Examples of Equivalent Dimensioning & Tolerancing Schemes

[pmarc]

Hi,

Throughout the years I have come to a conclusion that in general there are not many examples where changing dimensioning and tolerancing scheme from one to another would keep the geometric requirements for the system unchanged.

One example where this conclusion would not be true is changing from perpendicularity wrt A to total runout wrt A when applied to a flat face normal to datum axis A.

Another one would be a simple bushing where its ID and OD are controlled with the same +/- tolerance, and then it does not really matter which of the features will be datum feature A and which will be controlled with position or runout relative to A.

I have some more, but I would like to see what others can offer. So could anyone share some examples?

 

[Sem_D220]

That is 100% correct.
What I was saying is that maybe, sometimes the CDRF is a tool that can be used to instuct that inspection fixturing is done according to function. Without it, like in the case of fig. 4-9, nothing seems to guarantee that the A|B-C|, or worse, |A|C|B| scenario won't happen physically, even when all is done according to drawing specification. Am I mistaken here? In real assembly for a part such as fig 4-9, there may be all kind if of charactersitics of the mating part(s) that may guarantee the correct function. For example - when the part mates with a sub-assembly that includes a degree of freedom. I'd also like to say again that I don't know how a CDRF may work for that purpose in this particular case (I think that DT modifier may be a better solution here, more in line with possible real function), but perhaps it may work for other cases, even if the DOF's specified for each datum are the same as dictated by the datum precedence order. That is only a suggestion.

 

[pmarc]

I'd also like to say again that I don't know how a CDRF may work for that purpose in this particular case (I think that DT modifier may be a better solution here, more in line with possible real function), but perhaps it may work for other cases, even if the DOF's specified for each datum are the same as dictated by the datum presedence order. That is only a suggestion.

No, it can't work in other cases. If CDRF does not override any degree of freedom dictated by the datum precedence order, then the only purpose it may serve is, as I mentioned 3 replies ago, clarification of what particular degrees of freedom are constrained by each datum feature simulator.

What I was saying is that maybe, sometimes the CDRF is a tool that can be used to instuct that inspection fixturing is done according to function. Without it, like in the case of fig. 4-9, nothing seems to gurantee that the A|B-C|, or worse, |A|C|B| scenario won't happen physically, even when all is done according to drawing specification. Am I mistaken here?

If in a real assembly there is a possibility (high or low) that the part from fig. 4-9 will not be oriented and located in A, B, C sequence, then it does not really matter if you apply the CDRF concept or not. As you said, it is not the datum system defined on a drawing that determines behavior of the part in real assembly. The assembly and its components need to be designed in a way that the components always work (are oriented and located) in a repeatable way.

 

[Sem_D220]

Accepted.
I was always in favour of simply specifying the right tolerance values and making sure by them and by Virtual Conditions for each datum feature and it's mating feature, that all functions "according to the plan", especially I'm in favour of a method you pointed out for me some time ago in one of the threads here - specifying a particular size of the datum feature simulator in the FCF, instead of the (M) modifier. That value can be the virtual condition of the mating pin for a cotrolled hole. This way, in my opinion, functional requirements are being addressed in the most direct way. However, I was trying to think here in which cases a CDRF might be practical. I now realize it's only intended to be used when the DOF constraints which are understood from datum precedence order must be overriden. But since CDRF doesn't allow non-basic LOCTION of datum feature simulators, would it be correct to conclude that it is only usable to "release" ROTATIONAL constrains of degrees of freedom for a given datum feature, where they are reserved for other datums?

 

[pmarc]

Well, if you take look at fig. 4-45 you will see that it is translational degree of freedom [z] that gets overriden.

 

[Sem_D220]

I see. In fig. 4-45, if it wasn't for CDRF, there would be no point to call out datum B at all. I suppose that's one type of relevant cases for CDRF.

Question about fig. 4-46: to address the customized datum reference frame as specified, is it allowed to make the square tab simulator for datum feature B free to rotate in the fixture?

According to para. 4.5.2 there is a requirement for datum feature simulators:
"(b) basic orientation relative to one another for all the datum references in a feature control frame"

I don't think that this requirement is different in principle from this one:

"(c) basic location relative to other datum feature simulators for all the datum references in a feature control
frame, unless a translation modifier or movable datum target symbol is specified."

Both requirements valid for a Customized DRF as well as for a non-customized one. Then, how can the condition for fig. 4-46 be achieved in a fixture?

 

[pmarc]

I see. In fig. 4-45, if it wasn't for CDRF, there would be no point to call out datum B at all. I suppose that's one type of relevant cases for CDRF.

That is correct. Fig. 4-44 exactly shows that datum B is not needed at all if there is no DRF customization.

Question about fig. 4-46: to address the customized datum reference frame as specified, is it allowed to make the square tab simulator for datum feature B free to rotate in the fixture?

Yes, that is exactly how the square tab simulator is allowed to behave. There is a figure in Y14.43-2011 standard for gages and fixtures (fig. B-24) that shows this.

According to para. 4.5.2 there is a requirement for datum feature simulators:
"(b) basic orientation relative to one another for all the datum references in a feature control frame"

I don't think that this requirement is different in principle from this one:

"(c) basic location relative to other datum feature simulators for all the datum references in a feature control
frame, unless a translation modifier or movable datum target symbol is specified."

Both requirements valid for a Customized DRF as well as for a non-customized one. Then, how can the condition for fig. 4-46 be achieved in a fixture?

That is a good question. In my opinion the wording of para. 4.5.2 is not precise and does not describe all possible behaviors of datum feature simulators caused by the usage of CDRF concept. Although I imagine that one might say that in fig. 4-46 everything is okay because the "axis" (intersection of two mutually perpendicular center planes) of the square tab is perfectly oriented to datum plane A, and the axis of the tertiary pin simulator is perfectly oriented and located to primary and secondary simulators.

As a matter of fact in my opinion it is not the only scenario not covered by para. 4.5.2. There is no subparagraph in 4.5.2 that would cover behavior of datum feature simulators in figs. 4-30(a) and 4-31(a).

 

[Sem_D220]

I see what you mean by datum feature simulator behavior in figs. 4-30(a) and 4-31(a). It seems like the behavior of those simulators is dictated by the profile tolerance. Looking at fig. 4-30(a) for example- what if there was no profile tolerance, and instead the face defined as datum feature B was one side of a directly toleranced feature of size? Then the simulator should have been understood as fixed at basic location? Or would it still be allowed to "progress"? Since the related paragaraph, 4.16.3 which describes the "progression" explicitly mentiones profile tolerance in relation to the concept, it seems like this kind of behavior is only allowed when this specific control applied (meaning it is some kind of exception from the norm), and something about it doesn't feel quite right to me.

But back to the topic of CDRF, maybe the fact that the praragraph listing the requirements from datum feature simulators doesn't describe clearly what the datums should act like in case of CDRF, people think, like I did previously (and I think the logic is understandable) that since the DRF is "customized", it is allowed to translate and rotate the datum feature simulators in the fixture as needed for the desired "customization" without any limitations.
Here is a page about the customized DRF from an educational GD&T website (that I personally sympathize very much). Note the described requirements from datum feature simulators B and C. Are they in line with the standard?

 

[pmarc]

Looking at fig. 4-30(a) for example- what if there was no profile tolerance, and instead the face defined as datum feature B was one side of a directly toleranced feature of size?

Not sure I follow. Not sure how exactly you imagine directly toleranced feature of size in case of a planar feature. Are you thinking of directly toleranced dimension from part's axis to the surface B?

Here is a page about the customized DRF from an educational GD&T website (that I personally sympathize very much). Note the described requirements from datum feature simulators B and C. Are they in line with the standard?

In that example I agree that in order for the B to constrain only [x] and [y] translations, the simulator B must be able to rotate about Z axis. However, I do not agree that in order for the C to constrain [w] rotation and nothing else, the simulator C must be movable in Y direction. Just like simulator C is not movable relative to B in fig. 4-46.

 

[Sem_D220]

For 4-30(a) - I thought about a width feature of size symmetrical to the part axis, 30 mm wide and +- toleranced, the right side only defined as B datum feature. But probably it's not a very good example now that I think of it, because it doesn't meet the requirement of the standard to have a basic location between all datum feature simulators. There would be a basic zero implied to the center plane, but no basic dimension to the side face defined as datum B. Still something about the exclusive dependency on profile of a surface tolerance being specified for the behavior of the simulator to be defined - bothers me, rightfully or not. That is in addition to the more important fact that it's not entirely according to para 4.5.2. Based on the text of para 4.5.2(c), I would think that the behavior dictated by fig. 4-31(b) always applies. Do you share this opinion?

As for the example from that website, I don't see how the mobility of datum feature simulator C in [Y] can help to constrain [w] and nothing else. But that is not the point.
The important points in my opinion are:

1. Datum feature simulator for datum feature C is not allowed to be movable at all, according to para. 4.5.2 (c).

2.Unlike in fig. 4-46 where no other datum feature sets a particular orientation for the 4 faces of the square tab simulator around it's "axis" (centerplanes intersection), in this example datum feature B simulator must be basically oriented to datum feature C simulator (around all axes, as I understand), and therefore according to para. 4.5.2 (b) is not allowed to rotate as suggested by the author.

Essentially this leaves the CDRF concept inapplicable for the given example. Am I mistaken?

 

[pmarc]

For 4-30(a) - I thought about a width feature of size symmetrical to the part axis, 30 mm wide and +- toleranced, the right side only defined as B datum feature. But probably it's not a very good example now that I think of it, because it doesn't meet the requirement of the standard to have a basic location between all datum feature simulators. There would be a basic zero implied to the center plane, but no basic dimension to the side face defined as datum B. Still something about the exclusive dependency on profile of a surface tolerance being specified for the behavior of the simulator to be defined - bothers me, rightfully or not. That is in addition to the more important fact that it's not entirely according to para 4.5.2.

I assume the directly tolerance width 30 would have to be controlled with a position tolerance relative to datum A. If we take, for example, that the width size tolerance is +/-0.1 and the position tolerance is 0.3(M) wrt A, this means that the MMB of datum feature B (one face of the width) with respect to A is 15.2 = (30.0+0.1+0.3)/2. So the datum feature simulator B has to progress from 15.2 towards LMB of datum feature B until maximum possible contact with actual surface B is achieved. I do not see anything incorrect in this description provided that we agree/realize that para. 4.5.2 does not do good job in terms of description of all possible behaviors of datum feature simulators that one can imagine/define.

Based on the text of para 4.5.2(c), I would think that the behavior dictated by fig. 4-31(b) always applies. Do you share this opinion?

Yes, this conclusion could be drawn. But, as I have been trying to point out and as shown in figs. 4-30(a) and 4-31(a), this is not always true.

As for the example from that website, I don't see how the mobility of datum feature simulator C in [Y] can help to constrain [w] and nothing else. But that is not the point.
The important points in my opinion are:

1. Datum feature simulator for datum feature C is not allowed to be movable at all, according to para. 4.5.2 (c).

2.Unlike in fig. 4-46 where no other datum feature sets a particular orientation for the 4 faces of the square tab simulator around it's "axis" (centerplanes intersection), in this example datum feature B simulator must be basically oriented to datum feature C simulator (around all axes, as I understand), and therefore according to para. 4.5.2 (b) is not allowed to rotate as suggested by the author.

Essentially this leaves the CDRF concept inapplicable for the given example. Am I mistaken?

Yes to #1. Although I have to admit that it may look rather weird that on one hand I say that translation of datum feature simulator C is not allowed because para. 4.5.2 says so, but in the other aspect, that is mobility of datum feature simulator B in figs. 4-30(a) and 4-31(b), I claim that the same paragraph cannot be followed ;-)

As for #2, I am not sure I see a problem with the rotation of datum feature simulator B. Datum feature simulator B is not allowed to rotate relative to datum feature C (or I should rather say the opposite) in |A|B| DRF. This basically controls mutual location and orientation of both datum features relative to each other**. However, in the customized DRF, simulator B is allowed to rotate relative to C.

** The problem with the example is that since both holes have been also controlled with profile to A, they are subject to simultaneous requirement and the simultaneous requirement controls their mutual orientation and location in the first place. To me this makes usage of position tolerance for hole C at least questionable.

P.S.: Looks like the discussion about CDRF concept and related figures hijacked the thread a little bit (maybe even more than a bit). If you don't mind, semiond, I would like to bring it back to the right track. So could you please start new thread if you want the CDRF topic to be further explored? Thank you.

 

[greenimi]

Let me toss it out there for the discussion sake:

Total runout = concentricity + cylindricity

Circular runout = concentricity + circularity (roundness)


So far extremly informative thread. I do not want this thred to die or at least not anytime soon.
Keep it alive.

 

[Sem_D220]

I don't know why anyone would want to do it but here is what came to my mind:
- A cylinder of size 5+-0.1
- A cylonder cylinder of size 5+-0.1, cylindricity within 0.2, and a note stating "PERFECT FORM AT MMC NOT REQUIRED"

semiond

 

[pylfrm]

greenimi,

You weren't very specific so I won't provide counterexamples, but none of your proposed schemes can be equivalent unless the tolerance values are zero.


Sem_D220,

Your second scheme allows a feature with UAME size of 5.3, which would not comply with the first scheme.


pylfrm

 

[Sem_D220]

pylfrm,

I say the first scheme will accept it too.

Note how the actual local size is not greater than 5.1 everywhere on the perimeter.

 

[greenimi]

https://www.youtube.com/watch?v=NArW09DFhf8

I watch this video from Tec-Ease and, as you can see, this scheme is shown.

Their disclaimer has been that “for all practical purposes” (they are using different verbiage such as “picking up of a fly poop”) the “equivalency” is quite good.

I am posting this as I would like to understand the details when this idea fell apart. In other words I would like to be nit-picky.
I did not provide an example, but I would think that for the sake of discussion we can come up with some.

 

[chez311]

semiond,

Since your first scheme is subject to rule#1 it must fit within a cylinder of perfect form at MMC. This would be 5.1 - the deviations you show in your sketch would not be allowed under rule#1, they would be violating that cylinder of perfect form at MMC even though the local size is not greater than 5.1

 

[pmarc]

greenimi,

I agree with pylfrm. These are not equivalent schemes.

I hope someone will do it earlier, but if not, I will try to post a sketch later today to show this.

semiond,

I agree with pylfrm on that one too - the two schemes are not the same.

In the first one, the cylinder produced with all actual local sizes equal to 5.1 (MMC) must not have any cylindricity error, otherwise Rule #1 is violated. In the second scheme the cylinder produced with all actual local sizes equal to 5.1 might still have cylindricity error of 0.2, meaning that the size of its worst-case outer boundary will be 5.3.

Your sketch shows a condition of the cylinder that would not be allowed in the first scheme - the actual contour violates the envelope of MMC size = 5.1.

 

[Sem_D220]

chez311,
You are absolutely right.
I can't believe I totally forgot about rule #1 and didn't realize it even after being pointed out at it by pylfrm.
What confused me is the fact that rule#1 is supposed to apply a form tolerance equal to the size tolerance. But right now I'm confused. The inner boundary for scheme #1 seems to be 4.8 , MMC is 5.1 . Is it possible that the maximum form error is 0.3?

Edit: pmarc, I just saw your last post. I realized my error, but as you can see still confused

 

[Sem_D220]

Ok, I think now I got it.
The inner boundary for scheme #1 is 4.9 4.6 (another error ). The total allowed form error is 0.2. In the second scheme, if the note PERFECT FORM AT MMC NOT REQUIRED wasn't included, the two cases were the same but the cylindricity tolerance would be redundant. Sorry for the bad example

 

[chez311]

semiond,

Actually I believe the inner boundary would be 4.7 as per 2.7.1 (rule #1):

"Where the actual local size of a regular feature
of size has departed from MMC toward LMC, a local
variation in form is allowed equal to the amount of such
departure."

So at LMC the form error allowed would be 5.1 - 4.9 = 0.2 which means the inner boundary is 4.9 - 0.2 = 4.7

I think you are correct though, if "PERFECT FORM AT MMC NOT REQUIRED" was omitted from your scheme#2 the cylindricity would be redundant as it is the same as the form error allowed at LMC (0.2), if it were smaller it would no longer be redundant.