Sr GDTP Y14.5-2009 Exam Review Ch-8 Part-II Sep2021

[metrologic]

Sr GDTP Y14.5-2009 Exam Review Ch-8 Part-II Sep2021

I'm in the middle of section 8, Tolerances of Profile. 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 is the second set of questions I had from this section:


Q1. 8.3.1.5 Limited Segment of a Profile. Figure 8-6 details a part with three adjacent equal bilateral profile tolerances that control the entire continuous curvy outline except for a flat surface on its bottom. Between points A and B the tolerance is 0.12, between B and C the tolerance is 0.1, and between C and D the tolerance is 0.05. The flat bottom of the part serves as datum feature F. All the profile tolerances share the same datum reference frame |E|F|. The tolerance zones associated with the profile controls in this example are illustrated in Fig. 8-11(a). As can be seen from that figure, the tolerance zones "jump" from one zone width to another at the transition points B and C. The transitions are abrupt with tolerance zone ends that butt in to the next adjacent tolerance zone. However, where the true profile wraps around and intersects the flat bottom of datum feature F, the profile zones are trimmed off by datum plane F. Shouldn't the profile zones hang below the level of datum plane F? How does one know whether a tolerance zone should end square to the true profile or be trimmed flush with an adjacent feature?

Q2. 8.3.1.5 Limited Segment of a Profile. Back in subsection 8.3.1, Y14.5 said: "Where a profile tolerance encompasses a sharp corner, the tolerance zone extends to the intersection of the boundary lines." What happens if two profile segments with different tolerance zone widths converge at a sharp corner? Are the zone boundary lines intersected like in figure 8-12? Or do the zone boundaries feature an abrupt change from one tolerance to the next as indicated in Fig. 8-11(a) at the transition points B and C for the adjoining limited profile segments.

Q3. 8.3.1.5 Limited Segment of a Profile. "...if some segments of the profile are controlled by a profile tolerance and other segments by individually toleranced dimensions, the extent of the profile tolerance shall be indicated. See Fig. 8-7." The true profile in Fig. 8-7 is defined by some arbitrary basic dimensions and swoops from left to right between points D and E. Its extents are limited on either end by the two opposing faces of a regular feature of size. In other words, as the feature of size grows from its least material condition to its maximum material condition, the profile caught in between must naturally be enlarged in some manner. The distance between points D and E surely cannot be fixed in this example. How does one extend the true profile in these situations to fully define the tolerance requirements as the feature of size expands?

Q4. 8.3.2.2 Non-Uniform Zones to Smooth the Transitions. This subsection contains a note at the end: "NOTE: A profile per unit length, similar to that shown in Fig. 5-4 for the control of straightness, may be used to control abrupt transitions that occur when the profile tolerances are specified on adjoining segments of a feature." Profile per unit length seems like it would be a lot more complicated than a straightness or flatness per unit area. How exactly does one specify and verify a profile per unit length? What control's the orientation, location, and shape of the profile per unit length tolerance zones?

 

[Burunduk]

Q1
In this example, it would be impractical to show an extension of the tolerance zones pass datum plane F. The part will be rested on a datum feature simulator such as a surface plate (in case of manual inspection) and there will be nothing to control/inspect below the simulated datum. With that said, I would agree that it can be different when a CMM or vision system is used without physical datum simulation, but the only thing that may hang beyond the simulated datum is some burrs that are usually removed or controlled by other means.

Q3
the answer to that lies in the fundamental rules found in section one of the '09 version of the standard. According to one of them, the tolerance zone should cover the entire feature. So it is no problem that the span of the feature is influenced by the width between the vertical faces, the profile tolerance will simply have to be extended as much as necessary to cover the actual top surface. By the way, it is so even if the width between the vertical faces exceeds its tolerance limit.

 

[Burunduk]

A finding related to your Q1:
In the newest version of the standard (2018), the tolerance zone ends are shown both trimmed by datum plane F as in the '09 figure you were asking about, and hanging below the way you would expect, on different related figures. This is the kind of things that you shouldn't be overanalyzing, as there is apparently not as much thought put into the creation of the graphics as you may expect.

 

[metrologic]

Hi Burunduk,

A finding related to your Q1:
In the newest version of the standard (2018), the tolerance zone ends are shown both trimmed by datum plane F as in the '09 figure you were asking about, and hanging below the way you would expect, on different related figures. This is the kind of things that you shouldn't be overanalyzing, as there is apparently not as much thought put into the creation of the graphics as you may expect.

My hope is that someday Y14.5 could function in a manner that shares some key characteristics of computer programming languages. Programs are usually written and then compiled into instructions for specific computer architectures. This process is essentially hopeless if there's any ambiguity in the interpretation of the high level language used to write the program. Is it too much to expect that geometric tolerances should 'compile' into, as an example, a CMM inspection routine?

 

[Burunduk]

metrologic,
I agree, but this is a more general concern I have regarding the definitions in the text, less about the figures. In the case of tolerance zone extent, the standard is pretty clear, and based on it a CMM inspection routine could be programmed to extend the tolerance zones as necessary to make sure the entire actual feature is covered per the fundamental rules (with that said, when physical datum feature simulators are used they may make the extended tolerance zone inaccessible and that has to be considered too).