Plane surface datum feature MMB located basically from a higher precedence datum

[Burunduk]

Fig. 4-31, ASME Y14.5, illustration (c):

"Datum feature B must remain in contact at a minimum one point."

With this condition, It looks like if the profile tolerance applied on datum feature B was more than the shown 0.2 and large enough, or the position tolerance for the holes was less than 0.3(M) and tight enough, a pair of holes positioned accurately and passing inspection per the (a) and (b) cases, may be forced out of tolerance by rotating the part all the way to meet the requirement of minimum one point of contact with the datum feature simulator of B at MMB. Perhaps this can be prevented by having the datum feature B toleranced considerably more accurately than the holes controlled with reference to it, but it still seems weird; It looks like MMB and the minimum contact requirement in this case actually make it more difficult to meet the tolerance - unlike for example the case of 4-30 (b) where the scheme makes sense because there is actual datum shift. MMB in fig. 4-31 seems to impose a restriction rather than allowing shift. Where am I wrong?

 

[Belanger]

Agreed. And thanks go out to Burunduk for bringing up this little discussion.

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

 

[Burunduk]

Dean and others,
perhaps I am missing something and sorry if this was already addressed - but why do you think that MMB as in the 2018 standard can't be a functional requirement? Suppose that the mating part has a cylindrical pocket that mates with the 40 diameter, and there is a feature or part fixed inside the cylindrical pocket that sits near datum feature B with some clearance maintained or just contacting it at worst case without creating interference. At this mating part, the basic dimension from the center of the 40 internal diameter to the feature that "mates" with datum feature B could be 5.3 and there could be a profile tolerance controlling it within 0.2 with reference to the internal 40 diameter.

The way I see it, BSC is actually the problematic one. What if the B datum feature was produced at 5.09 normally offset from A center axis as the profile tolerance allows? Won't it cause the part not to fit with the BSC gage without having to rotate the part in a way that may force the holes out of tolerance?

 

[pmarc]

In my opinion it doesn't really matter whether datum feature B is referenced RMB, BSC, MMB, LMB. It is just not a feature that is able to fully constrain rotation of the part around datum axis A without introduction of additional artificial rules like the rule for MMB that changed between 2009 and 2018.

 

[Dean Watts]

Burunduk,

With the part in Figure 4-31 that you posted, I agree with you that BSC is probably not a good choice. The offset datum feature needs a greater offset distance to make BSC likely to model function. One way you could use BSC on this part is if you applied a unilateral and fully inside the material profile to datum feature B. Then the true profile and MMB would be the same, so BSC works even if there is no offset to the datum feature at all.

The other case, with MMB applied to datum feature B and following the new requirement in Y14.5-2018, I don't agree.

I said in an earlier post that the requirement would be the same for MMB or LMB, but that wasn't quite correct. While the boundaries are the same for LMB and MMB, the element of the datum feature that must be within the boundaries is different. For MMB Y14.5-2018 says that a "maximum material extremity" must remain between MMB and LMB. If LMB is specified then it's a "least material extremity" that must be between the same limits. A problem I have with this is that each of these cases would put other points on the datum feature surface beyond a boundary. If MMB is specified, then low points on the datum feature can go beyond LMB. If LMB is specified then high points on the datum feature surface can go beyond MMB. This seems odd to me, and makes me think there must be a better approach.

After some more thought, I can think of cases for which datum feature B could be functional if it is entirely within MMB and LMB (acting with other features in the assembly to block light or stop the movement of another part in the assembly in the direction parallel to datum A, for instance), but I think the tolerance requirement would be better if the DRF shift rules kept all points on the datum feature within the tolerance zone boundaries. I still think MMB or LMB should not be allowed with an offset datum feature, but maybe imposing both requirements would make sense. Then all points on the datum feature must remain between MMB and LMB.

Does this make more sense, and if so, what's the best way to specify that requirement?

Dean

http://www.validate-3d.com

 

[Burunduk]

A problem I have with this is that each of these cases would put other points on the datum feature surface beyond a boundary.

The 0.2 profile tolerance ensures the entire surface is within boundaries as it is required to be with reference to datum A only.
The position tolerance verification with datum feature B at MMB as shown in the 2018 standard may be intended to simply allow datum shift for the holes in the |A|B(M)| DRF, mimicking the conditions at a proper assembly. And since |A| (with reference to which the extremities of datum feature B being within boundaries is required) and |A|B(M)| are different datum reference frames used to control different features, why should there be an issue?
The design could intend the holes to be clearance holes for fasteners that float within them and the feature that mates with datum feature B could be a key of sorts that drives the part around the OD axis, can be assembled with some allowable initial gap and can be brought to contact only after the rotation starts (it would be necessary to analyze that the clearance between the holes and the fasteners is large enough not to override the function of the key).

 

[Dean Watts]

Burunduk,

Yes, of course the 0.2 profile would be met if the part is conforming. And then when |A|B(M)| is established we can push all but one point on the datum feature surface outside those tolerance zone boundaries. Isn't there a better way to determine, and then use boundaries?

I still think it makes no sense to impose a rule the will allow all but one or two points on the datum feature to be outside a tolerance zone boundary. We can meet the functional requirements you describe without imposing this rule. Why is that rule better than a rule that would require all points on datum feature B to be within LMB and MMB when evaluating the tolerances related to |A|B(M)|? A rule that requires all points on datum feature B in this example to remain within MMB and LMB may lead to a different profile tolerance value being applied, but the overall picture seems to make much more sense to me. I acknowledge that the rule I am proposing is more restrictive, but the requirement seems logical and also more clear.

Said another way, rather than have such a subtle difference between |A|B(M)| and |A|B(L)|, it seems better to me to have a rule that prohibits those specs and instead requires all points on datum feature B to be between its tolerance zone boundaries.

Edit: Maybe we could impose |A|B(M)(L)|? It would look odd, but I think it makes more sense then the 2018 rule.

Dean

http://www.validate-3d.com

 

[Burunduk]

Dean, I think that the difference between |A|B(M)| and |A|B(L)| is that |A|B(M)| as shown in the 2018 figure posted by Jacob allows more datum shift counterclockwise (until all points but one are above LMB) while |A|B(L)| would allow more datum shift clockwise (until all points but one are below MMB). I'm not sure how "subtle" this difference is; perhaps one might find one of the specs preferable over the other under certain circumstances.
The point you brought up made me think of it some more though, and what I am more worried about now is: can a part with datum feature B produced at MMB still have nonzero datum shift for the position of the holes when the tolerance is specified with reference to |A|B(M)|?

 

[Dean Watts]

Burunduk, Yes, I think that is a valid concern. Being able to rotate the part counterclockwise such that a point on datum feature B that is at MMB moves 0.2 mm sway from MMB, putting all of the rest of datum feature B beyond LMB is not modeling, in a clear and efficient way, any function that I can think of. I think pmarc's description of the new rule in 2018 as artificial is valid.

Dean

http://www.validate-3d.com

 

[Burunduk]

Seems like the MMB scheme gives 0.2 datum shift or even more for the most of the surface, regardless of the datum feature produced at LMB, close to true profile or at MMB. In the '09 standard, there was no datum shift at all regardless of how the feature was produced because of the forced minimum one point contact.

This figure has issues in both the 2009 and 2018 standard versions.

I also agree with pmarc's description of the new rule in 2018 as artificial, but the 2009 rule was artificial too.

 

[chez311]

In the '09 standard, there was no datum shift at all regardless of how the feature was produced because of the forced minimum one point contact.

Maybe not shift in the traditional sense and as noted does not have some of the qualities that we expect from datum feature shift, but the behavior is notably different than the RMB case which progresses and seeks the ambiguous "maximum contact". Comparing the two, I think the MMB case as defined in 2009 is actually not so artificial as I can imagine an assembly where the part mimics the simulation shown in Y14.5-2009 4-31(c) - the part is mated to A and then rotated towards a fixed boundary (feature on a mating component) until it makes a minimum of one point of contact (the physical analog/description would be to simply rotate it towards the boundary/mating feature until it stops). More shift could be available if the primary A were specified MMB as well.

Thus the RMB case seems more artificial to me at least as it relates to function, implying the mating feature must progress from MMB to LMB and seeks the ill defined "maximum contact". If a similar scheme is specified with the offset datum feature B as secondary at RMB, I think its less likely that it actually mimics assembly and more likely that it is the intent to create "nice" neat datum feature/simulator geometry. I'd be interested if someone has a case where this makes more functional sense.

Additionally, it seems a bit murky as to the difference in behavior between the MMB and [BSC] case in Y14.5-2009 4-31. The verbiage is different in the means this portion ie: "no translation or rotation of datum feature is allowed" vs. "datum feature B must remain in contact at a minimum of one point". It was mentioned a few times, but not directly addressed - I'm interested in what others view would be, if any, difference between datum feature B specified in the following manner (per Y14.5-2009). Seems to me they should be almost identical.

Basic 5 distance combined with |PROFILE|0.2|A| referenced |A|B(M)|
Basic 5.1 distance combined with |PROFILE|0.2(U)0|A| referenced |A|B[BSC]|

 

[pmarc]

I also agree with pmarc's description of the new rule in 2018 as artificial, but the 2009 rule was artificial too.

Agreed.

 

[Dean Watts]

chez311,

I think those two cases you describe are identical if using Y14.5-2009. We'll agree that the harder one to duplicate is LMB, since that addresses a low point instead of a high point. We don't have a way to alter the profile tolerance zone to make BSC model LMB, that is equivalent to your suggestion for making BSC and MMB equivalent for a particular case.

After more thought about Burunduk's original post here, I am thinking that the Y14.5-2018 approach, with the offset datum feature's high point constrained between MMB and LMB when MMB is specified, or the offset datum feature's low point constrained between MMB and LMB when LMB is specified, isn't so bad. I'll agree that it still seems a bit odd, but if a functional gauge is built with all datum feature simulators at MMB and boundaries for all other features at VC (better said MMB also, but not "proper", since they're not datum features), then with a feeler gage in hand, the requirement for MMB in Y14.5-2018 seems possibly sensible and useful.

If the functional need for a part that is similar, but a rough unmachined casting, then all the same for LMB, but with some more difficulty ensuring that the datum feature's low point doesn't go beyond MMB or LMB seems functional enough.

I'll still say that MMB or LMB for an offset lower precedence datum feature is odd, but I'll back off from my former position of not recommending the application at all. Y14.5 is intended to be a complete "toolbox", so the 2018 approach to this may be the best solution.

Dean

http://www.validate-3d.com