【转帖】looking for some information on gd

yang686526

looking for some information on gd&t
hi all,
i'm really struggling with a quandry at work, and am hoping that you can share some information with me so i can make a better engineering decision. basically, i've joined a new group where the senior engineers like to dimension everything using coordinate dimensions rather than geometric dimensions (gd&t or y14.5). i'm a younger engineer who came from a firm that extensively used y14.5, so i believe it's the only way to do aerospace drawings.
here's my questions:
- is the use of y14.5 (or european equivalent) growing or shrinking? some of the engineers talk about how it was the fad of the '90s, sort of like 6s or just-in-time.
- the machine shop can't read gd&t very effectively, so everything with gd&t costs three times as much. (classic example: they look at the number of decimals places in a basic dimension to determine cost and tolerance.) any good suggestions on how to work with them on it?
- should i be very concerned in the first place about coordinate dimensions? after all, they pay me, and if they want me to do coordinate drawings, i suppose that is what i should do. but i'm skilled at gd&t, and it's just really hard to be ok with coordinate geometry.
thanks!
find a job or post a job opening
i know exactly how you feel about this topic. i am a bit frustrated at how drawings are dimensioned today too. i have been in the field almost 30 years and miss the days when drafting standards were adhered to closely.
it appears to me that with the ease of cad drafting just about anybody thinks they can do it.
- it does appear to be shrinking to me, which is really surprising. i thought it would become more popular as the younger generation picked up on gd&t. it has been many years since the ansi standards group came up with some new and i do not know why that is.
- maybe take some time and explain to them what the tolerances mean. the part can actually be designed cheaper when gd&t is used correctly.
- coordinate dimensions really aren't that bad when used correctly, and gd&t can be used with them as well. i use them mostly for sheet metal flat patterns that are eventually formed. i do think some places go a bit nuts using solely coordinate dimensions - they can be just taking the easy way out. maybe when you have a year or two in at this company then start introducing better things to them.
in my opinion people have just been too lazy at adapting things like y14.5 and gd&t, thinking that they are saving money by getting it out quicker - but those who know this stuff well know the contrary.
i think both have their place, a bit like two employees getting to work one might live 50 miles away and choose to drive, one might live around the corner from the office and choose to walk, would you make them both use the same mode of transport?
the downside of letting everyone do what they want is drawings become very non-standard and checking for clearance etc becomes a real nightmare when different approaches are used.
i am not sure there is an easy one size fits all solution for all industries/ situations.

imo it's 'shrinking' because people have become lazier and just don't care anymore.
i suggest having a gd&t consultant come into your company and conduct some training classes...for every one.
you will be surprised how much time and $ could be saved using gd&t correctly.
without the training, you're company will be lost and continue to pay the high costs of ignorance.
chris
solidworks/pdmworks 08 3.1
autocad 08
i agree with ctopher on this one. i have seen where some designers have all dimensions covered with gd&t without consideration to the design needs. we have discussed having default gd&t in notes here before so i won't get into that topic.
get training as ctopher suggested but also make sure that after the training, personnel in your company can contact the trainer for gd&t consulting on your application. i have been in the business for over 20 years and this really surprises me. there just is no follow up after the training. a few years later, the same company calls me back to do the same old training again with the same people. go figure??   
dave d.
folks-
while i'm preaching to the choir, bad drawings and weak dimensional controls are just bad business.  if you have a vendor who charges you more for a drawing that is toleranced more loosely than one using bilateral tolerancing then don't send them rfqs anymore; find one that has a takeoff quoter with at least a cursory knowledge  seeing how that vendor has probably been seeing drawings with gd&t terminology since the early eighties, that vendor is obviously unwilling to adapt with the times or too cheap and undetermined to train and evaluate their staff to the new method.  as always, it is the accuracy of the manufacturing methods that has produced parts that fit for these vendors not their ability to understand their customers' needs.
  
tunalover
tunalover,
   i actually do not know how well our vendors understand gd&t.  my only experience so far was when a sheet metal shop questioned my 0 positional tolerance at mmc.  i pointed out to him that my diameter tolerance was extremely sloppy, and that the 0 tolerance did not apply at lmc.  
   is the op discussing co-ordinate dimensions, or co-ordinate tolerances?
   co-ordinate tolerances are less clear and more expensive to achieve than positional tolerances, at least in an environment where the fabricators understand gd&t.  co-ordinate dimensions take up less space on the drawing, and they are easier to inspect.  they are easier to fabricate if the fabricator is not using cnc.  you can apply positional tolerances.
   machining is an accurate process.  machinists fairly easily achieve the tolerances need to make the parts assemble.  i have seen so many drawings with .125" holes intended to clear 4-40unc screws (.112"), located to ±.005".  the parts all work, but you can do the math and demonstrate that conforming parts may not work.
   of course, this is dangerous if there is any chance the designer has to work with weldments or castings.
                           jhg
if you are in aerospace then i'm a little surprised they don't use it at least some of the time, even if not as religiously as some places.
by aerospace do they have contracts with govt or the big oem places?  if so (& in the us) i'd have though contractually they have to work to 14.5, which at least at the 95 version all but mandates use of positional or profile for features or size.
i know a lot of the older guys (especially designers/checkers) who learnt gd&t in the 80's are either retiring, or being replaced by cad jockies and/or younger engineers doing their own drafting who don't have a clue how to draft, yet alone do tolerancing or use gd&t.  modern courses tend to teach people how to use a cad system or two rather than generically how to draw/convey design intent etc.
take a look at these vaguely relevant posts of mine.  there are also some other good ones debating use of gd&t over coordinate d&t.
we just went though gd&t training here. this was mandated by unclear drawings at a sister company causing a million dollar mistake. i have started using it more, not enough time to see the effect on vendors, etc. if your vendor balks at it, get him trained.
peter stockhausen
pollak switching products
find vendors that understand gd&t.
if your company suddenly starts using it and your current vendors see it, they may (probably will) raise your cost.
chris
solidworks/pdmworks 08 3.1
autocad 08
thanks guys, especially keant. i think that your information is all correct and real good.
while i don't want to post my company name here because i don't want to do a search and have people find it, all of you know which company i work for. we are an aircraft manufacturing company based out of seattle.
before starting here, i surely thought that gd&t would be real important in our work. it is important in the production engineering side. however, i work in a different division where our parts are installed temporarily on the aircraft so people feel that it's ok that the drawings are sloppy because it's only temporary.
ah well, live and let live. if sloppy drawings is the rule of the day, i will create sloppy drawings. at least i'm working mandatory overtime rather than getting laid off.
thanks!