GD&T: True or False? 3

[stevepmd]

The following questions are not intended as an indictment of GD&T, but rather to provoke some critical thought on current theory and application.

T or F: The only reason for applying GD&T specs on component drawings is to reduce mfg costs by allowing relaxation of linear tolerances.

T or F: The application of GD&T specs on component drawings are rarely accompanied by relaxation of linear tolerances.

T or F: If the sum total of costs associated with learning, interpreting and implementing GD&T in the workplace exceeds component mfg cost saved, then GD&T is not cost-effective and thus has no legitimate usage in the engineering workplace.

T or F: GD&T theory has been "sold" to engineers by guru "disciples" in an evangelical manner - a body of commandments that must be "believed in" rather than implemented based on any practical merit.

T or F: The fact that interpretation of GD&T callouts often evoke heated discussions, indicates that GD&T theory as currently defined is flawed and possibly fundamentally defective.

T or F: The fact that "none", "a little" or "a lot" of GD&T can be arbitrarily applied to a component drawing - and still produce identical components - points to the arbitrariness of GD&T as it is currently defined and applied.

T or F: In the absence of a note to the contrary, all GD&T specs on a drawing require mandatory inspection.

T or F: Many GD&T specs are uninspectable and are thus ignored by shop and inspection personnel.

Word Problem: A component drawing with only linear dimensions/tolerances and no GD&T specs is submitted to the shop for fab. A second drawing of the same component with GD&T specs applied is also submitted to the shop for fab. After fabrication, the components are inspected and found to be geometrically identical. Question: what value has GD&T contributed to the fabrication and/or "quality" of this component?

Multiple choice: GD&T specs on a drawing are instructions to a) the machinist; b) the inspection department.

T or F: GD&T specs do not inform or instruct the machinist because component defects as covered by GD&T are a function of fabricating machine precision and thus beyond the control of the machinist.

 

[ewh]

As with most things, YMMV.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[ptruitt]

T of F: Technology can not advance competently without the additional design control that is provided by GD&T.

T of F: There are fewer and fewer easy answers to the challenges society faces.

Peter Truitt
Minnesota

 

[drawoh]

stevepmd,

My big GD&T experience was about 25 years ago when I designed a welded steel tube frame for an op[tical system to go into an aircraft. I knew that welding was not an accurate process. I don't think I had yet heard the term "GD&T". I was trying to control the outline of the frame, using ± tolerances on the linear and angular dimensions. It was a mess.

We finally got the thing built, but I understood that I was having troubles...
[ol]
[li]...specifying easily fabricated tolerances, ...[/li]
[li]...and providing a clear, unambiguous record of what I was willing to accept from the fabricator.[/li]
[/ol]

When I first saw articles on GD&T, I paid attention. This is why I like profile tolerances.

The really imporant thing about standards like ASME Y14.5 is that they allow unambiguous definition of your parts, GD&T or no GD&T.

Here is an old post of mine -- thread1103-261904. See if you can...
[ol]
[li]answer my question,[/li]
[li]get everyone else here to agree with your answer to my question.[/li]
[/ol]

Another thing you can do is Google Quest for Imperfection. and read the article in Design News magazine. I claim that the Japanese refered to in the article are preparing drawings with realistic, fabricatable tolerances, and they using these to work out accurate tolerance stacks. Their assembly procedures reflect this knowledge.

JHG

 

[legrand01]

T or F: The only reason for applying GD&T specs on component drawings is to reduce mfg costs by allowing relaxation of linear tolerances.
False - There are some features that cannot be captured by linear dimensions which GD&T can. Example: Total runout. I have no idea how I would callot out this on a drawing without having to attach a note to it.

T or F: The application of GD&T specs on component drawings are rarely accompanied by relaxation of linear tolerances.
No opinion. Does it matter?

T or F: If the sum total of costs associated with learning, interpreting and implementing GD&T in the workplace exceeds component mfg cost saved, then GD&T is not cost-effective and thus has no legitimate usage in the engineering workplace.
False: Tools that further the ability to create functional parts have a legitimate use. If you decide, however, to train everyone and make one part after gaining all this knowledge, then, yes, the ROI will be abysmal. You have to look more long term than just "today".

T or F: GD&T theory has been "sold" to engineers by guru "disciples" in an evangelical manner - a body of commandments that must be "believed in" rather than implemented based on any practical merit.
In my opinion, false.

T or F: The fact that interpretation of GD&T callouts often evoke heated discussions, indicates that GD&T theory as currently defined is flawed and possibly fundamentally defective.
False: Heated discussions arise from misinformation or lack of knowledge (training).

T or F: The fact that "none", "a little" or "a lot" of GD&T can be arbitrarily applied to a component drawing - and still produce identical components - points to the arbitrariness of GD&T as it is currently defined and applied.
False: Standards and feature importance dictate its usefulness. If one does not have standards of application, then of course it looks arbitrary, just as linear dimensioning can be seen as arbitrary.

T or F: In the absence of a note to the contrary, all GD&T specs on a drawing require mandatory inspection.
Do not all linear dimensions on a print require inspection as well?

T or F: Many GD&T specs are uninspectable and are thus ignored by shop and inspection personnel.
False: I know of none that are uninspectable, and trying to say they are incapable of being inspected points to ignorance.

Word Problem: A component drawing with only linear dimensions/tolerances and no GD&T specs is submitted to the shop for fab. A second drawing of the same component with GD&T specs applied is also submitted to the shop for fab. After fabrication, the components are inspected and found to be geometrically identical. Question: what value has GD&T contributed to the fabrication and/or "quality" of this component?
Example is too hypothetical. It was not stated if the final parts in both cases are functional. It is entirely possible that the GD&T called out a feature that was of functional importance to the part.

Multiple choice: GD&T specs on a drawing are instructions to a) the machinist; b) the inspection department.
c) BOTH

T or F: GD&T specs do not inform or instruct the machinist because component defects as covered by GD&T are a function of fabricating machine precision and thus beyond the control of the machinist.
False - If they understand GD&T, it will give them information regarding how to hold onto a part, how much care / importance to assign a given feature.

 

[btrueblood]

"T or F: GD&T specs do not inform or instruct the machinist because component defects as covered by GD&T are a function of fabricating machine precision and thus beyond the control of the machinist."

False. Implicit in the bonus tolerance scheme, e.g. for hole positioning is the idea that a clearance hole can be bored out to the LMC, and the hole pattern/location thus brought into compliance. Try implying or specifying that in less than a paragraph of note text.

 

[fsincox]

stevepmd,
I suspect you will not agree but here is my attempt:

T or F: The only reason for applying GD&T specs on component drawings is to reduce mfg costs by allowing relaxation of linear tolerances.

False, I believe it was originally intended to fill "gaps" in the old linear dimensioning method of part definition, while also, standardizing terminology based on a primarily symbolic based method to replace the need for lengthy notes. I do believe the relaxation of tolerances would be a good reason, I too, do not believe that has happened as much as it really should have.

T or F: The application of GD&T specs on component drawings are rarely accompanied by relaxation of linear tolerances.

False, properly implemented, and assuming the original part linear tolerances were specified to insure correct mating fit, it should result in greater tolerances for round features assembling in to round holes. As I have stated above, I do suspect most other tolerances have not changed much. I suspect you do not believe this either. This is not a fault of GD&T but really more a result of lazy engineering practices.

T or F: If the sum total of costs associated with learning, interpreting and implementing GD&T in the workplace exceeds component mfg cost saved, then GD&T is not cost-effective and thus has no legitimate usage in the engineering workplace.

False, I believe, your implied basic assumption is that an accurate manufacturing based definition of a part is really completely adequate. It also assumes production processes will never need to change and an attempt to define the part's quality in terms of functional requirements will never be desired/required. I feel this is only true from a “what do I need now” point of view. I believe there is an inherent value added by documenting the true functional requirements from an engineering historical perspective to the longer term system operation.

T or F: GD&T theory has been "sold" to engineers by guru "disciples" in an evangelical manner - a body of commandments that must be "believed in" rather than implemented based on any practical merit.

False, No business manager would do that, it is generally practical problems that lead one to GD&T as a possible solution only after multiple exhausted attempts at alternatives. Form my 30+ years in the business the story is never that simple. Managers know there are problems and are hoping (and possibly are being “sold”) to find a solution to problems they already have.

T or F: The fact that interpretation of GD&T callouts often evoke heated discussions, indicates that GD&T theory as currently defined is flawed and possibly fundamentally defective.

False, I would say it is the result of bad training and understanding the actual standard, combined with a natural desire to oversimplify complex real world problems.

T or F: The fact that "none", "a little" or "a lot" of GD&T can be arbitrarily applied to a component drawing – and still produce identical components - points to the arbitrariness of GD&T as it is currently defined and applied.

False, it means you have been lucky so far. It would imply you mo do not farm out a lot of work and have excellent communication between the shop and engineering groups (generally only possible in a small time operation).

T or F: In the absence of a note to the contrary, all GD&T specs on a drawing require mandatory inspection.

False, No more than a linear dimension does. My favorite example is the envelope principle a concept developed before the turn of the last century, implemented in all versions of the "American" standards and almost always ignored if parts are not being checked with gauges.

T or F: Many GD&T specs are uninspectable and are thus ignored by shop and inspection personnel.

False, it is generally not completely ignored by inspection or they would not be complaining about them. Runout is an attempt to document an inspection method.

Word Problem: A component drawing with only linear dimensions/tolerances and no GD&T specs is submitted to the shop for fab. A second drawing of the same component with GD&T specs applied is also submitted to the shop for fab. After fabrication, the components are inspected and found to be geometrically identical. Question: what value has GD&T contributed to the fabrication and/or "quality" of this component?".

About the same value as most of the rest of the engineering functions contributes to these.

Multiple choice: GD&T specs on a drawing are instructions to a) the machinist; b) the inspection department.

I do not think of a drawing as only an instruction sheet, the engineering department is attempting to document part requirements to everyone including for their own historical perspective. Really in my opinion it is: c) all of the above, including the engineering department

T or F: GD&T specs do not inform or instruct the machinist because component defects as covered by GD&T are a function of fabricating machine precision and thus beyond the control of the machinist.

False, I do not believe a real skilled machinist has no control of the quality of his work, also he should look at the tolerances specified and have some idea of what processes will be required to produce a so specified part.

 

[ewh]

Very good points, fsincox.
A drawing is not an instruction sheet. A drawing is a part definition. How to achieve that definition, while it should be considered in the design, is really a matter for further documentation.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[CheckerHater]

T or F: The only reason for applying GD&T specs on component drawings is to reduce mfg costs by allowing relaxation of linear tolerances.
FALSE: The reason is to help Manufacturing to make fewer mistakes than usual.

T or F: The application of GD&T specs on component drawings are rarely accompanied by relaxation of linear tolerances.
SEE ABOVE

T or F: If the sum total of costs associated with learning, interpreting and implementing GD&T in the workplace exceeds component mfg cost saved, then GD&T is not cost-effective and thus has no legitimate usage in the engineering workplace.
FALSE: In the long run it pays back, for short-term gain old-school personnel is usually laid off.

T or F: GD&T theory has been "sold" to engineers by guru "disciples" in an evangelical manner - a body of commandments that must be "believed in" rather than implemented based on any practical merit.
FALSE: GD&T professionals see themselves rather as lawyers of Engineering, charging suborbital fees for arguing about meaning of the word “Radius”.

T or F: The fact that interpretation of GD&T callouts often evoke heated discussions, indicates that GD&T theory as currently defined is flawed and possibly fundamentally defective.
FALSE: It only happens in groups where less than 5% of participants are GD&T-savvy.

T or F: The fact that "none", "a little" or "a lot" of GD&T can be arbitrarily applied to a component drawing - and still produce identical components - points to the arbitrariness of GD&T as it is currently defined and applied.
TOO VAGUE: Some parts being produced in different shops in different quantities may require different level of GD&T specified on the print.

T or F: In the absence of a note to the contrary, all GD&T specs on a drawing require mandatory inspection.
FALSE, But for not GD&T-related reason: Everything on the drawing requires inspection at some point.

T or F: Many GD&T specs are uninspectable and are thus ignored by shop and inspection personnel.
FALSE: There is a fine line between uninspectable and prohibitively expensive to inspect. If you try really hard you can inspect practically anything.

Word Problem: A component drawing with only linear dimensions/tolerances and no GD&T specs is submitted to the shop for fab. A second drawing of the same component with GD&T specs applied is also submitted to the shop for fab. After fabrication, the components are inspected and found to be geometrically identical. Question: what value has GD&T contributed to the fabrication and/or "quality" of this component?
The second drawing was quoted extra 5% for every FCF.

Multiple choice: GD&T specs on a drawing are instructions to a) the machinist; b) the inspection department.
None of the above. It serves all departments, at least it used to be.

T or F: GD&T specs do not inform or instruct the machinist because component defects as covered by GD&T are a function of fabricating machine precision and thus beyond the control of the machinist.
FALSE: They give machinist a warning when it comes to re-fixturing complicated part.

 

[powerhound]

T or F: The only reason for applying GD&T specs on component drawings is to reduce mfg costs by allowing relaxation of linear tolerances. False. It is one reason but the only reason.

T or F: The application of GD&T specs on component drawings are rarely accompanied by relaxation of linear tolerances. I don't know what you mean here.

T or F: If the sum total of costs associated with learning, interpreting and implementing GD&T in the workplace exceeds component mfg cost saved, then GD&T is not cost-effective and thus has no legitimate usage in the engineering workplace. I tried to answer this but there are so many variables to your hypothetical that I'll just leave it to the others that took the time to answer.

T or F: GD&T theory has been "sold" to engineers by guru "disciples" in an evangelical manner - a body of commandments that must be "believed in" rather than implemented based on any practical merit. False. GD&T is not a theory...unless I'm mistaken in what I believe a "theory" is. GD&T is absolutely practical.

T or F: The fact that interpretation of GD&T callouts often evoke heated discussions, indicates that GD&T theory as currently defined is flawed and possibly fundamentally defective. Very false. These arguments usually stem from ignorance of GD&T, not knowledge of it.

T or F: The fact that "none", "a little" or "a lot" of GD&T can be arbitrarily applied to a component drawing - and still produce identical components - points to the arbitrariness of GD&T as it is currently defined and applied. False. GD&T does not produce parts, machinists do.

T or F: In the absence of a note to the contrary, all GD&T specs on a drawing require mandatory inspection. False. They don't require inspection any more than a non-GD&T dimension

T or F: Many GD&T specs are uninspectable and are thus ignored by shop and inspection personnel. False. Name one "uninspectable" GD&T dimension.

Word Problem: A component drawing with only linear dimensions/tolerances and no GD&T specs is submitted to the shop for fab. A second drawing of the same component with GD&T specs applied is also submitted to the shop for fab. After fabrication, the components are inspected and found to be geometrically identical. Question: what value has GD&T contributed to the fabrication and/or "quality" of this component? This question is a set-up.


Multiple choice: GD&T specs on a drawing are instructions to a) the machinist; b) the inspection department. Both

T or F: GD&T specs do not inform or instruct the machinist because component defects as covered by GD&T are a function of fabricating machine precision and thus beyond the control of the machinist. False, and your reasoning is flawed. Where do you get the idea that "component defects as covered by GD&T are a function of fabricating machine precision and thus beyond the control of the machinist."? Perhaps this isn't your reasoning.


Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[ptruitt]

T or F: The only reason for applying GD&T specs on component drawings is to reduce mfg costs by allowing relaxation of linear tolerances.

True only if you exclude the "...by allowing relaxation of linear tolerances", and adopt the cost that Dr. Taguchi includes in his definition of quality.

T or F: The application of GD&T specs on component drawings are rarely accompanied by relaxation of linear tolerances.

I don't know. You can lead a horse to water... What's with the inclusion of 'linear' in the questions? It implies the author does not have proper training, if any training, in GD&T.

T or F: If the sum total of costs associated with learning, interpreting and implementing GD&T in the workplace exceeds component mfg cost saved, then GD&T is not cost-effective and thus has no legitimate usage in the engineering workplace.

True: But it implies that an organization, somewhere, and it's associates are inept and should immediately close their doors.

T or F: GD&T theory has been "sold" to engineers by guru "disciples" in an evangelical manner - a body of commandments that must be "believed in" rather than implemented based on any practical merit.

Not true in my experience, but it implies that organization somewhere is inept.

T or F: The fact that interpretation of GD&T callouts often evoke heated discussions, indicates that GD&T theory as currently defined is flawed and possibly fundamentally defective.

False: It implies that an organization somewhere does not have proper training, if any training, in GD&T.

T or F: The fact that "none", "a little" or "a lot" of GD&T can be arbitrarily applied to a component drawing - and still produce identical components - points to the arbitrariness of GD&T as it is currently defined and applied.

False: If specifications of any kind are arbitrarily (arbitrarily!) applied to two designs that subsequently result in what someone might determine to be identical (identical by what definition I can only imagine), it may be coincidental, but it certainly does not point "...to the arbitrariness of GD&T as it is currently defined and applied."

T or F: In the absence of a note to the contrary, all GD&T specs on a drawing require mandatory inspection.

False: Read the Standard.

T or F: Many GD&T specs are uninspectable and are thus ignored by shop and inspection personnel.

Invalid: The question makes a faulty statement which makes the entire question invalid.

Word Problem: A component drawing with only linear dimensions/tolerances and no GD&T specs is submitted to the shop for fab. A second drawing of the same component with GD&T specs applied is also submitted to the shop for fab. After fabrication, the components are inspected and found to be geometrically identical. Question: what value has GD&T contributed to the fabrication and/or "quality" of this component?

Invalid: GD&T definitions are not equivalent to non-GD&T definitions, so they can not be "geometrically identical".

Multiple choice: GD&T specs on a drawing are instructions to a) the machinist; b) the inspection department.

Excluding, ISO GD&T, ASME Y 14.5 GD&T is goal-oriented, not process-oriented. Calling ASME GD&T symbols 'instructions' would not be accurate.

T or F: GD&T specs do not inform or instruct the machinist because component defects as covered by GD&T are a function of fabricating machine precision and thus beyond the control of the machinist.

False: GD&T symbols define designs that may or may not be manufacturable. Since they are more concise than non-GD&T symbols it is easier to determine if a given manufacturing process will be able to accomplish the goal. The question indicates that an organization somewhere does not have proper training, if any training, in GD&T or is generally incompetent.


Peter Truitt
Minnesota

 

[fcsuper]

In my experience, many engineers have an irrational fear of GD&T that is based on falsehoods. The problem? It's not taught in any detail to engineering students. More time is spent on the topic in drafting trade schools and JC drafting programs than in the Engineering programs.

Also, in my experience, most vendors that receive GD&T on a drawing know GD&T far better than most engineers. So, don't worry about a vendor jacking up the price on you. There's a reason you send out for competitive bids. If one vendor doesn't know how to use it (or thinks you don't), then they will reveal themself with a high bit that will put them out of competition for the job.

Matt Lorono, CSWP
Lorono's SolidWorks Resources & SolidWorks Legion
Follow me on Twitter

 

[powerhound]

stevepmd,

Aside from the critical thought you were trying to inspire, was there a point to your quiz? You said it was not an indictment of GD&T but it kind of was. Every thought you were trying to elicit was a negative one regarding GD&T. There were also some false statements mixed in.

I'd really like to get your answers to your questions along with your reasoning as we all did here.

Thanks,

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[KENAT]

Or, you could look at some of the previous threads that touch on this type of topic.

thread1103-192933
thread1103-261325
thread1103-152475
thread1103-160822

Posting guidelines faq731-376

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(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[fsincox]

powerhound,
Correct me if I am wrong stevepmd, my impression, just based on the wording alone would be that his responces would be "true" in all the T/F cases.
Frank

 

[ptruitt]

The pattern in the T/F questions is familiar: Someone makes a statement that is erroneous or off-base sound as if it is a proper statement and then goes on to elicit discussion on it as if it is a proper basis for discussion, cementing in the initially unfortunate statement. It is an insidious way to wreck a potentially honest and valuable discussion.

Peter Truitt
Minnesota

 

[drawoh]

stevepmd,

Aside from the critical thought you were trying to inspire...

He sounds like a troll. I treated his questions as rhetorical. He has not logged in and answered mine.

JHG

 

[btrueblood]

I am in the process of finishing a drawing package for a certain off-shore vendor. At the boss' and vendor's request, all GD&T has been removed, and replaced with verbal descriptions of the requirements. The drawings have grown by several sheets to make room for the text, and this without even attempting to explain the concept of bonus tolerances or LMC/MMC conditions. I have a new appreciation for the Y14.5 standard. I'd propose the same excersize to anyone else who discounts the value of the standard, the training required to understand it, and the people who claim it only adds cost.

 

[fsincox]

btrueblood,
Thank you for you personal insight, I had suspected as much.
Then try to get people to agree on what it means. Parallel for example: I have had many shop people tell me a part is parrallel if it looks like a snake. I actually came up with a concept for it based on my misinterperetation of the old ISO parallelism shown with leaders to separate surfaces and no datum reference stated. I called it "equidistant". I had figured it was so common that we should have a term for it.
Frank

 

[KENAT]

btrue, some of us might have been smart asses and just transcribed big chunks of 14.5 onto the drawing in face of such a request.

Posting guidelines faq731-376

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(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​