【转帖】asme - where to star

yang686526

asme - where to start?
hi, i am interested in learning asme gd&t standards and obtaining a certification.  what is the best and most efficient way of achieving this?
any advice from people who have attained their certification would be highly appreciated.
i would check with any local community colleges.  also, check with technical consultants inc
asme has a "conference" once or twice a year.  this is to take the class and the test and get certified.  i don't have the direct link or information, i just know that 2 of my coworkers are going next week or first week of december.
tec-ease (
i would suggest that you get trained through someone who is a gd & t professional and i don't believe that the trainers in all colleges are certified. also get someone who knows how each symbol is measured and the standard does not reflect this.
i have attached the web site that goes directly in asme so that you might find a suitable trainer.
i think that this should help.
dave d.
dave raises an excellent point; don't assume you're instructor is qualified in gd&t just because they're teaching it.  i've seen many college and "professional" instructors that aren't certified by asme as gdtp or gdtp-s, and that leads to bad information.
one other point, you need to have 5 years of experience in gd&t before you can get certified.  if you haven't had any training yet, get started...it's a steep learning curve.
jim sykes, p.eng, gdtp-s
profile services
cad-documentation-gd&t-product development
relative to this discussion, i took the test a year ago and failed with a 73. i studied for months prior to the test using the body of knowledge as a guide and i really thought i was ready. i was deeply disappointed to discover that a large amount of the test was based on verbatim memorization of certain sections and things that i thought were inconsequential such as whether the datum "sucker" attached to the lower left or upper right. it seems that roughly half the test was actual practical application of gd&t and the rest was filler material. i spent way too much time trying to nail down the function of gd&t and not enough time on the minutiae that i didn't think mattered in the grand scheme of things. i re  
the test definitely focuses on the theory and concepts, with little in the way of applications.  from what i've seen in industry and what i've heard from veteran gd&t'ers, people hit a perception wall as soon as they see an application from outside of their specialty (e.g. a machinist may not know anything about bearings, and blanks out because of it).  as i recall, the failure rate's over 80% of those writing the senior-level exam.  
while i empathize that many of the questions seem inconsequential to the "real world", keep in mind that those subtleties are important and the successful completion means a combination of practical and theoretical knowledge.  i've worked with and debated with a number of veteran gd&t'ers, and mostly they are correct; the differences often come in the subtleties and the extension of principles that you get from knowing the standard.
jim sykes, p.eng, gdtp-s
profile services
cad-documentation-gd&t-product development
yes the amse gd & t test is pure theory and there is a high play upon words. yes there was very little practical application and absolutely nothing about measurement analysis just imaginary lines around planes, etc.
i needed the certification since i train in the subjecct so i wrote the technologist level first, passed it and finally the senior.
powerhound - work on it. i went through the standard until i couldn't look at it any more and i after i wrote the exam, my brain was fried
good luck.
dave
dave, looking back from the senior level perspective, was there value for you in writing the technologist level first?asme - where to start?
hi, i am interested in learning asme gd&t standards and obtaining a certification.  what is the best and most efficient way of achieving this?
any advice from people who have attained their certification would be highly appreciated.
i would check with any local community colleges.  also, check with technical consultants inc
asme has a "conference" once or twice a year.  this is to take the class and the test and get certified.  i don't have the direct link or information, i just know that 2 of my coworkers are going next week or first week of december.
tec-ease (
i would suggest that you get trained through someone who is a gd & t professional and i don't believe that the trainers in all colleges are certified. also get someone who knows how each symbol is measured and the standard does not reflect this.
i have attached the web site that goes directly in asme so that you might find a suitable trainer.
i think that this should help.
dave d.
dave raises an excellent point; don't assume you're instructor is qualified in gd&t just because they're teaching it.  i've seen many college and "professional" instructors that aren't certified by asme as gdtp or gdtp-s, and that leads to bad information.
one other point, you need to have 5 years of experience in gd&t before you can get certified.  if you haven't had any training yet, get started...it's a steep learning curve.
jim sykes, p.eng, gdtp-s
profile services
cad-documentation-gd&t-product development
relative to this discussion, i took the test a year ago and failed with a 73. i studied for months prior to the test using the body of knowledge as a guide and i really thought i was ready. i was deeply disappointed to discover that a large amount of the test was based on verbatim memorization of certain sections and things that i thought were inconsequential such as whether the datum "sucker" attached to the lower left or upper right. it seems that roughly half the test was actual practical application of gd&t and the rest was filler material. i spent way too much time trying to nail down the function of gd&t and not enough time on the minutiae that i didn't think mattered in the grand scheme of things. i re  
the test definitely focuses on the theory and concepts, with little in the way of applications.  from what i've seen in industry and what i've heard from veteran gd&t'ers, people hit a perception wall as soon as they see an application from outside of their specialty (e.g. a machinist may not know anything about bearings, and blanks out because of it).  as i recall, the failure rate's over 80% of those writing the senior-level exam.  
while i empathize that many of the questions seem inconsequential to the "real world", keep in mind that those subtleties are important and the successful completion means a combination of practical and theoretical knowledge.  i've worked with and debated with a number of veteran gd&t'ers, and mostly they are correct; the differences often come in the subtleties and the extension of principles that you get from knowing the standard.
jim sykes, p.eng, gdtp-s
profile services
cad-documentation-gd&t-product development
yes the amse gd & t test is pure theory and there is a high play upon words. yes there was very little practical application and absolutely nothing about measurement analysis just imaginary lines around planes, etc.
i needed the certification since i train in the subjecct so i wrote the technologist level first, passed it and finally the senior.
powerhound - work on it. i went through the standard until i couldn't look at it any more and i after i wrote the exam, my brain was fried
good luck.
dave
dave, looking back from the senior level perspective, was there value for you in writing the technologist level first?  i.e. did it help you significantly in preparing for the senior level body of knowledge?  i recall a bit of overlap, but the bok for the senior level was far more extensive and in-depth.
keep at it, powerhound.  i studied my butt off for the exam for a couple months, and did a number of practice exams todatum locationo.  good luck.
jim sykes, p.eng, gdtp-s
profile services
cad-documentation-gd&t-product development
  i.e. did it help you signifdatum location
does anyone know the proper way to add a datum point to an imaginary point?
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and just how is the imaginary point being established please?
it will be a center point(not centerline)of a closed slot.
patatmoog,
   your datum must be a real feature.  a basic rule of gd&t is that the fabricator and inspector must use your datums for fixturing.  it helps to visualize what this fixture looks like.
   the width and length of a slot can be picked up by a cross shaped pin.  this is necessary if you have a separate datum that prevents your part from rotating.  
   on your drawing, the width and the length of your slot would be two separate datums, i.e. you will need four datums to locate your part.  both datums need to be specified at mmc.  whenever i specify a slot, i make the length sloppy. sloppy features are not good feature-of-size datums.  
   if you need an x/y locating feature, you should make it round, and slot all the other sloppy features.  this makes the gd&t on your drawings, and your fabricator's and inspector's lives, easier.
   another possibility is to use your slot to control rotation as well as x and y.  this will work if your slot has an accurate width and it is long.  otherwise, this is a bad idea.  your maximum angle error in radians is your maximum clearance divided by your slot length.   
               jhg
there is no proper way to do it, because it isn't proper to do.
datums have to be real, tangible features.
v
you can add an imaginary datum to it.  
chris
solidworks/pdmworks 08 3.1
autocad 08; catia v5
this will maybe confuse, but it is a critical distinction.  a datum is a theoretical point, line (axis) or plane.  a datum feature is a real, physical feature on the workpiece, which is used to establish the datum (point, axis or plane).  the datum simulator would be the pin or key (in this case) or other representative component that would be used to represent the datum.
jim sykes, p.eng, gdtp-s
gentlemen,  
are we exercising proper descriptions in our discussion?  if not shouldn't we?
i was taught that datums are theoretical points, planes and lines.  that is rather imaginary is it not?
datum features on the other hand do exist on the parts.

ringster,
   the op is asking how to addatum placemend a datum point.  i am interpreting this as how to specify such a datum on a drawing.  my cross-shaped pin will locate the centre of a closed slot, at mmc, with the limitations noted above.
   i suppose my first paragraph is not well written.  how about...
   your datum must be defineddatum placement
is it legal to use a symmetrical centerline as a datum?
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i would say "no".
no - a datum is reflected using features on the part that would develop the centre line. how is the c/l developed? was it the outside width? hole? one needs to reflect the feature(s) that developed the c/l from a design perspective.
dave d.
i would think it would be ok.  unless i am miss-understanding the situation.  looking through the gd&t material i have aquired over the years, i think that would just be a datum plane.  in "geometric dimensioning and tolerancing" by david madsen this is shown as an ok practice.
but i could be mistaken...
asme y14.5-1994 para 4.3.2 states: "the datum feature symbol identifies physical features and shall not be applied to center lines, center planes, or axes except as defined in paras. 4.6.6 and 4.6.7.", which refer to datums defined from datum points.
the answer is nofff">.  no exceptions, "what if", or "i would think".  a datum must be a feature.  if you use a centerline, it must be the centerline of a feature.
put the datum flag on the feature.  if the feature has a centerline, then that centerline is used for certain types of callouts (position, runout).
honesty may be the best policy, but insanity is a better defense.
this is all very helpful. thanks a bunch!
ahh, but there is an exception, just not one pertinent to the op.  "...except as defined in paras. 4.6.6 and 4.6.7." is verbatim from the standard.
the only legitimate way to attach a datum symbol to a centerline is to include "axis of" or similar qualifier with it.
ewh - 4.6.6 defines establishing a datum from datum targets while 4.6.7 again establishes a datum from datum targets when there isn't a solid feature to create, as an example, a plane.
i don't think it applies in this situation.
dave d.
i agree, and stated such, with both "which refer to datums defined from datum points" and "just not one pertinent to the op".  i was just being facetious about tick's post.  i'll try to be more serious in the future.
i think this is at the heart of the debate as to whether or not symmetry even belongs in the gd&t.  sym has been dropped and picked up again, but i'm sure it will be up for reconsideration yet again next time the standard gets updated.  instead of using sym, see if their is some other gd&t method to accomplish what you wish.
matt
cad engineer/ecn analyst
silicon valley, ca
by a real feature.   
               jhg
my guess is that the orig post was to inquire how to identify a datum point.  if that is the case i believe it is covedatum planesred by the standard.
asme y14.5m-1994 should cover how and where to create datum points.
matt lorono
cad engineer/ecn analyst
silicon valley, cadatum planes
hello to everyone!
i apologize for the question....but i have never understood this issue
if i have a flatness surface (datum a) e the i use two holes for datum b and datum c, which mutually normal planes i must consider?
could you help me, please?
thanks in advance
luke
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can you tell us the sequence in the fcf? that will be the determining factor.
perhaps a sketch of your problem.

usually the primary datum is a plane and most often is called datum a. if another hole is called datum b and a third hole is called datum c, here is how it probably is referenced in the feature control frame - a|b|c.
if that is the case, then datum a is a plane and datum b is the intersecting point of 2 perpendicular planes while datum c only orients the part. now we have 3 mutually perpendicular planes - one created from datum a and 2 more from datum b.
hope this helps.
dave d.
there is a figure showing exactly dingy2 example in asme y14.5m-1994.  i don't ahve my copy but take a look.  i think it's probably in section 3 but i could be wrong.
kenat,
thanks to everyone!
last week i started to study the asme y14.5.m-19 and, with the help of this forum, i have understood the main issue.
now, when i have a drawing with three datum planes it's clear the meaning but, when i have a different drawing it's not clear the meaning for me.....
can i have always three mutually perpendicular planes also when in the drawing i have, for instance, a flatness plane (datum plane a) and two holes (datum b and datum c)?
also three holes can "fix" my part in the space but which planes i must consider like "datum planes"?
in attachment a drawing of the situation.
luke
the datum is the plan or center line that you fixture to when inspecting the dim.  it is ok to have commonly perp datums a long as there is a reason.  however, in the case of your drawing, i'm inclined to suggest against it.  if those holes are related, then they should be judged by the same process.   
matt lorono
cad engineer/ecn analyst
silicon valley, ca
i referenced the wrong section, though i would have hoped you could have found it looking through 14.5.  figures 4-8 & 4-9 and more importantly the relevant paragraphs (especially 4.4.3) explain what i understand your original question to be.
on your sketch though, the holes aren't perpendicular to your primary 'a' datum which changes things.  in fact, as set up i don't believe your datum structure works.  you'd have to look at it again based on function to try and determine what's really required.
kenat,
i think that i have just a little confusion because i have never understood what a datum plane rappresents. are they determinated by the function or by the inspection method?
which datum planes (secondary and thirtiary) i can use if i have a surface with only a little flatness face?
in addition.....(i don't have a big practical experience about it)....for every part i produce should i have a specific tool that simulate the datum planes re-called in the drawing part?
is also for this reason that is better to have three mutually perpendicular planes?(reduce cost linked to a lower number of tools)
thanks
luke
the primary datum plant on a noncylindrical part is its mounting surface when assembled. the secondary and tertiary datums are usually holes since they are more important to the assembly than sides. the secondary hole should be the first operation in the assembly. the part mounts on datum a and a bolt or shaft is next installed in a hole. that hole will be the secondary datum. all dimensions will come from that hole since it is the intersecting point of 2 perpendicular planes. the tertiary datum is probably another hole used in assembly and it only orients the part.
if you part does not have holes used for assembly, then the primary datum is the largest surface while the secondary ddatum reference frames fo composite tolerencesatum will be the longest side and the tertiary datum will be the end of the part.   
dave d.
dingy2,
a minor clarification, if i mdatum reference frames fo composite tolerences
question: can you have the following callout? first tier - profile of a surface w/in .030 to a-b-c. second tier - profile of a line w/in .010 to a-b-c.
i believe that the second tierwould cancel out the top. please give me your thoughts.
tim martinez
anaheim, ca
find a job or post a job opening
since you are calling out datums on the profile of a line tolerance, you are overriding the profile of a surface callout.  with datums, the entire surface is being controlled by location (position) to the specified datums by the profile of a line callout.  without datums, profile tolerances act as form controls.  you could omit the datums on the profile of a line callout and have a valid callout.
marcelino vigil
gdtp t-0377
what we have is an extrusion shape. the entire surface needs to be w/in .030 to a-b-c and each section cut line of inspection points needs to be w/in .010 holding the same datum orientation but not locked to position. how would this be called out?
profile of surface w/in .030 to a-b-c
profile of a line w/in .010 to ?
which datums do we need to use to lock the rotation but allow it to move positionally?
thanks in advance,
tim martinez
profile of a surface is three dimensional while profile of a line is two dimensional.
i can see the possibility of profile of a surface to a,b & c which controls the shape and location of the surface and is usually perpendicular to datum a.  
profile of a line is two dimensional and controls the shape of the shape and the location of the feature at its edge. one usually should never see profile of a line referencing three datums.
yes, one could see profile of a surface .030 to a,b and c while the second level is profile of a line .010 (component of profile of a surface) to b & c. (not datum a).
   
dave d.
tim,  
what you are describing sounds like exactly what this gd&t tip addresses. go to this link and study what it says and it would be a good idea to bookmark the link too. there's a lot of good information there. from what i can see, there is no need for a "profile of a line" specification. just refining the lower tier as required while still using the profile of a surface callout.
ay, to your previous.  two planes will intersect in a line and not a point.  
however, the line (secondary datum) is the axis of the hole thru which a set of mutually perpendicular planes may be passed.
ringster:datum reference style old  new on y14.5m standard drawing
right on - i stand corrected since it is an an "axis" per 4.4.2 page 55 rather than a point.
dave d.
datum reference style old & new on y14.5m standard drawing?
we are using asme y14.5m-1994 (r2004) on our detail drawings and i have a question.  i am being told that the standard says one can use the old style datum reference (i.e.: -a-) under the new standard.  is this true?  i have not been able to find this statement in the standard yet.
gary ashby
designer
ug/nx v8 thru nx5
solidworks 2007
autocad 2002 & 2008
4mranch1,
   i cannot find it either.  the change history notes that they changed the datum feature symbols over to the iso version as part of the 1995 revision.  perhaps they missed one.
   i looked at the start of chapter_4, as well as at section_3.2.2.
                           jhg
a5 seems pretty definite about "the universal (iso) datum feature symbol is adopted and replaces the previous one."
and even in the body it doesn't use the weak 'should' which allows people so minded to ignore the intent of the standard.
while i can't be 100% sure i'm tempted to say you're being told wrong.
kenat, probably the least qualified checker you'll ever meet...
i cannot find anything in the standard stating that the old datum symbol can be used and i have studied the standard quite extensively.
i would suggest using the new one if you state in your drawing "complies to asme y14.5m-94".
dave d.
thanks all for your input on this. i pointed out to this individual that our drawing titleblock stated "interpret drawing per asme y14.5m-1994 (r2004)", but he insisted that it was an acceptable practice.  i was thrown by the statement and came to this forum for some back-up.
if anyone can find anything that would make this practice allowable please let me know so i can confront this mis-statement with confidence.
gary ashby
designer
ug/nx v8 thru nx5
solidworks 2007
autocad 2002 & 2008
turn it around on the offender, ask him to show you where in the standard it says it is permissable.  you could back this up by showing all the places it says to use the iso symbol.
of course politically this may not be a good idea but if i was good at politics i'd be a manager by now!
kenat, probably the least qualified checker you'll ever meet...
"acceptable practice" and following standards are not the same. that means he is saying "i don't know the standard and don't care, this is how i do it".
stick to the new standard.
i don't know if anywhere it says the old standard is acceptable in the new standard.
tell him that the next time he gets the wrong part replaced in his car that it is "acceptable practice".
chris
solidworks/pdmworks 08 3.1
autocad 06/08
appendix a touches on, and appendix d (neither being part of the asme y14.5m-1994 standard) illustrates the former practice. these are the only references i noticed between the asme front and back covers.
due to cad system nueances when it came to switching standards, i have occasionally left them on the print.
weavedreamer,
do i understand that you have changed an existing drawing that was prepared to an earlier version, and updated to the 1994 version?.  if so, what is the value added and what statement is made on the drawing with regards to the 'prepared in acccordance with...' note?
if nothing else the forum posters here sure cover the bases.  thanks all for your help.  i will address this person with diplomatic finess and see if i can turn this injustice around.
gary ashby
designer
ug/nx v8 thru nx5
solidworks 2007
autocad 2002 & 2008
ringman,
the old, 'make it the same as, except' routine. the note claims in accordance with asme y14.5m-1994, while the biggest value added is the composite callout clarity provided by the 1994 standard over the 1982 compendium.
when addressing the feature of size, i've used a dimension of the feature showing the -datum- rather than the dimension for repeats. technically, the datum callouts should have been changed.
a paper written on the subject of going from 1982->1994 was written by the president of eti:
items to consider when converting a drawing from the
ansi y14.4m – 1982 to the asme y14.5 m – 1994 standards
by alex krulikowski
5/1/1998
many companies have switched from using the 1982 version of y14.5 to the 1994 version of y14.5. making new drawings to the standards is challenging, but there are many training courses that help the designer to use the new standard. when it comes to converting an existing drawing made to the 1982 asme standard to the 1994 asme standard there are not many (or any) materials to guide the designer.
hundreds of thousands of drawings are in existence, many are in accordance with the 1982 asme standard and may need to be updated. some designers think that simply changing the note that specifies which standard applies is enough, but that idea is a huge oversimplification. many subtleties must be addressed or the updated drawing will describe a different part.
if you want a drawing made to the 1994 version of the standard to say exactly what the drawing said when it was presented in the 1982 standard, the following items should be considered:
1. update the note that invokes the dimensioning and tolerancing standards.
2. determine if any detail dimensions need to be repeated on assembly drawings.
3. review the part configuration to determine if any shapes that were features of size now become features and need additional controls.
4. revise the datum specifications to the new datum symbol.
5. evaluate the radius specifications to determine if they should be controlled radii.
6. evaluate position tolerances for correct modifier specifications.
7. update projected tolerance zone specifications.
8. evaluate multiple single-segment and composite feature control frames to ensure their interpretation is as intended.
9. evaluate the use of a "boundary" note on elongated holes to remove axis interpretation.
10. evaluate composite profile tolerances to determine if orientation-only interpretation is desired.
11. in profile applications, replace the note for "between points" with the symbol for between.
12. evaluate concentricity callouts for desired part requirements; they may need to be replaced with position callouts.
13. determine if separate gaging should be specified for some position and/or profile specifications.
14. determine if the free state modifier should be specified.
15. evaluate orientation controls for application of the tangent plane modifier.
  
note: the explanations below assume the reader has a basic understanding of the y14.5 1982 & 1994 standards.
explanation of items
1. update the note that invokes the dimensioning and tolerancing standards.
updating the standards referenced note is probably the most obvious change that needs to be done. the note that specifies which standards applies to the drawing needs to be updated to read, "per asme y14.5m – 1994."
2. determine if any detail dimensions need to be repeated on assembly drawings.
the y14.5 1982 version did not specify if a detail dimension applies on an assembly of the detail. some companies used the interpretation of the standard to be that the dimensions on the detail drawing also applied at the up assemblies of the drawing. the y14.5 – 1994 version states that a dimension only applies at the drawing level in which it is specified. if a dimension is to apply at the assembly level, the dimension must be stated on the assembly drawing (not as a reference dimension).

3. review the part configuration to determine if any shapes that were features of size now become features and need addition controls.

in the 1994 version of y14.5, the definition of a feature of size was revised to require that the sides of a parallel plane feature of size are opposed. all dimensions that were feature of size dimensions on parallel plane features of size in the 1982 standard may not be features of size when the 1994 definition is applied. the major impact is the automatic application of rule #1. additional controls may need to be applied to control the form of part features that were automatically covered by rule #1 in the 1982 standard.
4. revise the datum specifications to the new datum symbol.
the datum symbol has been revised in the 1994 standard. it now matches the datum symbol from the iso standards. particular attention needs to be paid to the location of the triangle when converting datum identification symbols.

5. evaluate the radius specifications to determine if they should be controlled radii.
the definition of a radius has been revised in the 1994 version of y14.5, and a new controlled radius symbol has been added. a radius specified on a drawing to the 1982 standard is actually a controlled radius on a drawing to the 1994 standard.
6. evaluate position tolerances for correct modifier specifications.
rule #2 has been revised in the 1994 version of y14.5. it now states that all geometric controls apply rfs, unless otherwise specified. in the 1982 version of y14.5, rfs was specified in certain cases. each drawing should be reviewed and updated accordingly. rfs symbols may need to be removed from the position callouts.

7. update projected tolerance zone specifications.
in the 1994 version of y14.5, the way a projected tolerance zone is specified has been revised. each position symbol on the drawing should be reviewed to determine if it needs to be updated.
8. evaluate multiple single-segment and composite feature control frames to ensure their interpretation is as intended.
in the 1982 version of y14.5, the rules for composite positional tolerances in industry were interpreted two different ways. the 1994 version of y14.5 eliminates one of the ways composite tolerancing was interpreted in industry. each position symbol on the drawing should be reviewed to determine if it needs to be updated.

9. evaluate the use of a "boundary" note on elongated holes to remove axis interpretation.
in the 1982 version of y14.5, the tolerance zone for elongated holes was interpreted two different ways – axis and boundary. the use of the word "boundary" limits the interpretation of the tolerance zone of elongated holes to the boundary interpretation. each application of an elongated hole should be reviewed to determine if the tolerancing is correct.
10. evaluate composite profile tolerances to determine if orientation-only interpretation is desired.
in the 1994 version of y14.5, a definition for composite profile tolerances was added. it explains the use of the lower segment of a composite profile callout to be orientation-only. each composite profile used on a drawing based on the 1982 standards should be reviewed to determine if it is acceptable to use the orientation only-interpretation.

11. in profile applications, replace the note for "between points" with the symbol for between.
in the 1994 version of y14.5, a new symbol was added to specify that a tolerance applies between two points. it replaces the word "between," which is commonly used on drawings made to the 1982 version of y14.5. when converting the drawing, the new symbol should be used to indicate between.
12. evaluate concentricity callouts for desired part requirements; they may need to be replaced with position callouts.
in the 1994 version of y14.5, the definition of concentricity has been revised. concentricity now applies to the mid-point of a two-point measurement of the toleranced feature. if a concentricity symbol was used on a drawing made to the 1982 standards, it should be reviewed to determine if position rfs should be used on the drawing made to the 1994 standards.
13. determine if a separate requirement (gaging) should be specified for some position and/or profile specifications.
the rule for when a simultaneous requirement (gaging) applies has been expanded. it now applies to single features, and both position and profile callouts. when a drawing is converted, position and profile callouts should be evaluated to determine if any separate requirements automatically change to simultaneous requirements.
14. determine if a free state modifier should be specified.
in the 1994 version of y14.5, a new symbol has been added for free state. this symbol would only be used if a drawing contains a restraint note. the use of this symbol would most likely be replacing notes from an earlier drawing.
15. evaluate orientation controls for application of the tangent plane modifier.
in the 1994 version of y14.5,datum selection based on manufacturing or upper level assemb a new symbol has been added for tangent plane. this symbol actually allows more tolerance when it is used. if the flatness of a feature that is toleranced with angularity, perpendicularity, or parallelism does not have to be held to the same tolerance of the orientation datum selection based on manufacturing or upper level assemb
just wanted an opinion on this one...
this is my thought on the subject.
plate with a hole in the center.
the center hole is created by holding the bottom surface and 2 sides.
the plate is assembled by placing a bolt to the hole and clamping against the large bottom surface
i would do the following:
bottom of the plate has primary
the hole as secondary
possibly one edge has tertiary (if needed)
some people follow the manufacturing method.
bottom as primary
side as secondary
side as tertiary
any thoughts on this??
if the hole is effectively the critical feature i'd probably use it.  
we have cassettes that are located by a pin through a hole.  on the drawings of the cassettes we use the hole as the secondary datum if i recall correctly.
function/inspection of the part is more important in the dimensioning scheme than manufacture.
you could perhaps make the face and sides datums a, b & c then make the hole datum d and relate other features to it.  depends on the function/design of the specific part though.
but....  the datums should be defined by part functionality.
not in an attempt to mimic the manufacturing method.
that was the point i was trying to make and didn't mean to imply otherwise.
hierarchy in dimensioning is:
function
inspection
manufacture
of course as there's no point putting a requirement you can't inspect or otherwise verify it almost ranks equal to function.
you are correct by using the hole as a secondary and the mounting face as the primary datum. if the part is assemetrical, then a tertiary is a must and probably another hole should be used rather than a side.
manufacturing must somehow follow your datum set and not change it.
dave d.
i use the datums based on how the parts are assembled to each other, not per manufacturing.
chris
solidworks 06 5.1/pdmworks 06
autocad 06
datums should always be developed by the function of the part. that is it!!
dave d.datum setup
of course!
but you need to look at how the part is assembled in relation to a mating part ... not to a machine.
it is all i was trying to saydatum setup
it seems there are a lot of problems on the attached print, but i will focus on the datum callout. here is my understanding on the designer's intent:
the bottom surface is primary datum a
datum b is a pilot location hole
datum c is another pilot location hole which is established base on datum b
b-c create the axis orientation
datum b will be the original point for cmm measuring.
my questions are:
1.    can we allow two datum symbols (datum a and datum d) on a coplanar surface?
2.    is it a right way on datum c callout?
3.    which one is correct on the datum reference frame callout
primary datum a -> b-c -> datum b
primary datum a -> datum b -> b-c
primary datum a -> b-c (without tertiary datum)
thanks for all comments
seasonlee

i reviewed you drawing and:
1 each surface could be the datum but having a datum a on one surface and a datum d on the other surface does not make sense. one needs a phantom line between both surfaces and then both are datum a. your drawing in not quite correct here.
2 datum c should be reflected in a feature control frame referencing datum a (bottom surfaces) and datum b (hole) at mmc. the .000 tolerance is fine. it just states that the size of the mmc and the virtual condition size are the same.
3  if you change the bottom surfaces to datum a (not a & d), the reference datums should be a| b mmc| c mmc. when one has a hole as the secondary datum, it is the intersecting point of 2 perpendicular planes. all dimensions come from datum b while datum c is only to orient the part (square it up).
hope this helps.
dave d.
dingy,
i believe that 2 planes intersect in a line, not a point.
"dual datums" like "b-c" are only appropriate for runout.  not appropriate for position or profile.  use |a|b|c| for primary|secondary|tertiary.  the implication is that b positions the part and c clocks the part into position.
|a| and |d| can be individual datums because hey are separately identifiable features.  inspectors will need to block part to ensure part does not rest on wrong portion.  better to use datum targets to define datums with more clarity.
why is there a |d| datum?  it's not used for anything.
thanks for all comments.
when should we need to use dual datum like "b-c"?  where can i find it in the standard y14.5m ?
i believe the designer intent to specify the profile tolerance .020 is to control the flatness after forming, there is a note "two surfaces" below the profile control, so it must be a coplanar surface,i agree datum d should be deleted.
seasonlee
ringster:
i agree with that statement about line rather than point. actually, the standard states "axis".
thetick:
one can have 2 planes as a primary datum. one surface could be datum a while the other surface is datum b. when referring to the primary datum in a positional tolerance, one would state "a-b".
it all depends on how the part mounts on the mating part. above, it the part contacts on 2 surfaces at the same time - thus, datum a & datum b. if the part only contacts on 1 of the surfaces, then we only have a datum a.
  
dave d.
seasonlee,
multiple datum features is defined per 4.5.7 - 4.5.7.1 - 4.5.7.2
fig 4-19 shows a mdf with tolerance of position.
fig 6-21 illustrates a mdf with surface profile.
fig 6-49 & 6-51 shows a mdf with runout and total runout.
i found fig 4-8 in the standard y14.5m is quite similar as the one i posted earlier, the datum reference frame should be i a i b mmc i c mmc i for sure.
thetick :
1.    would you please advise where i can find out the rule of "dual datums like b-c are only appropriate for runout" in y14.5m.
2.    dual datums b-c not appropriate for position or profile. this makes me recall one part i inspected two years ago, all profile tolerance with dual datum b-c in the drf as shown on the attached
all comments will be appreciated
seasonleedatum shift on a restrained gdt par

the example you just posted showing datums b & c as a feature of size taken from theoretical intersections makes complience with 4.3 quite the stretch of the imagination.datum shift on a restrained gd&t part
datum shift on a restrained part - can you legally use 鈥淐heck in a restrained condition鈥?note with a profile or position callout with mmc (shift)?  can you have shift on a restrained part? sounds like an oxymoron or contradiction in terms at first look.  is there a way to restrain a part and shift at the same time with fixtures (like clamping on a movable plate) or in ccm?  
all parts are checked or confirmed in a "free state" unless otherwise noted as per asme y14.5m-94 .
yes, you legally can check a profile tolerance in a restrained condition but one should note the restrained force on the drawing and where the part is to be restrained which will most likely  be on the datum targets.
but
if you have positional with both the tolerance and the datums in mmc, do not use a restrained state since the part should be allowed to float.this condition simulates assembly.
if positional tolerance are in rfs, then restrained state could be used.
dave d.
i don't like my original answer in one area.
i keep thinking about a thin part with positional tolerances at mmc including the datums. the part could have an "oil canning" situation where the centre is not quite flat.
once the part is placed in a checking fixture the person confirming the positional toleraces would push the part flat and the insert the positional checking pins simulating an assembled condition. the company in this situation stated on the drawing that the positional tolerances are to be checked in an "assembled condition" but make sure the customer agrees with this. it this note was not stated, the part is confirmed in a free state.
i feel better about this answer.
hope it helps.
dave d.
we inspect composite parts in the restrained condition, and note such on the part drawing.  if we didn't, the part springback would cause many parts to fail, even though they are good parts when assembled.
can you have datum shift on a restrained part?  short answer - yes.
if a non-rigid part needs to be clamped into a fixture (i.e. restrained), there can still be clearance on one or more datum features (e.g. the part's holes on the fixture pins).  this means that the part could be clamped into the fixture in more than one position.  if the optimal position is not found the first time, the operator can unclamp the part, shift it, and reclamp.  this accomplishes the datum shift - the part does not have to "freely float" on the fixture.
evan janeshewski
axymetrix quality engineering inc.

as to the illustration posted 07feb09, the b(m)-c(m) is legal, but does not provide 'predictable' repeatability.
see
if dual datum b(m)-c(m) is legal in the post dated at 2-7-09, may i ask what is the difference with this drf a i b (m) i c (m) ?
thanks again
seasonlee
.
chris
solidworks 06 5.1/pdmworks 06
autocad 06
theedudenator,
what is it that you are looking for thoughts on?

  datum shif
control, the tangent plane modifier may be a worthwhile consideration. (the use of a tangent plane modifier would not be typical in a drawing conversion, but its use may reduce cost.)
conclusion datum shift ?
i have one dwg which has 3 holes on the plate.
in a dwg, they defined a
datum point a1, a2,a3 as the centre point of the hole.
datum b is defined as centre plane of one of the hole
in a feature controlled frame for datum b
   block 1 has 0.5 positional tolerance with 0.5 mmc
   block 2 has mentioned datum a
datum c defined as centre plane of one of the hole & there is attached featured controlled frame
     blcok 1 (in feature control frame) has 0.5 mmc of     positional tolerance
     in block 2 there is only mentioned datum a
     block 3 has datume b with mmc condition
now my question is
1, there is nothing like datum a defined but only the datum points a1, a2, a3  so how could i define the datum b& datum c using reference from datum a?
2. while defining the datum c ; what does the mean of block 3 in feature controlled frame " datume b with mmc condition" ( i think it is a datum shift - but not sure about the understanding or how to calculate tolerance stack)
could you explain me in detail.
thank you very much in advance,
  datums b and c do not appear to be correctly defined.  what exactly is "the center plane" of a hole.  how is it oriented?  how is it inspected?  the part may be acceptably dimensioned if it weren't refering to those center planes.  datums b and c should be the hole features, not some arbitrary planes along their centerlines.
  if your part consisted of irregular surfaces (as opposed to a flat plate), then datums b and c need to reflect flat planes.  datum a is correct in as it defines the primary datum plane consisting of three points.  datum b should be the secondary datum consisting of two points, and should be perpendiculat to datum a.  datum c should be the tertiary datum, consisting of one point and mutually perpendicular to datums a and b.
  as to your second question, datum c is being defined as having 0.5 mmc relative to datum a (regardless of the material condition of datum a) and datum b (at mmc of datum b).  this allows additional tolerance to be added to the positional tolerance of datum c as datum b approaches lmc.
   here is how i would approach it (others may have better suggestions).  if your part is a flat plate, and you are only trying to control the hole pattern and aren't concerned with its location relative to the part edges, i would define datum a as one of the surfaces.  make one hole datum b and control its perpendicularity to datum a.  make the second hole datum c, true position relative to datums a and b.  the third hole would be true position to datums a, b and c.
  a couple of additional comments...
  datum a would have no modifiers as it is a plane.
  datums b and c can be defined using the points that established datum a because those points in and of themselves are not datums.  datums b and c are relative to the plane established by those points, not the points themselves.
having datums a1, a2, a3 is confusing. i have never seen that before. which one is the primary datum?
chris
sr. mechanical designer, cad
solidworks 2005 sp0.1
pts a1, a2, and a3 are not datums.  they define datum plane a.
thanks for clarifying
chris
sr. mechanical designer, cad
solidworks 2005 sp0.1
thank you very much for your post,
let me define datum again as i did little bit mistake last time:
datum points a1, a2 & a3 are the centre point of hole
but, all the holes are at different level (means at different height)
datum b is attached the hole dia of one of the hole   
   block 1 has perpendicular toelrance
   block 2 has diametric symbol, 0.5 with mmc
   block 3 has mentioned datum a
datum c is attched to the one of the hold dia
     block 1 has positional tolerance symbol
     blcok 1 (in feature control frame) has 0.5 mmc     
     block 3 there is only mentioned datum a
     block 3 has datume b with mmc condition
1. now, my question is as you mentioned that datum b is perpendicular to the datum a which is not the case b'coz all datum points a1, a2 & a3 are at different height ?
2. generally, i didn't see modifier with primary datum plane - does any reason for that?
3. also, if you able to understand the datum b & c  - could u explain & also let me know if they are appropriate?
4. i understand primary, secondary & tertiary datum plane, but when i see the product its difficult to say anything so if you could give info about how to recognize datum & how to define datums while designing?
thank you,
that person has used their imagination of how the gd&t works instead of being actually knowledgable and fluent. i see this silly stuff all the time. found the best approach is ask them to point out the relevant sections of the asme y14.5-1994 book that justifies their technique of application, which they can't do. usually i find if they even have a copy of the asme book it's sitting on a bookshelf collecting dust but otherwise in pristine condition from never having been opened. unfortunately applying it poorly is apt to be more confusing then not using the gd&t system at all.
-keith
  i have to agree with type26owner that this tolerancing scheme is apt to add confusion.
  if datum hole b is to be perpendicular to datum a, and the points defining datum a are at differing heights on your part, then it cannot be normal (perpendicular) to the part surface.  this means that your holes are angled relative to the part surface.
  primary datum planes do not require a modifier.  modifiers are limited to features of size, such as your datums b and c.  your datum a is not a feature of size, but a perfect plane through 3 points.
  if the intent is to have the holes skewed, then datums b and c are called out properly.
  the best approach to defining datums while designing is to use the primary, secondary and tertiary scheme, while considering how the part will be made and inspected.  it is also very important to recognize the function of the part itself.  if you establish a, b and c using features that are easily made and inspected, you can always add additional datums to control features which are critical to the parts function (such as controlling runout on a drive shaft or the position of mounting holes relative to a pattern center).
  i hope this helps you to understand gd&t a little better.  the best way to understand it is to use it, either with hands-on machining experience or inspection experience.  design experience is also a good way to learn it, as we don't all have the opportunity to get our hands dirty.
i don't know if you folks are aware of the appendix e section of the asme book. there is an extremely handy condensed tool there that's called the 'decision diagrams for geometric control'. it should help here somewhat. suggest you peruse fig e-7 for the essence of datum selections.
i've copied those seven pages and have them hung up above my monitor because i got tired of opening up the book.
-keith
par1,
   let's see if i can interpret this datum scheme properly and work out an inspection fixture.
   datum a is a plane defined by the three datum targets.  a1, a2 and a3 are called datum targets. these would be modeled by three pins, slightly bigger than your holes,  sticking up out of a base, each with its own elevation apparently.  your part would sit on top of them, oriented to tdatum splihe same orthogonal coordinates as your drawing.
   datum b is the hole.  since it is inclined with respect to datum a, the actual datum point is the intersection of the hole's cylinder with the datum plane.  this is actually not an issue, since a vertical pin models it correctly.  this locates your part in x and y.datum split
i have a base plate where the bottom of it is datum -a-.
there is a groove along the entire length of the bottom of the base plate.
can it be implied that datum -a- is the total bottom face, as if the groove is not there, or do i need to deal with it as two bottom faces?
whatever surfaces are implied or dimensioned with that datum indicating it, that is the surface (or plane) that the datum references.
the surfaces on both sides of the groove will be the datum -a-, unless there is some offset in your design.
the groove surface is separate and can have a separate datum referencing other features/dims.
a picture may be more useful.
chris
solidworks/pdmworks 08 3.1
autocad 08; catia v5
assuming you're working to asme y14.5m-1994 look at section 4.5.7.1 & figure 4-20.
if the groove is relatively narrow you may get by without making the distinction since 4.5.7.1 does say separate identification if the 'groove' in your case is of "of sufficient width".
another option is to show an extension line across the groove but this may not work for a narrow groove.
i've sometimes seen a note "2 surfaces" placed next to the datum id to clarify that the datum is derived from the 2 surfaces but i'm not sure this explicitly in the standard.   
kenat,
place a phantom line across the groove to both surfaces which indicates that it is now considered one surface. that's all.
dave d.
dingy, i nearly suggested a phantom line and it's something i've done before but noticed that 14.5 says 'extension line'.  
per asmm y14.2m-1992 section 2.8 extension lines are 'solid lines' (though with short visible gap from the part out line asme y14.5m-1994 1.7.1.4).
a phantom would probably work, but if you'r a stickler may not be correct.
also if it truely is a narrow groove then neither phantom or extension will work well unless you do a larger scale view or detail showing the groove.
kenat,
kinat:
i don't know where in 14.5 stating a extension line should be used and 1.7.1.4 reflects crossing dimension lines.
i would suggest that you go to page 172 and 173 fig. 6.20 and 6.21 where it does show a coplaner condition and a phantom line is used in both examples.
dave d.
dingy, the reference i gave in my first post is where it states to use extension lines, 4.5.7.1.
penultimate sentence "where appropriate, an extension line may be used to indicate a continuation of one datum feature across slots or obstructions."
the examples you give with phantom lines are talking about using profile tolerance for coplanar surfaces, not explicitly about simulation of a single datum plane.  also, as has come up before, 1.1.4 means we can't overly rely on just figures.
i was just trying to share something i'd noticed.
  
kenat,
extension line
good to know you got shoes to wear when you find the floor."fff"> - robert hunter

some additional info on datums:
kenat is correct that 4.5.7.1, which specifically addresses this situation with respect to datum definition, says to use an extension line. he is further correct that y14.2m say that an extension line is solid.
however, dave is correct that the standard "shows" in figures 6.20 and 6.21 that a phantom line should be used. datum target -- help
my opinion in this is that you should defer to the section 1.1.4 that states that the figures are only intended as illustrations of the text. therefore the text in 4.5.7.1 is correct and the figures are wrong. though i would probably use a phantom line for clarity's sake regardless of datum target -- help?
asme y14.5 did not tell how to dimension the datum targets. i tried to tell people it should be dimensioned relative to each other. but they continue to dimension it from part features. would anyone know where to find such guideline except "datum targets are ... dimensioned relative to each other" - drawing requirements manual for departments of defense and commerce, 3rd edition.
thanks.
eng-tips forums is member supported.
for what it's worth (little), it is also in the 5th edition of the drawing requirements manual
y14.5-1994 is vague on this question.
believe it if you need it or leave it if you dare.fff"> - robert hunter
datum targets typically describe how a fixture will contact the raw casting, stamping, or forging. that processing fixture or its equivalent inspection or assembly replicate is used to orient, align, and locate the rigid rough structure or pliable elastic structure for first cuts etc. they are also used to provide stack path links to the rf structure or define functional clamped assembly evaluation of the finished contour. the finished feature alignments, orientations and locations are dependent upon how the structure registers with those targets not visa-versa... therefore the targets should be defined in a system 3-2-1, or 4,1,1 or pliable 6,2,1 or whatever the functional or surrogate system is within its own datum structure so that it can be defined without subsequent feature process and predicted stack variation and so it can be simply used to create processing, inspection and/or assembly fixtures!!!
some designers reuse secondary or tertiary datum targets in subsequent processed operations thinking that the registry is equivalent and disregarding the surface's form effects in their stacks but that mistake can bite you if those actual errors are significant.
bottom line... define the targets amongst themselves...tolerance the processed features from them...and then "as functionally reflective" create "tolerance" other functional datum features.
paul   
wow paul, my head is spinning from all that complex intermingling of vocabulary... i'm fluent in the english language and i barely understood any of that. i don't think dho's mother tongue is english so he/she is likely to skip it altogether.
were you saying that he is right and "they" are wrong? if so then i would agree that the datum targets should be relative to each other and not based on the location of any features that the part may have.
  
powerhound, gdtp t-0419
production supervisor
inventor 2008
mastercam x2
smartcam 11.1
ssg, u.s. army
taji, iraq oif ii
datum targets are dimensioned from each datum. you must have a minimum of 3 mutually perpendicular planes to create a datum reference frame. the datum reference frame is theoretical (doesn’t exist on part), so we simulate the datum’s from actual features of the parts.
simple example: a rectangular shaped casting.
datum targets a1, a2, a3 are all on datum –A- (shown in a view perpendicular to –A-),
they and are dimensioned from –B- & -c- (shown in views perpendicular to –B- & -c-).
note: all datum target dimensions will be basic (boxed dimensions with no tolerance).
datum targets b4 & b5 are on –B- and dimensioned from –A- & -c- as noted above.
datum target c6 is on –C- and dimensioned from –A- & -b- as noted above.
complex example: a casting with 3d contour and no flat planes.
actually the same process, except datum target points will also have a dimension from the datum they simulate.
datum targets a1, a2, a3 are all at different distances from datum –A-, the dimensions will be shown in views perpendicular to –A-, -b- and –C-.
the same process is used for b4, b5 & c6.
the datum target points are then used by manufacturing & inspection for the production of the product.
thanks to all who are helping me.
this is the part "they" assigned all datum targets. the problems here are 1) they missed z dimension for tp2a and tp1c. 2) tp1a, tp2a, tp3a and tp1c are going to "define" datum plane a. (see attachment)
1) yes. the z can be calcualted from the norminal dimensions. but it should not.
2) a physical part "sits" on the fixture will never touch those four point as the same time. only three.
comments?
i don't understand how there can be two datum a's.  the datum points a1-a3 define datum a, and it is hard to imagine how it can be located .250" from itself.
if you do indeed have a planar surface for datum a, why use datum points to redefine it as shown?
believe it if you need it or leave it if you dare.fff"> - robert hunter
there is only one datum -a-. tp3a is on a stepped surface. see asme y14.5 para 4.6.3.1.
thanks.
what are "they" trying to achieve?  cause this sorry, is a mess.
the example shows both tp2a and tp3a as being offset, but i'm skeptical that they could be considered as being on a stepped surface as shown in the y14.5 example.
i could be easily wrong though (and have been) as i haven't used stepped datums such as this before.
it seems to be a very awkward datum setup.
believe it if you need it or leave it if you dare.fff"> - robert hunter
a picture is worth...anyhow, here is a suggestion.
small changes:
1. all datum points are dim'ed with basic box (no tolerance).
2. added other dim's for complete definition of part.
datum changes: (there are a tdatum target areason of different ways, and the product desired end result drives the requirements, but, try to keep it simple, and, more importantly, easy to produce & inspect.)
1. put a1 & a2 on same plane. put a3 on center of shaft.
3 pt's will always align. datum target areas
hello,
i am trying to use datum target areas to define primary datum a on a sheet metal part. i have some questions about datum target areas.
1. can i choose three non-co planar surfaces as my datum target areas?
2. can i and how do i dimension and tolerance the distance between those data target surfaces?
thanks a lot in advance.
eng-tips forums is member supported.
you can dimension between parallel, non-coplanar surfaces to create what is called a "stepped datum".  the dimensions between are basic, with no tolerance.  this would be applied by having a fixture that touches the part with posts at the various heights.
ye368,
first, do you have a copy of asme y14.5m?  the spec goes into great detail how to define datum targets, the associated rules and possible applications.  in general, datum targets designate specific points, lines or ares of contact on a part that are used in establishing a datum reference frame.  these are located on a drawing with a datum target symbol.
datum target points - is indicated by the symbol "x" which is dimensionally (basic dimensions) located on a direct view of the surface.
datum target line - is indicated by the symbol "x" on an edge view of the surface, a phantom line on the direct view or both. the length of the line must be controlled, its length and location are dimensioned (basic dimensions)
datum target areas - where it is determined that an area or areas of flat contact is necessary to ssure estblishment of the datum.  the diameter of circular area is given in the upper half of the datum target symbol.
best regards,
heckler
sr. mechanical engineer
sw2005 sp 5.0 & pro/e 2001
dell precision 370
p4 3.6 ghz, 1gb ram
xp pro sp2.0
nivida quadro fx 1400
      o
  _`\(,_
(_)/ (_)datum9s0 on formed tube
"coming together is a beginning, staying together is progress, and working together is success." - henry ford

yes - absolutely but make surdatum(s) on formed tube
is there a recommended method for specifying datums on formed tubing, such as formed hydraulic tubing or formed exhaust tubing.  i care about the profile along the length of the tube and i care about the location of the ends with respect to each other.
i would use total-runout or cylindricity with profile tol.
look up asme y14.5m 1994.
chris
sr. mechanical designer, cad
solidworks 05 sp1.1 / pdmworks 05
datum targets....three points define a plane
best regards,
heckler
do you trust you intuition or go with the flow?
there is/was a milspec for tube and bar bending, try researching mil-d-9898c.
it delt with assigning points on the tube, like "position 0" for starting end, position 1 for first bend, etc.  it also mentioned bend angles, straight length dimensions, direction of rotation (cw or ccw) before next bend, etc.
i think there is a world market for maybe five computers.
thomas watson, chairman of ibm, 1943.
a tube _drawing_ is easier to understand if you orient the longest straight piece with the drawing's coordinate system.  then you can dimension the ends relative to the coordinate system and any tertiary datum along that length of tube.
there are also good reasons for orienting the part relative to one end, or the extreme tangent of one end, which is what i think you need in order to program a cnc bender.
third choice, sometimes the least awful, is to use the coordinate system of the assembly into which the tube fits as your datum coordinate system.   this can get messy fast because in the general case every important point requires three linear dimensions and three angles.
the best datums, imho, are not imaginary like centerlines, and not arbitrary like tube walls at particular planes, but are the actual features at which the tube touches the remainder of the world, e.g. flange faces and holes in hanging features.
mike halloran
not speaking for
deangelo marine exhaust inc.
ft. lauderdale, fl, usa
cpdpeckh,
   if it were me, i would use one end diameter as my primary datum, the end face as the secondary datum, and a point out on the tube somewhere to control rotation.
   how are you going to inspect this thing?  a sketch of your inspection fixture might tell you how your datums should work.
   i do not think there is a recommended procedure anywhere here beyond making sure your part is constrained in six degrees of freedom, and that your datums are some sort of functional feature if at all possible.
                   jhg
thank you for all the comments and suggestions.
does anyone use true position to control tolerance between the two ends of a formed tube?
don peckham
no
chris
sr. mechanical designer, cad
solidworks 05 sp1.1 / pdmworks 05
an inspection fixture should be easy to set up to inspect true position between the ends, so i would think that this would be acceptable.
the datums used to define the fixture should also represent the functional datums for the finished part.datums  hole patterns
i wouldn't use total runnout you would never get good parts.  i would use a profile tolerance providing your datum scheme is thoughtout.
cpdpeckh,
   a positional tolerance soudatums & hole patterns
if i have two holes the same diameter, and i want hole 1 to be the b datum (4-way) and hole 2 to be the c-datum (2-way), is there a difference between having one size & position tolerance called out off of the hole 1 with a prefix 2x, and the datum b leader coming off that size c/o (see attachment)
and
both hole 1 and hole 2 each having their own callouts (but with the same size and pos tol values)?
thanks,
jeff
when i have had situations like this, i give the holes separate callouts (your 2nd case) and control datum c relative to datums a and b. they can still carry the same size and position tolerance. we have done this with double dowel pin callouts, (though often the 2nd dowel pin is controlled tighter to the 1st) and when in-line, as yours seem to be, they can be used to establish a b-c axis.
also not sure about your 4 way, 2 way concept. application wise your getting any way within the 0.05 mm tolerance diameter. don't know your application, but if you really want 2 way on datum c, maybe it should be a tight slot??
i've done it similar to what checkerron suggests.  if two separate hole callouts with two separate datum callouts, it would be to control the second hole to the first.  if this relationship isn't critical, i would call out both holes with the same dimension; however, i would not tie the datum to the dimension, but to the hole(s) separately (similar to your datum c).
believe it if you need it or leave it if you dare.fff"> - robert hunter
jmarkus,
   i assume you will call up datums as |a|b|c|.
   you can apply your datum symbol directly to your second hole.  i understand why you are doing it the way you are, but it is the tertiary datum.  by definition, all it can control is rotation.
   whatever goes through hole_c should be diamond shaped so that there is contact only with the sides.  otherwise, you should slot the hole.
                        jhg
i concur with ewh's clarification. i was also thinking of datum c relative to the hole, not the dimension.
drawoh brings up an interesting point too. i really like to use a diamond pin in a 2 pin tertiary datum situation, but most of the commercially available ones i see start at 6mm (.250") diameter.
many of my applications are smaller than this.
has anyone seem purchased diamond pins smaller than a .250
dia or 6mm?
checkerron,
   i would like to know about small diamond pins too.  meanwhile, it looks like slotted holes!
                          jhg
don't know your application but a hole-slot is always preferable for a functional assembly.  
i get the felling that you a) are trying to put your own style on the way you call this out b) don't fully understand datum and the order of precedence c)that you are trying to apply a midplane datum to a cylindrical feature.  
in any case, just put a fos callout on each hole and apply a datum to each.  as long as neither is not primary,then  reference one with respect to the other you have than achieved  what i believe you are looking for which is locking the remaining 3 dof, x and y translation are locked with one hole and last is the z-rotation with the other.  this should be clear to all who have a basic gd&t understanding.   
an item of note is that if your pattern of two holes is located in a manner in which you can't tell them apart, since they are equal in size, then you are opening yourself to some more confusion not to mention r&r issues.
advice, use a slot and apply the tertiary datum to the slot fos.  it doesn't have to be much of one, and be done with it, your manufacturing/assembly will thank you in the end.

funny, i always dimension them separately but i took a look at asme y14.5m-1994 figure 4-8, it actually has a very similar layout.
kenat, probably the least qualified checker you'll ever meet...
but figure 4-8 uses the full "diameter" of b & c, as opposed to using one as a 4-way and one as a 2-way.
the problem i have with 4-8 is that in this method the tertiary is not positioned with respect to the secondary.  that's why i like to call them out separately.  
this maybe contradicting myself but if a hole-slot can't be used than, the question is if one hole is functionally more important than the other, if not, then a pattern datum should probably be considered, instead.  if this is what is chosen then create a basic dim scheme from one of the holes which will become drf origin.  this will lock the three remaining dof's which i believe is the op's objective.
nds good to me.  you want the other end of the tube located somewhere, and the positional tolerance defines that.  i am sure there are other ways to do this, but i would not spend any time worrying about it.
   if i were inspecting your drawing, the primary thing i would look for is your tolerances.  if they are not loose enough, the fabricator will assume you do not know what you are doing.  bending tubes cannot be a very accurate process.  this thing will be somewhat flexible, right?
                      jhg
e that manufacturing engineering agree with the target locations. design engineering should reflect the datums per assembly of the part while manufacturing engineering should decided the method of producing the part and their datum targets.
hope this helps.
dd

  
datums in multiple views
2. put c1 @ 90 degrees to -a- on center of shaft.
reason: datum target points hold a product in relation to the datum reference frame. try to make the points as strong in the degree/vector of the datum as possible, i.e.; the example shows a2, a3 & c1 at datums in multiple views
i have a drawing where it would be convenient to show 1 datum in multiple views.  this is because i have to dimension off that datum in multiple views.  what is the practice for this?  is the datum shown in one view and then labeled reference in the other views?  is the datum just shown "normal" in each view i dimension from it?
thanks,
anthony
i have always shown it "normal".  it would not be reference because you are dimensioning from it.
believe it if you need it or leave it if you dare.fff"> - robert hunter
per asme y14.5m-1994 3.3.2 "where the same datum feature symbol is repeated to identify the same feature in other locations of a drawing, it need not be identified as reference."
as i understand it, the datum (so long as applied to the same face/feature) can be shown in as many relevant views as you like.  
it's not like double dimensioning.
in fact i've had checkers encourage me to show it more than once when it makes the drawing clearer.
kenat, probably the least qualified checker you'll ever meet...
just realized all 3 posts are from you avc8130.  do you have a copy of 14.5?
it's worth spending some time looking at it as the more familiar you get with it, the easier it is to find what you need.  don't get me wrong, i still sometimes can't find stuff & have to ask but i usually try and look before i ask.  if nothing else i sometimes see other things that i re  
honestly, i have a copy.  there are actually 3 of us looking at the same copy!  two of us are young engineers new to this and the other is a 30 year veteran who is well set in his ways.  
i have only asked the questions that we could not find concrete answers to.  "equally spaced" wasn't referenced anywhere so i didn't know if it was still allowed.  
we couldn't find the answer to this no matter how much we searched.  the old-timer was convinced it had to be shown as ref.
the unilateral tolerance only shows that the amount of decimals should be the same as the amount shown in the nominal.  the assumption must be made that the nil value must be shown.  we have 1000's of military drawings that show .500+.005.
thanks,
anthony
i didn't mean to say you were being lazy or anything, just trying to help you out.  i rarely (never?) just sit down and read it but, if while looking for one thing i see something else of interest i'll maybe read it and make a mental note for when it comes up.
"the assumption must be made that the nil value must be shown"  no i dont' think so, 2.3.2 (which i typed in the other thread) seems quite clear on this to me.  'its dimension shall be expressed' hence the 0 to however many dp should be shown.
kenat, probably the least qualified checker you'll ever meet...
show the datums where needed, no ref. only same dims that are shown more than once should be ref.
chris
solidworks/pdmworks 08 2.0
autocad 06
anthony,
you are asking good, valid questions.
i've been doing this sort of work for almost 30 years, and often have a hard time justifying why i do things a certain way.  eq sp is an example.  i've always used it, but could not point to a standard where is is specified.  i can usually find documentation to back me up, but not always.  as you have noted, "old-timers" tend to be set in their ways.  just because i've always done something a certain way doesn't make that way valid, and i try to be open to differing opinions.  if that differing opinion can't be backed up with documentation however, i generally stick to my guns.
keep on asking questions.  keep us on our toes.
believe it if you need it or leave it if you dare.fff"> - robert hunter
amen, ewh. after all, engineering is a constant task to keep up with change and improvement. both young and old engineering people teach each other when both are amenable to change.
argh.  yeah, you are right, ewh, and i too will start putting basic angle callouts on my hole patterns.  unless i can figure out how to make inventor draw a box around the text "eq. spc."....
btrueblood,
you may not have to change afterall.  i think that the concensus is that eq sp is a valid callout.  see
less than 45 degree's to each other on two different surfaces. the odds are that those three points will not all make contact because we are already resting on a1 (4 points strong in the -a- vector). c1 is not strong in the -c- vector, so it will not control -c- location as strong as we would like.
how the text actually reads.   
david
kenat:
4.5.7.1 does state an extension line and it does make sense. the figures i previously mentioned do not follow the standard but words do supersede drawings here. going to make a couple a changes in my training book.
got to give you a thanks on this one. hats off to you!   
dave d.

   datum c is the other hole, which keeps your part from rotating.  this would be modeled by a vertical diamond pin.
   the only deficiency of your drawing is that the datum targets ought to be specified with a diameter.   the actual contact point, the centre of your hole, does not exist, at least, not in the sense of possessing material.  your drafter needs to define the diamter of the a1, a2 and a3 pins.
                        jhg

this list of items to consider when converting a drawing using the 1982 standard to the 1994 standard is based on my experiences when converting drawings. the list may not totally encompass all the items to be addressed. if you know of addition items that should be added to this list, let me know and i will update the list.   


icantly in preparing for the senior level body of knowledge?  i recall a bit of overlap, but the bok for the senior level was far more extensive and in-depth.
keep at it, powerhound.  i studied my butt off for the exam for a couple months, and did a number of practice exams too.  good luck.
jim sykes, p.eng, gdtp-s
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