Customized DRF Example

[pmarc]

This thread has been inspired by some comments (recent and past) that implied that the usefulness of the customized DRF concept is questionable at best.

So I would like to ask for opinions about the attached example. Do you think the customized DRF for the pulley (primary conical datum feature constraining 4 and not 5 DOFs, and secondary planar datum feature constraining 1 translational DOF) in this case would not make sense?

https://files.engineering.com/getfi...0ce7-4992-9e8d-465cf7807937&file=assembly.JPG

Thank you.

 

[greenimi]

Just to open up the discussion I would say it is a valid one.

The only caveat that I am seeing (and I am not sure I am seeing it correcly either) is that the tolerance value for the cones (and I would say for both mating cones in red and in blue) to be adjusted as such to make sure the translation degree of freedom (DOF) is not stopped by the cones but by the left vertical small flat surface of the pulley. So, I guess, we have to calculate the profile tolerance on both cones and adjuste both tolerances that the cones will stopp 4 DOF and not 5 (as would be capable of stopping).
Does it make sense? Am I wrong?

I know you have something in mind when you open this interesting subject (and not very well known ----called customized DRF----) and I do not want to fall in any trap you might have set there (if I haven’t falled already)

 

[3DDave]

This is a tricky one - the part, as installed, is not the same shape as it is in the free state and, with enough force, the free state axis of the cone feature won't be the deciding orientation control.

 

[chez311]

pmarc,

It should come as no surprise that I'm glad you're bringing this up - I've been sort of fascinated by custom DRFs but I've been hesitant to suggest their application because they seem to divide people so much as well as risks creating an artificial DRF that doesn't truly represent functional intent, depending on how it is specified. Not to mention the fact that for such a complex topic there is so little information on it in the 2009 standard - many discussions break down into disagreements about what exactly it does and what the DRF/datum feature simulators should look like. My typical suggestion is to exhaust all other options first and utilize it as a last resort.

In regards to your example, I would have to agree with greenimi and 3DDave.

Depending on the sizes/tolerances and installation forces (ie: clamp load applied by the bolt) involved, exactly how many DOF is constrained by each feature may change. Slight changes in the size within a given tolerance for the tapered feature could mean it would either constrain all available DOF (when it is installed the pulley "stops" before it contacts the adapter vertical face) or only translation (the pulley fully seats on adapter vertical face). Additionally increased installation forces could radically change what DOF are constrained by what features as it forces the pulley on the taper (ie: force the pulley to make increasing contact with the vertical face with increasing force) and deforms the components.

Basically I think in this instance while utilizing a custom DRF acknowledges that the situation cannot quite be captured by a traditional DOF sufficiently, it may also be more complex than a custom DRF can possibly handle as well.

 

[mkcski]

pmarc:

We routinely rehabilitate very large hydro-turbine machinery made before WWII. The use of tapered couplings was prevalent in that time period. Sometimes we have to reuse parts that assembled with tapers like your example. We do everything possible to eliminate tapers and change (remachine) them to cylinders to avoid dealing with the question you are asking. Where we must maintain the taper coupling, we take great pains in a trial and error process to check the contact between the conical surfaces. This doesn't answer your question directly, but it suggests interchangeable parts are questionable with tapers and adding notes about custom fitting the assembly and then checking "runouts" on the features impacted by the fit between the parts - the pulley grooves in your example.

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[pmarc]

Thank you for the comments.

There is no trap in this question, at least I did not mean to set any (maybe I even set one for myself). Perhaps I should, but I did not really think about any change of the shape of the part once it is installed. Also, my intention was not really to promote this kind of interfaces vs. cylindrical pin-hole mating pairs.

The picture, as you might have already figured out, is a modification of fig. 4-36 in Y14.5-2009, and it was prepared really just to show the idea of a conical feature that, in my opinion, may not necessarily constrain all possible (from mathematical point of view) degrees of freedom in a real assembly.

So if we assume that in this example the tolerances for both conical features have been defined to make sure that the pulley can slide over the adapter with a small amount of loose (play), so that in fact it will be possible to bring the pulley in contact with the bearing surface on the adapter and that the clearance between the two conical features is small enough to believe that it is the conical bore that orients the pulley in the assembly, could this be valid reason to apply customized DRF concept?

 

[CheckerHater]

I don't think this particular design will benefit from custom DRF. Once you force one cone into the other, it will even constrain one more rotational DOF - due to friction

Now, if some piece functionally allowed to float (something like this,

Then it makes more sense.

I personally see custom DRF as a way to counter the “Can, May, Must” rule: Whichever degrees of freedom can and may be eliminated by a feature, they must be eliminated by that feature

We are given a way to explicitly specify that THIS feature shall not constrain THAT degree of freedom.

Just my ¢2

"For every expert there is an equal and opposite expert"
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[powerhound]

Here's a good application. Without customizing the DRF, that hole would have to be located axially from the apex of datum A.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[greenimi]

Wow, that’s a small picture. I’ve tried to blow it up, but become blurry.

 

[SeasonLee]

Here is the big picture, snapshotted from "The GD&T Hierarchy" by late Prof. Don.

Season

 

[3DDave]

I suppose we will be left to guess the need to dimension from a non-interface surface and the lack of a diameter symbol in the tolerance section.

 

[greenimi]

Thank you Season. I did not know the picture is from Tec-Ease training material.
One comment I have about this picture: profile withn 0.3 to A is not needed. Only perpendicularity within 0.05 to A will suffice.
After Season posted his "big picture" I watch the video where Don explain that profile is a carry-over, same concept as provided in the "small picture" posted by John.

Back to pmarc's question:

.....could this be valid reason to apply customized DRF concept?

As I said before, I would risk a YES to his question. Anyone else?

 

[chez311]

3DDave - as well as the lack of basic dimensions for both the hole and relative location of datum feature B to A.

greemini - why is profile not needed? I thought this was the one thing missing from the original (small) figure. You need some way to locate that face to datum feature A, right? Maybe you could link to the video, I'm interested as to how perpendicularity alone would locate datum feature B..

As far as the original question, I would agree with CH a custom DRF would mimic a "floating" assembly pretty well. With rigid components, I think maybe it gets you closer than perhaps a standard DRF would but I think there would be quite a bit of actual sharing or changing of DOF constraint between the datum features with even minute changes in tolerances and assembly forces (even with pmarc's caveat that the tolerances would be held such that they wouldn't - I am wondering if that would even be possible). Maybe thats good enough though to say it would be a good candidate for custom DRF?

 

[greenimi]

why is profile not needed? I thought this was the one thing missing from the original (small) figure. You need some way to locate that face to datum feature A, right? Maybe you could link to the video,

Tec-Ease premium site has a brown bag meeting where your question has been discussed.

First datum is the axis and then the plane oriented to the axis (oriented to the first datum) and then you LOCATE anything else FROM these two (axis and plane). You are not going to locate these two (axis and the plane), but you are going to locate FROM these two "original" datums

Premium site on Tec-Ease worth the money. My opinion.

 

[3DDave]

The vertex of the cone should have a dimension to locate what is identified as datum feature B. I suppose 'this drawing is incomplete' applies.

 

[greenimi]

The vertex of the cone should have a dimension to locate what is identified as datum feature B.

May I ask you why is that? Could you, please provide more details?

 

[3DDave]

The cone vertex is the only invariant location on the cone. Some substitute a gage diameter to find some other location, but that just adds another step. Without this dimension the surface is left without a defined location.

 

[greenimi]

Without this dimension the surface is left without a defined location

Still not convinced.

I would say you locate anything else from it you don't neccessary locate the cone. Am I missing something?

 

[3DDave]

Yes. I said the surface used as B needs to be located from the cone vertex. Perhaps this word order will work.

 

[mkcski]

My I recommend the "gang" refer to figure 4-3, pg 50 in Y14.5-2009 for the datum established by different features

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