"Position" of a "V" groove on a block

[jassco]

Hi, Folk:

Recently I noticed that "Burunduk" asked a good question about position of a shallow cut on OD of a shaft. This makes me wondering if it is OK to use position FCF to locate a "V" groove on a block.

According to definition of position in ASME Y14.5 - 2009 or 2018 standard, position should be applied to features of size. But a "V" groove is apparently not a feature of size.

So, what is your thought? Actually I have another question on datum using "V" groove. I think it is related. But I'll save it in another post.

Best regards,

Alex

 

[Kedu]

Alex,

There are many discussions about this very same subject here on this forum. I guess we don't need another one unless you are providing something that has not been considered before.

Use the search function and see what you can find....learn and then post your specific question to have a value added post

ex 1:

https://www.eng-tips.com/viewthread.cfm?qid=460720

ex 2:

https://www.eng-tips.com/viewthread.cfm?qid=444710

 

[Burunduk]

I personally think it is a good question that has its place here.
It appears that if a (small?) "V" angle was due to a draft on a casting and the drawing was referencing Y14.8, controlling the groove with a position FCF would not be considered an issue, as indicated by examples that were brought up during a discussion that evolved in the same thread you are referring to. Then why would this not be a valid specification for a larger V angle on a machined part? Is there any essential difference? I have no good answer.

 

[3DDave]

Given a similar task I wrote an inspection document detailing how a gage sphere would be used to inspect the location as the mating part used a tooling ball to determine the location of the mating part. This was to produce sub-milliradian alignments for a precision radar system.

It's a peculiarity that a centerplane between the two nominally intersecting surfaces can be seen as a datum feature, but not be able use that same plane as an RFS reference for position or perpendicularity.

A trouble arises in precision use as small irregularities can cause noticeable shifts, which is why defining exact mating part analogs is needed. For example, if the faces of the groove is concave but the mating part has small contact, then defining where the midplane is may not match where the mating part ends up; if they are the same the mating part will change height in the mid-plane. If a least-squares method is used, it would displace the apparent location of the simulated surface from where a large contact mating part, riding the highest points, will end up.

Using a profile tolerance can control orientation and location. Use flatness on each face to refine deviations. I would still recommend just creating a document that expresses exactly what results are required/expected.

The version of ASME Y14.8 that is specific about position tolerances on tapered features is not yet released. the 2009 version seems neutral on the subject, not mentioning the topic.

 

[Kedu]

3DDave,

The version of ASME Y14.8 that is specific about position tolerances on tapered features is not yet released. the 2009 version seems neutral on the subject, not mentioning the topic.

I think in the other discussions you have some examples on how Y14.8 supports position on a conical features (drafted features).
I guess the argument was (and still is) if the position is applied to 2D circle section and not to the full feature.
If the full feature is used then somehow become invalid, if MMC/ LMC modifier is used together with full freature and draft (minus or plus)

https://www.eng-tips.com/viewthread.cfm?qid=474434

Figure B-1 as shown ......Kedu (Mechanical)17 Jan 21 19:35

 

[3DDave]

Kedu,

That is true - however there is nothing in the text and the figure is in the non-mandatory section. There is a specific paragraph dealing with profile; no paragraph dealing with position. That's why I say it is neutral as it offers no explanation.

Is there some other example in the released version of Y14.8 that says anything about a 2D circle? The only use of "2D" is in reference to drawings, not geometry; the same was true of the Y14.5-2009 version.

 

[jassco]

Hi, Kedu:

Thanks for the links! I saw those links before. But I am still puzzled by the followings:

1. If you drill a hole, you have a position to control;
2. But if you drill a taper hole with, let's say, 1 degree, then you can't use a position FCF.

Same things happen to a "V" groove.

1. If you mill a slot (a "V" groove with an angle of ZERO degree) on a block, you can control its position with a position FCF.
2. But if you mill a "V" slot with, let's say, 1 degree, then you can't use a position FCF.

I don't feel profiles FCF are good for this application as I want to only control location and straightness of the groove.

Have a nice weekend!

Alex

 

[3DDave]

For purposes of discussion - the following is based on the '2009 version of ASME Y14.5.

Profile controls location and straightness.

A position tolerance requires establishing a true position, which requires:

true position: the theoretically exact location of a feature of size, as established by basic dimensions.

So, what is a feature of size?

regular feature of size: one cylindrical or spherical surface, a circular element, and a set of two opposed parallel elements or opposed parallel surfaces, each of which is associated with a directly toleranced dimension.

So where is there a problem? No definition for "opposed" or "element" is given. For my purposes "opposed" is a synonym for opposite, as in the vector that is normal to the nominal surface of the indicated feature has a vector that is aligned with it and is of opposite direction to the opposing surface. Some ignore the "opposed" part of the definition and suggest that any two points or "elements" define a regular feature of size. However, there is no definition of what constitutes an element or how they can be "opposed."

Location tolerances on tapered features are an unsolved problem that goes back 50 years. A quick glance at the latest ASME Y14.5.1 Mathematical Definition of Dimensioning and Tolerancing Principles does not appear to me to deal with applying location tolerances to tapered features. Perhaps I missed it. It hasn't stopped people from creating their own solution by creating compound dimension references to locate gaging diameters/widths and saying that deviations from those are controlled by position tolerances, though it is not supported up to 2009. Users are certainly welcome to put into writing any dimensional controls they see fit to apply; that ability is recognized in the 2009 and many earlier versions of the Y14.5 standard.

 

[Burunduk]

There is the regular feature of size category under which tapered features do not fall.
Apart from that, there are two types of irregular features of size, both involving an unrelated actual mating envelope, which is the Y14.5 assigned axis/center plane simulator for checking whether the position or orientation of a FOS is within the tolerance zone, or for simulating a primary datum axis/center plane.

Per the definition, a UAME of a nominally tapered feature must be tapered too.
So in the end, it comes down to whether a UAME is defined for a tapered feature or not, Because the general definition of an AME implies a certain behavior - expansion (for internal features) or contraction (for external features) until being constrained by the feature:

envelope, actual mating: a similar perfect feature(s) counterpart of smallest size that can be contracted about an external feature(s) or of largest size that can be expanded
within an internal feature(s) so that it coincides with the surface(s) at the highest points.

That behavior is irrelevant for tapered features because it does nothing meaningful, but it doesn't mean that it cannot be realized by an expanding tapered pin or wedge (physical or virtual), just to go through the motions if the concern is compliance with the standard. The results should be identical to using a fixed size axis/center plane simulator (physical or virtual). So if one is only concerned about the practical aspect of things, the UAME concept can be ignored, but that approach may cause some controversy as it did in the past in this forum.

 

[jassco]

Hi, Folks:

Please take a look at attached concept drawing. What do you think of this positional FCF callout for the "V" groove? I know it does not conform to ASME Y14.5 standard as the angle (15 degrees) is not a feature of size. But does it make any sense? I could have applied profile of surfaces FCF, but I don't think it will do the same thing as a position if it were allowed.

Best regards,

Alex

 

[Burunduk]

jassco, I think that the problem is with the directly toleranced 15°. Had you made that dimension basic and applied profile "between" without datum references to control the angle, it would make sense to use a wedge-shaped gage as the center plane simulator and I think the position callout could be inspectable, even though not strictly conforming to Y14.5.

 

[jassco]

Hi, Burunduk:

Could you please explain this profile "between" you mentioned?

Also, if you don't use datum references for this profile, then position of this v-groove won't exist.

Best regards,

Alex

 

[Burunduk]

Hi, jassco,
The Between symbol looks like this: D <-> E, where D and E (or any other two letters of the alphabet) designate the points or lines at which the profile tolerance is intended to start and end on the view that shows the true profile. The symbol is placed below the profile tolerance feature control frame.

A profile tolerance without datum references with the between symbol that covers the entire tapered slot will control its form and angle and will replace the directly tolerance 15°. Then if you apply the position tolerance, as you showed with datum references, the angle of the wedge-shaped element that will be used to simulate the center plane for position evaluation is defined. Otherwise, if the angle is not basic, the nominal angle dimension that has a direct tolerance does not constitute the theoretical exact angle or a design/manufacturing goal, so there is no reason to set this angle for the center plane simulation wedge.