GD&T question: Measuring basic dimensions

Kingsld1

Involved In Discussions
#51
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.
I agree that the standard is the standard and official terms are official terms.

However, the point I was trying to make is that the word dimension in the term adds to the confusion of people trying to learn and understand GD&T. The choice of words brings in unintended difficulties as people try to relate their previous training to a new concept. By just using the word Basic the implications of the additional word dimension are not brought to mind. :2cents:
 
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Jim Wynne

Staff member
Admin
#52
I agree that the standard is the standard and official terms are official terms.

However, the point I was trying to make is that the word dimension in the term adds to the confusion of people trying to learn and understand GD&T. The choice of words brings in unintended difficulties as people try to relate their previous training to a new concept. By just using the word Basic the implications of the additional word dimension are not brought to mind. :2cents:
"Basic" in this sense is an adjective, and without a noun to modify it makes no sense. Turning adjectives and verbs into nouns is almost never a good idea. Aside from the grammatical issue (and my own pedantry in that area), there is nothing in the word "dimension" that suggests that a tolerance must be applied. The same idea applies to reference dimensions. In fact, the use of the adjective "basic" or "reference" tells you not to apply a tolerance. I think most people are capable of grasping the concept without undue confusion.
 

Jim Wynne

Staff member
Admin
#53
Hi,

Is this Due to Implied Tolerances for Normal dimensions or Is there any other Reaons

Regards

Kumar
Basic dimensions describe the ideal position or form of physical characteristics. The feature control frames describe how far from ideal the dimensions may vary. It has nothing to do with implied dimensions, which generally apply only to such things as 90-degree angles.
 

Stijloor

Staff member
Super Moderator
#54
I agree that the standard is the standard and official terms are official terms.

However, the point I was trying to make is that the word dimension in the term adds to the confusion of people trying to learn and understand GD&T. The choice of words brings in unintended difficulties as people try to relate their previous training to a new concept. By just using the word Basic the implications of the additional word dimension are not brought to mind. :2cents:
A competent GD&T Instructor/Consultant always addresses this during the GD&T training course. Our job as trainers is to explain and clarify the requirements, eliminate confusion; and part of this is getting rid of very bad tolerancing habits and misinterpretations.

People are very thankful when they see the GD&T "light."

BTW, the GD&T Standard has to be taken at its own merit, we can not deviate from it, or substitute terminology. ;)

Stijloor.
 
Y

yogurt mahony

#55
Hey. We have recently switched to GD&T format drawings, and we have much more control over the form fit and function of our parts, which is great...But there is one issue that keeps rearing its ugly head with regards to basic dimensions.

Our sheet metal suppliers have no CMM capability and limited fixturing for datum set-up. My question is this: if a part requires an in-process inspection (using a sampling plan), I'm pretty sure that GD&T type control is redundant. For example, if a part comes off a press brake, and the length of a newly formed flange needs to be measured prior to following operations (hardware insertion, deburring, final QC etc), this quick measurement needs to be taken with a calipers. This is because there is no CMM or no fixturing available in the press brake area, so there are limited resources at this time.

Our problem is that the length of this flange is only called out in the drawing with a basic dimension and need to be calculated back to the feature control frame (eg outside profile). But this is not practical in this particular scenario, and the operator should be allowed to quickly measure the flange dimension to a given tolerance. It would make complete sense to me to calculate the tolerance based on the GD&T control and use the basic dimension as the 'target'. This 'basic dim' plus 'tolerance' could then be documented on the work order/quality plan. If this is wrong, then I'd have to say that GD&T is very limited in terms of mass producing parts and in-process control. Any thoughts?
 
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