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Radius as a datum
3

Radius as a datum

Radius as a datum

(OP)
Hello,

Can a radius be a datum?

RE: Radius as a datum

Depends, most folks interpret 1.3.17 of ASME Y14.5M-1994 1.1.17 as meaning that to be a feature of size the radius needs to be more than 180° in order to have opposing points.

Quote (ASME Y14.5M-1994 1.3.17)

Feature of Size.One cylindrical or spherical surface, or a set of two opposed elements or opposed parallel surfaces, associated with a size dimension.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Radius as a datum

In theory, a datum is a plane, axis, or point (or a combination of those). So as long as the radius can repeatably derive an axis, yes, it can be the datum feature. If you have the Y14.5 standard, an example would be datum B in Fig. 4-29.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

RE: Radius as a datum

Quote (J-P)

So as long as the radius can repeatably derive an axis, yes, it can be the datum feature
Radius shown in fig. 4-29 does not derive an axis. It constrains rotation of two mutually perpendicular planes associated with datum axis A derived from datum feature A.

RE: Radius as a datum

OK, a datum is a plane, axis, or point.
Radius shown in fig. 4-29 does not derive an axis.
So what IS datum B on 4-29?

RE: Radius as a datum

I would say datum B is still an axis, but it is a different datum axis from datum axis A. It clocks down the mutually perpendicular datum planes derived from datum axis A. Precisely, it derives mutually perpendicular planes, but in DRF we only see the one passes through both axes, the one coincides with one of the datum plane derived from datum axis A. We don't see the other one.

RE: Radius as a datum

What kind of radius? Turned part? Hole? Fillet?

RE: Radius as a datum

Datum B sure seems to be an axis.
Pmarc -- sure, it constrains the rotational degree of freedom, but if it's not an axis, what does the 28 mm dimension originate from?

Maybe we're getting sidetracked on another academic thing. ImnotfromMars -- can you provide more info about your situation or a sketch?

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

RE: Radius as a datum

Wait -- my mistake, pmarc. The 28 is going from datum A to B (I was looking at it backwards). That still doesn't solve the dilemma: according to the standard a datum must be a plane, axis, point, or combination thereof. Which of those is datum B, if not an axis?

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

RE: Radius as a datum

I see at least two reasons why it is not 100% correct approach to consider datum feature surface B in fig. 4-29 as a feature used to derive a datum axis from it (although I admit that bxbzq's interpretation makes sense in this paricular case):
1. Explanation given in para. 4.11.4. Notice that datum axis/center plane interpretation applies only to features-of-size type of datum features [subparas. (d) and (e)]. When it comes to the surfaces, there is no such interpretation, which sounds reasonable to me because of... see point 2.
2. What if surface B was an irregular basic contour (like for example a set of basic radii tangent to each other), so that it would be impossible to clearly identify a center of datum feature simulator B? Would you search for a datum axis or a datum plane in that case? Would this be convenient interpretation at all?

So trying to answer to CH's question -- I am leaning towards saying that THERE IS NO datum B in fig. 4-29 - at least this is how I understand the letter of the standard. There is just datum feature B that is serving (through its datum feature simulator) as a rotational constraint for 2 planes derived from datum axis A, but that is all.

Does it sound reasonable at all or should I prepare for attack? smile

RE: Radius as a datum

pmarc,
Well, you know I don't likre it!
:)

RE: Radius as a datum

Interesting stuff, pmarc. Although the foundational items you give make sense, the conclusion drawn from there doesn't make sense: There is a datum feature B on the part but no datum B?

Turn the page: There is no datum B in Fig. 4-30 either? But most of us would say that there is a datum B (a plane) for that part.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

RE: Radius as a datum

Well, unless I am missing something, fig. 4-30 and accompanying paragraphs say not a word about datum plane in this case.

RE: Radius as a datum

I find the idea of datumless datum features fascinating, but…
From Para. 3.4.3 Y14.5-2009:
“Where more than one datum is required, the datum feature reference letters are entered in separate compartments in the desired order of precedence”
As long as letter B has its own “separate compartment”, there is a datum B – that’s the letter of the standard.

RE: Radius as a datum

Quote:

Well, unless I am missing something, fig. 4-30 and accompanying paragraphs say not a word about datum plane in this case.
Right, so I'm asking what you would say that datum B is in Fig. 4-30. Or is it another case of no datum B?

I admit that the radius in Fig. 4-29 is not a feature of size. But there has to be a datum (else why have a datum feature?) and that datum must conform to the definition of a "point, axis, line, plane, or combination thereof" (para. 1.3.13).

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

RE: Radius as a datum

Okay, I agree that the most intuitive interpretation of datum B in fig. 4-30 is to name it as datum plane, but then I repeat my question: what if the surface was not nominally flat, but a kind of really irregular contour? Would we be able to clearly identify datum?

RE: Radius as a datum

According to 2009 you can have “mathematically defined” feature. (Para 4.31)
To me surface that has axis and the radius is mathematically defined enough.

RE: Radius as a datum

I did not say that mathematically defined contour could not be used as datum feature. I asked if it is possible to clearly identify datum derived from that irregular datum feature. Figs. 4-29 through 4-31 show really simple features (regular single radius and flat surfaces) used as secondary datum features. In such simplified cases datum axis or datum plane interpretation works nice, or like I said before, is the most intuitive method. But if the datum feature is of more complex geometry, so that is difficult to clearly indicate datum point/axis/plane of datum feature simulator, this approach does not give satisfactory results. At least in my opinion.

RE: Radius as a datum

Actually the uglier the feature the better.
Standard recommends using coordinate system for “disfigured” features.
So if your drawing (or better yet CAD data) shows origin with X, Y, and Z sticking out; what could be easier than derive point, axes and planes from that?

RE: Radius as a datum

If an irregular hole can be a datum feature and derive a datum axis, so can portion of irregular hole, like an oval hole or the one shown in 8-19. But indicating the origin of DRF should be necessary.

RE: Radius as a datum

Pmarc -- if an irregularly contoured surface is identified as a datum feature, I would still say that the true datum is a plane.

See Figure 4-28 and notice that the datum is a plane, although offset from the part by the 44.4 dim. (Or is this a case where you would say that the standard doesn't use the word "datum" in its explanation of that graphic?)

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

RE: Radius as a datum

2
I still go back to, if it functions that way, I do not care if it is easy to find, it is. Then the limitation is ours, self made, and the standard needs to evolve to handle it.
Frank

RE: Radius as a datum

OK. I am convinced.

RE: Radius as a datum

Hi All,

Interesting discussion. Very interesting.

I agree that examples such as this expose the limitations in Y14.5's current datum reference frame theory. The plane/line/point datum concept makes things certain things easier to understand for very simple cases, but breaks down very quickly for more complex cases.

Y14.5-2009 provides a table in Figure 4-3 showing simulators, datums, and DOF constraint for various types of datum feature geometry. But these only apply to primary datum features. There is no explanation of how datums work for lower precedence datum features, where certain DOF's have already been constrained. Hence the confusion over Figures 4-29 and 4-30, in which a secondary datum axis or datum plane constrains only one rotational degree of freedom. How does this work exactly? We're left to wonder how an axis that is capable of constraining 4 DOF's only constrains one.

I think that pmarc is on the right track by questioning things, and exploring what the datum would be (if any) if the surface was irregular. I would say that if an irregular surface was referenced as a secondary datum feature, the datum would be a plane/line/point combination as defined in Figure 4-3. But exactly how that datum constrains only certain degrees of freedom, I'm not sure. The only way I've been able to make sense of it is to forget about the datums, and focus solely on the datum feature simulators. I agree with pmarc that when proposing this type of idea, one should prepare for attack ;^). It clashes with the core Y14.5 principle that a datum reference frame is a three-plane coordinate system derived from datums.

The way I like to look at it is that the reference frame is the "space" that the datum feature simulators are defined in. The coordinate system can be placed anywhere in this reference frame - it is arbitrary. The datums are arbitrary as well - they are only a conceptual convenience. This is why there are so many examples in Y14.5 in which the datums are difficult to figure out, or seem arbitrary. Figure 4-26 with the datum axis derived from a hole pattern is one of the worst. Don't get me started on that one!

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Radius as a datum

Thanks for support, Evan. I couldn't describe it better.

RE: Radius as a datum

Evan,
The hole pattern seems to be a very functional type of datum. What are we trying to do, define functional requirements, or make things easy to check? We can't always have both. I hear it all the time, the management always says they want both: quality and schedule, it is pretty obvious to me which drives and which suffers.
Frank

RE: Radius as a datum

I am interested in your POV because to me 4-26 looks like the ultimate functional requirement (without knowing the actual requirement, but projecting one), why would we not want that?
Is it not what we have been striving for?

RE: Radius as a datum

When everything is made by Stereolythography (sp)2 (or some such process) and machining is a rare occurrence will anyone care much where the edges are anymore?

RE: Radius as a datum


Quote (fsincox)

The hole pattern seems to be a very functional type of datum.
Frank,
I was always wondering how car industry was doing it for last 100 years.
To make sure you have good wheel, you have to check runout of the rim in relation to pattern of 4 or 5 conical holes used to bolt the wheel to the car.
Has anyone ever seen a blueprint?

RE: Radius as a datum

CH, I wish I had copied some of those auto company prints from back then (when I worked in an industry that supplied them machine tools).
Tapered cones is something always debated here, but has long been the foundation of the machine cutting tool industry. It was always an acceptable datum to me, I did not need ASME's OK to see that.

RE: Radius as a datum

Frank,

Let me explain further.

I don't have an issue with a hole pattern being specified as a datum *feature*. I agree that many parts mate this way and therefore referencing the holes together makes sense and captures the functional requirement. No problem there.

My problem is with the *datum* that Figure 4-26 shows as being extracted from the pattern of holes. The datum is described as an axis, which does not make sense.

Going by the feature types in Figure 4-3, the pattern of 4 parallel cylindrical holes falls into the category of Linear Extruded Shape (f). Thus the datum should be an axis and center plane. The axis alone would only control the X and Y degrees of freedom and would be insufficient to control w (clocking). I can't begin to count the number of questions I have heard over the years, on the meaning of the "datum axis" in Figure 4-26 (and the similar figure in Y14.5M-1994).

Even if we use the correct datum type in Figure 4-26, I still question the usefulness of the datum. Section 4.12.3 describes the pattern of datum feature simulators based on virtual condition boundaries, which is great. Then it states that the origin of the datum reference frame, which is based on the datums, is established either at the center of the pattern or at any other location defined with basic dimensions. In other words, the datum feature simulators are the important entities and the datums and DRF are just reference entities tacked on for convenience.

The wheel with 4 or 5 conical holes fitting simultaneously is a different can of worms.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Radius as a datum

Quote (axym)

The wheel with 4 or 5 conical holes fitting simultaneously is a different can of worms.
This is what I find the most disturbing.
People were making wheels and bolting them to the cars/carts before GD&T was invented.
And yet in 21st century part that is produced literally by millions for several decades is still considered nuisance, can of worms, etc. Isn’t that a shame that we can figure the way how to make and inspect part, we just cannot figure out how to draw in on the piece of paper?
Is it just me thinking that today’s GD&T is suffering from severe disconnect from reality?
This is definitely subject for separate thread, but could it be the reason for today’s slow rate of GD&T adoption?
Sorry, this isn’t Friday yet; I must be getting grumpy ahead of time.

RE: Radius as a datum

And what is really the problem with GD&T in case of a car wheel?

RE: Radius as a datum

The difficulty that I see with the wheel is that the 5 conical holes center on the 5 fasteners simultaneously. In order for this to happen, something has to give (deform or bend) and act as a non-rigid part. This goes beyond the default Y14.5 assumption of perfect rigidity. So it can be difficult to establish a DRF on the 5-hole pattern in a way that will duplicate the final assembly condition, without actually mounting it physically. Even then, the final DRF will depend on the exact geometry of the mating part or gage. There is some unpredictability there, and some sort of averaging or approximation is often used.

So it's not that we shouldn't apply GD&T here, it's just a challenging application for DRF establishment.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Radius as a datum

I am probably missing something, but why would one want to establish DRF from a pattern of 4 or 5 or 6 holes and not from center hole in the rim? As far as I understand this is the centering feature.

RE: Radius as a datum

Not on all wheels, especially steel ones.

RE: Radius as a datum

So the article said that some wheels center on hub and some not.

How is it different from my statement? And how does it help with GD&T issues?

I suspect I will never be able to convince you this way. Apparently you and I were changing tires on different cars, so we have different feelings about it. There are several designs out there. Have you ever seen wheels that are first roughly located by pins mounted to the hub and then bolted down with tapered screws?

Here is another automotive application: truck half-axle being mounted and centered using tapered holes/fasteners. Feel free to deny.

RE: Radius as a datum

All I was trying to say is that for hub-centric wheels I see no major problems with proper GD&T application.
And I was not challenging your statement in any way.

RE: Radius as a datum

Sorry if my tone was a bit harsh. I am not trying to start a war, just to convince that there is legitimate case. Also, in my understanding FUNCTIONALLY it is conical holes/fasteners that do all the heavy work, holding the weight of the entire vehicle. Just a thought.

RE: Radius as a datum

OP has not logged in since posting this question, and has not provided any additional information to clarify the situation. I think she just likes watching you guys bicker.

RE: Radius as a datum

RE: Radius as a datum

Indeed, Tick. I suppose I muddied the waters by mentioning Fig. 4-29 back near the beginning.
But that's the fun of these discussions. I await the insights about the recent wheel runout question.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

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