GD&T question: Measuring basic dimensions

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David DeLong

#42
Believe it or not, there are times when people from different departments work together to solve problems. Sometimes these people understand that getting it right is a shared responsibility.
I absolutely agree that departments should work together with Quality having an input in the resolution.

I have seen so many times when there are problems such as out-of-position hole as an example, when Manufacturing looks at the Quality and says "What do you want me to do?" Quality is a service department and supports Manufacturing. Manufacturing would make the final decision with input from Quality.
 

Jim Wynne

Staff member
Admin
#43
I absolutely agree that departments should work together with Quality having an input in the resolution.

I have seen so many times when there are problems such as out-of-position hole as an example, when Manufacturing looks at the Quality and says "What do you want me to do?" Quality is a service department and supports Manufacturing. Manufacturing would make the final decision with input from Quality.
While I understand that in practical terms there are companies that operate this way, where the prime motivation is avoiding blame when something goes wrong, I'm not sure it's a good idea to vote in favor of it. The responsibilities of all departments, not just production and quality, are decided by company leadership, and if there is leadership, there will be shared responsibility for problem solving.
 
J

JohnForeman

#44
Reporting the basic dim along with the positional tolerance can and more often than not will be used to trouble shoot machining/manufacturing problems. This issue points out one of the main concerns I see with most CMM softwares that have GD&T capability. Most CMM programmers/operators are used to programming the systems in a way that can be used to report machining problems. But the purpose of GD&T is to clearly define the engineering intent, not the manufacturing/machining process. This creates a general conflict between how a part is toleranced and how a part is inspected. More often than not, the inspector wants to 'inspect' the part in such a way they can trouble shoot a problem, but the GD&T is not conducive to provide the results in such a way. I have been in an ongoing debate on exactly how my companies software should output GD&T results in such a way that may help with both requirements (engineering intent and manufacturing). This is not an easy debate especially since everyone in my group understands CMM output as it pertains to alignments and origins. They have a hard time separating alignments, origins and datums. Throw transformations on top of this issue and at the end of the day, they all think i'm crazy. :) All this to say that there is a place and time for outputting basic dimensions with the GD&T results. In fact, thats how I found this thread. I was looking for any industry driven standard for outputting GD&T results. This would also require some work on the DMIS standard to incorporate some of the more complex issues that come with reporting the transformation matrix's.
I've said way to much. I prolly should have started a new thread.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#45
But the purpose of GD&T is to clearly define the engineering intent, not the manufacturing/machining process. This creates a general conflict between how a part is toleranced and how a part is inspected.
Yes, as the old saying goes, "Just because you can draw it, doesn't mean you can make it. Just because you make it, doesn't mean you can measure it." You are right, GD&T is great for verifying product final dimensions to a print (engineering intent). For process dimensions and control, other, better - maybe more restrictive - dimensioning may be required. There is nothing wrong with that, as long as the final result fits into the finished product print zones. It is not one size fits all, that is for sure!
 
K

kumarkangve

#46
If basic dimensions require A tolerance thru feature control frame, Why the dimensions has to Be basic..they can give Normal dimension & Add the fetaure control frame.Does that wont work?
 

Kingsld1

Involved In Discussions
#49
I really dislike the term "Basic Dimension". I prefer to just call them Basic's.
Basics are extremely different than dimensions. Basics identify precisely where the feature control frame tolerances are to be applied. The feature control frame tolerance is how far the feature can vary from the perfection of the Basics. Adding the word dimension implies that there is a tolererance to the numbers in the box.
 
D

David DeLong

#50
I really dislike the term "Basic Dimension". I prefer to just call them Basic's.
Basics are extremely different than dimensions. Basics identify precisely where the feature control frame tolerances are to be applied. The feature control frame tolerance is how far the feature can vary from the perfection of the Basics. Adding the word dimension implies that there is a tolererance to the numbers in the box.
The term "basic dimension" complies with the North American standard on GD&T, ASME Y14.5M-94 and the newest standard of 2009. I don't believe that one should not change it for personal preference.

You are correct that, among other applications, basic dimensions reflect the true or theoretical location of a feature while the feature control frames provide the tolerances.
 
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