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Position Refinement 1

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jimbod20

jimbod20

Aerospace
Sep 8, 2010
75
US
I have a question regarding position control/refinement. I provide a sketch.

D, E, F are cast datums. H, A and B are machined datums.

I locate machine surface H with a basic dimension and profile tolerance from cast datum D. I locate machine surface A with a basic dimension and profile tolerance from cast datum F.

I now want to locate machine diameter B with position to H, A and the cast datum E (cast datum E is secondary and consists of two .250 inch diameter buttons). I hold a position of .050 H, A and the cast datum E. The part is fully constrained by datums H, A, E. I need tighter control of position to machined datums H and A so I 'refine?' the position of B to H and A with a position of .015. Is this specific feature control frame correct if the part is not fully constrained by a tertiary datum? I add an additional 'refinement?' of perpendicularity of .0005 to surface A. Is my interpretation of refinement correct? Is this composite feature control frame consistent with GD&T language?

 
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Evan,
I am just thinking out loud:
Is it really true that an orientation tolerance cannot be used "because the feature isn't a planar surface or a regular feature of size"?
I am looking at paragraph 6.4.5(a) in '09 explaining surface interpretation of orientation tolerances at MMC applied to a hole. It refers to example showing cylindrical hole, but is it solely limited to that?
For fig. 8-24, would it be so incorrect to refine positional tolerance (applied all around) by perpendicularity tolerance at MMC wrt A|B|?
 
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John-Paul,

I thought that this one would make you squirm a little bit ;^). Y14.5 doesn't provide a "general orientation" tool that can be applied to any type of feature. Instead, we have to get by with non-intuitive applications of other tools. I know that this will leave a bad taste in your mouth, but I see the lower segment of composite Position FCF as the equivalent of an orientation tolerance for a pattern of features (and by the way, just to muddy the waters a bit more, Y14.5.1M-1994 says that a pattern can consist of only a single feature). In the same way, the lower tier of a composite Profile FCF is the equivalent of an orientation tolerance for a general surface or group of surfaces. I know that Y14.5 emphasizes the "feature relating" aspect of the lower segment, but we can relate features just as well with a single-segment FCF. To me, the main distinction of the lower segment (and the main reason that a composite FCF should be specified) is that the tolerance zones are not located to the DRF, only oriented. If all we want the lower segment to do is relate the features in the pattern to each other (with no datum feature references), then a composite FCF isn't even necessary and an additional single-segment FCF would be sufficient. I know that there is also the overriding of simultaneous requirements with the lower segment of a composite FCF, but that is another discussion.

pmarc,

Section 6 is quite specific on the type of tolerance zones that an orientation tolerance can specify (see 6.4.2). So it would be very difficult to apply Perpendicularity to an irregular boundary without directly violating some of the statements in Section 6.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
 
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First, I would differ with your statement that the standard doesn't provide a general orientation tool for any type of feature. It does -- profile of a surface with toleranced dimensions back to the datums.

However, the only reason that can't be used in Fig. 8-24 is that an orientation tolerance also controls form of the surface, but this figure wants to have a form tolerance of 1.2 mm.

I think it's funny that you are OK with bending and tweaking the statements of 7.5.1 to suit our situation, but then hold strictly to 6.4.2 when that suggestion is made. Either way, for what you propose we are going to have to bend something!

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
 
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Evan,
In that case 6.4.5 seems to violate 6.4.2, don't you think?
 
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Jim -- wouldn't angularity then control form? Check out Fig. 8-24; the form/size is supposed to be allowed a tolerance of 1.2 mm.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
 
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J-P,
I do not think it would.
Angularity (or perpendicularity) tolerance at MMC would be a requirement for not violating a boundary which size would be defined by collective effect of tolerance values of profile of surface and chosen orientation control. Very similar to how position is interpreted in this example, except that the boundary for orientation would not be constrained in location wrt A|B|C.
 
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I have one more question related to fig. 8-24:
The standard in paragraph 8.8 says that: "For an internal feature the boundary equals the MMC size of the profile minus the positional tolerance, and the entire feature surface must lie outside the boundary."
When I look at the explanatory pictures for 8-24 this rule looks pretty nice for height and widht of the hole. 2x0.25 = 0.5 = positional tolerance value, is substracted from MMC sizes of the profile. But how about the basic radii: R10 and R3? From what I see on the graphic the boundary in the area of R10 is 10-0.6-0.25 = R9.15. Am I correct? So is it really in line with the statement cited above? Or the radius is R8.9 (10-0.6-0.5)?
 
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pmarc -- is a radius a "size"? That might be one way to dodge the difficulty, since the formula refers to the "MMC size." Shrug.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
 
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J-P,
That was my thought too...
So another question: can features other than containing opposite parallel features be controlled by position at MMC/profile combo? Could a hole comprising for example 4 tangent radii of different values and no pair of nominally flat and parallel faces be controlled by such combination?
 
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The idea with the combined controls is that the virtual condition boundary is offset from the "maximum material boundary at basic location" by a distance equal to half the Position tolerance. The maximum material boundary is defined by the inner boundary from the Profile tolerance, set at the basic location. So the choice of the term "MMC size of the profile" to describe this in Sectioin 8.8 is quite unfortunate and misleading, because the feature is not a regular feature of size and hence does not have a size or a maximum material condition in the usual sense.

The combined Profile/Position technique does not have to be applied to regular features of size (although it could), and is really intended to handle closed shapes with irregular geometry. The shape can be fully irregular with no opposing parallel features, made up of combinations of tangent radii, or free-form mathematically defined curves. So to deal with 8-27, I find it easiest to dismiss any notions about "size" in the conventional sense - width, diameter, radius, none of these really apply. The basic geometry of the feature is a closed shape, and the tolerance zones and boundaries are offsets of that shape.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
 
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This is where I was heading to, Evan. If a hole is fully irregular, for position/profile combo we cannot think in terms of size, but in terms of boundaries and the offset on each portion of the hole's surface equals to half of profile tolerance value (assuming equal bilateral zone) plus HALF of positional tolerance. If the wording of 8.8 somehow included the "HALF", I would not have any problems. Actually this would also perfectly suit to the radii on the feature from fig. 8-24.
 
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True, Jim. I just couldn't see angularity applied to this as a FOS (irregular). But that makes sense.

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