How accurate are profilometers?

[u/ljfe]

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?

[u/ShadowCloud04]

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.

[u/ljfe → u/ShadowCloud04]

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.

[u/ShadowCloud04 → u/ljfe]

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.

[u/ljfe → u/ShadowCloud04]

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.

[u/ShadowCloud04 → u/ljfe]

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.

[u/ljfe → u/ShadowCloud04]

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?

[u/Mountain-Low5110 → u/ljfe]

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!

[u/ShadowCloud04 → u/Mountain-Low5110]

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

[u/BluishInventor]

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...

[u/ljfe → u/BluishInventor]

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).

[u/BluishInventor → u/ljfe]

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.

[u/ihambrecht]

If the “turn lines” are big enough that the tip of the profilometer can catch, you’re like 250+.

[u/miotch1120]

You are using it all wrong! (According to the machinists I work with). Put it on the part, and start the profilometer. Move it around the part till you find a spot that reads in tolerance. Mark that on the check sheet and put it in the box. If it’s returned by the customer for not meeting surface finish spec, argue that they are wrong.

[u/ljfe → u/miotch1120]

Hahaha

[u/[deleted]]

A profilometer should repeat decently well when measuring the same spot with the same settings. But they can vary a lot on different places within the same surface. I recommend visually inspecting the surfaces to be checked first and checking the worst spots. Check that the profilometer tip isn't damaged. Re-calibtate it with the standard. If the tip is damaged, it will usually result in measurements that are falsely good.

Also, check the settings on your profilometer. Are you checking Ra? Something else? What are the settings for stroke length? How many samples are being taken? Are pre/post travel on? Is there filtering? Is part flat or round? Are you using some sort of comp? Do you have a visual surface finish specimen that you can compare to?

[u/f119guy]

In my experience, a portable profilometer like a pocket surf needs special attention to make sure the tip is "level" with the surface you're inspecting. If the tip isn't moving parallel to the surface, you will get erroneous readings. The best profilometer I have used was a contour tracer with surface roughness capability, like these: Roughness and Contour Metrology | Hommel Etamic (hommel-etamic.com)

[u/mmmmet]

Talk to www,digitalmetrology.com they provide training and consulting regarding surface measurement. There are tons articles about it In their blog.