Sr GDTP Y14.5-2009 Exam Review Ch-5 Sep2021

[metrologic]

Sr GDTP Y14.5-2009 Exam Review Ch-5 Sep2021

Hi Everyone! I'm up to section 5, Tolerances of Form. As you may recall, I'm working through the Y14.5 standard in preparation for upgrading my GDTP certification from the tech level to the senior level. Here are some questions I had from this section:


Q1. 5.4.1.1 Cylindrical Features. This subsection discusses the straightness of line elements on the surface of cylindrical features of size. "The straightness tolerance must be less than the size tolerance and any other geometric tolerances that affect the straightness of line elements. Since the limits of size must be respected, the full straightness tolerance may not be available for opposite elements in the case of waisting or barreling of the surface. See Fig. 5-1." What geometric tolerances other than size could affect the straightness of line elements in this context? Perhaps total runout? Also, why is "waisting or barreling" highlighted here? I don't see waisting of barreling as a special case. Given that surface straightness tolerance is unavailable beyond the boundary of perfect form at MMC, I would think the part could be bent in any number of unremarkable ways that result in the loss of usable surface line element straightness tolerance.

Q2. 5.4.1.2 Violation of MMC Boundary. This subsection discuses straightness as it relates to the derived median line of a feature of size, not to be confused with the subject of the prior question. "Where necessary and when not used in conjunction with an orientation or position tolerance, the straightness tolerance may be greater than the size tolerance. Where the straightness tolerance is used in conjunction with an orientation tolerance or position tolerance value, the specified straightness tolerance value shall not be greater than the specified orientation or position tolerance value." Why does an accompanying use of orientation or position tolerance trigger a requirement for the straightness to be less than the size tolerance, and also less than the orientation or position tolerance? I'm kinda assuming this is to help us make meaningful statements about maximum material boundary requirements, but I'm not really sure what the exact logic behind this is. (Also, what if the part is nonrigid and measured in the free-state? Does the straightness tolerance still have to be smaller than the orientation or position tolerance value?)

Q3. 5.4.1.4 Straightness of Line Elements. This is more of a complaint than a question. "Figure 5-6 illustrates the use of straightness tolerance on a flat surface. Straightness may be applied to control line elements in a single direction on a flat surface; it may also be applied in two directions as shown. Where function requires the line elements to be related to a datum feature(s), profile of a line should be specified related to the datums." Subsection 5.4 indicates "form tolerances are not related to datums." And this generally makes sense for most features. But what about straightness of line elements on a surface? If the feature in question is a cylinder, Y14.5 goes into great detail regarding the manner in which such line elements on the surface are identified and analyzed for conformance. Subsection 5.4.1.1 requires that "Each longitudinal element of the surface must lie between two parallel lines separated by the amount of the prescribed straightness tolerance and in a plane common with the axis of the unrelated actual mating envelope of the feature." There is no corresponding guidelines for the application of straightness to line elements of a flat surface. Maybe this use should be dropped from Y14.5?

Q4. 5.4.3 Circularity (Roundness). In Fig 5-13 and 5-14, a circularity tolerance is directly associated with a size dimension. Can circularity be applied on an MMC or LMC basis?

Q5. 5.5.1 Specifying Geometric Tolerances on Features Subject to Free-State Variation. "Where an individual form or location tolerance is applied to a feature in the free state, specify the maximum allowable free-sate variation with an appropriate feature control frame. See Fig. 5-13. The free-state symbol may be placed within the feature control frame, following the tolerance and any modifiers, to clarify a free-state requirement on a drawing containing restrained feature notes, or to separate a free-state requirement from associated features having restrained requirements. See Figs. 3-21 and 5-14." Let's say I have a drawing of a nonrigid part that does not contain any restrained feature notes. If the intention was for the part to meet its tolerance requirements in the free state, is it necessary to use the free-state symbol? Would selective use of the free-state symbol in this context imply that a restraint requirement should be used for cases where the free-state symbol is not used? --and if no restraint requirements are provided for such examples, would this be considered a drawing mistake? I thought "Unless otherwise specified, all tolerances apply in a free-state condition." (4.20) Is there some requirement that I am not aware of that mandates nonrigid parts be measured in a restrained state unless the free-state modifier is applied?

Q6. 5.5.3 Average Diameter. In this subsection Y14.5 states: "Where form control, such as circularity, is specified in a free state for a circular or cylindrical feature, the pertinent diameter is qualified with the abbreviation AVG. See Fig. 5-13." Is it necessary to use the free-state modifier when specifying average diameter for nonrigid parts? There's nothing in Fig 5-13 indicating how the part should be restrained. I would interpret this as a nonrigid part. And since simulated restraint requirements are not listed, I would assume it must be measured in the free state, regardless of whether the free-state symbol was present or absent. Hence according to subsection 2.7.2, the part is "subject to free-state variation in the unrestrained condition", requiring an exception to rule #1. And if rule #1 does not apply, than it makes logical sense to stipulate AVG or some other methodology to compensate for the lack of a boundary of perfect form at MMC.

 

[Belanger]

Just picking the easy one for now:
Q4. 5.4.3 Circularity (Roundness). In Fig 5-13 and 5-14, a circularity tolerance is directly associated with a size dimension. Can circularity be applied on an MMC or LMC basis?

No. Even if the FCF sits under the size callout, as in those figures you mention, circularity is still a surface control, not a FOS control. Thus, it shall never be modified with MMC or LMC.

 

[powerhound]

Even if the FCF sits under the size callout, as in those figures you mention, circularity is still a surface control

metrologic,

This is a point of contention I've always had with associating circularity and cylindricity with the size tolerance. It gives one the impression that it means something different (as it actually does in the case of straightness and flatness) but it doesn't. Apparently I'm not the only one with heartburn about that because in ASME Y14.5-2018 this practice is now prohibited. In accordance with that version, the FCF and the size dimension must now be separated.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[metrologic]

@powerhound
I didn't realize it was prohibited in ASME Y14.5-2018. That's good to know.

 

[Belanger]

Your Q3 is a well-know beef. Straightness of surface elements on a cylinder doesn't pose a problem, because the direction being traced follows the longitudinal direction of the cylinder (although there are some nuances we can discuss there, I suppose).
But straightness of line elements on a nominally flat surface seems to cry out for a datum. The trite answer in the past was that the elements being traced should be parallel to the plane of the view where the FCF is shown. But what if the front face and back face aren't parallel to each other?
Profile of a line is indeed the better choice.

 

[Burunduk]

Q5. 5.5.1 Specifying Geometric Tolerances on Features Subject to Free-State Variation. "Where an individual form or location tolerance is applied to a feature in the free state, specify the maximum allowable free-sate variation with an appropriate feature control frame. See Fig. 5-13. The free-state symbol may be placed within the feature control frame, following the tolerance and any modifiers, to clarify a free-state requirement on a drawing containing restrained feature notes, or to separate a free-state requirement from associated features having restrained requirements. See Figs. 3-21 and 5-14." Let's say I have a drawing of a nonrigid part that does not contain any restrained feature notes. If the intention was for the part to meet its tolerance requirements in the free state, is it necessary to use the free-state symbol? Would selective use of the free-state symbol in this context imply that a restraint requirement should be used for cases where the free-state symbol is not used? --and if no restraint requirements are provided for such examples, would this be considered a drawing mistake? I thought "Unless otherwise specified, all tolerances apply in a free-state condition." (4.20) Is there some requirement that I am not aware of that mandates nonrigid parts be measured in a restrained state unless the free-state modifier is applied?

I think you answered your own question with that 4.20 quote. If all you have is a drawing of a non-rigid part with no restrain requirements, then in essence the drawing user shouldn't even care if the part is rigid or non-rigid, he should treat it the same as any other part. Whether applying the free state modifier to tolerances on such part be considered a drawing mistake, it may not be strictly per the standard, but it would be nonetheless wrong from the aspect of having redundant and useless stuff on your drawing. How do you imagine this implemented? A free state modifier after every single geometric tol. on your drawing? A free state symbol in the drawing notes related to all features? If not, what requirements will get it if they are all to be treated the same?

 

[metrologic]

@Burunduk

I think the drawing user does need to know if the part is to be treated as rigid or non-rigid. If you think about it, isn't the real issue here whether rule #1 applies to the feature at hand or not? But the free-state symbol doesn't convey this as far as I know.

 

[Burunduk]

metrologic,
Rule #1 is not automatically nullified for every part that "looks" non-rigid. If Rule #1 has to be nullified, it is up to the drawing maker to communicate this. One of the two default exceptions to Rule #1 ("default" in the sense that it doesn't need to be overridden by other means) is features subject to free state variation in the unrestrained condition. So the free state symbol does convey it, but it is used only on drawings where there are restraint instructions/requirements *. Other ways Rule #1 could be nullified is the independency symbol or if you specified a form tolerance for a FOS that controls a derived median line or derived median plane. If none of it happens, it doesn't matter if the part is rigid or not, in the sense that Rule#1 applies as for any other part.

* This one isn't explicitly stated in the standard, but it is the only sensible way to use the free-state symbol. Because, as you already saw, ALL requirements are applicable in the free state condition unless otherwise specified.

 

[metrologic]

Happy holidays Burunduk!

I agree completely, the drawing maker should be the one to determine whether or not rule #1 applies. That is the only way to avoid confusion. But I would think the free-state symbol might also be applied to rigid parts or features, and might not necessarily trigger an allowed violation of an MMC boundary. For example, perhaps a fairly hefty mounting flange is required to be bolted down to verify some critical characteristics somewhere else on the part, but the flange thickness size must still be subject to rule #1 in the free state. How would one clarify this feature of size requirement? Could one apply the free-state symbol directly to the feature of size, or would that imply that rule #1 does not apply?

 

[Burunduk]

metrologic,

In the ASME Y14.5-2009 standard, subparagraph 2.7.2 lists exceptions to Rule#1 (cases where the rule doesn't apply by default). There are two cases and one of them is:
"(b) parts subject to free-state variation in the unrestrained condition. See para. 5.5."
Sub-para. 5.5 provides further details on how the free state symbol is to be used.

So, bear in mind - where the free state modifier is used, the feature being controlled in the free state is NOT subject to Rule #1 per the standard. If something else is required, it should be specified by a note.

Happy holidays!

 

[metrologic]

Thanks Burunduk! I think that clears up the ambiguity I was reading into the use of the free-state symbol.