【转帖】asme y14.5m application and general drafting stds

yang686526

asme y14.5m application and general drafting stds
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
eng-tips forums is member supported.
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."fff"> - george bush, washington dc, 27 october, 2003

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
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.
well said evan!
paul
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'.

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."fff"> - robert hunter

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,
if possible, pls re-post the graphic.  i'm curious what seems so offensive.  tks.
  
jim sykes, p.eng, gdtp-s
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"

ref 3d model showing tube datums for a-b
how is there a possible connection
designbiz
"quality is in the details"