", so a slab of material with parallel vertical sides and say, horizontal top and bottom bounding surfaces, I think we will all agree that the two fully opposed vertical faces of the slab with a directly toleranced size dimension between them form a a feature of size...  Now, please make the slab a bit shorter...  I think we would have no argument regarding the fact that it is still a feature of size...  Keep making it shorter until it is 0.05 mm tall.  It is still a feature of size, I believe.  Now make it so short that its height is not measurable...  Convert it to ink lines drawn on a piece of mylar...  It is a 2D feature of size.

If you think that a 2D width cannot be a feature of size, then please tell me just how short a feature of size can be before it becomes too short to be considered the same type of feature.  Where do you draw the line?

As with all measurement, there will be uncertainty in determining the intersections that form the boundaries of the 2D feature of size.  A CMM could be used and a vision system can make this sort of measurement pretty easily.

I would probably recommend profile of a surface for the sides and also bottom face of the v-slot, but to respond to the question in the OP, I think the 2D feature of size and position of its "center line" (a 2D center plane..?) is valid.

Dean
www.d3w-engineering.com
 

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

Dean,
You are correct we did get off of the OP. I did not want to comment on the FOS part originally because I am not a FOS expert (kind of guy!). I do like your concept of FOS and hope you win the day. My guess would have been NO from the conversations we have been in on positioning gage lines of cones, because it seems if the concept is valid for this part then it should work their, too. It really is the same concept, just wrapped.
Frank
 

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

Dean, the "datum center plane" established by Datum-B would be perfectly perpendicular to datum-A.  The "2-point FOS" at the base of the inclined sides is not adequate to establish a centerplane; a center-line, yes, but not a centerplane.  You need some vertical aspect to establish a plane.  As to how much, that's a good question for debate; the taller the surface, the more accurate the centerplane.

Sorry, Frank; I started to follow the Spinoff2 thread as it unfolded, but didn't get involved due to time.  As for a knife-edge locator, you'd need it at two locations to have a controlled centerplane, and again the centerplane would be subject to the actual surfaces ... hence, profile.  A line-contact V establishes a single centerline which has planes associated with it; the rotational constraint of the plane thru the centerline is wide open though.

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

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

Jim,
I figured this was only one of the (2) "V" blocks as it's length is not signifigant unless the part is equally as short, then I might agree with your concern.
Frank

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

Jim,
I wouldn't call the feature in question "2-point", but I would call it "2D"...  Since it is 2D of course there is no center plane.  A position tolerance, with the tolerance zone having 2 parallel planes should be no problem to control its center line.  It may be unconventional, but a 2D feature of size should be acknowledged as valid, otherwise someone needs to identify just how short a feature of size can be before it is no longer a feature of size.

We may be left without a clear definition of how to establish the center line of the feature...  If it were a slot then an "unconstrained AME" would be fit to the feature to find its center plane.  We would be better off with this example if Y14.5 clearly described an "unconstrained AME" for a 2D feature.  How to define the mating envelope's orientation is an issue.  The same can be said for a very short 3D feature of size.

Regarding a 2D feature of size, in general, how about a circle printed on a planar surface...  With a size tolerance applied to, for instance, the circle's outside diameter is there a perfect form boundary at MMC?  In order to accommodate the form error of the planar surface the boundary would need to a cylinder rather than a circle..?  If instead we were dealing with a cylindrical feature that was 0.05 mm tall, we would probably need to get the mating envelope's orientation from the surface that the very short cylinder projected from, or from a bounding surface on the other end of the short cylindrical feature.  I think any difficulties with a 2D FoS also exist for a very short conventional FoS.  There is an extension of all of this into Y14.8's material...  When a size tolerance is applied to the mold line of a casting, is it essentially applied to a 2D FoS?  Is there a perfect form boundary at MMC?  If a position tolerance is placed below that size dimension does it apply to the center line between the two mold lines, or to the center point if the mold line is circular?  I don't think any standard addresses whether that position tolerance applies to a 3D element such as a center plane or axis, or to a 2D element, such as a center line or center point (Y14.8 is currently discussing this issue, and in my opinion it is the best standard to resolve the issue as it applies to tolerances applied to mold lines).

For the example in the OP, is there a perfect form boundary comprised of two parallel planes separated by .850?  I say planes rather than parallel lines to acknowledge the form error of the two intersections that form the "2D FoS".  None of this is absolutely and explicitly addressed in Y14.5, but for the example of the OP I'd have a hard time arguing that there were not "opposed elements" present to which a size tolerance can be applied.  They can't constrain a mating envelope (other than I 2D mating envelope, possibly)...  Anyone have thoughts I this?

Dean
www.d3w-engineering.com
 

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

Dean, I still don't buy the 2-D FOS at the points; technically questionably legal (IMO), but functionally not useful in any way.  On the other hand, perhaps I have been thinking of this in the "old fashioned" way.  Looking at '09, we have the option of using / defining an irregular FOS.  Perhaps that is the way to go?  Thoughts?
 

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

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

I am uncomfortable with the idea for various reasons.
First, I am not sure about corners being perfectly sharp, so dimension is taken from "virtual sharps", that is, distance between 2 imaginary lines. In most GD&T texts "feature" refers to physical object, something you can touch.
Second, I don't see how said 2 imaginary lines establish "envelope" to embrace or to be embraced.
Third, I believe (and you may argue with that) that to be "opposing" elements somehow have to be directional, as in "opposing sides" or "opposing faces". No luck either.
So, I don't see "opposing", I don't see "feature", and sadly I don't see anybody asking "how mating part looks like?" And yet everyone has some idea on how to gage the part in question.
Let say, we CMM sides of the V and bottom flat. Then we can possibly calculate position of "virtual sharp" (I am not an expert on CMM, just assuming). Or we can go old-fashioned way and drop round pin into V, and then treat that pin as genuine FOS (works for decades for threads and gears). Both methods will require dimensioning scheme different from shown in OP.
To summarize, right now drawing looks like a puzzle and pieces are not falling together (yet).
 

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

CheckerHater,
I think all of your points here are good ones.

On the other hand, all the same problems exist with very short features of size...  They're not really physically capable of orienting an axis or center plane in a repeatable manner, but we can still apply a position tolerance to them.

How short can a feature of size be before it cannot be considered a valid feature of size?

If a cylindrical hole is punched in a sheet metal part, it's pretty common to see position applied, which will control the hole's location and (?)orientation(?)...  Does perpendicularity get applied often to a hole in a sheet metal part?  Through about half of its depth its a cylindrical feature isn't it?  Why can't perpendicularity be applied to it?  Maybe because the cylinder is too short, so it can't really orient an axis..?  Why then can't position be applied to a circle printed in ink on a planar surface?

Dean
www.d3w-engineering.com
 

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

Dean,
I am OK with Position applied to circle.
Will you apply Position to 2 dots printed on surface?
 

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

CH,
If I applied a size tolerance to the diameter of a dot printed on a surface, I think a position tolerance could be used to control the dot's center point.  I acknowledge possible issues with the orientation and form imperfections of the surface that the dot is printed on...  Some explanation may need to be added in a flag note.

Since you're asking about two dots, I assume you may be referring to a dimension applied between their outer or inner tangencies (not between their centers though)...  Maybe those could be considered opposed elements, but if there was a desire to control the center point between the two dots I would be very careful and at least add a flag not to explain the feature element being controlled and the tolerance zone.  If the center point between the two dots was the desired entity to control I think with enough care, position might be the best choice...  There are also times that a given case is so specialized that GD&T isn't the best method.  I've seen this once on a an ink jet printer assembly and once with an optics application...  Can my answer be "it depends"? smile.

Dean
www.d3w-engineering.com
 

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

No, I mean will you consider 2 dots being 1 "feature of size"?

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

If a slot is to be considered a feature of size, it does need a size tolerance applied between the two opposed planar surfaces...  For the two dots, I wouldn't consider them to be a feature of size unless a size tolerance were somehow applied between their outer or inner tangencies...  How to you want to apply the size tolerance between the two dots?

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

Dean,
I found it. OK, should we use position for the sheet metal hole features at all, I know I want to say yes. I do see the problem with measuring perpendicularity of sheet metal holes, like Dean says, how do we make it so the shop doesn't scream? I admit in the past I have skipped using GD&T on sheet metal.
Frank  

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

Perpendicularity is applied to a hole in sheet metal not because we are literally concerned about the tilt of the hole.  It is because (usually) that hole serves as a secondary datum feature, and we want to tie it back to the primary datum surface.
So it's really about providing a relationship between the datum features -- especially is a functional gage will be used.

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

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

John-Paul,
Now we're back to touching on Frank's "Orientation tolerance controlling location" thread...

A tolerance on a secondary datum feature is not needed just because it's a datum feature...  The relationship between the datum feature simulators is present anyway.  For the sheet metal part the simulator for the hole as the secondary datum feature is required to be perpendicular to the simulator for datum feature A (for the part described above).

Dean
www.d3w-engineering.com
 

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

I may owe someone an apology big time! So Dean you are saying using datums without tolerances is OK it is not really cheating at all? I really have resisted the orientation tolerances only if we are going to use a gage, stuff, because in small production run companies we don't use gages for anything.
Frank
 

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

Hi All,

Here are some thoughts.

Many of the problems with applying Position or Perpendicularity tolerances to a very thin feature stem from having to find the feature's unrelated AME.  In many (almost all?) applications, we really don't care about what the unrelated AME is doing - we care about what the surface is doing.  So we should use the surface interpretation of Perpendicularity or Position and leave the AME out of it.

Let's look at the case of a nominally perpendicular hole in very thin sheet metal.  As J-P alluded to, a typical function of this type of hole would be to act as a secondary datum feature with the sheet surface as primary.  So the primary datum feature will constrain u and v rotation, and the hole only constrains x and y translation.  The hole will be interacting with a pin that is exactly perpendicular to the primary datum, and probably a fixed size (if the secondary datum feature is referenced MMB).  To function properly in this capacity, we need to control the hole's related AME so that it doesn't get too small to fit over the pin.  In many cases the angular tilt would be huge before the related AME got too small, but we need to have a tolerance.  So we're not applying a tolerance just because it'a datum feature.

It's a single hole and all we can control is orientation, so we specify Perpendicularity (not Position, right J-P).  Zero at MMC is appropriate, and a functional gage could be used to check it.  Again, we're controlling the orientation of the hole because we care about the surface and not the unrelated AME.

So I don't have a problem with specifying Perpendicularity on a hole in very thin sheet metal, provided that it's referenced at MMC.  It may be that the process used to make the hole is so capable that the chances of the hole being tilted too much are near zero.  I would say that the Perpendicularity tolerance still needs to be specified, and the person doing the inspection planning can choose to not have it inspected.

If the design is such that we actually need to find the unrelated AME of a hole in very thin material, then this is a red flag and the design must be questioned.  This would occur if the Perpendicularity tolerance was referenced RFS, or if the hole was referenced as a primary datum feature at RMB.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

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

The choice of putting an orientation control on a secondary datum fos on a thin part (metal, plastic, composite, etc.) comes down to the reality of the functionality.  If the hole acts only as a locator for whatever is going to poke thru it, then the reality is that the minimal height of the hole's cylinder will not establish an accurate axis anyway ... thus perpendicularity is irrelevant.  If the material is deep enough that the projected position of the mating part will be affected by an orienatation error, then you put the orientation control on.

I had a student argue this once in class.  He had been taught by his college instructor that it is an absolute that the orientation of the secondary datum feature (a hole) must be controlled wrt the primary (planar surface of sheet) because otherwise the parts would not function properly.  We went thru the assembly to see what each of the components did, how they interracted.  The 2"dia hole had an aligned hole several inches apart, after fabrication.  A rigid tube passed through both holes with a light-press fit.  The entire workpiece shifted to accommodate the tube.  There was no issue with the alignment that the hole could have contributed to in any way.   

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

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

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
www.d3w-engineering.com
 

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

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  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

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

Quote:

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
 

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

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..? smile.

Dean
www.d3w-engineering.com
 

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

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  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

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

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
www.d3w-engineering.com
 

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

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  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

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

Quote (Dean):

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.

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

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

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

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

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

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

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

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
www.d3w-engineering.com

 

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

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  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

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

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.
www.axymetrix.ca

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