How does polishing affect the friction between two sliding metal surfaces?

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Offline Alltracturbo

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All references to "friction" in this post are referring to sliding friction.

I am trying to figure out how polishing flat metal surfaces affects the sliding friction between them. Most of what I read online states that the finer you polish two flat surfaces the less friction there will be between them.

Then I read this statement, newbielink: [nonactive]
When you slide two extremely flat and highly polished metal surfaces together, molecular attraction adds to the coefficient of friction. In other words, the coefficient decreases as you reduce roughness up to a certain point where it then increases due to the molecular effects.

And this statement, newbielink: [nonactive]
If the surfaces that are in contact are rough, there is more friction. Hence to reduce friction, the surfaces in contact are kept smooth and polished. It is worth noting that if the two surfaces in contact are of the same metal, the friction increases on polishing the two surfaces. This is why the bearings and the axle are made of different metals.

I don't know if those two quotes are referring to the same effect, but I am trying to find a more detailed explination of this effect and at what point or level of polishing the metal surface will this increase in friction start.

Here is an example of where I have experianced this. I cut my own hair and I use the electronic clippers that they use when you go to get your hair cut if they do not use scissors. The clippers work by sliding two flat metal plates back and forth on top of eacher and they have teeth or blades that cut the hair. When they get dull, you just have to grind the two flat mating sides on a sharpening stone to make them flat again and they will cut.

I decided to sharpen the clippers and I ground them as needed and then I proceeded to polish them. I figured polishing the sliding surfaces would only reduce friction and make them work better. I brought them to a mirror finish at 0.3 micron and to my surprise when I put them back together they would not work. There was too much friction between the plates and they would not slide on eachother. Oiling the plates would help, but they would need to be oiled before evey hair cut to work. I ended up having to grind the metal back down to about 600 grit for them to work.

Can anyone give some more detail on exactly what is happening. Is there a known point (grit/ micron) where the metal will have the least ammount of friction? Is this point different for different metals?


Offline CliffordK

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Two pieces of glass can stick together to the point where they almost can not be separated. 

You likely would do ok with one surface polished, and the other slightly roughened. 

Air makes a good "lubricant" for a number of processes.  If you can exclude the air, and get a good seal, I think you are right that you would experience intermolecular bonds between the two polished surfaces, and they would not move well.