Looking for some information on GD&T
Looking for some information on GD&T
(OP)
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!
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!





RE: Looking for some information on GD&T
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.
RE: Looking for some information on GD&T
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.
RE: Looking for some information on GD&T
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
ctopher's home (updated Aug 5, 2008)
ctopher's blog
SolidWorks Legion
RE: Looking for some information on GD&T
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.
www.qmsi.ca
RE: Looking for some information on GD&T
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
RE: Looking for some information on GD&T
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
RE: Looking for some information on GD&T
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.
thread1103-193286: Drawing Checker/Approver Qualifications
thread1103-193705: Drawing standards only for Military work
thread1103-192933: GD&T part costing more
thread731-167631: What should i do?
At the end of the day, you get paid to do what they want you to do. If they don't want GD&T but you put it anyway then may lose patience. What you can try and do is educate where possible. Also make sure and do your tolerancing, if you end up with tight +- coordinate dimensions that GD&T would have helped with say something like "I'm concerned how tight this tol is, at a previous employer I'd have used GD&T which would have effectively loosened the tolerance but without using that I'm worried about cost/process capability".
As to the machine shop not knowing GD&T, the inspection department may be a more pressing concern.
KENAT,
Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
RE: Looking for some information on GD&T
Peter Stockhausen
Pollak Switching Products
RE: Looking for some information on 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
ctopher's home (updated Aug 5, 2008)
ctopher's blog
SolidWorks Legion
RE: Looking for some information on GD&T
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!
RE: Looking for some information on GD&T
So you're dealing with the IAM.
Bummer!
(you may email me if you like through your company)
Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 08
ctopher's home (updated Aug 5, 2008)
ctopher's blog
SolidWorks Legion
RE: Looking for some information on GD&T
KENAT,
Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
RE: Looking for some information on GD&T
I'll add my 2 cents here.
I was brought in as a checker for a German seat mechanism NA transplant company. All the drawings used coordinate tolerancing, had manufacturing notes, contained toleranced theoretical points, stuff like that.
The North American suppliers could not understand the drawings...I was asked to fix them all. The key is to redline the drawings...AND communicate what the GD&T symbols mean...I found most suppliers were very willing to learn.
I think coordinate toleranced drawings work in Germany because there are so many product engineers who are old toolmakers...for example they are able to decipher implied datums...we don't have that luxury in the US. We have engineers with less than 5 years experience who've only attended a two day GD&T course.
In the US...I'm pretty sure GD&T is here to stay...otherwise personally I would not have taken that 135 question certification exam.
But, sad to say, there will be NO shortage of those willing to mislead about what they do not understand.
Old Turkish Saying - "In the absence of a leader, the men of a village will follow their chickens."
Michael
RE: Looking for some information on GD&T
The moral of the story is in order for any tolerancing scheme to be effective you've got to have buy-in from all levels. It's key to know your audience. In this case the OEM descision had already been made and it was too late to pull the job so we muddled through. The language barrier didn't help, but I'm not sure we'd have been able to convince them even in Mandarin.
The other takeaway was that our team didn't have the GD&T chops to really be effective, but in this case I'd say we learned the easy way (we're rectifying the situation now).
Cheers
Adam
RE: Looking for some information on GD&T
To support the point, I'll add an example...
One supplier insisted that if there was GD&T that meant that a physical gage had to be made for the part and that for each FCF, a physical detail had to exist on the gage.
Of course, that's hogwash...the supplier was using the existence of GD&T symbols to justify selling us a more expensive gage.
Our purchasing deptartment was ready to go along with it too...until I found out.
All I could see was a bunch of men squawking around looking for pieces of corn on the ground.
:)
Michael
RE: Looking for some information on GD&T
Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 08; CATIA V5
ctopher's home (updated Aug 5, 2008)
ctopher's blog
SolidWorks Legion
RE: Looking for some information on GD&T
Just as much of a problem is when they try to create their own documentation instead of working with engineering to make drawing changes.
Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group
RE: Looking for some information on GD&T
How 'bout when purchasing sends out a drawing toleranced so that it cannot be made? And then chooses the supplier with the lowest bid.
The quality of the supplier is then chosen by natural selection.
Michael
RE: Looking for some information on GD&T
Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group
RE: Looking for some information on GD&T
Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 08; CATIA V5
ctopher's home (updated Aug 5, 2008)
ctopher's blog
SolidWorks Legion