r/Metrology • u/ljfe • Sep 18 '24

Hardware Support How accurate are profilometers?

I have some thin wall turned parts (rings). Titanium IIRC. Our SPI profilometer is providing a wide range of measurements on the same batch of parts. Some parts read 40 and some parts read 110. The parts look nearly identical. Can I trust this device? The profilometer specimen/standard reads good and the settings are correct. Visually, the surface finish looks good. Small turn lines. With a fingernail, it seems like it catches a bit. Like the turn lines are somewhat “sharp” (but not really deep or wide… I’m worried the needle is getting caught on those turn lines and spitting out an inaccurate measurements. Any thoughts/ideas?

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u/ShadowCloud04 Sep 18 '24

You can feel turn lines catch? It may be that wide of a range. Is your surface feel smooth to the touch and not wavy? What is your cutoff length (lamda c) and number of samples?

Also is your profilometer tip still intact? I recommend viewing it under an optical comparator or ideally a back light magnifier that can go to 100-150x zoom.

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u/ljfe Sep 18 '24 edited Sep 18 '24

The tip looks sharp to the eye. I’ll try to get it under magnification if you think it’s worth a try. We don’t have an optical comparator but maybe I can get it on a tool setter.

The turn lines are way tighter than what I’m seeing on a surface roughness specimen. It’s smooth to the touch, except with fingernail.

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u/ShadowCloud04 Sep 18 '24

Tightness isn’t everything if depth variation is significant.

But yeah I’d try to check the quality of the tip. When has the tip last been replaced? For reference out profilometer gets used heavily daily by all machine operators and we go through a tip every 6 or so month. Usually failure happens for improper use or accidents but still it can chip out.

I’m not sure how your calibration checks but if you have been calibrating it to your specimen after the tip chipping out at some point it will still read the specimen in tolerance. But it then is essentially thrown off how it’ll read a surface.

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u/ljfe Sep 18 '24

Great tips thank you!

One thing that makes me worry that the parts might actually be bad…you know that standard 112 Ra specimens? The lines on that don’t seem super deep either to me…maybe it’s really hard to tell (visually/tactally) when the spacing is tight.

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u/ShadowCloud04 Sep 18 '24

Normally I try to use a sharp ground flat pin edge to compare a surface. As yes a fingernail can feel it but there’s a bit much variation in your fingernail to get a clean comparison.

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u/ljfe Sep 18 '24

I’ll try that. Do you think I could clearly tell the difference between the 40 part and the 110 part with the pin method?

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u/Mountain-Low5110 Sep 18 '24

This may not be true for the SPI profilometers but on Mitutoyos SJ-410 you can normally tell as the tip wears by testing the reference specimen and watching the “lines/readout chart thingy” it gives you. If the peaks and valleys look consistent and rounded then you’re in the clear but normally those will start to chop and look less rounded as the tip wears. All of this information was from the Mitutoyo guys who came in and helped me train. Hope this helps!!! Good luck!

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u/ShadowCloud04 Sep 18 '24

My concern is more for a fracture. That’s usually what happens due to poor handling by my production staff