DRF Interpretation

[[To...]]

Relative to the attached sketch, when I create a DRF and have my settings set to ISO5459, Datum B shows a relationship constraint to Datum A, which is fine. But, Datum C only shows a relationship constraint to Datum A. After I think about how I would create a datum simulator, I suppose this is correct but I was wondering if you all agree and maybe one of you can direct me to a reference in ASME Y14.5.
ABC Question.pdf

[[Th...]]

I'm guessing it's because Calypso can't determine perpendicularity of the C plane relative to the cylindrical face of B? Which makes sense, I suppose.

[[Br...]]

What calypso is best trying to emmulate with the ISO5459 are how datum simulators come into contact with the physical datum features to establish a DRF.

In this case datum feature B doesn't control any rotational degrees of freedom. That's what those "Constraints" in calypso are effecting. It's forcing your datum feature C plane to be perfectly perpendicular to Datum Feature A. Its "origin" location is where that perfectly perpendicular plane contacts the highest point (using outer-tangential). Datum Feature B only constrains two degrees of translation. Any Degrees of Rotation that it could've constrained are already taken by the higher precedent Datum feature A.

Para. 4.4. of ASME Y14.5-2009 explains how datum features referenced constrain certain degrees of freedom.

para. 4.10.1 walks you through the development of a DRF and how the datum simulators immobolize the 6 degrees of freedom.

[[To...]]

Thanks guys. Brett, I saw those pages in my Y14.5 book but was hoping I just missed something regarding the specific application. I do understand what you're saying. And, also the Datum B simulator is already perpendicular to A, so I get the perpendicular relationship of C to A from a simulator point of view. Like I said, it seemed to make sense, but I just needed to shake out a few cobwebs...LOL

[[Br...]]

Ha ha. yeah they couldn't possibly do an example of every DRF imaginable. but there are quite a few different examples laid out, and once you get those principles down (which it sounds like you do but just wanted someone to confirm for you.), you should be able to take that to every DRF imaginable.

[[Za...]]

leaving this here for anyone else looking for explanation

[[An...]]

_

[[To...]]

Andreas, C can only rotate about B, so only the 2 highest points on C would be used. Similar to 3 orthogonal planes. The primary would use 3 highest points, secondary would use 2 highest points and tertiary would use single highest point.

And, according to Calypso and it's ISO 5459 setting, it implies the same.

[[Me...]]

That's incorrect, Andreas. The whole principal of ISO 5459 is based on 3 perpendicular planes. Look at the picture on the Form Datum tab. That's the whole point of the setting. It's not that Datum C is assumed to be perfectly perpendicular. It's that the analysis is using a perfectly perpendicular plane moved until it contacts the surface and stops the rotation.

Robert

[[Me...]]

Just to elaborate... the part is held on a perfectly flat surface on Datum A, 3 DOF. A perfectly perpendicular cylinder to A is around Datum B and contracts until it contacts the surface, 2 DOF. A perfectly perpendicular plane to Datum A is move in from the side until it contacts Datum C, 1 DOF.

[[Br...]]

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Just to clarify, I wasn't saying that Datum Feature C constrains any translational degrees of freedom in this particular DRF. I was explaining how calypso determines how a perfectly perpendicular plane makes contact with datum feature C to control rotation. I probably shouldn't have used the word "origin" to describe the point of contact. I did mention that datum feature B controls two degrees of translation so if you read my comment in context you should come away understanding that I meant Datum Feature C ONLY controls one degree of rotation.

I completely disagree with your conclusion that you don't need to use an Outer Tangential evaluation for Datum Feature C. The datum simulator for Datum feature C is a plane that is perfectly perpendicular to datum feature A and progresses into the part until it makes maximum contact with datum feature C. In order for Calypso to do this, you have to use OuterTangential to make it contact those high points and it needs to be constrained to Datum A to properly constrain that final degree of rotation.

[[An...]]

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I'm pretty sure that I understood well.
In fact,you need to use OTE,but you have to regard always three points whenever the actual plane-geometry makes it possible!
In this special case (plane,cylinder,plane) datum C does only stop one rotatory degree of freedom.

It does not stop any translatory degree of freedom!

And therefore the datum C simulator is not constrained to A!
The angle of the three-point-Outer Tangential Plane (unconstrained) is the dimension
that defines the Planar Alignment of this part.

But, if we talk about a plane,plane,plane DRF, then we talk about a two-point OTE (datum B)
that is for sure constrained to datum A

The attached drawing may put some light into the discussion:
Bringing up a fourth datum changes the meaning of datum B. (my opinion)

Contribution_13_07_2019.pdf

[[An...]]

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Try to understand that there is a difference between:
Plane;Plane;Plane
and
Plane;Cylinder;Plane

I think that's the reason of Tom's question 😉

My personal point of view,see attached file.

Contribution_15_07_2019.pdf

[[Br...]]

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Tom's question was answered on the first page of this thread. A DRF always consists of 3 mutually perpendicular planes. It sound like you are confusing "Datums" with "Datum Features".

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Try to refrain from the "Try to understand" comments. Its offensive and it really looks foolish when you yourself are the only one who doesn't grasp the topic but continues to argue as if you can't comprehend what others have written. Either you are blatantly misrepresenting other peoples positions and attacking that misrepresentation(such as how I never said the Tertiary datum constrained any translational degree of freedom.), or your reading comprehension skills need much improvement. Its really getting old. Either way, you are doing no one any service by continuing this nonsense and disrupting the thoughtful dialogue that began in this thread.

[[Me...]]

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I need a Brett on stand by at work. I've been trying to explain perp of a face to a bore for the past 2 weeks to guys I work with and they refuse to acknowledge the difference.

[[Ri...]]

I love you guys.

[[An...]]

_

[[Me...]]

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In Tom's example, |C| only serves to orient the mutually perpendicular planes already established by |A|B|. I believe the more appropriate paragraph to cite would be 4.5.2. This supports the argument of constraining |C| plane to the primary datum, which is equivalent to projecting all surface points onto a line for use as the planar feature.

A debate worth having, should |C| (in this example) always be established using the outer tangential fit? I would argue that the minimum zone fit is a better option, especially if high form error is anticipated. This is due to the software's inability to optimize the coordinate system when datum features contain irregularities.

[[An...]]

_

[[Br...]]

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The reference to the standard was a direct response to the misunderstanding that the "3 perpendicular planes" that Robert was referring to and the types of Datum Features used in a DRF are two different things.

You are talking about the main topic here. And yes that's another good reference to the standard that supports Tom's conclusion. Specificaly 4.5.2(b) "basic orientation relative to one another for all the datum references in the feature control frame."

Now as far as using the Minimum Zone fit instead over Outer Tangential, I think we are going to part ways. Minimum zone is a Chebyshov algorithm. It's similar to least squares in that it finds a type of mean. This is extremely useful for form tolerances because it optimally situates the feature within its tolerance zone. For a plane feature, It would be similar to having two parallel planes collapsing about the collected data until they stop. This yields slightly different results than LSQ but even better represents the way the standard defines form tolerances.

So by using that particular fit, your datum reference frame created through calypso will use the center of the set of data you collected for that feature instead of the most outer high spots.

If you look through Zeiss's Cookbook you will see they consistently affirm this in all the DRF's used. They always suggest Outer Tangential, Maximum Inscribed, and Minimum Circumscribed for datum features.

[[Ch...]]

Andreas,

Please take no hard feelings. We appreciate what you bring to the forum.

We are all science guys here and being polite and graceful is not our thing. I don't want someone to refrain from telling me something because they are worried about offending me. I don't get on this forum to feel better about myself. I do it cause I hope to learn something and if you have to be a little abrasive in order to get your idea across, then that's fine by me.

[[Me...]]

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I apologize for not being more specific. I was referring to instances where an individual arbitrarily applies tangential fitting without regard to the sampling strategy. In certain cases of extreme form error, the outer tangential fit could become skewed if the surface has not been sampled appropriately (point density, coverage...etc.). In those instances, a different fit objective should be considered.
I would have to imagine that there are some on this forum looking for information, and do not have formal training, or a machine with a scanning sensor. In light of that possibility, I’m only attempting to start the conversation.

Example.pdf

[[Br...]]

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I see, the problem still remains though that you are using the center of the data set instead of the higher contact points. If you're seeing some outliers affecting your results, it's probably because you're not properly filtering it out. Outer Tangential is very susceptible to outliers. So when scanning large amounts of data, filtering and outlier elimination is a must. Minimum Zone is very susceptible to outliers as well so you will run into the same problem. If stability is your concern you should be arguing for LSQ. But once again LSQ and Minimum Zone will only use the average/mean of the data set and not the outer most contact points as the standard requires. What you will find a lot of times is that when you actually use hard gauging to simulate the datums(e.g. granite surface plate, Right angle blocks, Gauge Pins etc.), your values differ from the CMM. As form error increases, the difference in the evaluation fits also increase. So if your Datum Features are very close to perfect form, and they are extremely close in their orientation to one another, you may not notice a big difference between the different methods. In fact the LSQ might at times seem more accurate if you are not properly filtering. You are effectively using LSQ as a type of filter. (not exactly the same thing though.) I would encourage you to pick up a copy of Zeiss's Measurement strategy cookbook if you don't already have one. Their default filtering strategies work extremely well in most cases.

[[Da...]]

Thank you all for your effort on debates like this.
Thank you Andreas and Zach and Phillip for graphical presentations. I'm pretty sure that Plane with half of the form offset would do the trick in such case.
And of course appropriate filters and outlier elimination.

[[Da...]]

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And I failed to write, that I was referring to what Phillip presented in his pdf. Sorry.

Tom, are you happy now? 😃