Who's responsible for understanding GD&T besides the drafter/designer?
Who's responsible for understanding GD&T besides the drafter/designer?
(OP)
Drafters and designers are expected to have more than a basic understanding of GD&T, they're expected to have a working knowledge on how to apply it in order to produce fit and function at a reasonable cost. In addition to the basic courses, many pursue supplemental classes that teach advanced techniques that relate to specific problem solving that may have a direct impact on their design projects. In my years of working in the automotive and aerospace (spacecraft) industries I've yet to see another person outside our profession hold the same expertise we do. Sure, some inspectors have the basic understanding of single and multiple datum reference frames, or even understand the basic concept of datum precedence but are they expected to have the same level of expertise we have? What about the machinist? How many have taken advanced courses and maintained current in ASME or ISO standards? My brother is an estimator at a contract shop and confessed to me yesterday that most drawings that come in with a tolerance spec of .002 or under, are simply assigned to their best machine (The Hurco)that will insure compliance and produce the whole part better than required. If the drawing contains anything questionable, they'll call the originator for clarification. They will do whatever it takes to get the part out within specs on time but, they won't spend the extra time to find the virtual conditon of features if they have to take into account multiple variables such as Datum Shift thru size modifiers on datum references. Time is money and if the drawing requires a lot of it they lose profit. So to prevent losses, they'll simply run the part on their best machine....even if it means opening up a new shift. Do the inspectors have the same working knowledge we do? I may be wrong in thinking this way but are we wasting our time with calling out more than the basic GD&T needed to define our parts if we can simply add a note on the face of the drawing specifing to be made on their best machine or an overall tolerance their best machine is capable of maintaining since it that's what's likely to occur anyways?





RE: Who's responsible for understanding GD&T besides the drafter/designer?
Engineers, on the other hand, seem to only know the basics, if that ;)
One of the better companies that I worked for made sure that all effected personel received training in ASME Y14.5, even the engineers.
RE: Who's responsible for understanding GD&T besides the drafter/designer?
Anyone that works with the drawings, should have a understanding of GD&T. But, it's not real world. A lot of companies leave it off because they just don't care or understand it.
Chris
Systems Analyst, I.S.
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RE: Who's responsible for understanding GD&T besides the drafter/designer?
I have talked with machine shops that don't have a clue about how GD&T can actually save them time, money and materials. And don't get me started about functional datums.... I think everyone in the design and manufacturing process should have an understanding of how to apply GD&T and what it means. To me I consider it a language and a means of communication.
Best Regards,
Heckler
Sr. Mechanical Engineer
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RE: Who's responsible for understanding GD&T besides the drafter/designer?
Those in the Engineering, Design, and Drafting fields as a collective tend to further the advancement and understanding of GD&T by sharing ideas and techniques in an open forum such as this one. In addition, companies such as yours will pay for additional education in hopes that what was learned in class will eventually trickle down to enhance the overall quality of the product...if not, at least reduce the cost and/or ease the manufacturability of the product. The ROI is worth it to some, if not most companies. As time goes on, those in our field become experts in communicating thru the use of "ANSI Speak". We use this language to communicate on paper our design intent. The question is, if we're the ones fast pitching the good word, who's catching it at the other end? Do the people (machinist and Inspectors) speak it as well as we do....or is the reality of it is that they speak "broken ANSI"? How many of them have you encountered in your advanced courses? I have only met one inspector in my 24 years proficient enough to know what rules to break. Just pondering the reality of the catchers out there in our industry.
RE: Who's responsible for understanding GD&T besides the drafter/designer?
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RE: Who's responsible for understanding GD&T besides the drafter/designer?
In my case I only have a limited knowledge of it and today you have to be the engineer/designer/drafter all in one. You have to choose what things you can be an expert in. I don't put it on a part unless I absolutely have to. However, I like it for runouts and stuff related to rotating parts because you can pick functional datums and the callouts are easier to use than saying "must be square with the diameter." I try to use plus/minus tolerances instead of true position most of the time- the people in the shop understand it and can measure it.
RE: Who's responsible for understanding GD&T besides the drafter/designer?
Chris
Systems Analyst, I.S.
SolidWorks/PDMWorks 05
AutoCAD 05
ctopher's home site (updated 06-21-05)
FAQ559-1100
FAQ559-716
RE: Who's responsible for understanding GD&T besides the drafter/designer?
Thereafter, I used feature control symbols to clarify drawings, by replacing awkward text.. etc. But I had to be careful about it, because the skill of reading the symbols was not widespread.
It damn near killed off a medical electronics company that I worked for. It was implemented by stupid zealots. Example; a flat plate 'located' by 13 clearance holes. Worked just fine, until somebody ran the equations, and the plate ended up with ~.255 clearance holes around #10 screws. Then they needed a fixture to locate the plate, because the screws let it wobble around so much that it didn't align with anything. And they had to put oversize washers under the heads, so the heads wouldn't go right through the clearance holes.
In many cases, it's like giving a child a chainsaw.
Mike Halloran
Pembroke Pines, FL, USA
RE: Who's responsible for understanding GD&T besides the drafter/designer?
If they listened to the Designer, it would not have had happened.
Chris
Systems Analyst, I.S.
SolidWorks/PDMWorks 05
AutoCAD 05
ctopher's home site (updated 06-21-05)
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RE: Who's responsible for understanding GD&T besides the drafter/designer?
GD&T was used as it was intended--as a method of locating and controlling features that allows more parts to be accepted.
Very Sweet.
I have to confess that I have very little patience anymore with shops that say they don't understand GD&T nearly 40 years after it became a national standard. My response is usually, "Find a shop that's current. Do they still use dial mills too?" Precision tolerancing and gauging is only 108 years old. GD&T first hit the scene in the UK, before WWII! How long does it take for a competent understanding to take hold in the industry?
RE: Who's responsible for understanding GD&T besides the drafter/designer?
Chris
Systems Analyst, I.S.
SolidWorks/PDMWorks 05
AutoCAD 05
ctopher's home site (updated 06-21-05)
FAQ559-1100
FAQ559-716
RE: Who's responsible for understanding GD&T besides the drafter/designer?
Chris......I don't have the luxury of choosing the shop. That gets awarded to who ever won during the bidding process. We don't have any influence whatsoever.
I agree "cost will skyrocket if not used correctly", however, how do we know the people on the manufacturing side know how to read and correctly apply GD&T? How many of you on the engineering side encounter them asking questions about the same GD&T techniques we're asking of one another? How often do you see them on this forum concerned with advanced or one-off custom callouts? This is a concern if 'we' are the only experts singing soprano ANSI and know one else is buying tickets to the show.
RE: Who's responsible for understanding GD&T besides the drafter/designer?
RE: Who's responsible for understanding GD&T besides the drafter/designer?
If the shop that won the contract doesn't understand GD&T then shame on the business development, purchasing development or technical procurement folks at your company for awarding the contract to a substandard contractor. If you're company is ISO9000, AS9000 or QS9000 that should be part of your documented processes is making sure your vendors can comply to those standards.
Best Regards,
Heckler
Sr. Mechanical Engineer
SW2005 SP 5.0 & Pro/E 2001
Dell Precision 370
P4 3.6 GHz, 1GB RAM
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NVIDIA Quadro FX 1400
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"Coming together is a beginning, staying together is progress, and working together is success." - Henry Ford
RE: Who's responsible for understanding GD&T besides the drafter/designer?
I agree strongly. Your drawing tells the vendor what it is you are going to accept. To all intents and purposes, the dimnensioning and tolerance system defined by ASME Y14.5M-1994 is a language. If they do not understand the language, you are not getting conforming parts.
I am not even sure of the cost issue, at least with a competent shop. A lot of my weirder GD&T stuff occurs because I am trying to allow as much variability as possible consistent with a functioning part. This is critical when you stop designing machined parts and get into sheet metal, weldments and castings.
JHG
RE: Who's responsible for understanding GD&T besides the drafter/designer?
Heckler,
The business "shame" is placed upon whould be the U.S. government. I'm not going to defend the practices just state the fact of what I have to work with here.
EWH,
I'm hoping for suggestions to alternate methods from as many who are willing.
RE: Who's responsible for understanding GD&T besides the drafter/designer?
Drawoh,
Even with a competent shop, have you ever spoke ANSI with the machinest who's making your part? Find me one who knows as much as you or I and I'll add that shop to the list. Most of the time, if not all, they'll have your drawings taped to their CNC and recalculating the dimensions to ordinate or you'll recieve a call asking for the 3d cad model....how much time do you think is being paid to our wonderful ANSI concerto?
RE: Who's responsible for understanding GD&T besides the drafter/designer?
Sometimes the U.S. Government doesn't make the right decisions based on lack of knowledge. I was working on a manufacturing project about a year ago for the Navy. All the drawings called out revisions to material specs which is a big cost issue. Most material suppliers and Mills cert materials to the current spec. And these drawings dated back to the late eighties. It was a big paperwork exercise to get the Government to buy off on this when it was there mistake to begin with.
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
NVIDIA Quadro FX 1400
o
_`\(,_
(_)/ (_)
"Coming together is a beginning, staying together is progress, and working together is success." - Henry Ford
RE: Who's responsible for understanding GD&T besides the drafter/designer?
Heckler,
There will always be mistakes with documentation in our industry. It's a long road when trying to standardize the company you're working in. No doubt your experience reflected a lack of coordination with industry standards. In regards to the original question, are you familiar with any machinists or inspectors who are responsible for the making and checking of your parts? Have you walked away wondering if they new what your callouts really meant. I don't know of any that require to have basic, let alone advanced, ANSI courses on their resume'.
RE: Who's responsible for understanding GD&T besides the drafter/designer?
I've also worked at places that QA was a joke.....I had to explain a weld symbol callout. I even had a vendor not know what NS / FS ment....Near Side / Far Side.
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
NVIDIA Quadro FX 1400
o
_`\(,_
(_)/ (_)
"Coming together is a beginning, staying together is progress, and working together is success." - Henry Ford
RE: Who's responsible for understanding GD&T besides the drafter/designer?
Half the battle is having your Inspectors know GD&T as much as you do....consider youself blessed if they know more. The other half is having your Machinist know GD&T as much as you. The odds are pretty slim however since this is not the norm. All our contractors are AS9000 certified and yet, still find a large percentage of them asking for the 3d cad models. When this happens, the probability of having the machinist read and interpret GD&T is far less. One would be hard pressed to find both the machinist and inspector responsible for interpreting your drawing and making your part, able to read ANSI as well as you or I. Again, I've yet to see either of them with same expertise as those in our field who are continuously adding to their education of GD&T application.
I don't have an answer for the mis-match between the intended and the interpreted, I'm just wondering if the direction we're heading, unless everyone who's involved participates, is a correct one.
RE: Who's responsible for understanding GD&T besides the drafter/designer?
Tick,
When in doubt, spell it out.
RE: Who's responsible for understanding GD&T besides the drafter/designer?
Personally, I don't wait for mistakes to end up in "GDT court". Before steel gets cut, I'm in touch with vendors or machinists to make sure any arcane points are understood.
RE: Who's responsible for understanding GD&T besides the drafter/designer?
I have used several shops that understand most of GD&T. I have only ever met one person that knows it (or understands it) 100%.
If the shop does not understand something on the dwg, they call me and we go over it together. If we both can not agree with the understanding og the specific GD&T, we find help elsewhere until problem fixed.
I use GD&T on all parts. The more I use it, the more I understand it. Same for the shops and inspectors.
I get purchasing all the time try to change the dwg to remove GD&T off. If I lose the battle, I tell them to have someone else do the change because I will not be responsible for any parts that do not work.
My outlook on this? If you design parts, have some understanding of it. Find a shop that understands it. When purchasing goes out for bids, let them know that the understanding of GD&T is included somehow.
I agree that CAD/CAM makes it easier for a michinist to machine a part without much GD&T knowledge, but it still has to get inspected ... and fit.
What you put onto a dwg is what you get in the part.
Chris
Systems Analyst, I.S.
SolidWorks/PDMWorks 05
AutoCAD 05
ctopher's home site (updated 06-21-05)
FAQ559-1100
FAQ559-716
RE: Who's responsible for understanding GD&T besides the drafter/designer?
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
NVIDIA Quadro FX 1400
o
_`\(,_
(_)/ (_)
"Coming together is a beginning, staying together is progress, and working together is success." - Henry Ford
RE: Who's responsible for understanding GD&T besides the drafter/designer?
Do you recall the big push towards converting our coveted archaic inch system to metric? Back in the mid eighties, I was working for a Class 8 truck manufacturer that adopted the metric system on all cad drawings (we used Cadam back then). The problem we encountered time after time was that most vendors weren't embracing the new system so, as a favor to them, we referenced the inch equivalence in order to ensure no ambiguity occured when interpreting our drawings. The struggle to conform to something as simple as the millimeter never caught on, sad really. This is how I feel with most GD&T....not all mind you. It seems that we have to be there to clarify and interpret for them so there's no ambiguity and possible mis-interpretation.....remember time is money and if there are ANY errors on the fabricated parts, profits are lost! They learn only enough to get by to appease us and say they can read GD&T in order to be certified by the latest ISO. They need to do this to stay in business. We are still the only one's persuing it's advancement. I feel we are still holding their hand. Has anyone of you felt completely comfortable knowing you've applied, not just basic but, advanced callouts and had complete faith they've interpreted your drawing without ambiguity time and time again, year after year? It's been over 40 years....you'd think it would have caught on by now.
RE: Who's responsible for understanding GD&T besides the drafter/designer?
From my experience, there is more mis-interpretation of dim's and tol on a dwg without GD&T. With GD&T, there is only one way to interpret it. The problem is not with the standards, it's with the user’s mis-interpretation of it.
i.e. If you have a part with a hole thru it and a centerline drawn thru it, the hole is theoretically straight, the machinist will machine a straight hole, but is it 90 deg to a surface? It may or not be. But, because there is not any GD&T, it is OK, but may not work with mating parts. With GD&T, you can make sure it is positioned exactly the way you need it and it can be inspected. Without GD&T, inspector’s jobs can become very easy. IMO, if everyone would take the time to understand it more, we would be more ahead than we are now.
Chris
Systems Analyst, I.S.
SolidWorks/PDMWorks 05
AutoCAD 05
ctopher's home site (updated 06-21-05)
FAQ559-1100
FAQ559-716
RE: Who's responsible for understanding GD&T besides the drafter/designer?
As far as parts made out of house, the majority are tooling with complex lofted surfaces, and the solid model is the definition, so it goes out with the p/o. Since it is usually for tooling, we only specify a profile tolerance and let them determine any other tolerancing. This relieves much of the headache associated with trying to follow the standard.
RE: Who's responsible for understanding GD&T besides the drafter/designer?
I am not absolutely confident in our machine shop's understanding of GD&T, but usually, machine shops are not the problem. Machining is an accurate process. In most cases, a machine shop supervisor should be able to examine a drawing and satisfy themselves that everthing can be done easily with their standard tools. The machinist can be told the use the DXF file to generate the CAM model. Machine shops easily work to within +/-.005".
If they ship you a non-conforming part, you ship it back. They will learn.
Machining does a great job of covering up an atrocious lack of comprehension of drafting by both design and manufacturing.
The fun starts when you punch sheet metal, then bend and weld it for precision electronics. It appears to me that sheet metal shops are more sophisticated about GD&T. They have to be, as do designers specifying this stuff. My favorite shop bends sheet metal to +/-.015".
I do not send enough stuff out to welding shops and foundries to make generalizations about them.
JHG
RE: Who's responsible for understanding GD&T besides the drafter/designer?
Ctopher,
You hit the nail on the head. My concern is not everyone involved, that should be, has jumped on the bandwagon with us....which is why we're not further ahead than where we should be. In fact, we're slipping back every time they ask for a 3d cad file.
EWH,
The people in-house are most likely following your lead(and the people in your department making drawings) in order to be able to understand your language. But, ask yourself this, how many of those machinist and inspectors have taken advanced GD&T courses....how many do you encounter on the forums pursuing their own advancement of ANSI knowledge to further the application of thier use of the language we in our field tend to sing with? As far as sending out a solid model, the shop will unlikely even look at your control boxes let alone extrapolate percentages based on tolerances with or without bonuses. Why should they?.....their 5-axis will ensure your part not only meets but exeeds your specs....time is money.
RE: Who's responsible for understanding GD&T besides the drafter/designer?
Drawoh,
See my reply to EWH. The reason shops are able to cover up the lack of understanding is because their machines can make the parts using the dxf file accurate enough to exeed specs on the drawing. The drawing is most likely just used as reference at the machine shop. I worked at a machine shop when I was 18 as a jitterbug (we made all the Atari enclosures) and witnessed first hand what sheetmetal stretch and deformation really was. There's more geometry involved with bent parts....understanding GD&T can be more critical than throwing a hunk of 6061-T6 on a Hurco and letting it run.
RE: Who's responsible for understanding GD&T besides the drafter/designer?
I laugh when someone tells me that every dimension is critical, and I refuse to approve or verify that drawing.
I really laugh when I get floating centerline to obscure angle as critical. To those people I say, "If you need a coordinate measuring machine (CMM) to verify that dimension, don't make it critical"
RE: Who's responsible for understanding GD&T besides the drafter/designer?
Maybe saying dimensions are 'less critical' would work better.
Sorry, my soap box is starting to crack and splinter.
RE: Who's responsible for understanding GD&T besides the drafter/designer?
Chris
Systems Analyst, I.S.
SolidWorks/PDMWorks 05
AutoCAD 05
ctopher's home site (updated 06-21-05)
FAQ559-1100
FAQ559-716
RE: Who's responsible for understanding GD&T besides the drafter/designer?
RE: Who's responsible for understanding GD&T besides the drafter/designer?
Chris
Systems Analyst, I.S.
SolidWorks/PDMWorks 05
AutoCAD 05
ctopher's home site (updated 06-21-05)
FAQ559-1100
FAQ559-716