Surface Finish Stylus To Be Used

[beavers]

Hi,

We have to maintain an ISO spec of 20 uinch for the finish on a steel part .5" long, .062" diameter.

We had lengthy discussions on the interpretation of this spec. When the steel exhibits minor surface porosity, the measurement is greatly influenced by the type of stylus used on the surface tester.
With a point-style stylus the measurement is above the 20 uinch spec (the point will go into the small pore if care is taken to position the part with the pore towards the point). With a blade-style stylus the measurement is below the max spec of 20, even when the stylus travels on the porous section.

Should porosity be considered in measuring a surface finish? Or surface finish is strictly the finish obtained from a machining operation, regardless of porosity and inclusions in the material.

Any help is appreciated.

Thank you.
Sorin Seruna

 

[IRstuff]

Is the 20uin requirement peak to valley, or rms?



TTFN

 

[beavers]

It's Ra.

Sorin Seruna

 

[GregLocock]

I'd say it depends on the function of the part. If the porosity affects the function of this surface then it should be included in the roughness measurement.

In fact I stuggle to think of a justificiation for not including it.

So far as stylus shape goes, that should have been specified as part of the inspection standard. If it hasn't been specified and you are following industry practice then I guess you can stick with the blade, but if that component causes problems down the track then I think you'd be on shaky ground.

Cheers

Greg Locock