Sr GDTP Y14.5-2009 Exam Review Ch-4 Part-I Sep2021

[metrologic]

Sr GDTP Y14.5-2009 Exam Review Ch-4 Part-I Sep2021

Alright we're on Section 4, Datum Reference Frames. I'm gonna break this Q and A into three different threads. This part covers 4.1 through 4.11.14.

Q1. 4.1-4.8 There's no subsection here simply called Datums. If we've got datum features, and datum feature simulators, and datum reference frames -what exactly do we need the datums themselves for? The datum features are mated to the datum feature simulators. The datum reference frame is defined relative to the datum feature simulators and "...constitutes the three-plane dimensioning system used for dimensioning and tolerancing."(4.7.1) Datums are described as "theoretically exact points, axes, lines, and planes."(4.1) But what are they used for? Fig 4-3 illustrates a variety of common datum features and their associated datums: Why does a conical(e) datum feature create a datum axis and a datum point, and not an axis and a plane? Why does a linear extruded(f) datum feature create a datum axis and a datum center plane, and not two planes? Why does a complex(g) datum feature create a datum axis, point, and center plane, and not three orthogonal planes. It seems like the creation of these datums is rather ad-hoc, and with no definite purpose in mind.

Q2. 4.10.1-4.10.4 Y14.5 seems to provide quite a few examples specifying the location and orientation for datum reference frames (DRF). They are always shown originating at some confluence of datum axes, points, etc. Am I the only one that thinks the DRF could be placed anywhere? Are we not just talking about a coordinate system? I mean, sure, most of the time it's probably best if it is aligned to some key datum features, but I would think there would be plenty of exceptions. For example, in Fig 4-8 a cylindrical part and its DRF is illustrated. The DRF is shown originating from the bottom center at the intersection of the part's datums, which is a plane and an axis. What if I just ripped the DRF from the datum B axis, and moved it, say 10 inches over to the left? What am I breaking?

Q3. 4.11.3 Effect of Material Boundary Modifiers Applied to Datum Feature References. This subsection states: "MMB, LMB, and RMB conditions may be applied/implied to any datum feature reference in a feature control frame." Does RMB really apply to a plane surface that is controlled only by flatness? I feel like something is being oversimplified.

Q4. 4.11.4 Specifying Datum Features RMB. "Where a datum feature is referenced at RMB in a feature control frame, the datum feature simulator geometry originates at the MMB...." What if the datum feature is too small? Is there no way to continue with an inspection of any other dependent geometric tolerances? Why can't I "originate" the datum feature simulator at an unauthorized size beyond MMB?

Q5. 4.11.6 Determining Size of Datum Feature Simulators at MMB. This subsection states: "Datum feature precedence shall be respected, except in the case of a customized datum reference frame. See para. 4.22." I understand what a customized DRF is, but it's not clear to me how they effect the requirements for datum feature simulator sizes. Can somebody unpack this for me? It sounds like some of the typical datum shift resulting from the use of a material modifier is being disallowed for customized datum reference frames, but it's just speculation on my part.

Q6. 4.11.6.1 Determining the Correct Maximum Material Boundary (MMB). It seems entirely possible that MMBs and LMBs could be 0, or mathematically negative. What then? Is the design invalid? The feature can't be a datum feature referenced with a material modifier? What?

Q7. 4.11.6.3 Clarifying Applicable MMB. This subsection states: "The term "BSC" or "BASIC" may be used to indicate that the datum feature simulator is located at the basic location of the datum feature." Just location? What about size? Seems like it should be size too.

Q8. 4.11.10 Translation Modifier. "Where it is necessary to indicate that the basic location of the datum feature simulator is unlocked and the datum feature simulator is able to translate within the specified geometric tolerance to fully engage the feature, the translation modifier is added..." So the movement is limited to the available tolerance? (They use Figs 4-19 and 4-32 for examples.) If the translation is actually limited to a tolerance, what tolerance is available? This seems like it might be kinda tricky because I would think the tolerance would depend on datum reference precedence.

Q9. 4.5 Datum Feature Simulator. Amongst other things, it says here that datum feature simulator geometry shall be the inverse shape of the datum feature unless otherwise specified, and have perfect form. Is perfect form also required for material boundaries? All the examples so far illustrate material boundaries derived from a combination of an MMC or LMC condition and a geometric tolerance zone. The geometric tolerance zones were always based on cylinders or parallel planes. And therefore these MMB and LMB boundaries always ended up being a shape that matched the parent feature but different in size. What if the tolerance zone is oddly shaped? Wouldn't that produce an MMB or LMB shape that is not the same as the parent feature? And in such a case, what shape does the datum feature simulator default to? I'm assuming the datum feature simulator geometry still needs to be an inverse shape of the datum feature, but at a size that can contain or be contained by the MMB or LMB oddly shaped boundary. What does everybody else think?

 

[3DDave]

A4: The MMB is a defined boundary - it has a definite shape to start from so it makes sense to use that as the basis to progress from.

A5: The effects on the DRF in a customized DRF are not necessarily in order of the datum feature references. One could give the controls normally associated with the primary datum feature to the tertiary datum reference. The effects on datum shift are unaffected, it's just precedence that isn't.

A6: That's what the axis interpretation is for. In practice, it seems like the committee depends on typical practice to protect them from dealing with the consequences of the rules they create. oh, right, in practice that's a situation that is rare where the location of a feature is so much more poorly controlled than the size of the feature.

A7. If that's on the test there's a 50% chance the "correct" answer will be wrong.

A8. No surprise that a half considered addition to the standard results in half considered explanation. Solutions to this problem were already available, remain available and, except for saying that datum targets can be moved to capture the part, the translation modifier is not necessary. With datum target symbols, the movement can be indicated by the symbol, so even then translation isn't required.

This symbol is a result of a long standing fight between the fixture makers and the CMM software guys. To simulate this in software is cheap. To do this in a fixture is expensive. However, if the datum feature simulator doesn't move, the result is reversed. It's cheap to have a fixed diameter pin pressed into a hole and harder to examine in software the entire boundary to ensure it's outside the simulated pin.

A9: In practice ... I'd guess that it's rare for anything but a pin/hole or width to be used, so they don't deal with it. Look at figure 7-28 to see that they just punt on the surface interpretation suggesting what it would be like if one or more of those holes were selected as a datum feature.

 

[axym]

metrologic,

There are a lot of good questions here, and I've wondered about of the same questions myself and reached many of the same conclusions. You're poking into the subtle oversights and inconsistencies in Y14.5-2009. These are great for discussion purposes - it sounds like you would probably enjoy attending ASME committee meetings. There are plenty of hardcore GD&T nerds around (like myself) who love to talk Y14.5 minutiae with others who share a similar affliction. Informal discussions of issues like these spring up all the time in the hallways between meetings, at dinner, etc.

Anyway, these questions are great but pondering them won't serve you well for the GDTP exam. It will distract you from what is important - memorizing what it says in the standard (not why it says what it says or whether it makes complete sense or not).

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[metrologic]

@axym

Hmmm, yeah maybe I would enjoy the ASME committee meetings. Part of me wants to believe there exists a secret book of dimensional logic from which we only get to see a simplified cross section. And each new revision is just a slightly altered section cut.