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Old 2020-07-22, 02:07   #2784
Dr Sardonicus
 
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Curiously, the tests were done with angle grinders, drills, and water jets -- not abrasive cutting wheels. From the descriptions I've read of how the new material resists being cut, I'd guess the stuff might ablate a gritty cutting blade and wear down an abrasive cutting wheel faster than most materials would. For the purpose of cutting a lock to steal a bicycle, a "nice July-4th-worthy shower of sparks" might be contraindicated.

Silicon carbide (abrasive trade name, Carborundum) is IIRC harder than corundum (aluminum oxide) and only just less hard than diamond. Carborundum is quite brittle and easily pulverized.

Corundum occurs naturally, sometimes in large crystals with slight traces of impurities that impart color (red for rubies and usually blue for sapphires). Silicon carbide only occurs naturally in tiny amounts, generally as the result of meteorite impacts. The mineral is known as Moissanite, in honor of the chemist who discovered it. It has become possible to create large flawless crystals, which have refractive index and dispersion comparable to diamond, but at a much lower cost.

Another hard material used in implements of destruction is tungsten carbide. I saw a rather impressive such implement being used recently to grind down woody plants. It was a roller with big teeth on an arm, affixed to the front end of a skid-steer. It was raised high, then lowered, easily grinding the vegetation to mulch. (I think there are larger versions.) I talked to one of the workers who was preparing a chain saw to deal with something too big for the roller they were using, which, he mentioned, had chewed up a manhole cover on one of their jobs. I have occasionally mused about the receiving end of the resulting phone call:

"Yes, this is XYZ Communications.

"You're where? OK, and -- why do we need to replace the manhole cover?

"Your machine did what?"
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Old 2020-07-22, 04:06   #2785
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Quote:
Originally Posted by Dr Sardonicus View Post
Another hard material used in implements of destruction is tungsten carbide. I saw a rather impressive such implement being used recently to grind down woody plants. It was a roller with big teeth on an arm, affixed to the front end of a skid-steer. It was raised high, then lowered, easily grinding the vegetation to mulch. (I think there are larger versions.)
Those are basically the same tips used to grind an asphalt street (Macadam) for resurfacing. Yes there are larger machines. Seeing them work causes Al Gore to have an arrhythmia and Greta Thuenberg to cry.
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Old 2020-07-22, 23:27   #2786
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The reason SiC is better than just about anything else for contractor-style cutting blades is that, at least in industrial-grades, is very cheap to synthesize. Tungsten carbide is quasi-metallic once sintered from raw power and less brittle, but relatively expensive - that's why it's typically used just in the teeth of steel cutting implements.

Re. the shower of sparks - sure, one can use a slower cutting technique such as the water-lubricated cutting typically used to cut ceramics - but ceramics need that because they don't take localized heating well. Kryptonite-lock-style steel, OTOH, speed of cutting is more or less proportional to degree of local heating - the hotter the better, as long as the outer rim of the cutting wheel can take the stress. BTW, angle grinders fitted with SiC cutting wheels are excellent cutting implements - better than my circular saw becuase more versatile. Re. "cheap to synthesize", I see Harbor Freight selling a 10-pack of 4in-dia SiC cut-off wheels for a mere $10 - so bring on that shower of sparks, if it's saving you precious time. (Though this particular 10-pack is "for ceramic" - but I bet it slices through a K-lock just fine.)
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Old 2020-07-23, 23:02   #2787
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Update on Betelgeuse's unprecedented dimming and what it may portend. Please, please let us be among the humans privileged to experience a naked-eye supernova.
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Old 2020-07-23, 23:51   #2788
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Be careful what you wish for........
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Old 2020-07-24, 02:17   #2789
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Originally Posted by ewmayer View Post
Update on Betelgeuse's unprecedented dimming and what it may portend. Please, please let us be among the humans privileged to experience a naked-eye supernova.
That article is seriously out of date. Betelgeuse is now pretty much back to normal, AFAIK, and began returning to it at close to the expected date.

Direct imaging of the photosphere showed what looked like either an enormous starspot or, more likely, a dust cloud. IIRC, the total energy output of the star didn't change very much, it was just shifted out of the visible into the infrared.
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Old 2020-08-02, 08:08   #2791
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Did a supernova trigger the late Devonian extinction?
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