GD&T applied to reference dimension.

#1
We do wire forming and I have a part that is basically a horseshoe, but the ends are swaged to a radius for welding to a pipe. The customer has the thickness of the swage called out, but the width is a ref. dimension. Can they apply the GD&T to the centerline of the swage?
 
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Miner

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#3
So, they don't care about size (width), but location (centerline) is still important?
 
#4
So, they don't care about size (width), but location (centerline) is still important?
Correct, they care about thickness, but width is a reference. The A datum is also the one centerline, but I'm arguing it doesn't apply in the swaged area, only on the raw wire area.
 

John Predmore

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#5
Often times, "reference dimension" is used to indicate a dimension shown in one view is also shown in a different place, sometimes on a different drawing. In those cases, being a reference dimension doesn't mean the dimension is not important, it doesn't mean it can't be checked, doesn't mean there are no instructions how to measure it. Your questions about interpretation of design should go to the design-responsible engineer.

"It's not what you know that gets you in trouble, it's what you know that ain't so" -- Mark Twain
 

Sebastian

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#6
Reference means "Maybe someone wants to know, how much it is from this point to that point without Catia, or Cad, so here it is".
Usage of it, sometimes results in huge waste of time, as it requires explanations to less dimensional advanced people and in many cases might end in nothing, because e.g. "I am customer and it is on drawing, so it must be measured".
Same I see in case of theoretically exact dimensions.

When something is important do not mark it as "reference".
Next, give individual tolerance.
Next, mark with special characteristic symbol on drawing or show its significance in other product design documents, e.g. DFMEA if you are afraid to use SC symbols.

From my experience.
There was a warranty case.
Disassembled part was returned with counter part attached.
Counter part was made by other supplier.
We have seen in plant for a first time, how our part really works.
We recognized what dimension is related and reviewed our control plan.
It wasn't there.
We looked on drawing and found why.
It had general tolerance applied, while other ones had individual.
As other product design documents were missing, no, DFMEA was available in fact, but there was no "know how", so we were using policy of selecting dimensions to control, by checking what tolerance type was applied.
Individual were important and monitored.
General, some were subject of inspection during process development trials.
So our policy failed this time, but in fact someone else did not do his homework during design stage.

Conclusion - if you find it necessary, ask your customer to revise drawing and remove "reference".
Alternatively, ignore it even it may result in claim.
Then, drawing will be revised, for sure.
Keep one approach regarding dimensioning and remain consistent.
 
#7
In my experience, a Ref dimension on OEM drawing means this: it needs to be checked at lower level drawing, a lower level drawing is sub component drawing , or it can be supplier owned drawing, or it can be a purchased part drawing if that sub component is being purcahsed from tier 2 supplier( supplier of the supplier)
bottom line, the dimension still needs to be checked but on a different drawing.
Ref dimensions are also used by OEM, if that dimension cannot be checked at the assembly level, perhaps because at the assembly level you cant access that dimension to check , you would need to disassemble the entire assembly to check that dimension, which is not possible. so OEM will just mark the tolerances and make it a reference.
 

Jim Wynne

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#8
In my experience, a Ref dimension on OEM drawing means this: it needs to be checked at lower level drawing, a lower level drawing is sub component drawing , or it can be supplier owned drawing, or it can be a purchased part drawing if that sub component is being purcahsed from tier 2 supplier( supplier of the supplier)
bottom line, the dimension still needs to be checked but on a different drawing.
Ref dimensions are also used by OEM, if that dimension cannot be checked at the assembly level, perhaps because at the assembly level you cant access that dimension to check , you would need to disassemble the entire assembly to check that dimension, which is not possible. so OEM will just mark the tolerances and make it a reference.
That's the way it should be, but all too frequently even OEMs don't understand it and will mark something as a reference dimension when there is no antecedent to be found.
 
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