【转帖】gdt part costing more

yang686526

gd&t part costing more
i was talking to another engineer the other day and the old chestnut came up of.
鈥淲ell i had a part without gd&t which i鈥檇 got prototypes for.  i then had the gd&t guy take a look and add gd&t and when i sent the new drawing to the vendor it came back significantly more expensive鈥?鈥?(this is a paraphrase of a longer conversation)
in this case the gd&t guy was my checker, before he was laid off, who had 40 years of experience, had used gd&t for a long time, had training, been a level 3 checker in aerospace/defence etc.  so while not infallible he was pretty darn good, certainly the best i鈥檝e ever worked with.
now the only ways i can see this would have happened was:
1.    the vendor didn鈥檛 understand gd&t.  i鈥檝e seen/heard of places that seem to charge an extra % or more for each feature control frame.  basically if they see any fcf etc they increase the price, even if the tolerances are actually effectively 鈥榣ooser鈥?
2.    the part wasn鈥檛 originally dimensioned/toleranced adequately to ensure function and fit and so in the process of adding gd&t tolerances were tightened but due to functional requirements not inherently because of the addition of gd&t.
3.    the gd&t was added incorrectly based on the function of the part.  i can especially see this being the case if the person doing gd&t wasn鈥檛 familiar with the function of the part and the engineer didn鈥檛 provide all the necessary information/mating part details etc.
4.    (kind of a variant of 3)  the person adding gd&t didn鈥檛 have a clue what they were doing.
knowing the guy who did the gd&t i know it wasn鈥檛 4 and i doubt it was 3 so i鈥檓 guessing it was 1 or 2.  
it looks like it鈥檚 going to get taken to management, not specifically this case but the whole concept of drawing checking and what we鈥檙e trying to do with adopting standards and i know this kind of thing is going to be one of their main points.  saying it was 2 won鈥檛 be popular or win me any friends and if it鈥檚 1 they鈥檒l say 鈥渨hy use gd&t then鈥?rather than 鈥渨e better find some competent vendors鈥?
so i鈥檇 appreciate any input-
does anyone else have any possible reasons for the price difference?  
i鈥檓 sure some of you get this thrown up to you, what is your response?
thanks for the help.
kenat, probably the least qualified checker you'll ever meet...
check out our whitepaper library.
from my experience there is a lot of truth in the statement he made.  my experience goes back quite a few years, for that matter. 20 years ago there were no local tech schools which offered courses relating to y14.5. nasa and micom are/were 2 major users at that time.  
it is my opinion that many of the short courses offered are inadequate and serve only as an introduction to gd and t and are over-priced.
i once worked for a company that provided in-house training for all who worked with drawings. it was a major defense contractor.  they used a self-teaching book, which i thought was excellent for the time.  (1963)
i like your numbers 1 & 4.
i have seen some well know trainers state that all features of size should have positional tolerances and all surfaces should contain profile tolerances rather than adding fcfs only on fit and function. most drawings are a bit of over kill and the using the noted approach will cost extra $$$$.
if designers would only add gd & t on fit and function, then all of the areas noted with the exception of your #1 would be addressed. if you need it, apply it. that's it.
if you have positional tolerances on holes/pins where there is a fit and function, make sure that they are applied at mmc. if they reference another feature of size, make sure it is also at mmc. tell the vendor that they would end up getting more tolerances and it would not hurt the integrity of the drawing.
make sure that you have someone in your company quite proficient in this subject otherwise get outside assistance. it takes a long, long time to become proficient in this gd & t. it took me 10 years of training to become comfortable so get help if you need it.
explain to the vendor what the particular applied gd & t means and hopefully, the costs should be around the same.
good luck!!!  
dave d.
making prototype parts is not the same as production.  along with production and gd&t comes inspection, documentation, recordkeeping, process monitoring, etc.  most anyone can knock off a few protos with few controls.
honesty may be the best policy, but insanity is a better defense.
i have also experienced parts being priced much higher when gd&t is used. the people involved with the part, purchasing & machinist, do not understand gd&t. they think it requires inspection and a special expertise to make the part, so they add cost.
when gd&t is used correctly, it is usually less cost than without it.
chris
solidworks 07 4.0/pdmworks 07
autocad 06
i would say that #'s 1 and 4 are the most likely candidates with a lot of weight being given to #1. if the guy adding the gd&t was clueless he would have put a bunch of nonsense together and any shop that understood gd&t would have either no-quoted it or called with questions. i don't think #2 is it because if the part functioned correctly being built at the limits of it's tolerance based on the coordinate system then the new tolerances would at least appear looser. for example, if you have a hole position called out at 2.0 +/-.010 from an edge in both x and y, a conversion of the same tolerance into a cylindrical tolerance zone and put into a fcf would read "positioned within a diameter of .028 wrt a, b, and c." so instead of your maximum tolerance in one direction being .010, it could potentially be .014 depending on how far off of true position the other axis is, not to mention when you see a tolerance of .028 as opposed to +/-.010 it even looks like you have more. the exception would be if the part didn't function correctly when built at the limits of it's tolerances in which case they would need to be tightened anyway. as far as #3 goes, it's in the same boat as #4. incorrect gd&t doesn't make for a more expensive part, it just makes for an incomplete drawing. a shop that knows the difference will ask what you meant.
  i teach gd&t as a side job and while i still don't know everything about it, bb's like this help me out tremendously. that being said, i always present gd&t as a language. you can understand it perfectly but if the shop doesn't, you haven't communicated squat with your perfectly y14.5m compliant print. this entire post could have been written in grammatically correct spanish but what good would that do for those of you who don't speak a word of it?
dingy2,
  i have to disagree on your point about only putting gd&t where you need it. if the print states to interpret the drawing per asme y14.5m-1994 then shouldn't the print comply completely? if you pick and chose the components that you want to adhere to then your supplier might do the same to you, such as deciding that he doesn't think general rule #1 should apply to the part he just sold you. now you have a .250 pin that was produced at mmc but it has straightness error of .020 and it won't completely go into the .255 hole that you intended it to because all you put as a tolerance was +/-.005 and assumed he would comply with general rule #1. when you use the term "overkill" you give me the impression that you think that the addition of gd&t is more of a nuisance than anything. i contend that a fully y14.5m compliant print is cleaner, clearer, and is only open to one interpretation to those who fully understand it. specifying the standard invokes a heaping helping of fundamental rules and general rules that are designed to close loopholes that suppliers have used over the years to force their customers to buy parts that are screwed up but the condition is not addressed in the standard. for example 1.4(m) states that all geometric tolerances apply to the full depth, length, and width of the feature. now why do you think they would add this to the 1994 standard? it wasn't in the 1982 ansi standard. i believe it was because there were a lot of suppliers that insisted that the positional callout only applied to the top of the hole and not the bottom. asme probably heard about it and addressed it.
  anyway kenat, i think #1 is your most likely candidate. it happens more often than not and way more often than i'd like to see.
  as a side note, i go to take my asme gdtp-technologist level test in a month. wish me luck.
powerhound
production supervisor
inventor 11
mastercam x2
smartcam 11.1
ssg, u.s. army
taji, iraq oif ii
or,
5.  the vendor understands gd&t just fine.  but he doesn't get paid for preparing quotes, and he only gets 1/10 or 1/100 of the jobs he quotes on, so it makes no economic sense for him to put any effort into understanding what you mean with all your fancy symbols.  he eyeballs the part print and guesstimates a process based on the tolerances he can pick out right off the bat.  he doesn't have time to do fancy calculations to find out that you are giving him 'bonus tolerances'.  he quotes on what's there, and easy to figure out.  if he wants the business, he quotes based on breaking even with the eyeballed process and the tolerances he can find with a few minutes' inspection of your print.  he's going to estimate higher when gd&t symbols are present, just because he knows that he'll have to go three rounds with _your_ quality assurance people, explaining to _them_ what a bonus tolerance is.
if and when he gets the job, _then_ he goes over the print with a fine tooth comb, not looking for bonus tolerances, but looking for ways to fixture and/or process the part that will be significantly cheaper/faster than the process he assumed when he quoted.  if he finds a way to make it cheaper, that's his profit.
mike halloran
pembroke pines, fl, usa
powerhound:
good luck on the gd & t exam. study hard and you need a minimum average of at least 75%.
maybe i did not explain what i meant by "appying gd & t where applicable" and "over kill". i did not mean that the asme standard should not apply. it should. all drawings should state in notes that "this drawing is in compliance to asme y14.5m-94. that covers rule 1 and everything else in the standard including full depth.
but
i have seen so many drawings with all surfaces with a profile of a surface tolerance and all dimensions basic. is it really needed on all surfaces? what if a surface has no function and relationship to the mating part? could a +/- tolerance be sufficient?
i have seen in under notes various layers of profile tolerances where a +/- might have been sufficient. each time one in quality sees a profile tolerance, that surface is scanned using a cmm and this is costly. is it really needed?
this "over kill" on the application of gd & t does drive up the $$$.
each time a designer places a fcf, should they not ask themselves something like "what is the function and relationship of this feature". if there isn't any, probably co-ordinate or polar tolerances are sufficient.
i hope that this explains what i meant in my prior submission.
dave d.
hi dave,
  thanks for the quick reply and thanks for wishing me luck. i'd like for you help me clarify something. i don't really see the difference between calling out "profile of a surface" and a plus/minus when either one would work. for example, if you have a block that is one inch thick, you can call it 1.0 +/-.010 or you can call it 1.0 basic, call one side datum a, and specify the other side "profile of a surface within .020 wrt datum a. they both say exactly the same thing and should be checked exactly the same way. general rule #1 still limits the variation in size to +/-.010 at any point on the part. i know that applying a plus/minus tolerance gives the impression that it's not as critical and applying a profile of a surface gives the impression that it's very critical so if the overkill you were talking about was based more on perception than reality then i will agree with you on that point. the reality is that even if the tolerance is plus/minus, no part of the surface may violate the mmc or lmc boundary, pos simply orients the surfaces to a datum frame and +/- doesn't.
powerhound
production supervisor
inventor 11
mastercam x2
smartcam 11.1
ssg, u.s. army
taji, iraq oif ii
all things being equal, i do hafta agree that gd&t when using positional or profiling should be a little more expensive because of the methodology employed in following the set up requirements is more, even if the tolerance is actually looser.  
with simple d&t, the feature simply has to fall within a particular (usually square) tolerance zone.  however, the real feature size affects which tolerance is applied in positional gd&t, since the tolerance varies based on the feature size (material condition).  so, instead of just one tol zone, the vendor now as to know a range of tol zones.  a premium should be expected for this, but not a huge premium unless every dim was replaced with gd&t.  just my opinion though.
matt
cad engineer/ecn analyst
silicon valley, ca
so another option then:
5.  the vendor didn't bother taking the gd&t into account when preparing pricing to save time or for whatever reason.
while i'd have major concerns over using a vendor like this it's not my decision and it is an option, thanks mike.  it鈥檚 pretty close to #1 but i guess distinct enough to mention.
in general case #4 is a hot contender, in the particular case i detailed though this wasn't at all the case.  the guy who helped with the gd&t is really good.
maybe i should explain more about option #2.  from what i've seen/can gather a lot of the engineers/designers here don't do anything approaching even a minimal tolerance analysis on their designs.  in fact some even say that due to the level of precision we (or at least they think we) require on finished assemblies that we shouldn鈥檛 insist on anything near 100% interchangeability and should put up with having to match parts on the shop floor or just rejecting a significant number of components.  as such they鈥檒l happily design parts that don鈥檛 go together at anywhere near worst case, in fact even if you do some kind of statistical analysis, from the one time i re