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ASME Y14.5m application and general drafting stds

ASME Y14.5m application and general drafting stds

(OP)
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

RE: ASME Y14.5m application and general drafting stds

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." - George Bush, Washington DC, 27 October, 2003
 

RE: ASME Y14.5m application and general drafting stds

(OP)
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

RE: ASME Y14.5m application and general drafting stds

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.
www.axymetrix.ca

RE: ASME Y14.5m application and general drafting stds

Well said Evan!

Paul

RE: ASME Y14.5m application and general drafting stds

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'.

 

RE: ASME Y14.5m application and general drafting stds

Hey, how did KENAT get a star and his post isn't even here? winky smile

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

RE: ASME Y14.5m application and general drafting stds

2
I'm that good, just my reading the post is star worthywinky smile

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: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?

RE: ASME Y14.5m application and general drafting stds

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

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: ASME Y14.5m application and general drafting stds

(OP)
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"
 

RE: ASME Y14.5m application and general drafting stds

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

RE: ASME Y14.5m application and general drafting stds

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  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: ASME Y14.5m application and general drafting stds

(OP)
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"
 

RE: ASME Y14.5m application and general drafting stds

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  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: ASME Y14.5m application and general drafting stds

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

RE: ASME Y14.5m application and general drafting stds

(OP)
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"
 

RE: ASME Y14.5m application and general drafting stds

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.

RE: ASME Y14.5m application and general drafting stds

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  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: ASME Y14.5m application and general drafting stds

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


 

RE: ASME Y14.5m application and general drafting stds

(OP)


Thank you MechNorth and Ringster for your inputs !

You were able to view the 2 JPEGs I posted?

This is actually what I had hoped for from this website

People who are interested in ASME Y14.5m and can explain their position using the standard and geometric
construction. It is interesting that so much of the standard can be interpreted differently by knowledgeable people.
This was brought to my attention approx. 1996 when at Delphi in GDT training for the 1994 standard, the certified
GDT instructor mentioned that he and "his buddy" could debate some things aspects for hours. Boy, that blew my
naive thinking regarding when I had first been "given instructions", for 1973 stand by Don Buckman at GE Evendale,
that this was a common language for all to understand drawings the same way.
It looks like MechNorth that you are a certified instructor also. A future goal for myself.

At this point in my career I have been trying to go to the current standard and it's definitions for as simple interpretations
as possible. My problem still persists even though MechNorth does give a logical explanation as well as "food for thought"
regarding the incompleteness of the standard and the need for advanced callouts.
Axym originally makes this claim, however, this kinda gets me excited when leaving me
with a comment that the standard is lacking without suggesting a remedy. Ok, let's agree that it does not encompass all possibilities. Now what? Anyone can
apply their advanced callout without explaining it in an amendment? Might as well go back to the old Cartesian plus/minus system
at that point or else we have arrived at the tower of Babel! No offense meant to anyone. Actually I find the standard and people's interpretations
fascinating and "thought provoking regarding designs". I'll keep it with its flaws.

 Now back to my persisting problem regarding the A-B callout on
the drawing I posted. I look at the datum definition;  plane, axis or exact point. I see that a datum reference callout in a Feature Control Frame
is for a "single" datum. I see that I cannot geometrically create a single datum for two features of size at compound angles as in the JPEG I posted.
 From the  same JPEG I believe it can be seen that a DRF cannot be determined from those two cylinders which axis are at compound angels.
Can't find the DRF origin. At this point the callout cannot meet the standards definition, unless there is an amendment to our standard here which explains
as MechNorth endeavors to do  from his point of view.

My position is that advanced type callouts can only be used when amended to explain what is lacking in the standard for that callout
and/or deviated from.

FYI, my 9th grade Biology teacher informed us that "ex or X" is the unknown, and "spert or spurt" is a drip under pressure, thus;
Expert = an unknown drip under pressure. Just kidding !

It seems like a conic for all practical purposes should and could be added to the definition of feature of size.

I think I have concluded that our in-house "consultant" could be correct regarding the A-B callout if we would explain it in an amendment.
He doesn't want to do this though.... he fears the dreaded "committee" to get it into our company standards.

Any one up to a profile callout as a "general note". We have it, and guess what? I disagree.
 

DesignBiz

"Quality is in the details"
 

RE: ASME Y14.5m application and general drafting stds

Wow, interesting thread.  I have some comments about the original post's issue of the A-B datum feature and will comment on some of the other issues later.  

I agree with MechNorth that "multiple datum feature" is the correct term, not "compound datum".

The functional situation that MechNorth described makes sense.  The design intent might be to hold the part in a fixture that contacts datum features A and B without giving one of them precedence over the other.  In other words, the degrees of freedom are simultaneously constrained by the two datum features as a group.  It's the equivalent of the scenario in Fig. 4-21 of the standard, except with datum features that are not coaxial.  The key thing is that neither datum feature lines up exactly with its simulator (neither one constrains particular degrees of freedom on its own) .  The multiple datum feature approach is useful (and necessary) in this situation, because the desired constraint wouldn't be possible if one of the datum features was referenced as primary.  For example, if datum feature A was referenced as primary, it would be obligated to constrain 4 degrees of freedom.  Very different.  Some of the posts favored the "conventional" primary-secondary-tertiary sequence over the multiple datum feature approach, just because the features are skewed.  I don't agree with this.  I agree (again) with MechNorth that the intended function should dictate how the datum features are configured.  I'm no expert on tubes, but it would seem that tubes of this type would often be aligned at both ends simultaneously and not aligned to one end first.

So is it legal to reference a multiple datum feature comprised of non-coaxial cylinders?  This is one example connected to a more fundamental question.  There seem to be two schools of thought – one says "it's not legal because the standard doesn't specifically mention it or show an example of it" and the other says "it's legal because it's a logical extension of a Y14.5 principle, there's just not a specific example shown".  In this case, I would say that it's a valid extension of principle.  Paragraph 4.5.8 reads "multiple features of size, such as a pattern of holes at MMC, may be used as a group to establish a datum when part function dictates".  It then goes on to describe and illustrate one example with a square pattern of 4 parallel holes, referenced as a secondary datum feature at MMC.  

I would say that the two non-coaxial holes are another example of multiple features of size and that we may use them as a group if part function dictates.  In the situation described above, part function dictates.
 

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: ASME Y14.5m application and general drafting stds

(OP)
Axym,

I can agree with the "logical extension" as you put it, when the logic is relative to the part function and how it
should be restrained in some cases.

In the case of a hole pattern as a referenced datum, I can determine the center of that pattern geometrically
and create a DRF that fits the definition in section 4 of the standard.  In the case of cylindrical features of size at compound
angles to each other I cannot, unless someone explains it either like yourself or MechNorth does. I would wager that
if you showed this callout to your average technically proficient GD&T person that you would not get most to understand
it based on the definitions of the standard without further explanation.

My only point is that if you do something like this, it merits an amendment to provide that explanation. Not necessarily in
the standard itself, rather an amendment by the company who owns the drawing. After all the whole idea here is for everyone
involved to understand and inspect the part in the same light.

If you cannot agree with my thinking then I suppose we can just agree that we disagree.

Thanks to everyone for their input, I would like to start a new thread on "profile".
 

DesignBiz

"Quality is in the details"
 

RE: ASME Y14.5m application and general drafting stds

Ringster, no flaw in the reasoning, just shortcomings in the standard.  In the absence of clearly defined orientations for the first two mutually perpendicular planar datums (intersecting at the datum axis), the designer arbitrarily sets the orientation.  The designer also then arbitrarily sets the orientation of the third datum plane, mutually perpendicular to the first.  These "arbitrary" orientations need to be documented on the drawing, and manifested repeatably on the simulator(s).

Again, these are extensions (distant extensions, but extensions none the less) of first principles.  DesignBiz, don't worry about documenting this in an amendment; it belongs on the drawing itself.  1.1.4 Figures: ... The absence of a figure illustrating the desired application is neither reason to assume inapplicability, nor basis for drawing rejection. ...".  The best place to communicate design (in this case, the location and orientation of datum planes) is on the drawing, not in auxiliary documentation. (I'll sketch something up and post it soon).

I was able to view the second graphic; the first didn't even list on posting (for me at least).

Evan, good reference to 4.5.8; one note on that item though, the standard currently only supports patterns of features of size at MMC, not at RFS.  That, however, can be addressed / remedied in a corporate addendum ;~}

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: ASME Y14.5m application and general drafting stds

This is an exceptionally good thread. As can be expected, some really good input from Jim.
I do want to add a question for thought with regard to the co-datums as shown in the drawing.  How is that any different then using a "pattern" of features of size as a datum feature? Where is the "origin" of a circular pattern of holes all of the same size and size tolerance (just to keep things simple). And with all due respect, does the origin of the nice circular pattern have to be shown on the drawing?  No, it doesn't.
The point is that once the 6 degress of freedom have been restricted, the location of the origin can be anywhere. Of course, as I believe Jim mentioned, there becomes an issue when referencing a tertiary datum and just exactly what that may do.
I'm looking forward to this forum. Glad I found it.

Norm Crawford
GDTP-S
Applied Geometrics Inc.
www.gdandt.com
 

RE: ASME Y14.5m application and general drafting stds

(OP)
The difference between a pattern of holes referenced a single datum and the JPEG of the tube shaded image, with 2 cylindrical features of size, at compound angles to each other (both ends of the tube run) each end having an axis and 2 planes 90 deg to each other, is:

For a hole pattern referenced as a datum, I could have lets say, 4 holes basically dimensioned 2.0" inches apart vertically and horizontally. Lets say these holes
are labeled datum B and a perpendicular surface that they intersect is labeled datum A.  I can easily see that in a Feature Control Frame if primary is A and B is secondary, that I can construct a plane aligned with A and 2 planes at right angles to each other  and that are perpendicular to A. The origin would be at the center of the hole pattern. All easy to create geometrically and fits the standard's definition for DRF (3 planes perpendicular to each other). Each datum reference block in the Feature Control Frame refers to a single datum. In the case of A, a plane. In the case of B, 2 planes 90 degrees to each other centered between the basic horizontal and vertical dimensions.

In the case of A-B as called out in the "drawing JPEG" (one thread prior to the shaded "image JPEG"), there are 2 features of size which in this case
are 2 cylinders that are at "compound angles" to each other. If a person tries to create a "single datum" from these "2 features",  because they show up in one
datum reference block, it is not geometrically possible to create a single plane, axis, or exact point from the two of them, as the standard's definition of a datum would require. Nowhere that I can find in the standard does it state that a datum reference block in Feature Control Frame can be anything else than a single datum.
 

DesignBiz

"Quality is in the details"
 

RE: ASME Y14.5m application and general drafting stds

Tks Norm.  Good to hear from you.

Here's the link for a sample of what I've been discussing:
http://www.profileservices.ca/files/tidbits/thd1103_235809.pdf

As for the differences between multiple datum features & patterns of features of size, here's a big one; patterns of features of size must (currently, per Y14.5M-1994) be referenced at MMC, and thus a boundary method of control for related features is facilitated.  In that case, one doesn't really care where the origin of measurement is because it's essentially / primarily expected to be a go/no-go gauge.  For multiple datum features, there is no such MMC requirement, and therefore the datum features could be established RFS if beneficial to the design; hard-gaging is not possible in this case.

I've encountered situations where a pattern of features (RFS) is needed to establish the datum, so it was written into a corporate addendum or other supporting documentation.  The origins of measurement were then documented on the drawing, similar to what I've shown for the tubing example.

DesignBiz, the only time that you get a single datum is when you have a single planar datum feature.  As soon as you reference a cylindrical or conical feature, you get a datum axis which is the intersection of two mutually perpendicular datum planes.  In the case of a spherical datum feature, you get a datum point, which is the intersection of three datum axes, each of which is the intersection of two mutually perpendicular datum planes.  Thus, you conjecture that one datum reference means one datum is invalid.  The number of datum planes, lines or points established by any individual datum reference within the DRF, is established by the nature of the datum feature, and the precedence in the datum reference frame.   

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: ASME Y14.5m application and general drafting stds

(OP)
Jim,

I will take a look more closely in the morning. What you show seems understandable to me. If you look at the drawing that I posted above you will see a quite different drawing. One I dont understand nor can anyone here explain it to me. Nor can anyone tell me how it is inspected from our suppliers.

Would you possibly add a FCF callout to your example?

Thank you !

DesignBiz

"Quality is in the details"
 

RE: ASME Y14.5m application and general drafting stds

DesignBiz, I quite appreciate your dilemma with your drawing.  It needs to have the clarifying details added as I've done on mine.  Also, the FCF of the general profile tolerance is inconsistent with the A-B datum used elsewhere.  While it may be legal, it doesn't show a consistency of design intent.

There's a general surface profile in the title block of my drawing, which references A-B and no other datums as they're not needed.  Adding secondary and tertiary datums would be possible, but then I'd have to exclude constrained degrees of freedom from the primary A-B.  Another topic for another day.  The general surface profile tolerance has extra boxes in the DRF because I haven't mastered SolidWorks yet, and can't get the DRF to update the size & number of cells automatically.

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: ASME Y14.5m application and general drafting stds

If a pattern of FOS is irregular (like on an oil pan), where is the center of the pattern?

As for any pattern of features of size MUST be referenced at MMC, you got me. I honestly don't recall that requirement.

If Datum feature A, being a diameter, is correctly positioned with a FCF that references A-B where as B (a FOS)also references A-B all at RFS and IDEALLY are coaxial and as if the mating part is variable in how it grasps the two diameters to establish the Datum A-B rfs axis, isn't that a pattern? And isn't that a legal specification?  I am not trying to be difficult! I know I am coming into this thread late and maybe it needs to be a new thread.
But alot of what I read I like, but some makes me wonder.

Norm

 

RE: ASME Y14.5m application and general drafting stds

Norm, agree with the point you're making re irregular pattern of FOS establishing a datum; was a long term issue for me as well until I was shown to do what I usually refer to as "surrogate" datums / measurement origins, as I've done in the drawing.  Prior to that, I'd been told a variety of silly things like find the center of area, center of mass, etc., and that there could be no origins of measurement and hard-gaging was mandatory.

Re pattern of features being at MMC, to more specific, the standard only shows and represents how to do datums based on patterns of features at MMC; patterns of features of size not being used to establish a datum presumably can be done at RFS.  It doesn't specifically preclude RFS from use on datums established by patterns of FOS, but it is in no way communicated how to handle it RFS, and as I recall even the cert exam had a question on the MMC requirement.  

I think you're right though, that the discussion of your last paragraph should be kicked into a new thread.  I'll leave that up to you; pls add a graphic too.  My brain is starting to hurt, and I like pretty pretty pictures!

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: ASME Y14.5m application and general drafting stds

Jim,
I understand the "surrogate" datum technique. Nice drawing. I see you do like pretty pictures and so do I. But you know how I feel about "drawings". Right?
Frankly though, I like that sort of drawing to be in the measurement plane and not on the part drawing. Just IMHO.
I agree with your clarification on MMC for patterns. I just wanted the group to be aware too.  I'm always careful with the "must" word.
When I have time, I may start a new thread but basically, the last paragraph is simply discussing coaxial diameters that are far appart like on an axle or drive shaft.

Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com

RE: ASME Y14.5m application and general drafting stds

(OP)
Jim,
I completely comprehend and agree with your descriptions of possible datums which can be derived from various features. I don't have a problem with the DRF concept regarding what you have mentioned.  Interestingly too, as you point out for a pattern referenced as a datum, it would require a material condition.  For me under any of these scenarios you mention, in light of section 4 of the standard, I can apply basic geometry to agree with the callouts.
My continuing problem is to see the relationship of "multiple datum features"  defined by 2 cylindrical features of size "at compound angles" to each other.  They cannot meet the standards definition for a single datum geometrically.  The big question to me at this time is how would you explain the DRF concept under these conditions?  I understand the concept for a part to be constrained by both ends of the tube in this case and how the multiple datum concept captures all degrees of freedom, however it seems as if none of the standard's DRF requirements apply to this multiple datum feature concept. What does one do with section 4 of the standard considering the multiple datum feature concept we have been discussing?
If you would read 4.1 in view of our multiple datum feature discussion, it appears quite obvious to me that this concept requires total disregard of section 4 of the standard (Datum referencing).  Even section 4.2.2 regarding positioning the part refers to three mutually perpendicular planes to "position the part".
I don't think anyone can explain this better than you have. This concept of multiple datum feature part positioning seems to merit an entirely different section.  Maybe some of you are smart enough to glean the" extension" of the DRF concept, however I am not. Simply comparing the drawing that I posted to any part of section 4 does not make sense to me. Even if a simple note such as the; "perfect form not required at MMC" and this case maybe "Multiple Datum Features position part; DRF does not apply", would help me tremendously.
Thanks for your patience.
 

DesignBiz

"Quality is in the details"
 

RE: ASME Y14.5m application and general drafting stds

No problem, DesignBiz.  You're right that the standard doesn't explicitly cover this situation; I'm not sure if the next revision does either, but we'll see this year.  After sittin in on the Y14.5-2009 meetings, it's clear that in some cases many extensions have been considered and are going to make it into the standard because they've reached consensus; in other cases, no consensus and not going into the book.  Many, maybe even most of the extensions need extremely deep knowledge of the entire standard; that usually comes from senior level certification (GDTP-S), and even then it's not absolutely clear in some cases.  You'll tend to see Dingy2, NormCrawford, and myself debate some items and corroborate on others; we're senior-level certified on the '94 standard, and we still remind each other of subtle things that easily slip our minds.  Overall, if you're not comfortable with a solution proposed in this forum, don't use it; it's you that has to explain & defend it to your company.  The easiest alternative in the tubing case is to select one cylindrical end as the primary datum feature which garners an axis with two planes, then select an end face as the secondary datum feature, gaining you the third datum plane.  A surface on a bent segment can act as your tertiary datum which will arrest rotation.   

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: ASME Y14.5m application and general drafting stds

DesignBiz,

Again I applaud your efforts to make sense of this.  Trust me, the multiple datum feature concept does agree with the DRF concepts of Section 4.  I'm enjoying the challenge of trying to explain it.  I'm glad to see that Norm has joined the forum, as I know that Norm "gets it" as does MechNorth.  Hopefully between the three of us (and others)we can put it in a form that is understandable.  Any understanding that I have of these issues has taken literally years of pondering, "aha" moments, picking apart and questioning the statements in Y14.5, and endless discussion and debate with "buddies".  

I'll reiterate my opinion that part of the problem you're having lies in the 1.3.3 definition of "datum".  Trying to make sense of datum referencing while taking 1.3.3 literally is difficult or impossible - believe me, I've tried.  To me, the theoretically exact planes, lines, and points are only understandable for simple combinations of simple datum features.  Even then, the whole idea of extracting datums from the TGC's has become a conceptual side trip for me - I don't see it as a useful bridge to get from TGC's to a datum reference frame.  

The definition in 1.3.3 also states that "a datum is the origin from which the location or geometric characteristics of features of a part are established".  This isn't generally true.  It's only true in simple cases in which the datum happens to coincide with a DRF component such as a coordinate plane or axis.  To me, the DRF itself provides the coordinate system in which measurements are taken.

More tomorrow,

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: ASME Y14.5m application and general drafting stds

(OP)
Axym,

Thanks to your perserverence, as well as Jim and Norm, I maybe having an "aha" moment. Maybe a false alarm, let's see.

All along I have had trouble with the three mutually perpendicular plane construction from the 2 features of size at compound angles. I believe Jim made and interesting statement that the designer arbitrarily orients the planes as in the case of a cylinder.

If I can take the planes established by one cylinder and then can arbitrarily pick a "point" from the axis of the other referenced datum (eg A-B), now I can geometrically create the 3 mutually perpendicular planes of a DRF. Prior to this I was trying to do this with the combination of planes and axis of the combined datums. I had not been thinking of a point as a possibility.

I am getting on track here?

DesignBiz

"Quality is in the details"
 

RE: ASME Y14.5m application and general drafting stds

DesignBiz,

In short, yes! You are on track.

Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com

RE: ASME Y14.5m application and general drafting stds

DesignBiz,

I thought about your idea of arbitrarily picking a point from the axis of the other datum, and it doesn't work for me.  But if it works for Norm and Jim as well, perhaps I need to think about it further.  I sometimes see things differently than others here because I don't come from a design background.  I spent several years doing CMM programming, in which a primary task is to take imperfect features on a real part and construct datum reference frames around them.  This means that I sometimes see things in a way that most designers don't, and sometimes miss things that are obvious to any designer!

The main problem I have with the arbitrary point datum is that the origin and clocking of the datum reference frame would also be arbitrary.  For argument's sake, let's say that the first datum axis defines the Z axis of a coordinate system.  This axis gives us the orientation of the XY plane but not its location (i.e. the Z origin).  The first axis also gives us the intersection of the XZ and YZ planes but not their clocking.  If the second axis is at compound angles to the first axis, then different points on it will result in different Z origins and different clocking.  This just doesn't sit well.  Am I missing something?

Establishing the first 2 datum planes from just one of the cylinders in the A-B reference doesn't sit well either.  
 

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: ASME Y14.5m application and general drafting stds

Evan,
The key is in simulatneous requirements. The "arbitrary" may be a bit confusing, but in reality, the origin can be as long as it remains the same for the other requirements referencing the same DRF.  So, yes! You could have more then on DRF for inspection, but one or another has to be chosen and then stick with it during that particular inspection.
I have seen enough of your posts to know that you are familiar with all of this. I think some of the wording like "arbitrary" and "point datum" may be part of the problem here.  

Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com

RE: ASME Y14.5m application and general drafting stds

Evan,
The designer arbitrarily selects, but documents that selection on the drawing as shown, and relates it back to the basic CAD geometries by means of basic dimensions.  That is the only arbitrary choice made; after that, the inspector must fixture, or establish in the CMM, the origins of measurement specified on the drawing, and then inspect back to those origins.  While I use the term arbitrary, it usually / should include discussion with the inspection department so that the selection can facilitate the inspection wherever possible.  Most CMM operators have fixturing components (rails, parallels, 1-2-3 blocks, calibration balls, etc.) somewhat permanently mounted to their CMM beds, and hopefully the designer makes use of those in the selection.

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: ASME Y14.5m application and general drafting stds

Sorry to resurrect this thread after a few days of inactivity, but there were some things that were left unresolved (in my mind anyway).  After e-reading the last few posts, I'm not sure that we're all talking about the same thing anymore.

Norm,

The reference to simultaneous requirements was a bit of a surprise.  I assume that you're using the term "simultaneous requirements" in a generic sense and not referring to section 4.5.12 and its effects.  These effects become important if DOF's are left open or if datum features are referenced at MMC (or LMC) and that isn't the case here (assuming we're still talking about the A-B reference which is RFS).  I will agree that all FCF's referencing A-B should be inspected with a consistent DRF origin.

Jim,

I agree with your description of the arbitrarily-selected DOF origin and your example drawing illustrates it well.  I'm not so sure about the statements regarding inspection though.  As to whether or not the inspector must inspect back to the origins of measurement specified on the drawing, I think it depends.  If the inspector is going to be reporting X,Y,Z coordinates for actual points on a surface or actual axis endpoints, then the origins of measurement would be relevant.  But if the inspector is only going to report deviations relative to true profile or actual values of position, then the origins of measurement are not relevant.  The numbers would be the same regardless of what origins of measurement were used.

I'm also not sure how the designer would use information about fixturing components on the CMM bed when selecting the DRF origin.  Can you give an example?  The CMM operator would still need to create a coordinate system on the datum features, and transforming to a surrogate measurement origin wouldn't involve the fixturing components.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: ASME Y14.5m application and general drafting stds

Interesting point, Evan, and quite correct.  The origins of measurement become irrelevant if you're mapping deviations from the surface.  My efforts to convert the masses to using whisker diagrams and such have largely fallen on deaf ears as most of the suppliers that I've worked with have client mandates to provide +/- size and position errors.  I've had a couple clients that had halleleujah moments and passed the blessing on to their suppliers, but most are bound by higher-ups that don't understand the idea.  C'est la vie.

Most CMMs that I've seen in use have parallels, angle blocks & such affixed to the CMM bed for dealing with small to moderate-sized parts; they are typically used as datum simulators.  Their use makes it quicker and easier to repeatably establish the datums, and actually follow the standard by using simulators rather than probing directly on the feature surface.  Providing that the fixturing used to hold the workpiece is repeatably coordinated with the on-bed blocks, you have a direct and repeatable relationship established by basic dimensions.

I've also seen tooling balls mounted to workpieces (typically larger), and subsequently used as datum simulators of sorts to establish the datums.  Also seen them used to chain back to an "origin" when using movable arm-type CMMs on larger workpieces.  Not perfect, but often more repeatable than using the workpiece surfaces directly.

I'm tired, but I think that's a relatively complete answer.(?)

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

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