Flexible "Ring" Roundness Measurement

#1
Hi All,

Hoping for a little help on the best route forward for a reliable roundness measurement.

We are producing two PC rings sample #1 & #2 (Dia both around: 14" (+/-0.01" Roundness: <0.016)
Measurement device: Micro VU + touch Probe (also done same study with CMM)
Roundness: 16 points. Least Square Circle method.

Sample #1 - Ring has Internal support
Ring placed on fixture, location is positioned in x,y

Diameter pass GRR
Roundness requires part to be measured 3x and average taken - Passes GRR

Sample #2 - No center support
Ring placed on fixture, location is positioned in x,y
Diameter passed GRR
Roundness measured 3x and average taken - Can not pass.


I believe that the variation, is caused due to product loading into fixture, and in-part-variation. Due to if I repeat the measurement without replacing the part the results are incredibly consistent.

I know the clear thing may be looking to fixate the part better but I am wary to restrain the part too much to effect the result and i feel the part is incredibly sensitive to touch and any forces applied on it.


How do other people measure roundness on flexible parts that have in-part-variation?
Is 16 points too little based on my diameters?


Any advise would truly be appreciated.

Thanks,
 
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John Predmore

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#2
You say flexible part. I don't know what PC is, polycarbonate? Without knowing anything about your part beyond what you said here, yes, fixturing a part or even the weight of the part could cause a measurable deflection. In your case, I think the part should be laid flat, unfixtured, on a flat surface and use non-contact methods. It sounds like that is where you are at. Sixteen points is a good number to start, but if the measurement is not repeatable, then more might be needed. If you get a repeatable measurement in repetitions without moving the part, but measurements vary once the part is moved, that seems to indicate the method is capable but roundness changes when the part is moved. How can you handle the part without the forces of handling the part? Can you float the part on a liquid, then drain the liquid, so the part comes to rest with no external forces causing deflection? Can you float the part into position on an air table?
 

Matt's Quality Handle

Involved In Discussions
#3
How is the part used in its intended application? As a matter of practice, on flexible parts, I would always recommend fixturing in a similar manner to actual use conditions.

With the disclaimer that the drawing must explicitly allow constraining for measurement purposes. IIRC, ASME 14.5 says that dimensions apply in the free state at 68 degrees F, unless otherwise noted.
 
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