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Wringing (wikipedia.org)
356 points by leetbulb on July 5, 2019 | hide | past | favorite | 77 comments

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

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.

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?

> 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.

> 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!

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

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

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?

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.

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.

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.

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

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

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.

If you (like me) read the entire article without really getting an intuition for these "gauge blocks", here they are in action:



This is a really nice video if you are interested in measurement precision for manufacturing:

Machine Thinking: Origins of Precision and first project introduction https://youtu.be/gNRnrn5DE58

The first is an AvE video, the one that introduced me to Wringing.

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)

While a lot of analog measurement tools have been replaced by digital ones, Gauge Blocks are still often used as the de facto reference(Though CMM machines[1] are becoming more and more common).

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. https://i.ytimg.com/vi/2d-hGd_kYLc/maxresdefault.jpg https://images-na.ssl-images-amazon.com/images/I/61RhwssKaBL... https://i.ytimg.com/vi/n572qMo3g0A/maxresdefault.jpg

The 1 Inch block always seems to go missing.

[1] https://en.wikipedia.org/wiki/Coordinate-measuring_machine

Fun video from Sixty Symbols explaining the underlying forces at work - https://www.youtube.com/watch?v=mgL_qH70KAU

I knew AvE's video would show up :D

I love this guy's units. 1/4 of a bee's dick is my favorite.

This reminds me of a phenomenon called cold welding, where if the surfaces are flat and clean enough, the blocks of metal will fuse. Just touching two sufficienly thin gold wires will weld them.


Almost all you'll ever need to know about gauge blocks [1]. It's also a fairly decent introduction to the uncertainties of measurements, and other factors of taking measurements of objects. Of course it also includes a section on wringing. (and other sources on papers about the wringing phenomenon)

[1] https://www.nist.gov/sites/default/files/documents/calibrati...

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. )

what would you consider to be the flattest off the shelf metal commodity component available today, would it be a typical hard drive platter or something else?

To play with this effect yourself:

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.

> 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?

If you're interested in how gauge blocks fit into the history of precision engineering, a new book called "The Perfectionists: How Precision Engineers Created the Modern World" puts them into context, and presents a very interesting perspective on the events and developments that brought us into modernity. These blocks feature strongly into the industrial revolution. I strongly recommend the book!


Or for the lazy, here's Destin https://www.youtube.com/watch?v=T-xMCFOwllE

I saw this book at the bookstore precisely one time. Made a note to look into it in the near future. And then was never able to find it again. Will have to look a bit harder maybe.

This got me wondering, (and I know it's probably not a physics question). If I scrape two fingers and press them together for a week. How do the skin cells know which other cells to attach to when healing? Or will I have one super finger?

They can grow together but will take longer than a week. There's a man the had his hand degloved and they sewed it into his stomach to heal, after which they cut his hand back out and used his stomach skin to wrap around and reshape some resemblance of a hand.

Example 1: https://www.thesun.co.uk/news/4303135/surgeons-save-hand-sew...

Example 2: https://www.menshealth.com/health/a19517212/carlos-mariotti-...

Wow. Can you imagine how that would feel, to have your hand inside of you? I'd worry about accidentally breaking something inside me. Amazing though.

This is the origin of the concept of a skin graft, surely.

Try it and let us know!

At our physics school we had some super flat metal blocks. Much flatter than gage blocks. We were warned never to put them together or we'd never get them apart.

These blocks were used for measurement, but I can't remember exactly how.

They’re probably surface plates. They’re used as a flat reference for measurement.

This is part of the reason why you commonly see cymbal players in orchestras use an oblique motion for a crash rather than a head-on direct motion.

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.

Are you sure that's not just the air pressure difference? Cymbals often have a shape with many concentric ripples, so you might need multiple holes to equalize the air pressure between each ripple. Wringing only works with extremely flat surfaces, and it works even in vacuum.

Yep, admit I was a little taken aback after the drilled holes explanation. This seems correct, as no practically priced cymbal could be so precisely made.

I don't think they need to be precise; any reduction in the natural vibration can be detrimental to the sound.

They never get "stuck" together, however, imnsho.

Yeah, I was saying that getting the surfaces to adhere sans vacuum seems unlikely for such an imprecise object

I was wondering about (what I presume to be) this effect the other day with regard to things sticking to my bench top that I hadn't moved in a few weeks. One was a glass jar and the other a plastic lid. The bench is oiled hardwood. I was thinking that it seemed related to the two coefficients of friction (static vs moving). That is, the static part of friction could well be this phenomena on a less sticky scale. My thoughts led me to how it could be an electron transfer going on that bonded the two materials on some way...

(This was all backyard conjecture of course)

So, many thanks for this post: it shows how much better thought has gone into the problem.

It’s less the risk of them getting stuck together than it is the fact that they sound better if you don’t create that partial vacuum.

I grew up next to an abandoned granite quarry. You could, and still can --though they are beginning rarer as the quarry is a bit of a tourist attraction, find super smooth, polished rocks. We would do this wringing with them all the time as kids. Never over until now did I wonder if anyone else had ever done this or if it had a proper name.

Really? It seems unlikely to find ones flat enough in such a non-precision environment to exhibit the same behavior. Maybe they were flat enough and cupped so that you could create a slight vacuum like you can with the palms of your hands? Would love a picture if you could get one.

I believe that the difference is our stones were wet. But also, very smooth. The blocks ship with one side polished smooth. Still being wet probabny helped.

Is this what makes clear plastic card sleeves stick together? I always figured it was static electricity but maybe it's this?

I suspect OP was brought here by this excellent video which enjoyed some success over the past couple days on the topic of flatness.


You can see a similar effect with ordinary glass plates --- the production process[1] is such that it normally has a flat enough surface that two pieces will naturally "stick" together if placed upon each other and there's nothing else between.

[1] https://en.wikipedia.org/wiki/Float_glass

The manufacture process is fascinating:


This takes me back to a summer job I had 35 years ago, calibrating a set of gauge pins. I think it was a couple hundred of them. Visually inspect them for corrosion or scratches, wipe them down with WD-40, measure each end and the middle with digital calipers, and log the measurements and any comment on their appearance. Honestly it probably to0k 2.5 days but it felt like forever.

I wonder how stable is such a "wring joint" over time and over mechanical stresses like temperature change and vibrations. Could you use this feature to e.g. attach stuff as a product assembly technique, as a temporary/non-destructively modifiable alternative to gluing or welding?

See "cold welding". As far as actual gauge blocks go, they're relatively easy to separate by sliding after you're done with your measurement or set-up. (Good Jo blocks are horrendously expensive. You wouldn't want them to be single-use items.)

Cold welded optical components were a part of my ph.d. thesis (someone else did the wring joint trick). As far as I know, the bits are still wring jointed together after 20 years of abuse.

The joined blocks can be pulled apart with you hands. See the first video posted in this comment:


So the bond is not comparable in strength to gluing or welding.

I watched both. I know it's not comparable, but it seems plenty strong. In the case of the video, I'd guess if you'd double the contact surface, you could easily hang a coat on it. My question is, how long would the join last.

FWIW the Starret tips on maintaining gauge blocks says not to leave them wrung overnight.

For about as long as the parts on both sides of the join stay flat. And therein lies the rub. Permanent cold welding works just fine on small contact areas, but on anything large you're at the mercy of thermal movement, etc. How much can that be? Quite a bit, actually [1]. That's just breathing on a braced precision straightedge, and the difference in temperature across the piece is easily measurable.


I feel like the article is missing a section on uses. Is it strictly measurement?

You can use them in concert with a Sine bar to create precise angles


Check out this alternative version of a sine bar https://observablehq.com/@jrus/sinebar

Also includes some nice youtube videos inline for context.

They're most often used as measurement references. You inspect parts on a surface plate (table or block that's very flat) using a dial test indicator on a rigid base that can be slid around. The indicator has a very short range of measurement (say, 0.01" for an indicator with 0.0001" graduations), so to inspect a part surface that's supposed to be 1", you'd wring together a 1" stack of gauge blocks on the surface plate, adjust your indicator up or down on the base so that its probe is in the middle of its travel on top of the stack, and zero the indicator face. Now you can measure 0.9950-1.0050"... just slide the indicator stand to your part and sweep the indicator probe along its surface to measure.

Guage blocks are for measuring things. The "wringing" thing has been used in optics for precision alignment for centuries, though nobody calls it that.

Edit: Oops - somehow missed the other post mentioning this book. Sorry!

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: https://bookmarks.reviews/reviews/the-perfectionists-how-pre...

and the audiobook as pretty good too.

Casimir forces are probably involved- gage blocks work with flatness/separation (tens of nanometers) that is too high for intermolecular attraction (hundreds of picometers). Casimir attraction works at longer distances, but it's far from well understood. It's most likely a combination of all the common guesses, although the air pressure one is pretty weak.

There's a great Sixty Symbols video about gauge blocks and how they work: https://www.youtube.com/watch?v=mgL_qH70KAU

This is such a beautiful HN post :) thanks!!

Isn’t this the same effect as the screen protectors we all have on our phones?

Who knew intermolecular forces could be so strong? This blew my mind.

Johannes Diderik van der Waals!

Flat suction cups.

Yep, that's what I always thought was the mechanism behind this action. Separating such precisely contacting surfaces creates vacuum, resisting the separating force.

The Cody's lab video linked above demonstrates that they don't fall apart in a vacuum.

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