GD&T question: Measuring basic dimensions

Jim Wynne

Staff member
Admin
#21
By providing them with the proper convention for reference dimensions.

A dimension placed between parentheses is for reference only.

Hope this helps.

Stijloor.
There's still a problem with even properly identified reference dimensions. "Reference" shouldn't be used for "Don't measure this" or "Don't apply the block tolerance" or "This dimension is what it is." A reference dimension should be one that's controlled at a different level, or elsewhere in the dimensioning of the same drawing. An example of the former case is a dimension on an assembly drawing that's controlled on a component drawing. For the latter case, a proper reference dimension might be an overall length where there is subsidiary dimensioning (individually toleranced) that can be added up to show the overall length.
 
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bobdoering

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#22
There's still a problem with even properly identified reference dimensions. "Reference" shouldn't be used for "Don't measure this" or "Don't apply the block tolerance" or "This dimension is what it is." A reference dimension should be one that's controlled at a different level, or elsewhere in the dimensioning of the same drawing. An example of the former case is a dimension on an assembly drawing that's controlled on a component drawing. For the latter case, a proper reference dimension might be an overall length where there is subsidiary dimensioning (individually toleranced) that can be added up to show the overall length.
I would buy into the use of reference for "Don't measure this" as a way to locate a specific feature - more of a map function than a measureable. But, I agree it generally infers that as a measureable it is controlled by other features, and to give it a tolerance would be double dimensioning. As far as "Don't use block tolerance" - well, OK, I also agree there. Give me a stated tolerance for the dimension, do not call it a reference.
 
J

JaxQC

#23
To confirm, No “Doesn't the general print tolerance apply to a basic dimension?”

One item I didn’t see addressed is that when doing a dimensional report, the basic dimension should be reported. Since it will never be the “exact number” the amount it is off relates back to certain positional tolerances and is used in the Bonus calculations. Even if you figure Bonus at the time, it’s nice to have the data there a year later to understand/confirm how it was determined at the time of the check.

Same for reference dimensions, doesn't reject a part but should be recorded.
 

Stijloor

Staff member
Super Moderator
#24
To confirm, No “Doesn't the general print tolerance apply to a basic dimension?”

One item I didn’t see addressed is that when doing a dimensional report, the basic dimension should be reported. Since it will never be the “exact number” the amount it is off relates back to certain positional tolerances and is used in the Bonus calculations. Even if you figure Bonus at the time, it’s nice to have the data there a year later to understand/confirm how it was determined at the time of the check.

Same for reference dimensions, doesn't reject a part but should be recorded.
I disagree. What you report is the deviation from whatever the basic dimension indicates. Whether it is true position or profile. The reports, if not interpreted correctly, add to the tragic GD&T confusion. Trust me, I've been there...

Stijloor.
 

Jim Wynne

Staff member
Admin
#25
To confirm, No “Doesn't the general print tolerance apply to a basic dimension?”

One item I didn’t see addressed is that when doing a dimensional report, the basic dimension should be reported. Since it will never be the “exact number” the amount it is off relates back to certain positional tolerances and is used in the Bonus calculations. Even if you figure Bonus at the time, it’s nice to have the data there a year later to understand/confirm how it was determined at the time of the check.

Same for reference dimensions, doesn't reject a part but should be recorded.
I agree with recording of measurements that contribute to verification of feature controls, but there's generally no good reason to waste time measuring and recording reference dimensions. AIAG (in the PPAP manual) specifically excludes reference dimensions from dimensional reporting requirements.
 

Jim Wynne

Staff member
Admin
#26
I disagree. What you report is the deviation from whatever the basic dimension indicates. Whether it is true position or profile. The reports, if not interpreted correctly, add to the tragic GD&T confusion. Trust me, I've been there...

Stijloor.
I've also seen instances where dimensional reports were fudged, and you could tell because the requirements for a position tolerance didn't add up to the result in the report. I like to see the basic dimensions reported for position callouts.
 
S

sept68

#27
Another problem is that one of our customers who has design responsibility loves to use surface profiles. What so frustrating is that they apply this to the every print's general tolerance. I don't this is right or fair. And shouldn't be legal. They cover themselves by saying all Basic Dimensions included in this catch all. Depending on the feature we are expected check for parallelism, perpendicualrity, position, angularity, symmetry, flatness, etc if it "potentially" applies. It costs a fortune to layout product which frankly they don't pay that much for. The ratio of surface profile to other controls is at least 4 to 1.
 
J

JaxQC

#28
I'll still stand behind my answer that it's good to see the basic dimensions reported for position callouts for exactly the reason stated about you can tell the numbers were fudged. Also with reference dims. If they are not recorded, how do you compare them to future parts and how they match? Mostly for problem solving I admit.

I might have another option if I thought the design groups always had everything perfect rather than just making a dimension reference (even if it was important) just because they couldn’t hold it in mfg.
 
J

JaxQC

#29
I'd already responded before seeing Sep68's response. (if it "potentially" applies) I smell a scam. That's taking more than little liberty I'd say on their part.
 

Stijloor

Staff member
Super Moderator
#30
Another problem is that one of our customers who has design responsibility loves to use surface profiles. What so frustrating is that they apply this to the every print's general tolerance. I don't this is right or fair. And shouldn't be legal. They cover themselves by saying all Basic Dimensions included in this catch all. Depending on the feature we are expected check for parallelism, perpendicualrity, position, angularity, symmetry, flatness, etc if it "potentially" applies. It costs a fortune to layout product which frankly they don't pay that much for. The ratio of surface profile to other controls is at least 4 to 1.
Wait 'till you see the new Standard where now tolerances are specified relative to the CAD model. Meaning that "the model" is the basic dimension.

Very common now in automotive, which means that the supplier is expected to acquire the appropriate (CMM) software to inspect the part.

Stijloor.
 
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