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# Piezoelectric Rock Chisel

Fighting fire with fire. Only the fire is rock, and so's the other fire.
 (+3) [vote for, against]

Chiseling away at rock is something of an ancient pastime. The general process has remained the same since sphinxes were stylish additions to the built environment. You take a sharpened bit of metal, put it against some rock then bash the bit of metal with a heavy thing. If you're lucky, the tip will cause a localized fracture and a bit of rock will fall off.

If you get good at it, you may find that you can drive a chisel into the rock and split large sections off. This is often desirable as it's much less work. To enhance this effect I propose a chisel with an electrically actuated expanding/contracting tip. The idea is to narrow the tip, drive in the chisel and then expand the tip. A functional equivalent to how you get to the front at a busy bar.

To do this, the section just behind the very tip will be composed of two halves with piezoelectric crystals mounted between them. By applying a voltage to the crystals you can get them to change shape. This isn't a dramatic shift. Typically, it's around a 0.1% change in length. So a 10mm piezoelectric crystal stack might expand only 10 microns. This isn't much, but they generate huge forces. 30N/mm2 is normal, and this would give 30,000N or 3000kg for a 100x10mm strip in the middle of the chisel.

Now, how to control it. The cool thing about the piezoelectric actuators is that they're very fast. 30 microseconds or so for the shape change. Now the speed of sound in steel is about 4000m/s, that's not just noise, but the speed of the pressure wave from the hammer blow travelling down the chisel to the tip. That means we have over 50 microseconds to play with between hitting the thing and the force arriving at the tip-rock interface. If we can get the signal to the piezoelectric actuator in 20 microseconds we can narrow the chisel just before the pressure wave arrives.

So, the chisel is grounded the hammer is charged. Hammer hits the end of the chisel which is faced with a conductor connected to the piezoelectric actuator. The electrical signal and the force are delivered at the same time, but the electrical signal is way faster. The piezoelectric actuator narrows the chisel just before the pressure wave arrives and drives the chisel into the rock. When the hammer lifts off, the actuator changes to wide formation expanding the hole by a few microns. Since rock is very bad in tension and elasticity, you should be splitting off big chunks in no time.

 — bs0u0155, Nov 30 2017

Piezoelectric actuators https://www.piceram...cement-modes/#c2518
[bs0u0155, Nov 30 2017]

 //A functional equivalent to how you get to the front at a busy bar.//

Just what kind of a bar are we talkin' about here?
 — RayfordSteele, Nov 30 2017

Cold Chisels usually get ground and resharpened a few dozen times or more. How strong is the metal you're working with such that the piezo device doesn't become a part of the environment?
 — RayfordSteele, Nov 30 2017

Why not use shaped charges ?
 — 8th of 7, Nov 30 2017

 //Why not use shaped charges ?//

Works really well if you shape into something that has pointy teeth e.g mice, rabbits, gophers.
 — bigsleep, Nov 30 2017

No, they just do holes. Pretty much any size, but in the end, just holes.
 — 8th of 7, Nov 30 2017

 Hmm. So, the aim is to widen the chisel just after it has been driven into the rock?

I sort of get this, but chisels are normally wedge-shaped which surely accomplishes the same thing?
 — MaxwellBuchanan, Nov 30 2017

 //Why not use shaped charges ?//

Well, quite a lot of technology would be required to get explosive penetration of a rock face. In fact if an explosion were imminent, I think I would choose rock as one of the preferred things between me and it. I view drilling and explosives as very much complementary technologies. A drilled hole is dynamite's best friend when it comes to making a mess of a lot of rock.
 — bs0u0155, Nov 30 2017

 // quite a lot of technology would be required to get explosive penetration of a rock face //

 There speaks the voice of one with little or no direct personal experience of the use of high explosives.

This is not a criticism, merely a reasonable observation of a deficiency in your education and life experience, and one which, given the slightest opportunity, you would no doubt rectify with indecent haste.
 — 8th of 7, Nov 30 2017

I'm with [bs0] on this one. Miners, who blow up rocks professionally, always drill them first. I'm not saying HE is ineffectual (thinking of the Japanese bombardment of Corregidor and the RAF's grand slam bombs for U-boat pens) but, in peacetime conditions, when one's drill rig is moderately safe from RPGs, etc., it's for amateurs. Actually, in both the examples I quoted, the HE was backed by a fair amount of kinetic energy anyway. I'm not sure how much joy you'd get from a demolition charge that you'd just blu-tacked to a flat cliff face, or placed on the ground. But I daresay [8th] can enlighten me.
 — pertinax, Nov 30 2017

Piezoelectric crystals that are placed as facings on chisel- sides, being crystals, are likely to break when the chisel is hammered. Brittle, you see....
 — Vernon, Dec 01 2017

 // I'm not sure how much joy you'd get from a demolition charge that you'd just blu-tacked to a flat cliff face, or placed on the ground. //

 An immeasurable amount. Of course, it won't achieve very much other than making a tremendous amount of noise and mess and throwing stuff around, breaking glass,knocking things over, and generally upsetting people, but if you know a better definition of "joy" we'd be interested to know .

 Well, maybe not exactly joy ... more a sort of sociopathic ecstasy,blended with malicious glee and and atavistic fascination with destruction. But it's a very positive thing.

 // But I daresay [8th] can enlighten me. //

 Of course. Let's see ... microcrystalline pyrophoric magnesium powder ... thermite ... white phosphorus ... RFNA ... chlorine trifluoride ... er ... matches, matches, matches !

OK, ready. Where do you want to be enlightened first ? Any colour preferences ? We can add metal salts to change the colour ....
 — 8th of 7, Dec 01 2017

 //An immeasurable amount [of joy]. Of course, it won't achieve very much [...]//

 ... which nicely sums up "amateur", quid* est demoliendum.

*I refer of course to insurance premia.
 — pertinax, Dec 01 2017

 //Piezoelectric crystals that are placed as facings on chisel- sides, being crystals, are likely to break when the chisel is hammered. Brittle, you see....//

Steel remains the tip and sides. The piezos will form a core between two divided halves. The halves can be keyed together for longitudinal stability. The piezos are so tough you could weld a 1mm plate over them and they'd just make the plate bulge out. There's a few ways of doing it. I should underline that the crystals themselves take none or very little of the longitudinal load.
 — bs0u0155, Dec 01 2017

 Just use HMX.

 // which nicely sums up "amateur", //

 Not necessarily; it depends what you're trying to achieve.

If your objective is to randomly damage stuff and upset and frighten a lot of people, and you do that, how is it "amateur" ?
 — 8th of 7, Dec 01 2017

Because it's not a profession. More of a confession.
 — pertinax, Dec 01 2017

This is a scale thing. Want to get through rock, explosive. Want to carve a David , the chisel. Thing is, where you want a chisel, do you want extra uncontrolled force. Jack hammers already do extra force chiseling for the in-between need.
 — wjt, Dec 02 2017

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