Wringing • Hacker News Board

Re: Wringing

#71

Post by sirsar » Sat, Jul 06, 2019, 2:33 PM UTC

hwillis wrote: post #52

nabilhat wrote: post #51

Earlier quoted context omitted.

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. 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=Kj6jmQx…

> 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!

Re: Wringing

#72

Post by bhaak » Sat, Jul 06, 2019, 2:34 PM UTC

hwillis wrote: post #43

Bizarro wrote: post #11

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 grea…

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…

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.

Re: Wringing

#73

Post by Gracana » Sat, Jul 06, 2019, 3:36 PM UTC

kohtatsu wrote: post #15

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

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.

Re: Wringing

#74

Post by dredmorbius » Sat, Jul 06, 2019, 5:51 PM UTC

userbinator wrote: post #48

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:

https://youtube.com/watch?v=ig4G5WbOMLc

Re: Wringing

#75

Post by mietek » Sat, Jul 06, 2019, 6:17 PM UTC

robocat wrote: post #42

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?

Re: Wringing

#76

Post by mveety » Sat, Jul 06, 2019, 6:53 PM UTC

emmelaich wrote: post #54

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.

Re: Wringing

#77

Post by hwillis » Sat, Jul 06, 2019, 8:18 PM UTC

bhaak wrote: post #72

hwillis wrote: post #43

Earlier quoted context omitted.

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…

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.

Re: Wringing

#78

Post by pard68 » Wed, Jul 10, 2019, 12:34 PM UTC

defterGoose wrote: post #45

pard68 wrote: post #33

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.