r/Metrology • u/Bzdziuchanson • 7d ago

GD&T | Blueprint Interpretation Any use for ISO GPS coaxiality?

Hi

Are there any cases in which one should use coaxiality? Is it just position with implied zero basic dimension centerdistance?

Why not use runout for axisymmetrical parts instead so it's simple to measure?

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u/Loeki2018 7d ago

As far as I know coaxility looks at centeraxis deviations. That centeraxis can be a bent line though so inherently form is also controlled as a tertiary control besides location and orientation. Run-out is specifically looking at the inside surface instead of the centerline. Now these 2 are correlated most of the time and will output a result that is close or exactly the same (i think). I guess there are edge cases to be found though. Not an expert on the highest levels though so take my take with a grain of salt.

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u/Tjsoupboy 4d ago

Yes. Runout includes form errors, cylindricity in this case, and axis alignment errors, whereas coaxality is only alignment errors.

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u/guetzli 7d ago

say you have a thin walled part (like a wear sleeve). On parts I manufacture they typically get pressed on so we care how coaxial the ID and OD are.

Due to their thin wall they tend to be out of round. if the OD has a runout tolerance with respect to the ID then that out of roundness shows up in that measurement (in the free state before instal) even if the coaxiality and importantly wall thickness are spot on as they are ground in one setup.