Orientation of a center plane of a tapered feature 2

[Burunduk]

A designer applied perpendicularity feature control frame with the leader pointing to a centerline representing the center plane of an internal taper feature (a "pocket" with symmetrical non-parallel opposed surfaces). I don't consider this specification valid according to ASME Y14.5-2009 but I'm struggling to provide a good explanation of why perpendicularity shouldn't be used this way. The one use of perpendicularity I know when it is applied on a virtual, derived geometry (as opposed to an actual surface) is when a center plane/axis of a feature of size is controlled. It doesn't seem right in the context of a tapered feature, not associated with a size dimension - but I can't form a good argument why. A valid point is that a center plane can be derived from a tapered feature (for example, a datum plane derived from tapered datum feature), and I suppose that a way to evaluate the derived plane orientation relative to a DRF can be found. I need your help, please.

 

[chez311]

Burunduk,

I think the main issue is that the center plane of the feature which is to be controlled is the center plane of the UAME per Y14.5-2009 section 1.3.29

feature, center plane of: the center plane of the unrelated actual mating envelope of a feature.

A nominally tapered feature does not have a defined UAME.

 

[Burunduk]

chez311,
Thanks for the reply. I want to agree, but there is illustration (g) of fig. 4-3 that shows a "center plane" (in addition to point and axis) associated with a feature which is tapered.

 

[chez311]

Burunduk,

What is really important about fig 4-3 is showing the DOF which each type of datum feature constrains.

I am fairly confident that Y14.5 does not define a UAME for a nominally tapered feature. A feature must have a UAME to control the orientation of the related center plane/axis - I think you'll find this pretty clear cut.

A designer applied perpendicularity feature control frame with the leader pointing to a centerline representing the center plane of an internal taper feature

Additionally, your FCF must be attached to a physical feature so it is doubly incorrect.

 

[mkcski]

Burunduk, For what it's worth: of the (6) total motions, orientation controls limit rotation but not translation.

Certified Sr. GD&T Professional

 

[Burunduk]

chez311,
The designer told me that this method was implemented before, and when the part goes through inspection they assemble a wedge-shaped gage into the tapered pocket. The wedge-shaped gage is produced with an accurate wedge angle and has two parallel surfaces accurately centered on the bisector of the tapered faces. The two parallel surfaces of the gage are used to simulate a "center plane" of sorts, so essentially this method relates a "center plane" to the taper without involving the concept of UAME. Then they evaluate the perpendicularity error of the simulated plane to a datum feature on the part, and this is how the orientation of the considered tapered internal feature us controlled.
This is not something that's suggested or covered by the standard, but I still can't point my finger on anything that makes it illegal.

Additionally, your FCF must be attached to a physical feature

I also tend to think the leader should point to a physical feature, and that the only other option is when there is no leader and the FCF is associated with a size dimension. This case is neither of those options.
This seems like a symbology issue rather than something fundamental. Nevertheless is there an explicit requirement for this in the standard? I looked briefly through chapter 6 of the 2009 edition and didn't see a specification about it.

 

[chez311]

The designer told me that this method was implemented before

This is a common adage - "its been done this way for years" - this does not make it correct per the standard.

The two parallel surfaces of the gage are used to simulate a "center plane" of sorts, so essentially this method relates a "center plane" to the taper without involving the concept of UAME.

Well then its not orientation per Y14.5 and cannot be addressed simply by slapping an orientation symbol on it. If you wish to fabricate a note which describes this simulation process precisely then you are free to do so - just know that orientation strictly per Y14.5 does not suggest such a simulator or method.

This seems like a symbology issue rather than something fundamental. Nevertheless is there an explicit requirement for this in the standard?

If you are fabricating a custom note per the above all bets are off - apply the FCF however you see fit.

If you are asking about features/controls covered directly by the standard yes this is a fundamental concept. FCF's and controls must be applied to physical features. The theoretical points/lines/planes are thus derived from these physical features, not the other way around.

As far as what the standard exactly says about this, the rules surrounding how an FCF can be placed are pretty clear.

3.5 FEATURE CONTROL FRAME PLACEMENT
A feature control frame is related to a considered feature by one of the following methods and as depicted in Fig. 3-29:
(a) locating the frame below or attached to a leader directed note or dimension pertaining to the feature
(b) attaching a leader from the frame pointing to the feature
(c) attaching a side, corner, or an end of the frame to an extension line from the feature, provided it is a plane surface
(d) attaching a side, corner, or an end of the frame to an extension of the dimension line pertaining to a feature of size
(e) placing in a note, chart, or the general tolerance block

1.3.27 Feature
feature: a physical portion of a part such as a surface, pin, hole, or slot or its representation on drawings, models, or digital data files.

 

[Burunduk]

chez311, you are right about the need to fabricate a note to clarify what the feature control frame applies to. This is what I am going to suggest.

As a side note, it seems to me that the wedge-shaped gage they use could be used as a datum feature simulator if the internal flat taper feature was called out as a primary datum feature. Same principal as simulating a datum axis from a conical taper. Is this correct?

 

[chez311]

As a side note, it seems to me that the wedge-shaped gage they use could be used as a datum feature simulator if the internal flat taper feature was called out as a primary datum feature. Same principal as simulating a datum axis from a conical taper. Is this correct?

Yes, I would agree with that. The below might be an interesting read for you - pylfrm brings up some good points about why a cone/taper should not be considered a feature of size (either regular or irregular) as well as the similarities between a flat surface and a tapered feature for simulating a primary datum feature.

https://www.eng-tips.com/viewthread.cfm?qid=448819

Some highlights from that thread:

Consider the behavior of a conical feature with an included angle of 178 degrees or so. Don't you see some strong similarities between that, Fig. 4-28, and a flat feature?

Using these definitions, the conical feature (with or without the flat surface at the tip) is certainly not a feature of size. If you imagine the true profile starts out roughly aligned with the actual feature and an offset envelope progresses toward the material, it never reaches a point where it can't progress further. The feature just gets pushed away from the true profile.


If a conical surface is used as a primary datum feature as shown in Fig. 4-44, the datum feature simulator is coincident with the true profile. In this respect, a cone has more in common with a plane than it does with a feature of size like a cylinder. That doesn't mean it can't establish an axis, just that it does so in a different way.

 

[Burunduk]

chez311, thanks for providing a link to that discussion and quoting the highlights. An interesting read indeed.
For me - it raises more questions
Obviously, a cone is not a regular feature of size as the shape of these is clearly stated in the Y14.5 standard.
For irregular features of size, per the subparagraphs of 1.3.32, the only clear requirements seem to be being "directly toleranced" and being able to "contain or be contained" within envelopes of various geometrical shapes (shapes that distinguish between type (a) and (b)).
I never fully grasped these concepts.
"Directly toleranced" may mean a "Geometric Tolerances Directly Applied to Features" (per 2.2 (c)) such as a profile tolerance on a conical surface, or simple +/- toleranced dimensions associated with a cone as in figures 8-17, 8-18. So I guess one requirement is met.
Can a cone be "contained" within a cylinder? A sphere? Some other random closed-shape envelope? If not - why?
Is a conical datum feature that sits inside a conical datum feature simulator being "contained" by it? If not, why?

 

[chez311]

Burunduk,

If you read a little further up in the post I quoted by pylfrm (8 Feb 19 01:54) thats discussed as the "ability to escape".

The "ability to escape" condition was my rough idea for how the concept of containment should be interpreted in this context, but let me back up a bit. The definitions provided by ASME Y14.5-2009 are not sufficiently robust to provide a definitive answer here. To arrive at a reasonable answer, we can attempt to patch up some of the gaps. To that end, consider the following replacement definitions:

Unrelated actual mating envelope: A theoretical envelope outside the material, uniformly offset from a feature's true profile as far as possible in the direction toward the material. The relationship between the true profile and the actual surface is otherwise unconstrained. If the material of the feature does not provide a limit to the offset, no unrelated actual mating envelope exists.​

Feature of size: A feature with an unrelated actual mating envelope.​

I think these definitions generally produce the same answers for cases that are clearly defined in the standard, and provide some additional clarity for cases that aren't. Thoughts?

Using these definitions, the conical feature (with or without the flat surface at the tip) is certainly not a feature of size. If you imagine the true profile starts out roughly aligned with the actual feature and an offset envelope progresses toward the material, it never reaches a point where it can't progress further. The feature just gets pushed away from the true profile.

 

[Burunduk]

chez311,
That's an interesting interpretation of the "concept of containment", although it's based on "modified" definitions of the UAME and FOS concepts.

Another possible and simpler interpretation of "containment" is just the feature being surrounded by an envelope that maintains some amount of contact with it. One could imagine a conical feature that sits in a conical simulator as I already mentioned, or one could enclose a conical feature within a sphere - I doubt the cone will be "able to escape". Regarding the latter example - one could try to dismiss it saying a spherical envelope can't represent a UAME of a cone, but I'm not sure an actual mating envelope must always mimic precisely the shape of the feature. The standard doesn't seem to define how much "air" can be contained with the feature within the UAME envelope; I think that a cylindrical shank with 4 flats along it is considered type (a), isn't it? By the same token, googling for images of "irregular feature of size type A" will show you illustrations from various articles and training sites, among which there are 2D representations of triangles enclosed in circles. If people were asked what real-world applications they associate with these examples the common answer would probably be a triangular bar enclosed in a cylinder. But, why not imagine that the same images describe a cone within a sphere?

 

[pylfrm]

chez311, you are right about the need to fabricate a note to clarify what the feature control frame applies to. This is what I am going to suggest.

I would be interested to see the note you suggest.

As a side note, it seems to me that the wedge-shaped gage they use could be used as a datum feature simulator if the internal flat taper feature was called out as a primary datum feature. Same principal as simulating a datum axis from a conical taper. Is this correct?

Yes, although the interaction between datum feature and simulator is not exactly defined by either ASME Y14.5-2009 or Y14.5.1M-1994. This ambiguity doesn't really matter if the feature has sufficient form accuracy though.

Does the orientation tolerance in question have only one datum feature reference? If so, is it either a planar surface or regular feature of size? If so, would it make sense to swap the datum feature and toleranced feature to avoid the the non-standard tolerance and custom note?

Just out of curiosity, what is the included angle of the taper? What other tolerances apply to the pair of surfaces?


pylfrm

 

[Burunduk]

pylfrm,
Everyone wanted to keep the note short so it is just "TAPER BISECTING PLANE" under the feature control frame. It leaves some ambiguity, but it is better than nothing I guess.

Yes, although the interaction between datum feature and simulator is not exactly defined by either ASME Y14.5-2009 or Y14.5.1M-1994. This ambiguity doesn't really matter if the feature has sufficient form accuracy though.

I can see what you mean, I also think that the less accurately the angle produced and the more it differs from the basic angle, the less stable and repeatable the orientation of the derived datum plane will be.

Does the orientation tolerance in question have only one datum feature reference? If so, is it either a planar surface or regular feature of size? If so, would it make sense to swap the datum feature and toleranced feature to avoid the non-standard tolerance and custom note?

The orientation tolerance refers to two planner datum features which are part of the DRF of the part. There is a separate drawing that defines the form and angle of the taper by the means of a profile control but it doesn't orient or locate the tapered feature with reference to anything. The separate drawing is part of a product definition of a series of parts, that may have different configurations of datum features.

Included angle is about 30° I think.

 

[dtmbiz]

Curious (would like a graphic of feature being considered and relationship to the desired DRF)

Not sure why some statements were made above in light of ASME Y14.5 2009.

1) A cone is most certainly a Feature of Size type (b), and restricts 5 DOFs, and "complex feature" (1.3.8) 6 DOFs
2) A cone's and "complex feature's" axis can be defined and and so can a center plane of a wedge feature (pg 50 fig 4-3)
3) my interpretation of "directly dimensioned" is direct dimensions defining size vs. i.e. a square block with a square hole at its center
in which the hole has each of its sides dimensioned from the corresponding outer surfaces of the square block
4) Considering the OP's original description of concern is the "delineation" which if done properly would not require a "special note"
5) Extended principle of inclined surfaces may come into play

***Burunduk From what your posts describe
A) The designers "concept" appears to be correct, however delineation is a problem (a geometric control's FCF cannot be made with leader to center-plane or axis)
B) Inspections Wedge Gage for inspection appears to be correct for RFC, also a virtual condition Wedge Gage for MMC from a single gage piece.

1.3.32.2 Irregular Feature of Size. irregular feature
of size: the two types of irregular features of size are as
follows:
(a) a directly toleranced feature or collection of features
that may contain or be contained by an actual mating
envelope that is a sphere, cylinder, or pair of parallel
planes
(b) a directly toleranced feature or collection of features
that may contain or be contained by an actual mating
envelope other than a sphere, cylinder, or pair of
parallel planes)

 

[Burunduk]

dtmbiz,
Thank you for the very interesting contribution.
Sorry that I can't provide a graphic, but for a simplified visualization imagine a stepped shaft of 2 different diameters. The smaller diameter cylindrical feature has a flat on top, and the larger diameter feature has a tapered slot cut into it along the axial direction. Datum feature A is the smaller diameter cylinder, datum feature B is the flat, and datum feature C is the planar shoulder between the smaller and the larger cylinders. The perpendicularity tolerance applied on the tapered slot with reference to B(flat) and C(shoulder).

What would be your suggestion for the solution of the "delineation" issue?

my interpretation of "directly dimensioned" is direct dimensions defining size vs. i.e. a square block with a square hole at its center
in which the hole has each of its sides dimensioned from the corresponding outer surfaces of the square block

That helps me to finally understand what an "indirectly toleranced" feature may be; a feature that is fully defined but has no direct tolerances applied to it. The size (and form?) of the square opening in the center of the block is defined by the wall thickness of each of the 4 sides and the external widths! Excellent. Does that mean that the square opening in a part that is dimensioned this way is not a feature of size?

 

[chez311]

Another possible and simpler interpretation of "containment" is just the feature being surrounded by an envelope that maintains some amount of contact with it. One could imagine a conical feature that sits in a conical simulator as I already mentioned, or one could enclose a conical feature within a sphere

Concepts like "maintaining contact" and "maximum contact" are ambiguous, you can search this forum for a plethora of discussions which exhaust this topic. The actual definition for UAME involves the terms contraction/expansion - If I'm interpreting it correctly the "uniform offset" described by pylfrm I think is an attempt to put this in more concrete terms to cover features for which size has no meaning like a cone/taper. Concepts like maximum contact take us further away from the concrete and into the abstract.

A spherical envelope can be contracted around any external feature - so I guess any external feature in combination with a spherical envelope is an IFOS type A (not so with all internal features - ie: a sphere can be infinitely expanded within an internal conical feature). There are two cases where a similar such a boundary is useful - when (1) the mating part replicates the boundary which is specified (imagine a cylindrical sleeve assembles over the three pins in Y14.5-2009 fig 4-35b) or (2) you are utilizing the boundary for approximation purposes (ie: instead of controlling form/location/"size" of all three sides of a triangle with profile a circular boundary is contracted which only contacts the points giving an approximation of location/"size" and almost zero control over form). If your case does not fall into (1) or (2) I don't see how this is useful.

Gauge balls are sometimes used in conjunction with internal tapers, however this is not a UAME it is a simulator of fixed size.

In regards to a conical/tapered boundary expanded/contracted against (or uniformly offset toward) a conical/tapered feature unconstrained to any DRF it will allow infinite expansion/contraction (uniform offset). A conical/tapered boundary will not contain a conical/tapered feature.

 

[dtmbiz]

OK for description Burunduk, however now it is unclear to me about the 'tapered slot" vs. OP "tapered feature" "pocket"
1) is the feature open at one or both ends, like perhaps a key way that breaks through the shoulder of the two diameters ?
2) or if it does have enclosed ends are they or round ends ? (round I would suppose if it is a "slot")
3) From description not sure as stated it seems as if Datum Features B and C are parallel planes... not sure what that gets for you for part description and it doesn't sound like good DRF if both are used in
DRF ...more on that later
4) let me know more about geometric configuration and will suggest a possible dimensioning and geometric control scheme.
(I only have pptx to create sketch so it maybe a bit rudimentary.. miss NX right now... awesome design software for the last 30 years)

BTW
I am a stickler for staying with the actual language of the standard's text.
Workbooks and instructional concepts are very useful at times, however the standard is the governing document.

Gentleman, this is a football.

He said this to the team every season including the seasoned veterans... idea is to be grounded in the fundamentals and build on that... I keep reminding myself of that too..

Concepts like "maintaining contact" and "maximum contact" are ambiguous, you can search this forum for a plethora of discussions which exhaust this topic. The actual definition for UAME involves the terms contraction/expansion - If I'm interpreting it correctly the "uniform offset" described by pylfrm I think is an attempt to put this in more concrete terms to cover features for which size has no meaning like a cone/taper. Concepts like maximum contact take us further away from the concrete and into the abstract.

chex311
IMO
Like = "...exhaust this topic" (though don't believe topics can be "exhausted" in this forum ... someone always "picks up steam" until interest is lost or no longer practical"
Dont Like = "...cover features for which size has no meaning like a cone/taper." (not sure I understand what you mean... i.e. a surface feature; agree || a tapered or conical FOS ; disagree)

 

[chez311]

Dont Like = "...cover features for which size has no meaning like a cone/taper." (not sure I understand what you mean... i.e. a surface feature; agree || a tapered or conical FOS ; disagree)

ASME Y14.5.1-1994 section 2.3 only defines size (actual value and limits thereof) for FOS described as "one cylindrical or spherical surface, or a set of two opposed elements or opposed parallel surfaces, associated with a size dimension". This would limit us to RFOS and IFOSa.

Shapes other than these (tapered or not) will not have a unique actual value for size. The concept of "uniform offset from the true profile" expands (ha!) the concept of "expansion/contraction" to features/shapes of all types. A moot point anyway, because a cone/taper is not a IFOSb as I stated previously. Let me restate and modify part of my response from (14 Nov 19 16:15):

An IFOSb allows a boundary which contains or is contained by the feature/collection of features to be a shape "other than a sphere, cylinder, or pair of parallel planes" - in this case we are interested in a conical/tapered boundary. A conical/tapered boundary expanded/contracted against (or uniformly offset toward) a conical/tapered feature unconstrained to any DRF will allow infinite expansion/contraction (uniform offset). A conical/tapered boundary will not contain or be contained by a conical/tapered feature, therefore no UAME exists and it is not an IFOSb.

With which part(s) of that do you disagree?

 

[Burunduk]

dtmiz,
If that matters, in the OP (opening post?) I was talking about a pocket that is open at one end (where the end face is) and has limited depth. In the latter description I thought that for simplification, a through slot along the larger cylindrical feature will do just fine (the pocket was "deepened" until passing through the entire feature, i.e. there are 2 open ends). I don't think it matters as it is only the pair of angled surfaces that defines a center plane/bisecting plane (whatever term is valid). The two planar datum features are perpendicularular to each other as the flat along the smaller cylinder is parallel to the axis, and the shoulder between the two diameters is perpendicular to the axis.