| There's something at play that causes precision ground surfaces to stick together. Oil certainly plays a part, but there's a point that when two surfaces are flat enough, they'll stick hard in place, and there's a "crack" moment when twisting them apart. Around a decade ago, I worked in a machine shop.
Mill table surfaces are ground flat. Not precision surface flat, but good enough to be a reference for cutting tools. Mill vise clamping surfaces are ground flat. It's normal for an object that's at least cut (not even ground) flat on one side to get hydraulically suctioned via cutting oil to either of these. It's usually easy to slide the part off to the edge of the surface. Very flat surfaces do this as well, but with a little more encouragement to work out oil from between parts, they'll start to stick in place. The more finely ground and flat each side is, the more pronounced the sticking moment will be. Extremely finely surfaced blocks being wrung together will frustrate you by sticking before you have them lined up the way you want them. This made me suspicious of the role that surface tension and vacuum played. Hydraulically stuck things are easily separated by a quick blast of compressed air. Wrung blocks aren't as easily separated by a blast of air. I tried to clean blocks as devoid of liquid as possible and wrung them together. They still stick, but it's not as secure, and they come straight apart the moment they're twisted apart. After this, it seemed that wringing blocks together worked best with a trace of oil. Brief cleaning with a dry rag does leave a trace, and if the surfaces are flat enough, perhaps that trace is enough to fill some microscopic voids between flat-ground surfaces. Twisting blocks together encourages entrapped air to escape, and can shuffle trace oil into voids. The solution I came up with is that the metal of the two blocks does stick together somehow once it's in contact, and the surface tension provided by a trace of oil contributes additional sticking force where the surfaces don't meet. We didn't have ceramic blocks, though. It might be an interesting additional experiment to try this with mixed materials. Does wringing together ceramic and steel precision surfaces work the same way? Should it? What would that mean? |
Like you say, this is a hydraulic/air pressure effect- it's related to why you can float things on a reference plane: https://www.youtube.com/watch?v=Kj6jmQxZe8s
> Twisting blocks together encourages entrapped air to escape, and can shuffle trace oil into voids. The solution I came up with is that the metal of the two blocks does stick together somehow once it's in contact, and the surface tension provided by a trace of oil contributes additional sticking force where the surfaces don't meet.
Basically right, but the dominant theory is that the twisting/sliding motion creates vacuums. First a sealed pocket forms by bringing asperities close together so that they are attracted by stronger forces. Then as the blocks slide, they stretch out the voids and cause the pressure inside to go below atmospheric. I'm kind of skeptical of it, but the sliding does definitely prevent anything additional from being trapped between, and makes the oil film as thin as possible.
Additionally, there is an oil film on literally everything. It takes really serious equipment, like plasma chambers, to actually remove the thinnest layer of oil from a material. Oil just floats around the air constantly, and bonds like glue to basically everything: https://youtu.be/atVSxvbiPg0?t=39
> We didn't have ceramic blocks, though. It might be an interesting additional experiment to try this with mixed materials. Does wringing together ceramic and steel precision surfaces work the same way? Should it? What would that mean?
Indeed they do: https://youtu.be/_YVWdxr0E_g?t=155
It mostly just indicates that intermolecular forces don't have much to do with it. There's no real reason they'd want to stick together- the bonds in the ceramic are extremely tight and ordered. Ceramic blocks also wring more tightly than steel blocks, but shouldn't experience high intermolecular forces between each other.