Stick slip phenomenon in CMM

[Cos_BO]

Dear all,

I am currently trying to CMM the surface of a part by scanning with a probe. This part has a layer of machined Kapton on top. I have measured parts like this before, and had always thought the strange wave-like feature I observe was caused by machining errors, but now that we have received a similar part in aluminium with the same CNC program I start to doubt if it is machining problems, since the wave-like features are completely gone.

I expect the tribology between the ruby and kapton might be causing the probe to stick and slip, alternating from a lower measurement (slip) to a higher measurement height (stick) to a lower measurement again.
Is there a way to check whether this is the case?

Other people who have experience with scanning polyimide, or have had issues with stick slip?

Thanks,
Ben

[JEFMAN]

I would say that Kapton is too hard to give this kind of problems.
Maybe you could try to scan point to point with a high density, and compare.
The problem could come from the difference between machining aluminium and machining Kapton ?

[Cos_BO]

I think it is the machining of Kapton indeed, since scanning both ways, the waves are too consistent locationwise and the " pitch" of the waves is a bit too large.
Could try a high density measurement, thanks for the tip.

[Matthew D. Hoedeman]

is your probe perpendicular to the surface? If so, then, yeah, gonna get screwy data when doing that.

[Cos_BO]

I might try to place the part under an angle and scan the surface. The kapton seems to be 0.5mm (0.02") thick after machining.

[louisd]

If the kapton is adhered, even being 0.02" thick, it still might be thin enough to deflect with probe force and contribute to your wavy form error. Also if it's adhered, look into the process used to apply the adhesive (is the adhesive sprayed or brushed on? --Or is the kapton adhesive-backed from manufacturer?)

You could be measuring orange peel or brush marks of the adhesive.

[Cos_BO]

The Kapton is first added to the surface and afterwards machined into the correct shape so this should normalize the adhesive layer. However, perhaps during machining this adhesive layer is compressed? I will ask the manufacturer what kind of process is used for the machining of the Kapton!

[BKulpa]

Not familiar with this material, but can you put a film of lubricant on the surface to stop the slip / stick? Calibration techs do this on the calibration sphere when doing the scan test to lower the resistance.
What are you using for filters?

[Cos_BO]

The surface is used in the optics industries and all measurements are in done in cleanroom. So lubrications are unwanted in my situation. Non the less good advice though!