[Bizarro]

Great stuff. We need more of these types of science tidbits on HN. And good links by the commenters too.

I love Feynman's classic style of "explaining to a 5 year old" on Cold Welding.

The reason for this unexpected behavior is that when the atoms in contact are all of the same kind, there is no way for the atoms to “know” that they are in different pieces of copper. When there are other atoms, in the oxides and greases and more complicated thin surface layers of contaminants in between, the atoms “know” when they are not on the same part.

— Richard Feynman, The Feynman Lectures, 12–2 Friction

[hwillis]

Note that this is a big oversimplification- it doesn't apply to gage blocks. Cold welding and similar phenomenon can only truly happen with metallic bonds and very large grain sizes. Non-metallic bonds are structured and require precise orientations and conditions to reform. Metals are still structured -metals have crystal grains- and the microscale structure needs to line up in order to reform. In cold welding, you force that to happen with high pressures.

This doesn't really apply to gage blocks because tool steel has passivated surface oxides and complex crystal structures. There are very few metallic bonds exposed on its surface. It also has very nasty, often needle-like grains. There's no chance that the metal atoms are being attracted to each other. Not to mention that wringing works with ceramic gage blocks as well.

It is probably a complicated combination of effects. Adhesion (intermolecular forces) probably plays a minor effect at small regions where the surfaces are extremely close together. Casimir forces probably have an effect on most of the surface. Small amounts of grease probably form much stronger adhesions by "carrying" the forces between the surface oxides. Vacuum being trapped by grease probably plays a fairly large part in rougher blocks wringing in an atmosphere. Adhesion and Casimir forces are not well understood.

[nabilhat]

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?

[hwillis]

> 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.

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.

[sirsar]

> the dominant theory is that the twisting/sliding motion creates vacuums.

Unfortunately for this theory, gauge blocks will remain wrung together even in a vacuum!

[SiempreViernes]

> Indeed they do: https://youtu.be/_YVWdxr0E_g?t=155

What a fine example of the white labcoat as an authority prop!

[IgorPartola]

Wouldn’t it be fairly easy to eliminate water/oil and air from an experiment to see how it affects wringing? Air: try to wring the blocks in a vacuum or at high altitude to see if it is weaker/stronger than at sea level. Water/oil: clean the surfaces and do the experiment in a really dry environment.

Does wringing work with glass blocks? If so, could we then examine the interface with some type of instrument? Could we X-ray the ceramic blocks interface?

[bhaak]

Wringing doesn't work because of cold welding but apparently some gauge blocks are also subject to cold welding.

The German wikipedia article claims that you shouldn't let gauge blocks stick together for longer than 8 hours at a time as they are prone to cold welding.

[hwillis]

That's likely a myth. Even in space, cold welding doesn't happen under static loading: http://esmat.esa.int/Publications/Published_papers/STM-279.p...

It requires impact of fretting, and even then there's a lot of debate over whether it's really cold welding or just more normal galling etc.

[semi-extrinsic]

Well yes, wringing and cold welding are distinct phenomena. Breaking a cold weld will make the surface ugly, but getting two wrung gage blocks apart will not.

You are talking about wringing, Feynman was talking about cold welding.

[rhizome]

Now put it in terms a 5 year old can understand.

[rkagerer]

Here's one I posted today showing how tunnel boring machines work. More civil engineering than science, but I found it quite fascinating: https://news.ycombinator.com/item?id=20364474

[dang]

We've put that in the second-chance pool (described at https://news.ycombinator.com/item?id=11662380) so it will get a random placement on the front page.