【转帖】olerance analysis iso2768

yang686526

tolerance analysis iso2768
i鈥檓 in the us working to asme standards.  i used to work in the uk where we invoked our british standard version of the iso standards.
a few weeks ago i had to check a print of a new part which was a minor change from an existing part.  
one main component it interfaces with is german and on the drawing references iso2768 fein (fine, in case you didn鈥檛 guess)  i don鈥檛 recall ever working with this tolerance standard in the uk.  i found extracts that i thought gave me what i needed on the web.
i did some tolerance studies of the interfaces and came up with big clashes at worst case.
i went back to the designer and he said that while he couldn鈥檛 argue with the numbers the old version worked etc.  he contacted the vendor to verify my interpretation of the standard.
i have an email now from the vendor basically saying my interpretation is completely wrong, trouble is i don鈥檛 have iso2768 to be sure and the way the email is worded either a. the person writing it doesn鈥檛 really know what they鈥檙e on about or b. their english just isn鈥檛 fantastic.
for instance i have part of a hole pattern, two threaded holes in line.  the first is 36.66mm from the 0 ordinate.  the second is 103.34mm from the 0 ordinate.  nominal spacing therefore is 66.68mm.  from the extract of the iso i found both 103.44 & 36.66 are +-.15mm.  therefore i assumed that the spacing is 66.68 +- .3 (.012鈥? which i was using in hole position calculations as pos dia .033鈥?
the vendor is saying the +-.15 applies not only to the holes relative to the 0 ordinate but also their spacing to each other.  so if one moves away to be maximum distance from 0, the other at least has to be on nominal or further away from 0 etc.
does anyone know if the vendor is right, or can point to a website where this is explained?  (i need to give the designer an answer in the next few days and am not sure i can justify buying the standard or get it in time.)
further does this iso say anything about assumed coaxiality or position tolerance of circular features (holes)?
it鈥檚 bad enough trying to work out the drawing given it鈥檚 1st angle projection and my brains stuck in 3rd, the addition of this tolerance is just icing on the cake.
thanks for any assistance.
kenat, probably the least qualified checker you'll ever meet...
find a job or post a job opening
having looked at it i should clarify that the drawing only says fein, doesn't ref a letter grade which i believe is needed to invoke part 2 for geometric tols, please correct me if i'm wrong.
neither of the below gives me the answer.
i haven't worked much with iso standards but i have worked with it tolerance grades which may be similar to what you're looking at (maybe not), but either way, in all my time working in machine shops, the only thing that any tolerance was applied to was the exact dimension, not an extrapolated association between features that didn't have a dimension between them. my opinion is that the tolerance only applies to each dimension from 0 and not to the difference between the 2 features. in your case, the first hole could be 36.66 -0.15 and the next hole could be 103.34 +0.15. while this will produce a centerline difference of 0.3 from the nominal difference between the two, you can't extrapolate a dimension between the two holes and give it a tolerance of +/-0.3. if you nail the 36.66 dimension perfectly and then put the next hole at 103.04, it will be in tolerance with your extrapolated dimension of 66.8 +/-0.3 but it will be out of tolerance with the print dimension of 103.34 +/-0.15.
  my opinion is that both you and your vendor are incorrect. i've stated in the above paragraph why i felt that your line of thinking wasn't exactly right but if your vendor thinks that one hole should follow the other when the dimensions of the holes are originating from another point, independent of each other, he's incorrect as well.
powerhound
production supervisor
inventor 2008
mastercam x2
smartcam 11.1
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powerhound, you have a point i was being a dumb*** when i wrote that, thanks.
the 'equivalent position' tol should be based on the +-.15, not sure what i was smoking at that point  in fact that's the value i used in my calculations not .3.
however my point on the hole spacing effectively being 66.68 +- .3 on centerline is correct even from what you say.  this is what the vendor is arguing, he's saying that it's 66.68+-.15.
either the iso says something about this or he's smoking something even better than i was when i put my first post.
i just want to get a feel for which.  the designer i'm working with isn't the most cooperative at times so i want to try and make sure i'm right.
kenat, probably the least qualified checker you'll ever meet...
kenat,
  i absolutely agree that the tolerance is effectively +/-0.3 between the holes but what i was saying is that it won't work if you're trying to achieve the center distance between the holes as well as the dimension from the edge of the part (or whatever feature the holes are called out from).
  i can't imagine in my wildest dreams that the iso standard says something along the lines of ignoring a hard callout and somehow relating the position of one feature to another even though they aren't called out relative to each other. the way to relate the position of one hole to the other is to show a dimension between them, it's as simple as that. my new and improved opinion is that either (a) your vendor knows something that we don't or, (b) he's still wrong.
   
powerhound
production supervisor
inventor 2008
mastercam x2
smartcam 11.1
ssg, u.s. army
taji, iraq oif ii
thanks powerhound.  i just got another two of our german interns to take a look and they too think that you end up with +-.3 between the holes if dimensioned this way.  one's off to ask a couple of others their opinion.
kenat, probably the least qualified checker you'll ever meet...
ok, i now have 4 german interns who agree.
does your opinion, 4 interns & mine outweigh one vendors engineer?
ken
kenat, probably the least qualified checker you'll ever meet...
kenat,
the copy i have of iso 2768-1 (1989) doesn't make any specific mention of assumed coaxiality or positional control.  it's just a general tolerance for untoleranced dimensions.  i don't believe it's meant to take the place of properly added positional control.  (the spec does specifically state that is doesn't apply to basic dimensions).  i believe your interpretation is correct.
joe
sw office 2006 sp5.1
p4 3.0ghz 1gb
ati firegl x1
thanks jmarv.
i appreciate you looking, it confirms what the excerpts i've found on the web and what the interns tell me the german version of machineries hand book says.
the more i think about it the more i'm convinced what the vendor is saying isn't true.  i think it may be the language barrier but at the same time i meet so many people that don't seem to have a clue about tolerances that i'm loathed to make that assumption.
now i just have to work out a way to break it to the designer.  i think that can wait till after the holiday (labour day for non us types).
kenat, probably the least qualified checker you'll ever meet...
maybe you can get your vendor to explain his rationale for why he thinks the way he does about this issue. i'll be willing to bet that's it's nothing more than a notion he's had for years and has just never been challenged about it. on the other hand if he whips out the iso standard and shows that he's correct, we'll all be eating crow for awhile.
powerhound
production supervisor
inventor 2008
mastercam x2
smartcam 11.1
ssg, u.s. army
taji, iraq oif ii
if i had more time, i would but i'm out in few minutes and the designer needs an answer when i get back.  plus like i say i think there are communication issues so it would probably be painful.
plus i'm pretty sure this would just add fuel to the fire of this particular designer complaining about having his drawings checked, why are we invoking military standards etc. (you've seen my other posts).
thanks,
kenat, probably the least qualified checker you'll ever meet...