Tolerance of Positoin - feature positioned to itself ??? 2

[DesignBiz]

Please review attached drawing for comment on the tolerance of position callouts circled.

1. I do not understand how a FCF can reference the feature positioned to itself in a datum reference block. (e.g. datum A is positioned with reference to itself in A-B)

2. I believe the location tolerance and the basic dimension relavent to it should have the same number of decimal places. (Not shown on the drawing) (e.g. if tolerance in FCF is .XX then its basic dimensions should be XXX.XX NOT XXX) Is this in the standard?

DesignBiz

"Quality is in the details"

 

[ringster]

Jim,

I moved south from MO years ago. I wonder if you could illustrate how with JUST THE TWO datums A and B as shown you can establish a relationship between the two?

 

[axym]

Ringster,

I think that we might be running into some terminology problems here. When you say "datums A and B as shown", what exactly do you mean? Which of my diagrams are you referring to?

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[drawoh]

axym,

My understanding is that the OP's tubes are flexible. It is possible to fixture them to centre on Datums[ ]A and[ ]B. It is likely that the final installation actually works that way, making the envelope dimensions at inspection, very relevant.

If the tube is very flexible, it may be necessary to have a datum[ ]C to further control the flexibility. Read ASME Y14.5M-1994, Section[ ]6.8 on Free State Variation.

If the tube is not flexible and it were my drawing, I would use the centre of[ ]A as my primary datum, the end face of[ ]A as my secondary datum. I would then pick up the sides of[ ]B to use as a clocking feature, and my tertiary datum.

JHG

 

[MechNorth]

Ringster, who were you with up there / which ops?

I've done a "quick" drawing of what I'm talking about. I didn't put the basic dimensions on the fixture views 'cause I really hate annotating to sketch intersection points with SolidWorks.

http://www.profileservices.ca/files/tidbits/thd1103_237732.pdf

Personally, I'm not a fan of the A-B compound datum for this kind of workpiece. I'd prefer to use moving datum targets. The A-B compound datum is legal though, if that's the design intent.


Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[DesignBiz]

FYI,

The original tube dwg posted is an example of a rigid metal exhaust tube and how these rigid tubes (typ .045"-.095" wall)are dimensioned at this company.

DesignBiz

"Quality is in the details"

 

[drawoh]

DesignBiz,

If the tubes are rigid, then one of the possible errors is the angle between the two datums. MechNorth's fixture will not work properly unless the tube is flexible enough to bend to the fixture geometry.

An alternative is to assume an MMC condition of maximum tube size sufficient to fit inaccurately bent tubes into the fixture. This is silly though, since tube ODs are fairly accurate. There is no way for a supplier to take advantage of the extra slop. The real tube will be located with play.

You need to go with my datum suggesting above, or something similar.

JHG

 

[DesignBiz]

DrawOh,

I whole heartily agree with you and tried to convince some folks around here for the last several months. The template for tubes here remains the same, however I have convinced my project engineer that a callout with a DRF as you have also proposed is the way to go. He is okay with it.

Thanks again to all for your inputs !

DesignBiz

"Quality is in the details"

 

[MechNorth]

Drawoh, you seem to be assuming an intimate fit on both datum features. My graphic shows the basic geometries, not "actual" parts. I can assure you that this type of fixture is used, and does work as shown to me by formed-tubing suppliers. The workpieces are sort of "wedged" (for want of a better term) into the chucks for a best fit as opposed to an intimate fit. An intimate fit on both datum features would indeed require flexibility in the workpiece.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[drawoh]

MechNorth,

That is sort of the MMC approach I described above. The datum holes are large enough to accomodate some angle error.

I would not want to call this up on my drawing. I cannot visualize what would pass and what would fail inspection. How do I account for the oversized fixture?

JHG

 

[MechNorth]

Drawoh, the problem with MMC is that it allows the workpiece to float within the "slop" of the clearance so that it is only "held" when datum feature reaches its virtual condition. As you indicate, it would be difficult to validate the workpiece. For RFS, the simulator (jaws in my graphic) close down to actually hold the workpiece.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[axym]

DesignBiz,

I'm can't help but feel a bit disappointed that you've convinced your project engineer that the AB scheme is the way to go. Based on the descriptions of the part and its function, the AB scheme specifies DOF constraints that are very different from how a real part would be held during assembly. If the parts are constrained properly to the AB scheme during inspection, the results will disagree wildly with fitness for use. The surface profile will appear to be way off when it's really not. In my diagrams from yesterday, note how good the part looks (compared to the nominal centerline) in the A-B scheme. And how much worse it looks in the AB scheme, and how horrendous it looks in the BA scheme. It's exactly the same part geometry! This is what you can expect to see.

I hope that things work out okay in spite of this. Sometimes, the right design intent somehow gets through in spite of what the drawing actuallly specifies. Usually through some form of tribal knowledge, where the vendor or inspection department knows what to do to make it come out right. This can put the quality department in a precarious position, however, when applying the oversimplified GD&T correctly during inspection will result in massive nonconformances and the whole operation grinding to a halt.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[DesignBiz]

Evan,

Could you agree that the meaning of the A-B concept is not what you will find in the standard? I am referring to the definition of a DRF in section 4; the definition of a datum (1.3.3); and the instruction that a datum reference block in a FCF references a "single datum" (3.4.2 and 3.4.3).

Maybe I have a part in an assembly and later a different model assembly with a newer and slightly physically different cover plate than the original assembly is developed. This new cover plate is superior in quality, cost, appearance, durability than the original, however with the physical differences it cannot be retrofitted to the original model without modifications. Maybe I need to drill a couple of new holes in the mating part, open up an opening that the new cover plate has a tab that fits into it and deepen a well the plate lies in. No reasonable person in engineering would say it is simply good enough to replace the new cover plate in the old assembly, UNLESS there is proper documentation to modify the old assembly to receive it.

This may not be comparable to you but it is to me. My resistance to the A-B concept is primarily based on the unwillingness at this facility to document an amendment as to what the callout means. I would disagree with you (in absence of amending documentation) that my position is to use the simplest and not the best callout for these tubes. My position is that without the amendment this A-B callout defies the standard as written. (Wonder if the 2009 standard addresses this callout?)

I do like to read what you have to write. It seems as if you are passionate about your positions relative to the standard. I respect that and will say that you do have me interested in investigating the differences for the 2 fixturing setups. For a hard tube that is usually formed by starting at one end and proceeding thru the bends until the end is arrived at, I don’t believe that if you held the tube at both ends vs. holding it at one end and clocking with another feature that the tube would pass inspection based on how it is fixtured. If it was formed within dimensional requirements it will fit and function as designed regardless of how it is fixtured.

Don’t feel too disappointed Evan, I am interested in reading what you write and give consideration to it.


MechNorh,

This tube is rigid and nowhere on the drawing or in it's function are we supposed to constrain the part. It is obviously connected at both ends. This isn’t like a spring where one expects to use some force to position it.
No one is suggesting that it is plausible to force the part into a fixture are they? I didn’t believe the standard allows for restraining a part for inspection unless noted on the drawing. Correct?


DesignBiz

"Quality is in the details"

 

[ringster]

I may be inserting 'foot into mouth', but I DO believe that 2 skewed lines (axes) cannot by themselves be related to one another. To establish a relationship there must be another feature/s added that will provide a point on one or both axes.

 

[MechNorth]

DesignBiz,
Wording is again important here. To constrain something is to remove its degrees of freedom, which you do with the datums. To restrain something is to hold it in place, typically by applying forces at specified locations other than at the datum features (see 6.8). If you are specifying datum features of size at RFS, then you most certainly are getting both constraint & restraint. A set of jaws, for example, closes down on each end of the tube as best it can given the basic relationship between the datum features. The workpiece cannot move (i.e. it is restrained) and if the pipe ends are skewed wrt each other, then you get the part fully constrained as well. Try it physically if you are still dubious; this is not merely an intellectual exercise because real people are doing it with real parts in real production facilities. The real challenge should be reconciling your belief system to the physical reality, not whether or not it is legal. If you are going to apply GD&T based on the manufacturing method alone, then you miss the raison d'etre for GD&T which is to communicate design intent.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[axym]

DesignBiz,

The cover plate analogy is a bit of a stretch, but I think I see your point. If the meaning of the A-B callout may not be documented and will be misunderstood, then it isn't of much value in your organization.

Again I applaud your attempts to evaluate things in terms of the "rule of law" stated in the Y14.5 standard.

To paraphrase what Jim said, one should base the datum referencing method on the physical reality. In most cases, the physical reality can be closely approximated by the applying the tools provided in Y14.5. I some cases, it cannot. In the case of the bent tube, the physical reality is that the tube is fully constrained by two skewed cylinders without one taking DOF precedence over the other. I believe we've established that as a fact. But Y14.5 does not directly address this particular situation.

So what do we do? One option is to make simplifying assumptions about the situation in order to idealize it into something that Y14.5 does deal with. In this case we could make the assumption that the two ends of the tube are oriented and located very accurately relative to each other. This would mean that the tube would end up in approximately the same location whether it was fixtured on both ends simultaneously or on one end first. This would allow us to use the conventional AB scheme. Is this a close (or even decent) approximation of the physical reality? In the case of the bent tubes I would say no, because of the variation you described and the projection effects that I have tried to illustrate. You say yes, and you have direct knowledge of the parts and the process. So we can agree to disagree over this.

Another option is to take a Y14.5 tool and apply it in a way that is not described. In this case, applying the multiple datum feature concept is one possiblility. Others might be the pattern concept and the mathematically defined surface concept.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[ringster]

Possible solution to the OP?

If the end surfaces were designated as datum features, D on the end of dia B, and C on the end of dia A, as it is now.

We could use a DRF B[B-D[A-C[. This would provide an axis, B, a plane normal to B, and a point at the intersection of a and D.

In the world of Geometrics, B might be related to the axis of the earth, C the equator and D the Observatory at Greenwich England. All dimensions could then be related to this DRF.

??

 

[axym]

DesignBiz,

You had some questions on the A-B concept relating to datums, datum reference frames, and their definitions in the standard. I've been putting off answering them because what I'm going to say might be taken the wrong way if I'm not careful. Or it might portray me as someone worthy of ignoring in the future. Either of these is probably still likely even if I am careful!

Relating the "two skewed cylinders A-B concept" to datums is where the problems start for me. Here's the definition from 1.3.3:

Datum. A theoretically exact point, axis, or plane derived from the true geometric counterpart of a specified datum feature. A datum is the origin from which the location or geometric characteristics of features of a part are established.

Presumably, the purpose of datums is to provide geometry to orient and locate a coordinate system relative to. The idea is that the datums are extracted from the simulators and then the DRF is constrained to the datums in the specified order of precedence. This can work well for simple datum feature types:

Planar datum feature: datum plane
Cylindrical datum feature: datum axis
Spherical datum feature: datum point
Width datum feature: datum centerplane
Coplanar surface datum feature: datum plane
Coaxial diameter datum feature: datum axis

In certain combinations of these simple datum features, a DRF can be constructed on the datums in a fairly obvious way. Three orthogonal planar surfaces (figs 4-2 and 4-3) is one example. Primary planar surface - secondary cylinder - tertiary slot (figs 4-6 and 4-7) is another. Primary planar surface - secondary cylinder - tertiary cylinder (figs 4-8 and 4-9) is another. Y14.5 and a lot of GD&T textbooks tend to illustrate DRF construction using these simple cases. On the plus side, a good majority of part interfaces are covered by these cases. On the minus side, these are very special cases with unique simplifying properties that do not exist in more complex (but still very common) situations. Many important effects and complexities are masked.

The explanations in Y14.5 are also full of nasty pitfalls. There are several statements and depictions in Chapter 4 that are misleading, not generally true, or outright incorrect. The use of certain terminology is also imprecise and inconsistent and, IMHO, has led to a huge amount of confusion over the years.

One example, that I've mentioned before in another thread, is the diagram on the front cover. The lines labeled "datum axis" are not datum axes, they're coordinate axes of the DRF. The point labeled "datum point" isn't a datum point, it's the origin of the DRF. The planes labeled "datum planes" are datum planes, but they're also coordinate planes of the DRF in this special case of 3 orthogonal planar datum features. The datum planes are also labeled as origins of measurement, which is incorrect. The coordinate planes of the DRF are the origins of measurement, and again they happen to coincide with the datum planes in this special case. So misleading.

Another example is a statement made in section 4.4.2 regarding cylindrical datum features. It states that "a cylindrical datum feature is always associated with two theoretical planes intersecting at right angles on the datum axis". How much confusion has this caused for members of this forum alone? The problem is that the statement is not generally true - it's only true in certain special cases. One is a cylindrical primary datum feature. Another is a cylindrical secondary datum feature that is nominally perpendicular to a primary planar datum feature. A third is a cylindrical secondary datum feature that is nominally in line with a primary spherical datum feature. There may be a few other cases that I haven’t thought of. But that's it. In all other cases, the “two theoretical planes” on the datum axis conflict with DRF planes that have already been established. For me, trying to apply the idea of these two theoretical planes in a general sense has been a conceptual wild goose chase. I’ve tried to make sense of it but for the life of me I can’t. Based on posts we’ve seen on this forum, others have run into similar difficulties.

For datum feature configurations other than the aforementioned “special cases”, constructing a unique DRF from the datums just isn't possible, for a variety of reasons.

For simple datum features that are not orthogonal to each other, the datums are well-defined but the origin and/or clocking of the DRF is not obvious or unique. One example of this is the "two skewed cylinders" referenced A|B. The primary datum axis establishes the direction of the DRF's Z axis and the X and Y origins. Because the secondary datum axis is skewed relative to the primary, there is not an obvious and unique way to constrain the DRF's rotation about Z and or define its Z origin.

For non-simple datum features, the location of the datum is not well defined. As a consequence, the origin of the DRF is not well defined. One example is the hole pattern secondary/tertiary datum feature, where the location of the "datum axis" is arbitrary. For circular or rectangular patterns, the center is an obvious, but still arbitrary, choice. For other patterns, there is no obvious center or origin point. There have been many discussions of the “center of the pattern” issue on this and other GD&T and CMM forums.

Some GD&T books, as well as the recently released Y14.5M-2009, expand the definition of a datum to include combinations of a point, a line, and a plane. This allows datums to be identified for features like cones, extruded shapes, and complex surfaces. For example, the datum for a pattern of parallel holes would be a “line on a plane” and the datum for a complex surface would be a “point on a line in a plane”. This is somewhat useful for visualizing what degrees of freedom are constrained by a particular datum feature when it is referenced as primary, but that’s about it. There is still the problem of non-uniqueness. For the hole pattern, the locations of the plane and the line are still arbitrary. For our two skewed cylinders referenced as A-B, the datum is a point on a line in a plane. But the orientation and location of the plane, the orientation and location of the line in the plane, and the location of the point on the line are all completely arbitrary. So we’re not any further ahead as far as defining a unique DRF origin. I just don’t find it very useful. Don't even get me started on how these datum types would constrain DOF's when referenced as secondary or tertiary.

So where does that leave us? In most FCF applications, it doesn’t really matter where the DRF origin is because everything is relative anyway. If certain DRF axis directions and/or origin are desired by the designer for some reason, these can be annotated on the drawing or model in the form of a labeled set of coordinate axes related to the datum features by basic dimensions and angles. The model defines the relationship between the datum features and the DRF, and thus defines the relationship between the datum feature simulators and the DRF. When it comes time to build inspection fixtures or CMM programs, a unique DRF can then be constructed directly from the (physical or virtual) simulators. Without the need for extracting combinations of points, lines, and planes for each datum feature and trying to hang a DRF on those. I’m not saying that this “simulators to DRF” approach is easy in every case, but it is doable.

Comments?

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[ringster]

Evan,

I read then re-read your comments. I applaud your statements and you for being outspoken. I feel that I agree with at least 90 percent of your comments and have had feelings as to the inadequacy of geometric background on some of the committee members in producing the standard.

I have had some very strong misgivings about several items that you mentioned. I was in somewhat of a contest with another person (anonymous) in the input for the 1994 update corrections to the draft. My recollection is that some of your comments were addressed and subsequently rejected at that time.

 

[axym]

Ringster,

I appreciate the feedback. What was the 10% that you don't agree with?

My comments were on the standard itself, and not intended as any kind of commentary on the committee members. To be honest, I really don't know very much about most of them or what their backgrounds are. I attended a Y14.5 meeting for the first time last year in Albuquerque, when they were in the comment review stage. I got to see how the meetings work and how the committee operates. It sort of reminded me of the old joke about sausage and law - you don't want to see either one being made. Like anything made by committee, there are a lot of different viewpoints and interests to balance. There is definitely a compromise between the "new and improved" and the "tried and true".

My comments are based on the content of the '94 version. I don't have the final release of the '09 version yet, so I don't know for sure what the final content of the DRF section is. I've only seen the public review drafts. So some of these issues may have been addressed - we'll see.


Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[ringster]

Evan,

Reread the post "at least 90 percent".