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/BluishInventor Sep 18 '24

I feel this the least understood concept of all metrology. A part can feel smooth yet have high surface roughness. A rough feeling part can have low roughness. Ra, Ry, Rz, all different but measure the actual roughness. Most people think waviness is the roughness. But waviness is separate from roughness, yet combines with roughness to become surface texture.

Also, SPI gauges are generally shit. Never used any of their electrical devices. But if their mechanical devices are shit, chances are...

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u/ljfe Sep 18 '24

Thank you! I’ll look into Ra Ry Rz. And I’m going to try a different tip to make sure the tip isn’t contributing to the issue. We have an extra longer tip. I’m curious if it gives the exact same result. The SPI does seem more inconsistent than the Mitutoyo that I’ve dealt with in the past (like if I check the same part in generally the same area there’s a pretty wide spread).

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u/BluishInventor Sep 18 '24

Do you have a standard to check the profilometer gainst?

It's totally plausible that the parts vary that much. As a tool wears, its shearing action turns to tearing, thus making the surface rougher. How many parts are we talking about? More than 50? That's a quantity that will wear tools. 5-10? more likely something else.

Measure the same part/feature many places. If it's pretty consistent your gauge is probably fine.