ASME Y14.5m application and general drafting stds 3

[DesignBiz]

1. The attached tube dwg is the current template for tube dwgs. I find it obviously in error in many respects, in particularly the use of A-B as a single datum. (Ref ASME Y14.5m 1994 4.5.7.1 and 4.5.7.2) In this case the ends of the tube are A and B and these features of size are at compound angles to each other. The in-house consultant rationalizes this callout somehow in his thinking and claims it is easily understood according to ASME Y14.5m 1994. I couldn't disagree more.

2. The profile callout via default note completely defys ASME Y14.5m 1994 in my estimation. (ref ASME Y14.5m 1994 6.5.1 paragraph (a) regarding required view or section for a profile callout.

3. Basic drafting standards seem to be a foreign concept.

Comments invited.

D-Biz
Sr. Designer (Auto-Aero Mechanical)
NX4 / Team Center user
25+ yrs experience

 

[ewh]

I agree that there seem to be poor practices used on the drawing, but (from under one of your blocks) the drawing states that it is IAW ASME Y14.5-1994 AS AMMENDED...
Without knowing these amendments, it is difficult to know what is or is not correct on the drawing.


"The ambassador and the general were briefing me on the - the vast majority of Iraqis want to live in a peaceful, free world. And we will find these people and we will bring them to justice." - [small]George Bush, Washington DC, 27 October, 2003[/small]

 

[DesignBiz]

ewh,

Thank you for your affirmation. I think you are gracious to judge that this drawing uses "poor practices". In my opinion it is not interpretable according to ASME Y14.5M 1994 and violates use of defining datums. This was my 1st posting and I didnt realize the ppt image could be manipulated. JPEG's next time. However I will tell you that the amendments do not address the fact that to use two part features to generate a single datum according to the standard (Y14.5m), can only be 2 co-planar or 2 coaxial features. The in-house amendment in this case is virtually the same as the base Y14.5m standard but allows for a material condition to be added to the A-B. I have tried to be open minded about this call out but believe I can prove that 2 axis at compound angles (this is the case for many of our tube runs) to each other can NOT produce a single datum (plane, axis or exact point according to Y14.5m standard defintion).


D-Biz
Sr. Designer (Auto-Aero Mechanical)
NX4 / Team Center user
25+ yrs experience

 

[axym]

I don't see an attachment or link in the original post, but I'm going to comment on the compound datum issue anyway.

It is legal to reference two cylinders at compound angles to each other as a compound datum feature.

I agree that the two cylinder axes cannot produce a single plane, axis or point. However, they don't need to - the definition of "datum" in Y14.5 is incomplete. A datum can be, and often is, some combination of a plane, a line, and a point. For example, the datum for a conical datum feature is a point on a line, and constrains 5 degrees of freedom if referenced as primary. If the two compound-angled cylinders were referenced together as a primary datum feature A-B, the datum would be a "point on a line in a plane". This datum would constrain all six degrees of freedom of a datum reference frame.

Part of the problem is that Y14.5 doesn't explain this. It also doesn't explain exactly how to get from a set of datum features to a set of datums to a datum reference frame. There are many common misunderstandings relating to this, most of them (IMHO) caused by misleading terminology, definitions and diagrams in Y14.5. Even the diagram on the front cover of Y14.5 is misleading - the entity labeled "datum point" isn't a datum point - it's a point at the origin of the datum reference frame. This is only one example, there are many others like it.

I must give credit where it is due - I am aware of these things from working extensively with Bill Tandler, who is the grand master of datum reference frame construction.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[PaulJackson]

Well said Evan!

Paul

 

[ringster]

axym,

I hope that I am not splitting hairs, at least unnecessarily, but should your statement better read a point AND a line, rather than a point ON a line?

Otherwise 'right on'.

 

[ewh]

Hey, how did KENAT get a star and his post isn't even here? ;-)

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[KENAT]

I'm that good, just my reading the post is star worthy;-)

I'm guessing as my post was related to the security issue, which is now addressed, it was no longer relevant. Not sure it was star worthy though.

axym, your post is very interesting.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[MechNorth]

If possible, pls re-post the graphic. I'm curious what seems so offensive. Tks.


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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[DesignBiz]

My beef is with the improper use of the “A-B” according to the standard, in lieu of any fully definitive amendment. I am open to an amendment that can be fully explained and hopefully demonstrated with graphics. The idea that [a "point on a line in a plane"] restricts all degrees of freedom seems totally inadequate to me. I also have a problem with the two datums A and B being positioned to themselves. (see dwg).
My intention was to stimulate conversation and possibilities for proper callouts regarding the tube drawing I have originally posted. I will post another example drawing along with a 3D view of the datums for the cylindrical features of size (end runs of the tube) according to ASME Y14.5m 1994. Please comment according to the standard as written or proposal of an amendment could be interesting. My position is that the callout can be made with simple application of the standard in which both ends could be primary and secondary datums, along with the face of one end as the tertiary datum.
I do appreciate all comments. Special thanks to Axym and his creative thinking, though we don’t seem to agree on accepting ASME Y14.5 1994 fundamentals and definitions in some cases.
I am not sure where Axym is getting the information to validate the claim that it is legal to have a compound datum. I don’t believe it is in the standard. If it is then could you give me the section and paragraph for this definition and its application in the ASME Y14.5m 1994?
I will agree that the standard is not clear sometimes. It does state that all of the figures used as examples are not complete. I will also concede that it is not perfect. I have been involved with the standard since the ANSI 1973 version, the 1982 version and now its form today.
A position that the standard is incomplete is true in some respects, however my position is that the “rules are the rules”. Changes have been and will be made again, including definitions. If you reference the 1973 standard you will find that symmetry and concentricity do not mean what is in today’s standard (1994). In 1982 they were omitted from ANSI.
Okay, the ASME Y14.5m standard’s definition of a datum can be found in 1.3.3. The rules for what a ‘datum reference’ is, in a Feature Control Frame, can be found in 3.4.2 and 3.4.3. The requirement for mutually perpendicular planes in a DRF can be found 4.2.2.1. The rules for using “multiple datum features to establish a single datum” can be found in 4.5.7; 4.5.7.1; 4.5.7.2.
Axym’s point was confusing and from my view does not agree with the standard. I disagree with assertions that the standard’ definition of a datum is incomplete. It is what it is, based on acceptance of a plane, axis, or exact point by the engineering community at large. Could the definition be expanded? Possibly it could, but again it is what it is for the time being. We need to work within the frame work as written or write amendments acceptable to a particular organization’s need. It is common to write an amendment to the standard to explain deviations from it. At a minimum, the objective here is to convey design functionality to all involved and to have the component inspected with the same setup for each inspection for some unique circumstance.
I would disagree somewhat with the example of a conic as a primary datum. As a primary datum for a feature of size according to the standard, the axis of the conic is the datum and that axis is understood to be intersected by two perpendicular planes. On a drawing this would be the centerline of the conic. To simulate the datum in a gage setup could be done with a conic shaped gage pin or a chuck which would be position with a minimum of 3 points of contact. I do agree that it can restrict 5 degrees of freedom.
“Datums “are theoretical planes, axis, and exact points; “datum features” are portions of a part used to establish datums; “simulated datums” are established by “datum feature simulators” which are basically your inspection equipment (e.g. surface plate). These can be used to defined DRF’s which are related to features based on the functionality and relationships to other features. I believe the standard does an adequate job of showing the relationships of the terms and their use. It is mentioned in the standard that practicality should be considered. In my experience it helps to include common-sense.
BTW, the cover does correctly point to a “datum point” not to be possibly confused with a “datum target point”. It is the intersection of the DRF planes. It is theoretical and is the origin for defining and inspecting aspects of a feature(s). It can and is simulated with inspection equipment such as surface plates or defined with CMM. “Datum target point” can be found at 4.6.1.1


DesignBiz

"Quality is in the details"

 

[DesignBiz]

Ref 3D model showing tube datums for A-B
How is there a possible connection

DesignBiz

"Quality is in the details"

 

[ringster]

DesignBiz,

I do not believe that compound datums are appropriate in this case inasmuch as the 2 datum features are skewed. However, I do believe compound datums are acceptable if they meet the proper criteria.

I think this drawing would have been better defined with A, ID of tube, as primary, C, the end, as secondary, and B, a selected POINT on the dia at the other end, as tertiary.
NO COMPOUND!

ringster

 

[MechNorth]

Axym has the right idea, but his explanation stops short of explaining where the three mutually perpendicular planes are. This is also where the current standard comes up short. You must show on the drawing where the origins of measurement are, and also which degrees of freedom the compound-primary datum A-B is eliminating if you are planning on using secondary and tertiary datums as well.

Thinking about it logically; if the workpiece is engaged/held on Datum Feature A and Datum Feature B simultaneously, then the part cannot move in space; all degrees of freedom are eliminated. The only remaining question is then where to take the measurements from. The designer (hopefully in consultation with the inspector) decides & marks this on the drawing. This can be any convenient location in space as long as it is related back to the datum features by basic dimensions. Before anyone asks, no this is not shown in the standard; it is extensions of fundamental principles.

Consider a compound datum composed of 7 planar & parallel features at different elevations; this is legal per the standard (Fig. 4-20; 4.5.7.1 Simulation of a Single Datum Plane). Here, unfortunately, the standard doesn't tell you explicitly how to deal with more than two datum features. As defined in the standard, you actually measure from the datum simulator, not from a theoretical plane (the datum). Which of those 7 simulator surfaces do you measure from? Does it actually have to be from any of those 7? The standard doesn't say anything more than you measure from the simulator. Thus, any repeatable feature on the simulator, including an offset that is traceable back to the original simulator surface(s), is acceptable.

Consider also 4.4.1.1 Parts With Inclined Datum Features; (Fig. 4-4), the datum is not on the workpiece, but it is traceable back to the datum features by means of basic dimensions.

Throw in 4.5.10.1 Mathematically Defined Surfaces; again, the actual datum feature surface won't make contact with the datum simulator, however you will measure from the datum simulator.

It's a long way around, but that is how advanced GD&T applications are derived from Y14.5.

There is a valid argument for using the compound datum method if the pipe is rigidly affixed at both ends (i.e. welded or rigidly clamped). Otherwise, I would tend to use datum targets instead.

DesignBiz, to be correct conical datum features are not features of size, they are just datum features.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[DesignBiz]

Ringster,

I appreciate the response and agree with your assesment.

However I will be willing to go with a compound datum if I can be lead to where this term is defined in the standard and its application; and an explaination of how it is possible to geometrically create a compound datum in light of the standards definition for a datum being a plane, axis or exact point.

My point being that many drawings are in error relative to AMSE y14.5m 1994 due to use of terms that do not exist and mis-interpretations. The datum reference frame consists of mutually perpendicular planes. This compound datum term I hear referred to is really not true and if you interchange a datum definition (eg plane, axis, point) it becomes more obvious. For instance "compound plane' or 'compound axis' or 'compound point'. These are not even known geometric elements as far as I know. If we talk about orienting datum simulators at compound angles based on datum reference frames then I am good with that. Termonology is important if we are going to effectively communicate in the GD&T "language" we need to be careful to try and use the proper fundamentals, definitions, and rules according to the standard. I think many times people refering to "gage setups", use the term 'datum reference frame' (DRF) which causes confusion.

Thanks for your contribution !


DesignBiz

"Quality is in the details"

 

[MechNorth]

DesignBiz,
The DRF consists of datum references, which together establish the mutually perpendicular datum planes. Nowhere in the standard does it say that the datum references (i.e. the datums in the DRF) must be mutually perpendicular. Again, look at Fig. 4-4 in the '94 standard. The planes established by the DRF must be mutually perpendicular, but there is no such requirement for the datum features themselves.

The distinction between Datum Feature, Datum, and Datum Simulator are absolutely critical. A datum feature is a real, physical feature of the workpiece. A datum is a theoretical point, line or plane, represented by the datum simulator. A datum simulator is a representation of the datum, and makes contact with the datum feature.

I did a search of the '94 standard, and "Compound Datum" is not defined there; it is apparently a carry-over from the '82 standard where 4.4.5 Coumpund Datum Features was defined, but in '94 the same content was renumbered/renamed to 4.5.7 Multiple Datum Features. So, Compound Datum is a carry-over in lingo from the '82 version, which should properly be referred to as Multiple Datum Features when referencing the '94 standard. Otherwise, the substance and intent are the same.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[ringster]

My assessment of compound datums is that they should be derived from surfaces which are intended to be parallel, or inline. This of course is perfect condition,as design intent, but due to mfg tolerances,not probable.

ringster

 

[DesignBiz]

MechNorth,

You did get my attention and I will investigate your input.
I do question the following though.

I did write above, the following:
"The requirement for mutually perpendicular planes in a DRF can be found 4.2.2.1"

The standard wording in 4.2.2 Datum Reference Frame, is:
"...features are chosen to position the part in relation to a set of three mutually perpendicular planes, jointly called a datum reference frame."

Referring to the previous sentence that I wrote above, I did write about what a datum reference is in a "Feature Control Frame" trying to make a point that a datum reference in a Primary,Secondary, or Tertiary block following the tolerance does refer to a single datum. The standard's definition of a 'datum' is an exact point, axis, or plane.
I dont believe I wrote anywhere that all datum references must be perpendicular.

I believe I on am on the same page with you regarding that the the origin of the DRF is three mutually perpendicular planes, however other planes, axis, and points can be dimensioned to thru basic dimension from the DRF. I understand that these dimensions do not need to be perpendicular to the DRF.

In addition, regarding the conic as a 'feature of size' and it's axis. The defining diameters are measurable and an axis can be determined. To me this can be considered along the lines of a cylinder where when referenced as a datum we are actually considering the axis of the conic not its surface as in a flat surface that could be used as datum reference for a plane. Whether feature of size or feature, they become datum features when used to establish a datum.
My sentence structure was not the best as written above. I was clarifying as I read in the standard, in reply to Axym's; "For example, the datum for a conical datum feature is a point on a line, and constrains 5 degrees of freedom if referenced as primary." ; that while referencing a conical feature that the datum reference would be an axis.

I appreciate your inputs !


DesignBiz

"Quality is in the details"

 

[ringster]

Skewed lines are by definition, non intersecting as I recall and additionally not in the same plane. Therefore I am of the opinion that they cannot be used as a compound datum. You cannot derive a plane or a point with a combination of just these 2 features.

 

[MechNorth]

Apologies, DesignBiz. I missed and/or misunderstood some items in the earlier post. Glad that we have consensus on most items. Must agree with Axym that a conical datum feature, referenced as primary, will establish the axis of the cone and the point at the apex of the cone, whether projected (as when the cone is truncated) or physically present. Thinking of the simulator, you would use a conical taper of ideal geometry and size. When the workpiece is engaged with the simulator, the workpiece's translational motion perpendicular to the axis is eliminated, as its longitudinal motion along the axis. The theoretical representation of this longitudinal constraint is the third mutually perpendicular datum plane. It is very common to specify on the drawing (in a note) that the conical datum feature is not being used to constrain longitudinal motion along the axis.

A conical surface is not a feature of size. Per 1.3.17 Features of Size; "One cylindrical or spherical surface, or a set of opposed elements or opposed parallel surfaces, associated with a size dimension". While you can put a tolerance on a diameter at any cross-section of the cone, that does not make it a Feature of Size. As I understand it, there has been a fair bit of debate over including a "caliper rule" in the standard, to the effect that the directly opposed "flats" of a set of caliper jaws must make contact with all directly opposed points on the worksurface. For cones, you could measure this way for the largest diameter on an external taper/smallest diameter on an internal taper, but the diameter at the opposite end (or any section in between) would not pass the test.

It has confounded me that conical tapers, one of the most basic features, very commonly used as mating / datum features, is not covered adequately in the standard. At one time, I had an "industry expert" tell me that conical tapers could not be used as datum features, and that we (a mold making company) would have to change our designs to use cylindrical and planar datum features. Yeah, right.

Ringster, the only way to reconcile the issue is to accept that you "arbitrarily" define the datum planes and manifest them within the context of your datum simulation fixture. As the standard only reflects the agreed-upon knowledge base at the time of publication, many advanced applications of GD&T are extensions of fundamental principles and creativity that does not violate the standard. In cases where I've been driven in this direction, I've elected to use the axis of the datum feature simulator at one end of the pipe to establish my first two datum planes. I put an indication on the drawing (graphical and note) that those planes are to be used as the origins of measurement. The fixturing is thus accepted, and the origin of measurement is clearly understood. Hope that helps.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[ringster]

Jim,

I believe there is a basic flaw in your previous statement with regards to the 'datum feature axis simulator'.

Your statement as I read and interpret it establishes your first 2 datum planes. My understanding of Geometry says that there MAY BE AN INFINITE number of planes passed thru any line. From that infinite number we may choose any 2 which are perpendicular to one another. Normally we choose the set that are oriented by the tertiary datum feature.

ringster