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
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.
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.
Unfortunately for this theory, gauge blocks will remain wrung together even in a vacuum!
What a fine example of the white labcoat as an authority prop!
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?
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.
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.
You are talking about wringing, Feynman was talking about cold welding.
Origins of Precision and first project introduction
There's also a Cody's Lab video where he sees if they will wring in a vacuum: https://www.youtube.com/watch?v=jNEvS_bjKIo (using AvE's blocks)
You have a 'Master Set' of expensive Gauge Blocks that you use as a definite reference. Stacks of which can be combined with various tools for various purposes.
The 1 Inch block always seems to go missing.
Further more, if you're really interested in how modern precision engineering/industrial manufacturing came to be; I highly highly recommend you look into Foundations of Mechanical Accuracy by Moore. (it's a $150 book, pdf easily found by googling.)
(I'm a flatness/metrology nerd. I recently lapped three cast iron plates together just because I wanted to create as flat a surface as I could. Just because. )
1. pull apart an old multi-platter hard drive
2. Take the platters out
The platters are very flat, and they stick together remarkably well without any liquid between the surfaces. Specks of dust will stop them sticking together so well.
Be careful not to breath in any dust (or eat anything!) from the drive, as they do contain some exciting elements.
Can you expand on this? Which elements?
Example 1: https://www.thesun.co.uk/news/4303135/surgeons-save-hand-sew...
Example 2: https://www.menshealth.com/health/a19517212/carlos-mariotti-...
These blocks were used for measurement, but I can't remember exactly how.
Bringing two symbols together directly along their axis of rotation risks getting them stuck together like suction cups--the crazy part is that even when holes are drilled to release the near vaccum between two stuck cymbals, they remain glued together by the forces described in this article.
They never get "stuck" together, however, imnsho.
(This was all backyard conjecture of course)
So, many thanks for this post: it shows how much better thought has gone into the problem.
Super interesting stuff.
So the bond is not comparable in strength to gluing or welding.
Also includes some nice youtube videos inline for context.
These are covered with a charming personal story by Simon Winchester in his book The Perfectionists: How Precision Engineers Created the Modern World
It's reviewed here:
and the audiobook as pretty good too.