Virtual Condition Check at RFS 3

[dthom0425]

Hello all-

I am aware of how to perform virtual condition checks for MMC and LMC but I am wondering how does one verify (what is the math?) that the RFS true position applied to a hole will work in a mating condition.

Example:

C'sink with .150" thru hole @ (dia) True position .010" RFS.
mates to
#6 thread (.138) @ (dia) True position .028" RFS

How would you verify through math that your true position tolerancing is OK at RFS?

Thanks

 

[3DDave]

I think 4-21 (c) is a possible outcome, but not the only outcome of the allowable variations for that datum reference frame. Without knowing the length of datum feature A and width of the face it isn't possible to know that datum feature B will make full face contact when A is at LMC. Based on the proportion in the diagram, with the perpendicularity tolerance that is twice the diameter tolerance, the LMC surface would be binding against the MMB datum simulator at the same time there is face contact. This ignores that at LMC the feature could be bent enough to not allow rotation anyway and the face would also make no contact.

For me, the apparent problem with 'datum precedence' is that, aside from interference fits and planar contacts (I feel like I'm missing a case), there is nothing in real mechanisms that preclude shifting parts to accommodate clearances.

If, for no good reason, one datum feature has a large amount of clearance and one later in the chain has very little, the later one may be the one providing the greater control. But, when it comes time to install the part, the installer cannot tell the difference, so is there necessarily significance to the order they appeared on the drawing?

What was not in 4-21 was the bold move of suggesting |B|A(M)] and showing how to evaluate the MMB when A is secondary.

 

[Belanger]

3DDave -- I've always felt the same about Figs. 4-21(c) and (d). If the pin is made at a smaller size, then datum feature B sort of takes over as the main balancing feature. We don't know the length of the pin etc., so it's kind of a strange scenario.
Should I presume that your last comment was tongue-in-cheek? Because having B as the primary datum would make a lot more sense from a practical point of view -- and that's what they had shown in both the 1994 and 1982 standards. Maybe boldness is what they were after in 2009!

 

[3DDave]

Berlanger - we'll never know about the 1994 version since they excluded geometric characteristic controls from the datum features.

 

[Belanger]

Dave -- though a callout relating the two datum features wasn't given (i.e., perpendicularity), they clearly indicate "virtual condition" in the explanation.
Do we really need a number to communicate the gist of that page? ... No.

 

[3DDave]

Berlanger - I was pointing out that the committee cannot decide which is important. They were the ones to exclude the references in the 1994 version and then bring them, and firm numbers, back in the 2009 version.

My preference would be to make the flat surface datum feature A, and give a perpendicularity tolerance to the round and mark it as datum feature B; then work the rest of the diagram from there. This is particularly true as in the example (2009 version) of [A(M)|B], the reference to B is redundant for the pattern of holes.

 

[pmarc]

First of all, my apologies for late reply.

pmarc - the interest in re-ordering isn't to deal with the hole; it is to examine how to evaluate the way mating parts will interact. Perhaps "B" is fit over a shaft with a shoulder against "A" and "D" will provide a pivot for a lever that interacts with that shaft. It is useful to know how "B" varies in the reversed context in order to design that lever because "B" is a stand-in for the shaft at the part level.

3DDave, I thought you were specifically talking about the part from fig. 4-16, hence my remark. My apologies for misunderstanding your question.

I see the discussion has focused on fig. 4-21, but if you don't mind, I would still like to dig a little bit more in your virtual condition theory. As far as I recall, in fig. 4-16 option (c) you have been saying that the of MMB of datum feature D is not an internal cylinder of dia. 7.5 (as given in the standard), but a 7.3 X 7.5 elongated hole with fully rounded ends in the 7.5 direction and that this is all because of the perpendicularity callout |perp|dia. 0.2(M)|A| applied to the datum feature D. If the perpendicularity callout was not there, the shape and size of MMB of D would be exactly as given in the standard. Am I corect?

So let me ask you a following question:
In the light of your theory, what would be the size and shape of MMB of datum feature B for the position callout applied to datum slot C in fig. 4-16 assuming the perpendicularity callout applied to datum feature B was for example |perp|dia. 0.1(M)|A| and not |perp|dia. 0(M)|A|? In other words, is there anything in the theory that could make you say that the MMB of datum feature B in that case would not be a cylindrical pin of diameter 10.8, but something else?

 

[3DDave]

pmarc - I don't think so - the datum feature B is defined relative to A directly, so the MMB would be dia 11.0-dia tol 0.1-pos dia 0.1 -> dia 10.8.

Had the 4-16(c) example been [A|B(m)|C(m)] there would also be a direct path which would match the answer given for [A|B(m)|D(m)]

 

[pmarc]

3DDave,
Thanks for the answer.

In the original thread we used an example of an observer standing exactly at datum axis B and looking at the datum feature D. The way I understand your argumentation for 7.3 x 7.5 elongated shape of MMB of D in fig. 4-16(c) is that in |A|B(M)| DRF, due to the lack of a reference to the tertiary datum feature C, the observer would always be allowed to adjust/optimize his view direction (by rotating slightly about the datum axis B) so that in an extreme case (datum feature D at MMC and produced with maximum perpendicularity error of 0.2 wrt A) the datum feature D would be seen as creating a 7.3 (but never 7.5) wide boundary. And it wouldn't really matter that one workpiece could be produced with the datum feature D axis tilted by 0.2 relative to A and located 0.2 below the virtual horizontal line passing through the centers of B and C on the drawing, while the other workpiece could be produced with the datum feature D axis tilted by 0.2 relative to A and located 0.2 above the same virtual horizontal line.

So I am kind of surprised by your answer, because assuming that this is how the theory works, I would rather expect you saying that the MMB for datum feature B in my modified scenario is an elongated shape of 10.8 X 10.9, as this is the minimized MMB that the observer standing at plane A and looking vertically up could see. But I must be missing something here...

 

[greenimi]

pmarc,

Is your questions regarding fig 4-16 a continuation and extension of the discussion in this thread?

https://www.eng-tips.com/viewthread.cfm?qid=372340

If not, sorry for bringing it up. I am just trying to learn from each experts a little bit of something.

 

[pmarc]

greenimi,
Yes, that is the thread 3DDave and me have been referring to.

 

[3DDave]

pmarc - interesting thought, but there isn't anything that fixes the orientation of the short-dimension, so it's free to rotate; nothing forces the observer to align with it. It is free to rotate in relation to the slot, leaving the internal feature maximum encroaching surface as a cylinder of diameter 10.8.

In contrast, the orientation in the Figure 4-16(c) example is fixed between the datum features B and D; with that constraint the minimum surface is not a cylinder. If it wasn't fixed, then the proper evaluation of D would be a ring, but that would not be an external feature minimum encroaching surface.

(I don't see the images I put in as IMG links in the text in the preview. I will try to get them in; they look like they uploaded OK)
Image 1:

https://files.engineering.com/getfi...66a-442c-a9cc-dac0118c087a&file=4-16c_ABC.png

Image 2:

https://files.engineering.com/getfi...-4935-b489-3330a467e006&file=4-16c_2_to_1.png

Image 3:

https://files.engineering.com/getfi...-4495-90a0-dea2ddbab2b8&file=4-16c_1_to_0.png

Image 4:

https://files.engineering.com/getfi...59-befa-5399430dd846&file=4-16c_ABC_0perp.png

Image 5:

https://files.engineering.com/getfi...-4814-ab17-6f342b168295&file=4-16c_1_to_1.png

I've worked out a simulator for this that works as expected. It does two things. It first generates allowable position and perpendicularity variations that can be seen to be restrained within the position tolerance. Then it rotates each of these to align the center of the MMC feature axis to the X-axis. The result generally conforms to what I said. It clearly shows the width restriction for reasonable ratios of positional tolerance and perpendicularity. As expected, at perpendicularity of diameter 0 at MMC it is compressed to a straight line.

What's interesting is when the simulation allows the perpendicularity tolerance to near equaling or exceeding the position tolerance. Because of the way the perpendicularity variation is computed, it doesn't allow the axis to tilt more than the position tolerance limit or what remains as the position approaches the position limit. In these cases, when the transform aligns to the center of the axis, the ends of the axis can exceed the original boundary, but it never exceeds the expected limits that an encompassing rectangle would make.

The key to the images is that random colors are used for the each axis simulation to show them individually. The nominal position limit is a white filled circle with black edge.
Image 1:

is the way that datum Feature D is defined relative to [A|B(M)|C(M)]
Image 2:

is the transformation of a 2:1 position to perpendicularity ratio case to the [A|B(M)]D(M)] frame of reference. Two things are clear from the picture. One is that the perpendicularity limits the top and bottom of the range. The other is there appears to be leakage to the right and left.

The leakage comes from this - by way of example, if perpendicularity ends up with one end on the original boundary, at a point aligned (or nearly aligned) on the line between the centers of B and D, but the position tolerance center is not on that line, then when the center of the position is moved to the mutual line, the end of the perpendicularity tolerance is also moved, to a place outside the original boundary. Recall, all axes meet the original feature requirements. Choosing a different frame of reference results in a different boundary.

For example, had the datum reference for the hole been [A|D(M)] the result would be a virtual condition of the MMC diameter plus the perpendicularity tolerance alone - the position tolerance would not be included. Likewise in [A|B(M)|C(M)] the virtual condition would be the MMC diameter plus the position tolerance alone - the perpendicularity tolerance would have no effect.

One limit case is when the perpendicularity tolerance is zero: Image 3:

which is just the thin green line made up of points that indicate the location of the perfectly perpendicular position axis. This is the transformation of Image 4:

which doesn't have the lines of the previous ABC image that represented the radius of each perpendicularity axis as projected to A.

The other limit is when the perpendicularity tolerance equals or exceed the position tolerance. This maximizes the leakage. The software prevents any case where the axis of the feature exceeds the position constraint, so it acts the same as if there is no perpendicularity tolerance at all. In this case the maximum leakage is when one end of the feature axis is on the mutual line and the other is not. When the center is aligned it moves up or down out of the diameter. Again, notice that neither the vertical nor horizontal limits are exceeded and the virtual condition is a truncated American football shape: Image 5:

This particular result is from 5 million iterations.

I'll post the source code separately.

For certain, as long as a perpendicularity refinement is used, the simulator for the D(M) in the [A|B(M)|D(M)] datum reference frame is not a diameter; and if the perpendicularity refinement isn't used, it still isn't a diameter, though it's sort of close.

 

[pmarc]

Thanks for the detailed reply.

I get what you are saying. And I see now that I was approaching the problem in my modified scenario for calculating MMB of datum feature B from a wrong angle.

 

[Kedu]

I get what you are saying. And I see now that I was approaching the problem in my modified scenario for calculating MMB of datum feature B from a wrong angle

Pmarc,
Could you, please, provide more details? What do you mean "wrong angle"? What could be the "correct" perspective?

 

[Kedu]

And one additional question:

My preference would be to make the flat surface datum feature A, and give a perpendicularity tolerance to the round and mark it as datum feature B; then work the rest of the diagram from there. This is particularly true as in the example (2009 version) of [A(M)|B], the reference to B is redundant for the pattern of holes.

On the same token:
Is secondary datum feature B adding any value in the profile callouts in Fig 4-38; assuming that's not for simultaneous requirements?

 

[3DDave]

Kedu - that's a weird figure. I like the "EQLSP," which doesn't appear in the text. The use of the character/symbol "X" is suggested for equal spacing, per 1.9.5.2.

That aside, it seems like datum feature B is only for simultaneous requirements in the profile callouts. The standard typically only deals with that when a feature of size is involved that would affect the outcome - not because of rocking of a datum feature. Since "B" is referenced RMB, there's not even that case of potential influence of a feature of size, so I don't know why it's used in this case.

 

[pmarc]

Kedu,

Pmarc, 
Could you, please, provide more details? What do you mean "wrong angle"? What could be the "correct" perspective?

Imagine that there is for example 100 real parts (instead of 5 millions used by 3DDave) with datum feature B at MMC produced with maximum axis perpendicularity error for that material condition (dia. 0.1). When projected at datum plane A, the axes will be seen as straight lines 0.1 long, but each line may actually lie anywhere on the datum plane A and be at any angle to each of remaining 99 lines.

If I then try to put the straight lines together in such a way that all 100 mid-points of the lines meet at one point (which I am allowed to do because the only datum reference in the perpendicularity callout is datum plane A), I will get a collection of 100 radially emanating lines crossing at that one point and creating a circular boundary of dia. 0.1.

If the boundary is circular (cylindrical in 3D) that means the shape of the MMB of datum feature B is also cylindrical and its diameter is 10.9-0.1=10.8.

Not sure if that helps, but this is how I try to explain it to myself.

 

[Kedu]

Pmarc,
Yes it helps. Still need more time to digest the concept and its hidden details. Working on it. Thanks for the hint

 

[Kedu]

. And from that point of view I symphatize with 3DDave. But the undeniable fact is that the standard (rightly or not) uses terms Virtual Condition and Resultant Condition only in the context of geometric tolerances applied at MMC or LMC basis. For geometric tolerance applied at RFS basis the standard uses terms Inner Boundary and Outer Boundary.

As mentioned by J-P, one place in the document that somewhat proves that are pages 32-34. The other are figures 5-2 and 5-3 showing application of DML straightness tolerance - in fig. 5-2, that is where the straightness tolerance is applied at RFS, the extreme boundary is named Outer Boundary; in fig. 5-3, that is where the straightness tolerance is applied at MMC, the extreme boundary is named Virtual Condition

Pmarc, 3DDave,
I have noticed that in Y14.43 Gaging standard in Fig I-1 the terms used for holes located with position modified at MMC is inner boundary / outer boundary (should I say instead of virtual condition and resultant condition respectively?).
In my opinion, disagrements between related ASME standards does not help either. What do you think? Am I looking / seeing the stuff correctly?

 

[pmarc]

Kedu,
The differences in terminology between standards definitely don't help. In this case Y14.43 could have used Virtual Condition and Result Condition, but maybe someone decided that Inner and Outer Boundary better fitted to the convention of the standard?

Like I said in my first reply in this thread, it certainly does not help that the standard uses so many different terms for the extreme boundaries of the features in first place. If I had a power to change that, I would definitely do it or at least try.

 

[3DDave]

This is the software I used -

https://files.engineering.com/getfi...a-80cc-a268d645632e&file=figure_4_16c_pde.pde

The .pde file is a text file that Wordpad can format correctly.

It uses the MIT Processing environment available at

https://processing.org/

I think the comments are enough to work with, but the basic model is a pin that is at an X and Y offset from the origin, a position diametral tolerance, and a perpendicularity diametral tolerance. It supports two DFRs, the choice is set on the first line. To keep the resulting axes in view the generated coordinates are translated to move the nominal center of the pin to the center of the window.

The size variation of the pin is ignored as one might use each end of each generated axis as an MMC circle, creating an offset from the shown axes; it doesn't add anything to the understanding of the consequence of the change in datum sequence between the creation of the feature and the use of the feature as a datum feature reference.